Tectono-Stratigraphy of Upper Carboniferous to Triassic

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Tectono-stratigraphy of Upper Carboniferous to Triassic

Successions and Petroleum Geology of the Khorat Plateau

Basin, Indochina Block, Northeastern Thailand

January 2019

Tomonari MINEZAKI

Tectono-stratigraphy of Upper Carboniferous to Triassic

Successions and Petroleum Geology of the Khorat Plateau

Basin, Indochina Block, Northeastern Thailand

A Dissertation Submitted to the Graduate School of Life and Environmental Sciences, the University of Tsukuba in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Science (Doctoral Program in Earth Evolution Sciences)

Tomonari MINEZAKI

Contents

Page

ABSTRACT…………………………………………………………… iv

List of Figures…………………………………………………………. v

List of Table…………………………………………………………… vii

1. Introduction……………………………………………………….. 1

2. Upper Paleozoic and lower Mesozoic stratigraphy of the Khorat

Plateau Basin……………………..……………………………….. 9

2.1. General stratigraphy…………………………………………. . 9

2.2. Upper Carboniferous Si That Formation……………………... 13

2.3. Lower to Middle Permian Pha Nok Khao Formation………… 14

2.4. Upper Triassic Kuchinarai Group (Huai Hin Lat Formation).. .. 15

2.5. Upper Jurassic to Early Cretaceous Khorat Group……………. 16

3. Structural interpretation of seismic sections………………….. .. 18

3.1. Seismic survey and well data………………………………..... 18

3.2. Stratigraphic cross sections through the Khorat Plateau Basin 19

3.2.1. Representative cross sections in the basin…………………….. 21

3.2.2. Seismic profiles in the northern part of the basin…………….. 24

3.3. Structural mapping of subsurface horizons…………..………. 28

3.3.1. Structural mapping technique………………………………… 28

3.3.2. Top of basement map…………………………………………. 28

3.3.3. Base of the Lower Nam Phong Formation……………………. 29

3.3.4. Isopach map of the Upper Paleozoic section…………………. 30

4. Tectono-stratigraphy and structural elements of Indochina Block 34

5. Discussion for tectono-stratigraphy of Late Paleozoic to

Mesozoic rocks…………………….………………………..…….. 39

5.1. Structural elements and tectono-stratigraphic evolution of

the Khorat Plateau Basin……………………………….…….. 39

5.1.1. Basin rifting during the Late Paleozoic…………………….… 39

5.1.2. Indosinian I event during the Triassic and later tectonic events 40

5.2. Early Carboniferous to middle Cretaceous paleogeography of

the region surrounding the Khorat Plateau Basin……………. 43

5.2.1. Tectonic units in northern Thailand……………….…………. 43

5.2.2. Late Carboniferous to Middle Permian…………………….. 44

5.2.3. Late Permian to Late Triassic……………………………...... 45

5.2.4. Jurassic to Middle Cretaceous……………………………….. 46

5.2.5. Summary of the Late Carboniferous to middle Cretaceous history

of the Khorat Plateau Basin………………………………….. 47

6. Petroleum Geology of the Khorat Plateau Basin………………. 53

6.1. Petroleum geology and oil & gas industry in Thailand…...... 53

6.2. Petroleum systems in the Khorat Plateau Basin……………… 56

6.2.1. Petroleum systems………………………………………….. 56

6.2.2. Exploration and producing gas fields in the Khorat Plateau 58

6.2.3. Reservoirs………………………………………………….. 62

6.2.4. Source Rocks and maturation……………………………… 63

6.2.5. Migration…………………………………………………… 70

6.2.6. Trap…………………………………………………...... 71

6.2.7. Seal and preservation………………………………………. 71

6.3. Summary of petroleum geology…………………………... 73

7. Conclusions……………………………………………………... 75

Acknowledgements…………………………………………………… . 76

References……………………………………………………………... 78

Appendix (Geological description of the outcrops in the Khorat Plateau Basin)

iii

ABSTRACT

The late Paleozoic and early Mesozoic Indosinian Orogeny in Southeast Asia was characterized by subduction in the Paleo-Tethys and collisions among continental blocks and fragments. The Khorat Plateau Basin in northeastern Thailand covers much of the Indochina Block and was subjected to complex tectonic activity during the late

Paleozoic and Mesozoic. The Mesozoic sequence of the basin comprises thick, gently folded non-marine sediments that include red beds and is separated from the underlying upper Paleozoic sequence by the major angular unconformity of the Permo-

Triassic boundary, which we refer to as the “Indosinian I event” in the Khorat Plateau

Basin. Seismic and well data acquired for oil and gas exploration indicate that the upper Paleozoic sequence is heavily faulted and structurally complex. This study showed that in some areas about 2000 m of the upper Paleozoic sequence has been eroded at the Permo-Triassic boundary, although thicknesses of up to ~6000 m of upper

Paleozoic rocks remain in some areas. Previous interpretations of oil and gas exploration data attribute the erosional event to back-arc closure due to collision of the

Indochina and Sibumasu blocks during subduction in the Paleo-Tethys ocean.

However, the detailed structural mapping indicated that the Indosinian I unconformity was a consequence of the collision of the Indochina and South China blocks during the late Permian to Middle Triassic. It was considered that this collision initiated the

Indosinian Orogeny in mainland Southeast Asia.

At present there are two commercial gas fields producing gas from Permian carbonates in the extensive basin. It was considered by the evaluation of petroleum geology from viewpoints of reservoirs, source rocks, seals, migration and traps. As for the source rocks of these gases, it was assumed that there are mainly two potential source rocks, the

iv shallow marine shales behind carbonate reefs of late Paleozoic and the lacustrine shales in continental areas of the Triassic. Considering about petroleum systems in the basin, the shallow marine shales of the late Paleozoic much more contributed to the gas reservoirs.

Because it occurred the structural deformation many times during the Mesozoic to recent, it was considered the destruction of traps and leakage of hydrocarbon to the surface. The preservation for gas reservoirs is a critical issue for petroleum exploration in the Khorat

Plateau Basin. As for further exploration success, it is important to recognize the Permian carbonate reservoirs accurately by improving the seismic resolution acquiring 3D seismic survey.

Key words

Thailand, Khorat Plateau, Paleo-Tethys ocean, Indochina Block, Permian carbonates,

Indosinian Orogeny, Petroleum geology

List of Figures

Page Fig. 1. Major tectonic subdivisions of mainland Southeast Asia and late

Paleozoic and Mesozoic sedimentary basins…………….. 4

Fig. 2 Major tectonic elements of mainland Southeast Asia overlapping

the satellite image………………………………………… 5

Fig. 3 Tectonic subdivisions of the region surrounding the Khorat Plateau

Basin in the Indochina Block…………………………….. 6

Fig. 4 Map showing locations of selected exploration wells, gas fields,

lithostratigraphic correlation of seven wells, stratigraphic cross

sections, and seismic lines………………………………… 7

Fig. 5 Tectono-stratigraphic column for Paleozoic and Mesozoic rocks

of northeast Thailand…………………………………..…. 8

Fig. 6 Lithostratigraphic correlation of wells that penetrated the Upper

Paleozoic and Mesozoic rocks beneath the Khorat Plateau 12

Fig. 7 Tectonic elements in the Khorat Plateau Basin (Upper Paleozoic)

identified from seismic profiles and well data…………… 20

Fig. 8 Stratigraphic cross sections across the Khorat Plateau Basin

(a) Representative cross section B–B’ is ~400 km long and extends

eastward from an outcrop west of the Khorat Plateau to

the central area of the basin and then swings northeast toward its

northern margin………………………………………. 23

(b) Section C–C’ is oriented roughly N–S and crosses from the Dong

Mun Graben in the south to the Sakon Graben in the north of

the basin……………………………………………… 23

Fig.9 Seismic sections across the northern part of the Khorat Plateau Basin

(a) Roughly E–W composite section D–D’ across the Si That Platform,

Kuchinarai Graben, and Phu Phan Uplift…………..... 26

(b) Roughly N–S section E–E’ crossing from the Si That Platform

in the south to the Sakon Graben in the north……….. 27

Fig. 10 Depth contour map showing the top of basement

in the Khorat Plateau Basin……………………………… 31

Fig. 11 Depth contour map showing the base of the Lower Nam Phong

Formation (Indosinian II Unconformity) in the Khorat Plateau Basin 32

Fig. 12 Isopach map of the Permian section………………………… 33

Fig. 13 Tectonic trends in the Khorat Plateau Basin……………... 41

Fig. 14 Tectonic subdivisions of the Khorat Plateau Basin in the Indochina

Block and surrounding region……………………………. 49

Fig. 15 Schematic diagrams of tectonic evolution of mainland SE Asia 50

Fig. 16 Summary of the tectonic evolution of the region surrounding the Khorat

Plateau Basin from the Middle Carboniferous to the Early Jurassic 52

Fig. 17 Sedimentary basin map and fields location of mainland SE Asia 55

Fig. 18 Plan map and geological cross section showing the geographic

extent for the petroleum system…………………………. 57

Fig. 19 Geological cross sections of gas fields

(Phu Horm and Nam Phong)………………………….….. 61

Fig. 20 Geochemical basin modeling (Burial history diagram)…. 67

Fig. 21 Schematic cross section of the Khorat Plateau, showing the structural

and stratigraphic setting of exploration wells……………. 72

Fig. 22 Petroleum systems event chart…………………..…...... 74

List of Table

Table 1 Well results summary………………….……………….. 60

vii

1. Introduction

The Khorat Plateau Basin extends over an area of more than 200,000 km2 (about 500 km ×

500 km) across three countries in Southeast Asia: the largest part of the basin is in northeastern

Thailand, with the remainder covering parts of southwestern Laos and northern Cambodia (Figs.

1 - 3). In Thailand, there is an extensive cover of thick Jurassic and Cretaceous sediments on the Khorat Plateau that include red beds and dinosaur ; however, only through petroleum exploration seismic survey and drilling well has the subsurface presence of thick Upper

Paleozoic and Triassic successions been revealed. The Khorat Plateau consists of a hilly and

gentle terrane ranging in altitude from 150 to 500 meters, and is surrounded by narrow and

elongate mountain chains (Fig.4).

The upper Paleozoic and Mesozoic sequences in the basin have been explored for peroleum

and several gas fields have been discovered. Over the past 50 years there have been numerous

petroleum exploration programs in the Khorat Plateau Basin. To date, oil companies have

carried out seismic surveys over most of the Khorat Plateau region and drilled more than 40

exploration wells (some representative wells and fields are shown in Fig. 4). At present there

are two commercial gas fields (Nam Phong and Phu Horm) producing gas from Permian

carbonates (e.g., Booth, 1998; Sattayarak, 2005; Chantong et al., 2013).

The Khorat Plateau Basin represents a complex sequence of basins formed during five periods

of deposition from the Late Carboniferous to the end of the Cretaceous (Booth and Sattayarak,

2011). The five sequences are separated by unconformities related to major orogenic events

(Fig. 5). The Indosinian I unconformity is regionally most extensive and oldest of four

Mesozoic unconformities recognized in the basin and was followed by the Indosinian II unconformity in the Late Triassic. Between the Indosinian I and II unconformities, half-graben- filling strata (Kuchinarai Group) deposited during an extensional period are preserved. Jurassic

1 to Early Cretaceous thick continental deposits (Khorat Group), which form gentle long-wave folds, overlie the Indosinian III unconformity. The Khorat Plateau Basin began to form during

Late Carboniferous rifting, developed into a distinct horst-and-graben system during the

Permian, and was uplifted during the Early to Middle Triassic. In previous studies, Booth

(1998), Sattayarak (2005), Thongboonruang (2008), Polachan et al. (2010), Racey (2011), and

Chantong et al. (2013) described the stratigraphy, Permian carbonate reservoirs, hydrocarbon occurrences, and exploration potential of the basin. Booth and Sattayarak (2011) provided a detailed summary of the stratigraphy and structure within the Khorat Plateau Basin showing interpreted seismic sections, structural maps, and well columns, and established a geological framework for the basin. Kozar et al. (1992) summarized the tectono-stratigraphy of the basin on the basis of sequence stratigraphy derived from many exploration wells drilled by Esso.

Lovatt Smith and Stokes (1997), Cullen et al. (1997) and Hoang et al. (2013) described the stratigraphy, structure, and hydrocarbon potential including the occurrence of oil seeps of the part of the Khorat Plateau Basin that lies in Laos.

For this study, I used unpublished structural maps compiled by petroleum explorers, but added own interpretations in areas between their maps, thus completing the regional coverage of structural mapping of the basin. Seismic data can now be correlated between wells with reasonable confidence, particularly in the northern and central areas of the Khorat Plateau.

Commercial oil and gas accumulations are mostly found in large, deep sedimentary basins.

Subsurface basin analysis to determine the large-scale sedimentary and structural framework of such buried basins is important work carried out by petroleum geologists. Seismic data interpretation is a key component of these analyses.

In this paper, I first summarize the regional geological framework of the Khorat Plateau Basin.

I then present regional structural maps of the basin that are based on interpretations of seismic

data and correlations with well data, and discuss the basin’s tectonics based on the tectono- stratigraphic history of the Indochina Block. Furthermore, I use these data to identify the cause

of the major Indosinian I unconformity, which locally characterizes the Lower – Middle

Triassic hiatus. Finally, based on the findings for the tectono-stratigraphic history in the basin,

I conducted the petroleum geological study for the origin of gases.

Fig. 1 Major tectonic subdivisions of mainland Southeast Asia (West Burma Block, Sibumasu Block, Inthanon Zone, Sukhothai Zone, Indochina Block, and South China Block; after Metcalfe, 2013) and late Paleozoic and Mesozoic sedimentary basins (Khorat Plateau basin, Booth et al. 2011; Tonle Sap and Preah basins, Vysotsky et al. 1994; Kompong Som (Phuquoc) Basin, Fyhn et al., 2010). Strike-slip faulting on the Mae Yuam Fault and the precursor of the Mae Ping Fault separate the Inthanon Zone from Southern Thailand (Ridd, 2012).

Fig. 2 Major tectonic subdivisions of mainland Southeast Asia overlaid on a satellite image (ASTER GDEM).

Fig. 3 Tectonic subdivisions of the region surrounding the Khorat Plateau Basin in the Indochina Block. Interpreted basement faulting is overlaid on a regional tectonic map with normal (red) and reverse (purple) faults shown. Compiled from Metcalfe (2013), Ueno and Charoentitirat (2011), Charusiri et al. (2002), and Osanai et al. (2008). Following the mid-Cretaceous basin inversion of the Phu Phan Uplift, three sub-basins of the Khorat Plateau Basin were formed (Khorat, Sakhon-Nakon, and Savannakhet basins).

Fig. 4 Map showing locations of selected exploration wells, gas fields, lithostratigraphic correlation of seven wells (along A–A’), stratigraphic cross sections (B–B’ and C–C’), and seismic lines (D–D’ and E–E’) shown in other figures. Interpreted basement faulting is overlaid on a satellite image (ASTER GDEM) of the Khorat Plateau Basin region.

fluvial - bedded limestones - and lacustrine shales thick and fractured dolomites limestones including evaporites and coals Thick terrestrial red beds Mixture of terrestrial sandstone A seriesof half grabens Shallow marine massiveand The main gasreservoirs Calcareous shales and Shallow marine to littoral ・・・・・・I Unconformity II UnconformityII III Unconformity Group Khao Fm. Khao Fm. Group Source rock Reservoir Cretaceous Unconformity Cretaceous Nok - Indosinian Indosinian Indosinian S Mid Carboniferous Unconformity R Mid Si That Si Khorat Kuchinarai Pha R S S S R R R R Petroleum System Petroleum Lithology Lat Duk Phong Phong Phan Tok Wihan Khua Kruat Hin Kradung Sarakham Nok Nam Nam Phu Phu Sao Si That Phra Formation Basement Phu Khok Hua Na Kham Na Hua Huai Lw UpNam Pha Khao Maha stratigraphic summary of Paleozoic and Mesozoic successionsinnortheast Thailand Group - No Name Saraburi Khorat No Name No Name Kuchinarai

Late

Late Late Late Early Middle Late Late Early Early

Jurassic Cretaceous Triassic Permian Carboniferous 359 200 299 251 S Cb 145 Ma - -

S Fig.5 Tectono Fig.5

Paleozoic Mesozoic Chronostratigraphy

2. Upper Paleozoic and lower Mesozoic stratigraphy of the Khorat Plateau Basin

2.1 General stratigraphy

The Mesozoic sequence overlies a geologically complex upper Paleozoic sequence from

which it is separated by a major angular unconformity identified on the basis of seismic and

well data. Booth and Sattayarak (2011) named the Mesozoic sequence the “Khorat Basin” and

the upper Paleozoic sequence the “Isan Complex”. Here, I consider the “Khorat Plateau Basin”

to comprise both the upper Paleozoic sequence (above the mid-Carboniferous unconformity)

and all the Mesozoic sequence (Fig. 5). In this study, I used the stratigraphic framework

established by Booth and Sattayarak (2011).

Within the basin, the Upper Carboniferous and Permian sequence is predominantly shallow-

marine carbonates and shales but includes some terrestrial deposits in the Upper Carboniferous

Si That Formation. The Permian sediments comprise the Nam Duk, Pha Nok Khao, and Hua

Na Kham formations in ascending order, although they interfinger locally. Above the

Indosinian I unconformity, the Mesozoic strata consist of terrestrial sediments, including red

beds. The Upper Triassic rocks are the Kuchinarai Group and the overlying Lower Nam Phong

Formation, which are separated by the Indosinian II unconformity. Jurassic and Lower

Cretaceous sediments comprise the terrestrial Khorat Group, which is unconformably overlain by evaporite deposits of the Upper Cretaceous Maha Sarakham Formation.

Several exploration wells have penetrated the mid-Carboniferous unconformity and confirmed that sedimentary rocks beneath it were deposited in shallow-marine to littoral–

fluvial environments similar to those of Carboniferous rocks in the Loei area (Ueno and

Charoentitirat, 2011) (Fig.6). In the central area of the basin, the Yang Talat-1 well (location in

Fig. 4, lithologic column in Fig. 6) found cuttings of hornblende granite near the bottom of the

well that were dated to the late Early Carboniferous (329 ± 3 Ma, 40Ar/39Ar age; Booth and

Sattayarak, 2011). Although pre-Carboniferous rocks have not been reported from wells drilled

on the Khorat Plateau, Silurian–Devonian rocks exposed in the Loei Province (Wongwanich

and Boucot, 2011) may provide an analog for the basement rocks underlying the Khorat Plateau

Basin.

Lithostratigraphic correlations have been made linking seven wells over a total distance of

about 450 km through the Khorat Plateau Basin (Fig. 6). The lithological data and well

correlations I used are from unpublished well completion reports and the lithostratigraphic column published by Booth and Sattayarak (2011). The Kaset Sombun-1 well, drilled in the

Dao Ruang Thrust Belt in the westernmost part of the study area, confirmed the presence of thick sequences of Permian carbonates of the Pha Nok Khao Formation and presumed

Carboniferous siliciclastic sediments of the Si That Formation. In the central part of the basin, the previously mentioned late Early Carboniferous hornblende granite was encountered near the bottom of the section penetrated by the Yang Talat-1 well, but Upper Carboniferous and

Permian sediments were absent. The Si That-1 well in the north of the study area confirmed the presence of thick Permian carbonates on the platform. Kuchinarai-1, near the northeastern bounding fault of the basin, penetrated a thick sequence (~2000 m) of dominantly shales and sandstones of the Upper Triassic Kuchinarai Group. In the eastern part of the basin in the Non

Sung Graben, the Non Sung-1 well penetrated thick sequences of carbonates of the Pha Nok

Khao Formation (about 800m) and siliciclastic sediments of the Si That Formation (+250m).

The Indosinian I and II unconformities are correlated widely among the various wells in the basin. The stratigraphic units used in this study are described in sections 2.2 to 2.5 below.

A geological excursion was carried out in the Khorat Plateau area in December 2016 using examples from the field trip guide (Booth et al., 2014). The purposes of the trip were to examine the actual rocks and gain better knowledge of the sedimentary facies, and the actual scale the

10 of structures and basin. I attached pictures and descriptions of outcrops in the Appendix.

u a e t a l P t a r o h K e h t h t a e n e b s k c o r c i o z o s e M d n a c i o z o e l a P r e p p U e h t d e t a r t e n e p t a h t s l l e w f o . ) n 4 o i . t g a i l F e r r n i o ’ c A c – i h A p g a r n g o i l t a a r s t l s l o e h w t ( i L 6 . g i F

2.2 Upper Carboniferous Si That Formation

The name Si That Formation was first applied by Booth (1998). Booth and Sattayarak (2011)

described the formation was consistent with ages obtained from outcrops of the Wang Saphung

Formation of east of Loei (Ueno and Charoentitirat, 2011).

The Si That Formation consists mainly of calcareous shales and carbonates and interbedded sandstones in the Non Sung-1 well, and coal, chert, and evaporite in outcrops in the Loei area

(Ueno and Charoentitirat, 2011). The base of the formation lies on the mid-Carboniferous

unconformity. The carbonates in the formation developed in and around the Si That Platform

in the central part of the basin, whereas in other areas the sediments are more siliciclastic by

well data and seismic horizon. In the western part of the Indochina Block, two elongate

carbonate platforms (the Khao Khwang and Pha Nok Khao platforms) were developed during

the latest Carboniferous to Permian. The two platforms are separated by the Nam Duk Basin

(Fig. 3).

Surakotra et al. (2005) reported the presence of Upper Carboniferous (Moscovian) gypsum–

anhydrite deposits associated with carbonates in the Loei area and suggested that these

sediments represented shallow-marine deposition in a tidal flat to subtidal environment.

Kuroda et al. (2017) described the petrogenesis of evaporitic gypsum deposits in northeastern

Nakon Sawan and concluded that they were originally precipitated during the Serpukhovian

from hypersaline seawater in a shallow lagoon or shelf on the Khao Khwang Platform and that

the gypsum was converted to anhydrite during the early stage of burial diagenesis. Because no

evaporites have been reported in wells drilled in the study area, these deposits likely represent

a local event that occurred during the very beginning of the Late Carboniferous. Thus, the Si

That Formation was deposited in a shallow-marine to littoral–fluvial environment.

2.3 Lower to Middle Permian Pha Nok Khao Formation

The Pha Nok Khao Formation consists predominantly of shallow-marine massive and thick- bedded limestones, fractured dolomites, and thin-bedded shales (Chaodumrong and Burrett,

2014). For all of the company reports and publications, the Pha Nok Khao Formation name employs all of Permian massive carbonates encountered in the subsurface (Booth and

Sattayarak, 2011). The formation is more than 1000 m thick in several wells. Booth and

Sattayarak (2011) mentioned that there are in reality numerous, quite distinct and separate, carbonate platforms in the subsurface which probably developed at different times by correlating dating data of the wells. The Pha Nok Khao platform began to develop west of the

Khorat Plateau during the latest Carboniferous (as described in section 2.2). On the basis of carbonate facies analysis, Udchachon et al. (2014) reported that the sedimentary environment on the Khao Khwang Platform (Fig. 3) probably evolved from a rimmed platform in the Early

Permian to a ramp in the Middle Permian.

Seismic facies analysis and the distribution of wells that penetrated carbonates indicate that the carbonate platforms and reefs of the Pha Nok Khao Formation likely developed on a horst and their lateral equivalents were deposited in moderately deep water in a neighboring graben.

Because water circulation is usually restricted in a back-reef setting, the resultant deposits were presumably excellent petroleum source rocks, as indicated by geochemical data

(Thongboonruang, 2008).

The depositional environments of the Permian carbonates are widespread throughout the

Indochina Block, not only in Northeastern Thailand, but also in Laos and Cambodia. They include those penetrated by a well drilled on the eastern Khorat Plateau in Laos (Burnhill, 2010) and those observed in outcrop in the Sri Sophon area (Nildam et al., 2016) and in the subsurface in the Tonle Sap Basin (Vyosotsky et al., 1994) of northwestern Cambodia.

2.4 Upper Triassic Kuchinarai Group (Huai Hin Lat Formation) The Kuchinarai Group contains coarse, fluvial, alluvial-fans sandstones and organic-rich lacustrine shales interbedded with volcaniclastic rocks. The Kuchinarai Group is from 100 m to more than 5000 m thick, and its sedimentary facies are extremely variable because of the variety of depositional environments. It is therefore difficult to invoke stratigraphic correlations among the wells that penetrate it. In the Late Triassic, an extensional environment resulted in the formation of a series of half grabens that were subsequently filled with sediments of the

Kuchinarai Group. Because of variety of its sedimentary facies, Booth and Sattayarak (2011) that the name Kuchinarai Group is used to refer to the entire section between the Indosinian I and Indosinian II Unconformities to avoid confusion for stratigraphic correlation. The

Kuchinarai-1 well penetrated about 2000 m of these rocks (Fig. 6) in a graben close to the basin-boundary fault; these have been correlated with rocks of the Hua Hin Lat Formation that are exposed westward from the Nan Suture Zone in northern Thailand and are likely of Triassic

(Norian) age (Chonglakmani, 2011), although biostratigraphic evidence for this age is absent.

The Kuchinarai Group is overlain by the Lower Nam Phong Formation of Rhaetian age with the Indosinian II unconformity. Booth and Sattayarak (2011) suggested that by the end of

Indosinian II event the whole of northeastern Thailand was essentially a peneplain with remnant karst hills composed of Permian carbonates or cuesta scarps formed in more resistant rocks of the Si That Formation at the southern edge of the Si That Platform.

Triassic marine strata are widely distributed in Thailand, and deposited as the Lampang Group and the Song Group bordered on the west by the Inthanon Zone and on the east by the Nan-

Uttaradit Suture in the northern Thailand. (Chonglakmani, 2011). The Early – Middle Triassic

Lampang is made up of a succession of repeated carbonate- and turbidite-dominant facies

(Chonglakmani, 2011; Hara et al., 2017). The Late Triassic Song Group is mainly characterized

by turbidites with minor limestone interbeds (Hara et al., 2017). Kamata et al. (2009) identified

pelagic and hemipelagic cherts formed in several distinctive environmental setting within the

Paleo-Tethys Ocean. The major part of northeastern Thailand, including the Khorat Plateau, was uplifted in the Middle to Late Triassic, and the upper Paleozoic rocks were eroded to form the Indosinian I unconformity.

2.5 Upper Jurassic to Early Cretaceous Khorat Group

The Khorat Group consists of extensive and thick continental deposits that cover much of the

Khorat Plateau. It is 2000–3000 m thick and spans from the Indosinian III unconformity (which separates the Lower Nam Phong Formation and Upper Nam Phong Formation) to the Aptian–

Albian mid-Cretaceous unconformity. Both of the Lower Nam Phong Formation and Upper

Nam Phong Formation, which form its initial fill, thin markedly toward the edge of basin

(Booth and Sattayarak, 2011). Mouret et al. (1993) provide detailed study for the group that ties the section of Phu Phra -1 well and the outcrop data in the Phu Phan Uplift. The sediments are characterized by fluvial deposits, including red beds, which consist of conglomerates,

sandstones, and mudstones. In ascending order the sedimentary succession comprises the

Upper Nam Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, and Khok Kruat

formations.

On the basis of facies analysis, Horiuchi et al. (2012) interpreted the Phu Kradung and Phra

Wihan formations of the Early Cretaceous Khorat Group in the northwestern Khorat Plateau

Basin to have been formed by an anastomosing river system. The paleoclimate of the basin

during the Early Cretaceous was semi-arid to sub-humid with wet and dry cycles indicated by

the occurrence of calcretes (Horiuchi et al., 2012). Meesook (2011) mentioned that there was

uplift at that time in areas surrounding the Khorat Plateau and large volumes of sediment were

transported into the basin and that the dominant fauna indicates that the Indochina Block was

by that time part of a larger Asian continent including southern China. Racey et al. (1996) and

Canham et al. (1996) studied the sedimentology, sandstone petrography, petroleum reservoir

quality, and diagenesis of Khorat Group rocks and suggested on the basis of burial history

modeling that the basal part of the Khorat Group reached a maximum depth of 8000 m with paleotemperatures of 150–170°C. The Khorat Group sediments were deposited and deeply buried across the entire Khorat Plateau, but at about 65 Ma they were considerably uplifted, resulting in the erosion of 3000–3500 m of post-Middle Jurassic Phra Wihan Formation sediments (Mouret et al., 1993, Mouret, 1994).

3. Structural interpretation of seismic sections

3.1 Seismic survey and well data

In the early 1990s, a joint-venture exploration by Unocal Corporation (now Chevron Ltd.)

and Mitsui Oil Exploration Co., Ltd. obtained gas shows from Triassic sandstones in four wells

drilled in the northeastern Khorat Plateau Basin. To date oil and gas companies have drilled

more than 40 exploration wells, two of which have discovered commercial gas fields (Phu

Horm and Nam Phong; locations in Fig. 4). The data from the wells include sample descriptions, geochemical, biostratigraphic, and petrophysical analyses of drilling cuttings and cores, electrical log measurements of porosity (density), lithology (gamma radioactivity), and correlation of sonic logs with velocities obtained from seismic profiles.

The joint-venture partners interpreted the two-dimensional seismic reflection profiles by tracing the several key seismic horizons and faults, and compiled subsurface structural maps by contouring the level of horizon over parts of the Khorat Plateau Basin. As I mentioned, for this study, I used maps compiled by the joint-venture partners, but added own seismic interpretations in areas between their maps, giving a total of more than 15,000 line kilometers of interpreted seismic data, thus completing the regional coverage of structural mapping of the basin. I then converted the regional maps from two-way seismic time to depth. Seismic data quality was good enough to provide reasonable confidence in the accuracy of the mapping, particularly in northern and eastern parts of the Khorat Plateau Basin, although the data quality over some deep grabens was poor. Apart from a structural grain map compiled by Kozar et al.

(1992) and structural contour maps of several seismic horizons presented by Booth and

Sattayarak (2011), previously published interpretations of the seismic and well data do not

provide structural maps covering the entire Khorat Plateau Basin. In this study, I completed the

structural maps depicted the tectonic features consisting both of major faults and structural

contours covering the basin.

3.2 Stratigraphic cross sections through the Khorat Plateau Basin

The stratigraphic cross sections discussed herein are based on 2D seismic profiles and well correlations. Out-of-plane structural trends cannot be seen on the 2D profiles, so those trends mentioned in passing in this section are based on the regional structural maps discussed in

section 3.3. It is shown the tectonic elements in the basin based on seismic interpretation for

this study using the seismic profiles and well data in Fig.7.

Fig. 7 Tectonic elements in the Khorat Plateau Basin (Upper Paleozoic) identified from seismic profiles and well data.

3.2.1 Representative cross sections in the basin

To investigate the continuity of strata and the structure between the surface and subsurface sedimentary rocks of the Khorat Plateau Basin I used interpreted seismic profiles to compile a representative cross section (B–B’; Fig. 8a, location in Fig. 4, 7) of about 400 km length extending eastward from outcrop west of the Khorat Plateau to the central area of the basin and then northeast toward its northern margin.

The Permian Pha Nok Khao Formation, Upper Triassic Kuchinarai Group, and Jurassic sediments crop out at the western end of profile B–B’ and then plunge eastward in the subsurface. West of Khon Kaen, NNE–SSW-trending thrust faults of the Dao Ruang Thrust

Belt can be seen on the profile with throws of up to 1000 m. Northeast of Khon Kaen, basement rises and Upper Carboniferous sediments and the entire Permian section have been eroded. In the area of the Phu Phan Uplift, a NW–SE-trending anticline is evident both in the surface and in the Mesozoic section. In the deeper subsurface in this area, about 5000 m of Upper

Carboniferous and Permian rocks are cut by numerous NW–SE-trending thrust faults. Further to the northeast, the Upper Permian section has been eroded and a thin layer of Triassic sediments lies unconformably on the basement. Only a few faults of the Dao Ruang Thrust Belt and the Phu Phan Uplift cut the Jurassic section, whereas other faults in the Permian section terminate at the Indosinian I unconformity.

A roughly N–S profile through the basin (C–C’; Fig. 8b, location in Fig. 4, 7) shows a series of NW–SE-trending horsts and grabens within which the shales and carbonates of the Late

Permian Hua Na Kham Formation (recorded in Lam Pao-1, see lithologic column in Fig. 6) and the shales and intercalated sandstones of the Late Triassic Kuchinarai Group (recorded in

Phu Phra-1) have filled the half grabens formed by the main faults. Cross section C–C’ clearly shows the Indosinian I angular conformity; the faults in the Upper Paleozoic section are

21 truncated at the unconformity, except in the Sakon Graben, which is filled with a thick sequence of Upper Triassic Kuchinarai Group rocks. The Dong Mun (half) graben is bounded by a main syndepositional fault with several associated synthetic and antithetic faults.

Both profiles (B-B’ and C-C’) show the inverted structures through the Kuchinarai Graben to

Sakon Graben in the northern part of the basin.

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3.2.2 Seismic profiles in the northern part of the basin

The WNW–ESE seismic profile crossing from the Si That Platform, to the deep Kuchinarai

Graben and then to the Phu Phan Uplift in the northern part of the basin (D–D’; Fig 9a, location

in Fig. 4) clearly shows the angular unconformity of the Indosinian I event westward from the

Phu Phan Uplift, although there is little angularity at the unconformity farther to the west. The profile confirms the presence in its western part of about 5000 m of the Permian sedimentary rocks (including carbonates) penetrated by the Phu Lop-1 well. Southeast of the Phu Lop-1 well, the Permian section below the Indosinian I unconformity is strongly deformed by faults and folds of a stacked imbricated thrust system, the major thrust of which exhibits a small throw above the unconformity. The main thrust fault strikes NW–SE and is the only thrust associated with the Phu Phan Uplift that cuts the Jurassic section, presumably because it was reactivated after the Paleozoic. It is thus apparent that the tectonic histories of the Upper

Paleozoic and Lower Mesozoic sections above and below the Indosinian I unconformity are different.

The western part of profile D–D’ intersects the southern end of N–S profile E–E’ (Fig. 9b; location in Fig. 4), which from south to north crosses the Si That Platform, Kuchinarai Graben,

Phu Horm Platform, and Sakon Graben, all of which trend NW–SE in the northern part of the

Khorat Plateau Basin. The Si That-1 well (lithologic column in Fig. 6) confirmed the presence of at least 1100 m of limestones and dolomites of the Pha Nok Khao Formation. Profile E–E’ shows that the southern edge of the Si That Platform was eroded during the Indosinian II event

(presumably including the Indosinian I uconformity), forming a cuesta topography with a shallow dip to the north. The northern side of the uplifted Si That Platform is separated from the Kuchinarai Graben by a major reverse fault that formed a deep half graben that shallows from south to north toward the uplifted and complex faulted Phu Horm Platform, which lies

24 south of the Sakon Graben. The Phu Horm Platform trends E–W where it crosses profile E–E’, but swings to NW–SE and NNE–SSW trends east and west, respectively, of the profile.

r e n a h l o t i u t g n c e n o s a d n e e t a h i t a t m o g r n n e i n P a w e o s t h h i t s n , l u y l l e r c i a w e n l o 1 c t . - c , p t m e t o f e i t l L e s p y h u t s U h t g P s n n a u i e r h h w h t P t o f h d u o s e h t t P a p s c a a i d e r m n h b a t A u m , i o . n ) e S d 4 e b . t . a k f r c g i i l a G t F p s i U a n a i r f a n s o a n i n h s o h P d i c t l u a u o f c h K o P d l , ( n e m a r n h t i s o t s f l f t a u o a B l a t f P s u e e t a h a e w t t h a ) y l T y b P r i a d t S e d a r n e m o u h r t h o o f b s K e s d T e o r – h y t c l P a g f e ’ n o h o t t D r r ( t – a s t D p s n i e n n v r y o t i e e i t h I c t m r e r n s o o a . f n i e 7 n t n i e i . o s s h c g t o i o n p d F s u s n s I m I o a o r n e c c a d h i a e t n d s i W f i s n – v o o o E o i d y r t t n y p c i I l e s h m s e i r g h s o c t u i f n o n o w m i R o s t o i c l c ) e e n e a S b ( s u 9 . g i F

i t c d i n S n a g a f n o o l h m o t e e h o g r t o i d l h ( e B t i l . n l n r w e e o i n w h t i t a a u 1 l - o m t p r s a e o h n e F e h T t p o i t a S a a h e y n l K h l o i a T k t i t o . a n h N t m e r r s a o s o h e n F P t s e a a h n t h a w i T n i d m i r n S n e a l e P e i b a h a e t h r h f t T G o f n n o s r o k e s t k c e s a t o a i r S e t m h e t n o h r l a t t o o s n o i d t s f e d o h r n t a e u e l r o s o s o e h n e m w o h t t n e s i e h n t i d m e t i l n m m r e r f o v o o f f e t a I s l m I p P r 0 n t a 0 a a c i 1 s h n 1 i T a s t t s i o s a S e d e u n l e I c t h f t r a o o f m s d o o e n r t e e f a c n e n g o e h n t s i b s r e y r s a b p o c r t e c n a a h ’ h t i t E m d – d r e e E e t m P s n r e i f o f i g o t n g c o d u e c s e s s ) ) o S 6 1 p – 1 . . 0 m g N i 2 m o ( r F y c l o k f s n h t l i a g l a r i l u n a h o P y m t t a R s t u a t r l ) h a a o b T k S c (

3.3 Structural mapping of subsurface horizons

3.3.1. Structural mapping technique

Because of the geological complexity of the Khorat Plateau Basin, it was difficult to map all

of the identified formations. Thus, to obtain a meaningful representation of the structure of the

basin I compiled subsurface structural depth maps of reflectors representing the top of the acoustic basement (Fig. 10). The mapped reflectors approximate the uppermost Lower

Carboniferous (or older) rocks by correlated with the well data and correspond to the mid-

Carboniferous unconformity in areas where Triassic rocks do not directly overlie basement,

and the base of the Lower Nam Phong Formation (Fig. 11), which approximates the Indosinian

II unconformity. Depth conversion of the two-way-traveltime maps was achieved by using

velocity data from nine wells, most of which are shown in Fig. 4. The structural maps for this

small scale are demonstrated for mainly major boundary faults, and are not shown for the minor

faults. The isopach map of the Upper Paleozoic section is shown in Fig. 12 and the major

tectonic elements identified by the mapping are shown in Fig. 7.

3.3.2. Top of basement map (Lower Carboniferous or its older rocks)

The depth contour map for top of basement (Fig. 10) shows that the dominant trend of faults

in the northern and central parts of the basin is NW–SE, but it swings gradually to follow an

E–W trend in the south. Within the mapped area, basement is shallowest (about 2500 m) in the

north and northeast on the Nong Khai High, in the south (about 4000 m) on the Buri Ram High

and on the Kalasin High (about 3500m). Most of the basement faults were originally normal

faults activated during rifting since the Late Carboniferous (Kozar et al., 1992), but in the north

and westernmost areas they were reactivated as reverse faults during inversion tectonics. As

also shown on profile D–D’ (Fig. 9a), the areas northwest and southeast of the Phu Lop-1 well

on the Phu Phan Uplift have been intensely faulted below the Indosinian I unconformity and in

the areas of complex structure between closely spaced major NW–SE-trending faults. In the

central part of the basin basement highs at 3000–4000 m depth correspond to the Si That

Platform, Dong Mun Platform, and Kalasin High (Fig. 7). East of this structurally complex area, basement deepens to about 9000 m along a NNW–SSE trend, where I estimated the Upper

Paleozoic section to be about 6000 m thick (Fig. 12). Farther to be east, the basement reflector across both the southern Kuchinarai Graben and the Sakon Graben is unclear, but by extrapolation I infer a depth of several thousand meters in both grabens. In the southern part of the basin, basement is shallower, with E–W-trending normal faulting. In the Dao Ruang thrust belt on the eastern margin of the Loei-Phetchabun Fold Belt, NNE–SSW-trending thrust faults have lifted basement to about 2000 m depth in a NNE–SSW-trending anticlinorium bounded mainly by west-dipping thrusts.

3.3.3. Base of the Lower Nam Phong Formation (Indosinian II unconformity)

The depth contour map for the base of the Lower Nam Phong Formation (Indosinian II unconformity, Fig. 10) is defined by a strong band of reflectors that I tied to all of the available well data (Fig. 10). Above the unconformity, each of the formations of the Khorat Group manifest as parallel reflectors. In contrast to the faulting at basement level (Fig. 10), only a few faults cut the base of the Lower Nam Phong Formation (Fig. 11). Despite differences in structural complexity, the structural trends shown by the two depth contour maps, and those of surface structures, are similar: at all three levels, trends are NW–SE in the northern and central areas, E–W in the south, and NNE–SSW in the westernmost area. It is likely that the deep structures influenced the development of shallower structures.

The base of the Lower Nam Phong Formation is shallow (approximately 1500 m) in the area

of the Phu Phan Uplift in the Kuchinarai Graben, where it is cut by a thrust fault. In the southern

part of the basin, there is an extensive syncline (about 300 km long and 100 km wide) that

trends E–W to ESE–WSW and reaches a maximum depth of 5500 m, which is also the

maximum depth of this mapped horizon in the entire basin. In the westernmost area of the basin,

NNE–SSW-trending thrust faults and anticlines are associated with the Loei-Phetchabun Fold

Belt, and the structure there is concordant with that of the underlying basement. Chaviroj

(1997) interpreted the tectonic event in three episodes during the Late Cretaceous to Pleistocene

from the aerialphotographs and LANDSAT-5 which demonstrated the surface topography. The

major structural lineaments on his map by his interpretation are also similar with the map for

the base of the Lower Nam Phong Formation.

3.3.4. Isopach map of the Upper Paleozoic section

The isopach map of the Upper Paleozoic section was constructed by taking the height

difference between “Top of basement” map and “Indosinian I Unconformity” map generated by Booth and Sattayarak (2011) (Fig. 12). Northeastward from the Si That Platform, the basement deepens to about 9000m along a NNW-SSE trend (Fig. 10). It was estimated that the

Upper Paleozoic section attains to about 6000m thick (Fig. 12). In the Kalasin high (Fig. 7), it

was missing the Upper Paleozoic section based on the isopach map.

Fig.10 Depth contour map showing the top of basement in the Khorat Plateau Basin. The dominant trend of faults in the northern and central parts of the basin is NW–SE, but the trend gradually becomes E–W in the south. In the westernmost area, NNE–SSW-trending thrust faults have lifted the basement to about 2000 m depth in a NNE–SSW-trending anticlinorium bounded mainly by west-dipping thrusts.

Fig. 11 Depth contour map showing the base of the Lower Nam Phong Formation (Indosinian II Unconformity) in the Khorat Plateau Basin. Unlike faulting at basement level (Fig. 10), few faults cut this horizon. Despite differences in structural complexity, the structural trends shown at this level are similar to those at basement level; structural trends are NW–SE in the northern and central areas, E–W in the south, and NNE–SSW in the westernmost area.

Fig. 12 Isopach map of the Upper Paleozoic section Eastward from the Si That Platform, basement deepens to about 9000 m along a NNW-SSE trend, where the Upper Paleozoic section is estimated to be about 6000 m thick.

4. Tectono-stratigraphy and structural elements of Indochina Block

The Khorat Plateau Basin covers a large part of the Indochina Block, which is the largest

tectonic unit of mainland Southeast Asia. Here I describe the current understanding of the tectono-stratigraphy of the Indochina Block and the results of recent research on its tectonic evolution, and discuss the influence of the Indosinian I unconformity on the geological history of the block.

The Indochina and South China blocks separated from the margin of eastern Gondwana when the Paleo-Tethys Ocean opened during the Early Devonian, and following the separation of

Indochina Block, the Sibumasu Block drifted away from Gondwana in the Early Permian

(Metcalfe, 2011, 2013). Burrett et al. (2014) suggested the paleogeographic model of

Gondwana by the detrital zircon that place South China and Indochina blocks close to the western Himalayas. The Indochina Block extends north–south and is bounded in the northeast by the Song Ma Suture Zone in Vietnam and in the west by the Jinghong, Nan Uttaradit, and

Sra Kaeo sutures and a cryptic suture offshore of the eastern Malay Peninsula (Metcalfe, 2013)

(Fig. 1, 2). The magmatism, tectonism, and metamorphism associated with the collision of blocks have generated mineralized fold belts at the block margins; these include two major belts adjacent to the Khorat Plateau Basin (the Truong Son and Loei-Phetchabun fold belts;

Khin Zaw et al., 2014; Fig. 3).

I focus here on two tectonic regions: one to the west of the Indochina Block that trends NNE–

SSW along the Nan-Uttaradit Suture and another to the northeast that trends NW–SE along the

Song Ma Suture and includes the Truong Son Fold Belt (Fig. 3).

Adjacent to the Nan-Uttaradit Suture, Charusiri et al. (2002) identified the Nakhon Thai Block

between the Nan and Loei Sutures which trend NNE–SSW (Fig. 3). Hisada et al. (2005)

suggested that the Nakhon Thai Block developed in a marginal sea to the west of the Indochina

Block. The Loei Suture opened during the Devonian and then closed during the Carboniferous after the closure of the Loei Basin on the margin of the Indochina Block and amalgamation of the Nakhon Thai Block with the Indochina Block, which has been correlated with the mid-

Carboniferous unconformity (Barber et al., 2011).

As previously mentioned (section 2.2), two almost N–S-trending elongate carbonate platforms (the Khao Khwang and Pha Nok Khao platforms, separated by the Nan Duk Basin) developed in the western part of the Indochina Block during the latest Carboniferous to

Permian. Morley et al. (2013) suggested that the Khao Khwang Platform area is related to basins in the Khorat Plateau area but is somewhat different in that it lies close to the western margin of the Indochina Block at the Sra Kaeo Suture, which suggests that unlike the Khorat

Plateau, the Khao Khwang Platform was not positioned in an interior basin. Morley et al.,

(2013) and Arboit et al., (2014) pointed out that the folding and thrusting of the Khao Kwang

Platform and the Carboniferous - Triassic volcano-sedimentary unit trend approximately E-W, which exhibits the same trend as what can be seen in the southern part of the Khorat Plateau

Basin recognized in this study.

The Truong Son Fold Belt forms the north-northeast peripheral part of the Indochina Block

(between Khorat Plateau and Song Ma Suture) and is connected to Loei Fold Belt at its northwestern end (Manaka et al., 2014). The fold belt is generally characterized by the emplacement of Paleozoic-Mesozoic volcano-plutonic rocks which either intruded into or extruded onto early Paleozoic basement rocks (mainly Silurian granites and Carboniferous limestones at the northern part of the belt in Lao PDR) (Liu et al., 2012; Manaka et al., 2014).

Cromie et al. (2018) described the stratigraphy from Ordovician to Devonian of the Sepon

Basin near the current Sepon Mining area in the Truong Song Fold Belt. The successions consist mainly of shallow water siliciclastics and carbonates intercalating Silurian

volcaniclastic rocks.

Several NW-trending shear zones which are partly mylonitic cross the Truong Son belt of

Central Vietnam, along the Song Ma Suture, and north of the Kontum Massif, and the age of

synkinematic metamorphic minerals in different parts of the belt were dated to be 250–240 Ma

(Lepvrier et al., 1997; Maluski et al., 2005). They suggested the occurrence of ductile strike-

slip tectonics at the early stage of the Indosinian Orogeny.

Numerous analyses of the petrography and geochronology of metamorphic rocks have been

included in studies on the collision between the South China and Indochina blocks (e.g.

Lepvrier et al., 2004, 2008; Maluski et al., 2005; Osanai, 2008; Nakano et al., 2010, 2013;

Zhang et al., 2013; Kamvong et al., 2014; Zaw et al., 2014; Faure et al., 2014; Lai et al., 2014;

Halpin et al., 2016; Owada et al., 2016; Hu et al., 2017; Gardner et al., 2017, Qiu et al., 2017).

Kamvong et al. (2014) suggested that the Phu Kham adakites in the Truong Son Fold Belt were

emplaced during the Late Carboniferous (ca. 306–304 Ma) and were likely formed during the

initiation of subduction of the South China oceanic plate. According to Kamvong et al. (2014),

subduction continued until the middle Permian and initiated the collision between the South

China and Indochina blocks. This south directed subduction better accounts for the late Permian

intraplate magmatism in the Truong Son Belt (Liu et al., 2012; Faure et al., 2014; Manaka et

al., 2014). Qiu et al. (2017) studied major and trace element composition of Early to Middle

Triassic sedimentary rocks from the Youjiang Basin in the South China Block to constrain their

provenance and tectonic setting. They inferred that Early to Middle Triassic sediments in the

Youjiang foreland basin recorded the transition from late Permian and Early Triassic

subduction to Middle Triassic collision at the southwestern margin of the South China Block.

Furthermore, they suggested that an Indosinian Orogeny was triggered by the collision of two blocks rather than by intracontinental collision and orogeny driven by subduction of the Paleo-

Pacific Plate. Rossignol et al. (2018) also studied the Triassic stratigraphic framework

regarding both the depositional environments and age of the main formations for the Song Da

and the Sam Nua basins, northern Vietnam, which locates nearby boundary of the South China

and Indochina blocks. During the Middle to the Late Triassic, both of basins underwent erosion that led to the formation of a major unconformity, resulting from the erosion of the Middle

Trissic Indosinian mountain belt, built after an ongoing continental collision between the South

China and the Indochina blocks (Rossignol et al., 2018).

Osanai et al. (2008) proposed that the collision between the South China Block and Indochina

Block formed the Trans Vietnam Orogenic Belt, which extends from northern Vietnam (Red

River zone) to central Vietnam (Kontum Massif). The evidence for the collision for the two blocks were the presence of ultra-high pressure and temperature metamorphic rocks indicating a typical clockwise P-T evolution path and the granulite facies derived from a basal portion of the subducted Indochina Block beneath South China Block (Osanai et al., 2008). Owada et al.

(2016) indicated that the Kontum Massif of Central Vietnam was formed before 270 Ma during the collision of the South China and Indochina blocks. In the same area, metamorphism of a metagabbro that originated from magma related to the Emeishan plume was dated as 260–250

Ma. Nakano et al. (2010, 2013) reported eclogite facies metamorphism of Triassic age (234 ±

4 Ma) in rocks of the Song Ma Suture in Northern Vietnam. They concluded that the closure of the ocean between the South China and Indochina blocks was a consequence of continental collision initiated in the Late Permian to Early Triassic and subsequent exhumation during the

Late Triassic. Zhang et al. (2013) used a U-Pb age of 230.5 ± 8.5 Ma from ophiolite of the Song

Ma Suture Zone to constrain the timing of the collision of the two blocks, and concluded that it took place during the Middle Triassic and corresponds to the major episode of the Indosinian

Orogeny. Considering the ages of metamorphic rocks described above, the collision between

the South China and Indochina blocks was likely initiated during the Late Permian to Middle

Triassic.

Four sedimentary basins of Paleozoic to Mesozoic ages occupy the central part of the

Indochina Block: the Khorat Plateau, Tonle Sap, Preah, and Kompong Som basins (Fig. 1). The

largest of these is the Khorat Plateau Basin, where petroleum exploration programs have produced seismic survey and drilling data. There has been less exploration in the other smaller

basins so their subsurface geology is less well-documented. Vyosotsky et al. (1994) reported

on the stratigraphy and structure of the Tonle Sap and Preah basins. The Tonle Sap Basin is an

intermontane basin, and a complex system of grabens may fill with Upper Carboniferous to

Middle Triassic terrigenous-carbonate and volcanic units by analogy with the Khorat Plateau

Basin (Vyosotsky et al., 1994). Some oil companies including Mitsui Oil Exploration Co., Ltd.

and seismic survey company carried out the onshore 2D survey for the Tonle Sap Basin,

however, it has been less disclosed the data. The Preah Basin is confined on an east-west

trending trough up to 3000m of Mesozoic sediments (Vyosotsky et al., 1994). Fyhn et al. (2010)

provided a detailed account of the stratigraphy and basin inversion of the Mesozoic Kompong

Som (Phuquoc) Basin as below. The basin forms an elongated, more than 500km long and is a

Late Jurassic to Early Cretaceous foreland basin. They suggested that the Kompong Som Basin

and the Khorat Basin constitute the erosional remnants of larger basin that covered large parts

of SE Asia in Late Mesozoic time and subsequently became segregated during earliest

Paleogene inversion and erosion.

Regarding the tectonic units of mainland Southeast Asia, there are two tectonic domains: one

to the west of Indochina Block that trends approximately N-S (Paleo-Tethys domain) and

another to northeast of the block that trends NW-SE (collision between South China and

Indochina blocks) (Figs. 1 – 3).

5. Discussion for tectono-stratigraphy of Late Paleozoic to Mesozoic rocks

5.1 Structural elements and tectono-stratigraphic evolution of the Khorat Plateau Basin

5.1.1 Basin rifting during the Late Paleozoic

In the preceding section I noted three conspicuous structural trends in the Late Paleozoic sequence of the Khorat Plateau Basin, in particular in the basement map (Fig. 10), where the dominant fault trend is NW–SE in the north, changes gradually to E–W in the south, but is

NNW–SSE in the west.

Taking into consideration the basement map, the seismic facies on seismic profiles, and well data, I, in collaboration with joint-venture partners constructed a tectonic elements map that identifies the grabens, basement highs, horsts, and carbonate platforms or ramps (each element:

150–300 km long and 20–40 km wide) (Fig. 7). In a shallow-marine environment, basement highs are favorable locations for the development of carbonate platforms and ramps. The top of carbonate sequences on the seismic sections the joint-venture partners used show a strong and clear reflection pattern with internal downlapping reflectors that they interpreted to represent carbonate platforms.

In the central part of the basin, the Kalasin High adjoins the large Dong Mun carbonate platform on its northern side, where the Nam Phong field and Dong Mun-1 were discovered natural gases. To the north, the Dong Mun Graben deepens to the northeast between the Dong

Mun and Si That platforms. Farther to the north, the Phu Horm Platform and Kham Palai Horst are connected, but the trend of their distribution changes direction as a result of the interaction of the NNE–SSW and NW–SE trends of the horst systems in the northwest of the Khorat

Plateau Basin.

The westernmost part of the study area shows a dominant NNE–SSW arrangement of tectonic elements that is clearly parallel to the Loei-Phetchabun Fold Belt. Thus, I recognize two

distinctly different tectonic regions in the Late Paleozoic successions of the Khorat Plateau

Basin: one in the northern and central area covering most of the basin (with dominant NW–SE

to E–W trends) and the other in the westernmost part of the basin (with dominant NNE–SSW trends).

5.1.2 Indosinian I event during the Triassic and later tectonic events

Following an extensional rifting phase during the late Paleozoic, the Indosinian I event caused widespread uplift and deep erosion of the Upper Paleozoic section during a major compressional deformation event. The entire Permian section and the top of the Upper

Carboniferous section were deeply eroded, although the extent and degree of erosion across the basin is unclear. The original thickness of the Upper Paleozoic section is difficult to estimate because it thins on horsts and thickens in grabens. Nonetheless, I estimated that about 2000 m

of Upper Paleozoic section by seismic profiles was eroded in some places.

By considering the overall trends of the platforms, basement highs, and horsts, and the

grabens that separate them as described above, subparallel trends that rotate gradually

counterclockwise from NW–SE in the north of the basin to E–W in the south can be identified

(straight red lines in Fig. 13). Inversion of the grabens by reactivation of existing faults during

later shortening is evidence of a compressional stress regime in northern area. It demonstrates the structural inverted area where the most of compressional faults of the Upper Paleozoic section terminate at the Indosinian I unconformity developed through the Kuchinarai Graben to Sakon Graben in the northern part of the basin (Fig.13). I consider these changes to be the response to compressional stress from the northeast on a discrete tectonic region.

Fig. 13 Distinctly different tectonic trends characterize the Khorat Plateau Basin. In the northern and central parts of the basin, the overall trends of the platforms, basement highs, and horsts are subparallel, but rotate gradually counterclockwise from NW–SE in the north of the basin to E–W in the south. The area of structural inversion is demonstrated roughly in the northern part of the basin. In contrast, in the westernmost parts of the basin, the dominant trend is NNE–SSW.

The NW–SE trends in the northeastern part of the basin are parallel to the Truong Son Fold

Belt described in section 4 and the Song Ma suture, which forms the boundary between the

South China Block and the Indochina Block. Moreover, the NNE–SSW trends in the

westernmost part of the basin are the same as those of the Loei-Phetchabun Fold Belt, Nam

Duk Basin, Loei Suture, Nan-Uttaradit Suture, and other tectonic units in the Paleo-Tethys domain (Fig. 3). Considering the distributions of these two structural trends, it is plausible that the NW–SE trends are related to the collision of the South China and Indochina blocks, and the NNE–SSW trends are related to subduction of an oceanic lithosphere of the Paleo-Tethys

Ocean. Because the area of NW–SE and E–W trends covers most of the Khorat Plateau Basin,

I consider that the compressional stress that uplifted the basin was the collision of the South

China and Indochina blocks.

The age of the Indosinian I event is difficult to determine because of a lack of fossil evidence from the terrigenous sediments deposited during the Triassic and the deep erosion of the Lower

Triassic and Upper Permian section. Booth and Sattayarak (2011) argued that it might have commenced in the earliest Triassic and continued into the early Middle Triassic. By assuming there was a time gap between the collision of the South China and Indochina blocks and the closure of the Nan back-arc basin, I presume that the Indosinian I event commenced in the

Middle Triassic and continued into the early Late Triassic, which is similar to the timing of these events proposed by Morley et al. (2013). Barber et al. (2011) proposed that the Indosinian

I event represents the closure of Nan back-arc Basin during the collision of the Sukhothai Arc with the Indochina Block and that the Indosinian II event represents the closure of the Paleo-

Tethys Ocean during the collision of the Sibumasu and Indochina blocks.

The Indosinian II event marks the end of the extensional period during which the Kuchinarai

Group was deposited and was followed by basin inversion during the Late Triassic. Booth and

Sattayarak (2011) indicated that the lowermost unit of the Khorat Group onlaps the Indosinian

II unconformity; however, over most of the basin (except on basement highs) the boundary between the Kuchinarai Group and the overlying Lower Nam Phong Formation is conformable.

The structural stress that caused the Indosinian II event was considerably weaker than that of the Indosinian I event, which suggests that there was only partial basin inversion during the

Indosinian II event. Because the Indosinian III event (between the Lower and Upper Nam

Phong Formation) has not been identified on most of the seismic profiles in the study area, we have not considered it in detail in this study.

Mouret et al. (1993) and Mouret (1994) mentioned that an episode of major uplift and erosion began at about 65 Ma in the area of the Phu Phan Uplift, and that this event was contemporaneous with folding and inversion of thick late Paleozoic and Triassic sediments in the area. In the northern and central parts of the basin (including the part in Laos), there were periods of compressive inversion tectonics in the middle Cretaceous and middle Paleocene

(Lovatt-Smith et al., 1996). On their seismic profile that shows the mid-Cretaceous unconformity, Lovatt-Smith et al. (1997) interpreted the Upper Cretaceous section to onlap the underlying Khorat Group, and in the area of the Phu Phan Uplift, they identified a thrust fault that cuts Upper Carboniferous to Jurassic section and suggested that the fault might have been activated by two or more compressional movements.

5.2 Early Carboniferous to middle Cretaceous paleogeography of the region surrounding

the Khorat Plateau Basin

5.2.1 Tectonic units in northern Thailand

The late Paleozoic to early Mesozoic Indosinian Orogeny in Southeast Asia was generally characterized by subduction in the Paleo-Tethys oceanic plate and collisions among continental

blocks and fragments that form present-day SE Asia. In northern Thailand, a complex collage

of four, predominantly N–S-trending geotectonic units has been proposed: from west to east

these are the Sibumasu Block, Inthanon Zone, Sukhothai Zone, and Indochina Block (Fig. 1),

which are separated by the Mae Yuan fault, the Chiang Mai line, and the Nan-Uttaradit Suture,

respectively (e.g., Ueno and Hisada, 2001; Sone and Metcalfe, 2008; Ueno and Charoentitirat,

2011). Numerous recent studies support the view that there was closure of a back-arc basin (the

Nan Back-arc Basin) in this region in response to the collision of a volcanic arc (the Sukhothai

Zone) with a Cathaysian continental fragment (the Indochina Block) during the Early to Middle

Triassic (e.g., Morley et al., 2013). The combined Sukhothai Zone and Indochina Block then collided with the Sibumasu Block as the Paleo-Tethys Ocean closed during the late Middle

Triassic to early Late Triassic (Sone and Metcalfe, 2008) or at 230 Ma in the early Late Triassic

(Gardiner et al., 2016).

The key tectonic events associated with the evolution of the region surrounding the Khorat

Plateau Basin are summarized in the following sections 5.2.2 and 5.2.3, and Figs. 14, 15 and

16.

5.2.2 Late Carboniferous to Middle Permian (Fig. 15 a, b)

Subduction of the Paleo-Tethys oceanic floor beneath Indochina likely started in the latest

Carboniferous or very early Permian, initiating arc magmatism along the margin of the

Indochina Block (Sone and Metcalfe, 2008). Qian et al. (2015) reported that the Loei Ocean

Basin developed during the Early Carboniferous on the basis of zircon U-Pb ages of volcanic

rocks in the Muang Fuang (350–330 Ma) and Pak Lay (~315 Ma) areas of northwestern Laos

(Fig. 14). These ages suggest the presence of an Early to Late Carboniferous active continental

margin along the western margin of the Indochina Block that I interpret to indicate subduction

44 of the Paleo-Tethys oceanic floor (Fig. 16). The Nan-Uttaradit Suture Zone is a mélange composed of gabbro, tholeiitic metabasalt, andesite, and radiolarian chert, and the samples of gabbro and meta-basalt from the zone have yielded zircon U-Pb ages of 311 ± 10 Ma and 316

± 3 Ma, respectively, suggesting that the Nan-Uttaradit Ocean existed during the Late

Carboniferous (Yang et al., 2016) (Fig. 16).

In the northeast of the Indochina Block, subduction of the South China oceanic plate under the Indochina Block, which started in the Late Carboniferous and continued until the Middle

Permian, initiated the collision between the two blocks (Kamvong et al., 2014) (Fig. 15 a, b).

Following the erosional event that caused the mid-Carboniferous unconformity in the Khorat

Plateau Basin, Late Carboniferous rifting of sediments overlying the unconformity developed horst and graben systems during a period of extensional tectonics that continued into the Late

Permian. During the Middle to early Late Permian there was minor igneous activity within the

Sukhothai Arc and chert deposition in the inactive back-arc basin (Hara et al., 2017) (Fig. 15 b).

5.2.3 Late Permian to Late Triassic (Fig. 15 c, d, e, f)

The Late Permian to Middle Triassic collision between the South China and Indochina blocks dramatically changed the paleogeography and depositional environments of the Khorat Plateau

Basin. The NE–SW compressional stress of the collision resulted in uplift and erosion of extensive areas of the Indochina Block during the Middle Triassic, and the larger continental block created by the collision presumably activated subduction in the Paleo-Tethys Ocean, resulting in increased volcanic activity and formation of the Sukhothai Arc (Fig. 15 c). The compressional stress caused by subduction of the Paleo-Tethys ocean floor west of the basin resulted in the early Late Triassic closure of the Nan back-arc basin and minor uplift of the

Khorat Plateau Basin during the final stage of the Indosinian I erosional event (Fig. 15 d).

Hara et al. (2009, 2012) indicated that mélanges were developed within an accretionary

complex in the Inthanon Zone during the subduction event, and that clastic rocks within them

were derived from felsic volcanic rocks from the Sukhothai Arc and from quartz-rich

continental margin rocks from the Indochina Block. The Permian–Triassic forearc sediments were influenced by the volcanic activity of the Sukhothai Arc, and distinctive unconformities such as the Indosinian I event were not recognized in the Triassic succession. On the other

hand, a large volume of sediments of the Lampang Group that were eroded during the

Indosinian I erosional event in the Khorat Plateau bypassed the Sukhothai Arc via large channels and were presumably transported into the trench (Fig. 16). The widespread deposition of the non-marine Khorat Group over the Indochina and Sibumasu blocks indicates the possible closure of the Nan back-arc basin between the Sukhothai Arc and the Indochina Block during the latest Triassic (Hada et al., 1999) (Fig. 15 f). Hara et al. (2017) suggested that the Nan back- arc basin had closed by the early Late Triassic (Fig. 16), on the basis of the presence of Permian to Middle Triassic limestones, cherts, and hemipelagic sediments identified by Ueno and

Charoentitirat (2011), thus indicating that closure of the Nan back-arc basin was later than previously reported (Early to Middle Triassic)

After the Indosinian I event there was a short period of extensional tectonics during the Norian that formed a series of half grabens in which thick sequences of Kuchinarai Group sediments were deposited along the main faults (Fig. 15 e). At the end of the Triassic, the collision of the

Sibumasu Block with the Sukhothai Arc during the Indosinian II erosional event marked the final event of the Indosinian Orogeny in mainland Southeast Asia (Fig. 15 f).

5.2.4 Jurassic to Middle Cretaceous

During the Jurassic and Cretaceous, thick non-marine sediments including red beds were

deposited throughout the Indochina Peninsula. In the Phu Phan Uplift in the central northeast

of the Khorat Plateau Basin and in the Dao Ruang Thrust Belt to the west of the basin, the

thrusts associated with basin inversion cut the Jurassic section. At least two inversions have occurred on the faults that were active during both the mid-Cretaceous erosional event and the

beginning of the Himalayan Orogeny at around 65 Ma.

5.2.5 Summary of the Late Carboniferous to middle Cretaceous history of the Khorat Plateau

Basin

The long history of the Khorat Plateau Basin records a complex series of Late Carboniferous

to Permian extensional horst and graben systems that were deformed by thrusting and inversion

during and after the Triassic (Fig. 16). In this study I revealed that there are two distinct tectonic

regions that reflect different components of the Indosinian Orogeny (Fig. 14). I here propose

that the Indosinian Orogeny was initiated by the collision of the South China and Indochina

blocks as Qiu et al. (2017) suggested from the sedimentary records in the Youjiang Basin, South

China.

Two aspects of geodynamics and paleogeography during the Permian and Triassic require

further investigation.

(1) Permian carbonates are distributed over the entire Indochina Block, in the Loei-Phetchabun

Fold Belt and Khorat Plateau Basin in southeastern Thailand, and in the Tonle Sap Basin in

Cambodia. However, information is scare on terrigenous siliciclastic sediments that were

eroded from the large hinterland during the Middle Permian. The paleogeography of the

Indochina Block and the tectonic mechanism that created the series of horst and graben systems

in the Khorat Plateau Basin during the Upper Paleozoic are still not well understood.

(2) Although it is clear that the Indosinian I unconformity was the main consequence of the collision of the Indochina and South China blocks during the Late Permian to Middle Triassic, further investigation is required into the tectonic interactions between the Khorat Plateau Basin and the metamorphic belt consisting of the Song Ma – Red River Zone, Kontum Massif, and

Truong Son Fold Belt (Fig. 14).

Fig. 14 Tectonic subdivisions of the Khorat Plateau Basin in the Indochina Block and surrounding region (as shown in Fig. 3) showing the collision zone metamorphic areas of Osanai et al. (2008). The NW-SE trends in the northeastern part of the basin are parallel to the Son Ma suture, which forms the boundary between the South China and Indochina blocks. The NNE-SSW trends in the westernmost part of the basin are the same as those of the Loei-Phetchabun Fold Belt, Nam Duk Basin, Loei Suture, Nan- Uttaradit Suture, and other tectonic units in the Paleo-Tethys domain.

Fig. 15 Schematic diagram of tectonic evolution of mainland SE Asia (Referred Khin Zaw et al., 2014). a. Early Permian (330-290Ma): Subduction of the Palaeo-Tethys oceanic floor beneath Indochina likely started in the latest Carboniferous or very early Permian, initiating arc magmatism along the margin of the Indochina Block Nan-Uttaradit Ocean existed and was spreading from the Late Carboniferous Subduction of the South China oceanic plate under the Indochina Block, which started in the Late Carboniferous and continued until the Middle Permian b Middle - early Late Permian (270-260Ma): Developed horst and graben systems during a period of extensional tectonics that continued into the Late Permian During the Middle to early Late Permian there was minor igneous activity within the Sukhothai Arc and chert deposition in the inactive back-arc basin c. Early - Middle Triassic (250-235Ma): The Late Permian to Middle Triassic collision between the South China and Indochina blocks The NE-SW compressional stress of the collision resulted in uplift and erosion of an extensive area of the Indochina Block including Khorat Plateau during the Middle Triassic Basaltic-felcic increased volcanic activity in the Sukhothai Arc

Fig. 15 Continued d. Early Late Triassic (230-220 Ma) Early Late Triassic closure of the Nan back-arc basin and minor uplift of the Khorat Plateau Basin during the final stage of the Indosinian I erosional event e. Late Triassic (215-210 Ma) After the Indosinian I event there was a short period of extensional tectonics during the Norian that formed a series of half grabens in which thick sequences of Kuchinarai Group sediments f. Latest Triassic (205-200 Ma) At the end of the Triassic, the collision of the Sibumasu Block with the Sukhothai Arc during the Indosinian II erosional event marked the final event of the Indosinian Orogeny in mainland Southeast Asia

n a r i r c i e u s m w e n s s n o s g i e a k n S L u e r c o d y o t I w o i h n I u t l r H P a / e J b m n e 1 i r b 0 u a f a m i a n o i 2 s f n a n a n h i i i a t t n n o N e h i s . o o a o m s c o i h c w h l u o d b l k L n R b d r o n i u U I n . a S C S I p C n . G n e p i . e n o b i l ) a a G r i r e s r d 7 c e r a a i c g n o g d 1 n p n n n c e i i i s N a t f e p 0 s l k c h l x l u c l a e 2 c e m S o U o f E o ( F v r c H n i e . K u c n r l i h e o a t n s n s u a i t i l s k y e f e n M c m i t r s I a n o i a i o r o a l r r . n r C 5 r c f i e C r m n p 3 T a o v n 2 n j o i o i G a n a f H s i a s i c n n l i n a g n i s e o o d l n e M i a m o n v t a a d t E r o L o s n o c B p I i / d c r i s n n c t f l m u f a u i n d i e M l a I U f n s e d i a a - i p L n r c n e b i i s A 7 t t U a l h 4 u a a a r 2 l c C s S e O r B a P w h t B A c o t m I u r L i a a 2 s 5 c r h a a k i n 2 - S o K n o c h a c i i s a k t a o n g h s k l c l l o c n c N e i e b o o K p a e h F v r a a o u 0 k w e B L w t n 6 H i y u o t 2 h e i h t n n L b S v c i a t i a ) o o n g e c p t a d N o u a n h i l h s n i t a I s c r K i i d l d f l n o a d k n o u a . o o n H c u p G l C a N 2 n y - o 7 o G t a a v r 2 i i h r o n e v P a v u i e i m t t g s r b c c s N a a e n a d i n r I n o p o P c i t a i G i i u k l ( g n e n o u e v r r a a i r r a n s t D c u i c k g c G o l s s c e l i i a i m e o o e h r n r s f C a p I t - v u y d c n c N i t i f e t b e d n l t I t o a a a a n r c s x l a m n a a a o a E v B o n y 9 g S l i 9 a t h a 2 u p w m l a G a k d o r c k n p g o v n c l i o s u o e K a t B l i - t a u G B n n a k c a r a i i i a n t o h a i v a v r n m o l h T h S e c y c o r s o e T . s o t o r f S i f B i c n s d M n c i o n u n e n I n s h o t a r i s k o h r i t e P e a a i k e f y b i t h r N i w s h r n a t u o m 3 B a r J b 2 f r o 3 f C y a o n l C o r - y c a S d M r i n a E M U m e t ) h a n m t r u n o o i i n u u a t a h o i a n i a n a v S t s K a n e e a n p i t o a o i n a n i p h m v a i h i i n m t a g r a t i o k l i s k n a a l n 6 o e e s u a i m e o a m h P m i N F n 1 p u t m s h r t r s e t d s h c a z l u o . e i e a n a F N I G O k S K H o g M b r i i ======u o = d F S h I n M S K G O H S I K (

6 Petroleum Geology of the Khorat Plateau Basin

6.1 Petroleum geology and oil and gas industry in Thailand

The Indosinian Orogeny in Thailand is often recognized as having developed between strongly linear terranes, which today trend approximately N-S, and the terranes were

subsequently disrupted by later tectonics, particularly NW-SE trending Cenozoic strike-slip faults (Morley et. al., 2013) (Fig. 1, 2). Hydrocarbon-bearing sedimentary basins in Thailand has been controlled by these tectonic linearments. The petroleum geology of Thailand is divided into four main area as follows (Racey, 2011) (Fig. 17).

・Gulf of Thailand / Pattani Trough, Paleogene- Neogene, non-marine basins (gas-prone)

・Onshore Northern and Central Thailand / Phitsanulok Basin, Paleogene-Neogene, non-

marine basins (oil-prone)

・Onshore NE Thailand / Khorat Plateau Basin, late Paleozoic marine, overlain by Mesozoic

non-marine sediments (gas prone)

・Andaman Sea / Mergui Basin, Paleogene-Neogene, marine basin (gas-prone)

The petroleum exploration for offshore Thailand started in the Pattani Trough (Paleogene to

Recent) of the Gulf of Thailand in late-1960s, and it was successful gas developments and commenced large gas production from 1981. It has become a major contribution to economy and industry of Thailand. The Pattani Trough is the largest in a series of Cenozoic north-south trending basin, and large quantities of gas and condensate charged in the central trough and lesser oil accumulated in the flank area and northern trough. The gases in the Pattani Trough are derived from the thermal cracking of type III kerogens and the cracking of pre-existing oils, and two petroleum systems with two different source facies, the Oligocene lacustrine algal facies and Miocene fluvial coaly facies, are operating in the trough (Minezaki et al., 2001).

Before exploration effort in the Gulf of Thailand, Union Oil was granted exploration license in early-1960s in the Khorat Plateau basin (Mid Paleozoic to Paleogene) of NE Thailand.

100° 105° 110° CHINA 25°

MYANMAR

Hanoi ●

20°

Song Hong Yangon Basin ● THAILAND Phitsanulok Basin Khorat Plateau Basin M1o5°attama Basin

Bangkok ● Tonle Sap Preah Phu Khanh Basin Basin Basin Mergui Basin CAMBODIA VIETNAM ● Phnon Pattani Penh Trough Kompong Som 10° Basin Cuu Long Basin ?

Nam Con Son Basin Malay North Sumatra Basin 0 500 km Basin Tectonic subdivision and sedimentary basins of mainland SE Asia Paleozoic - Mesozoic Basin Oil Field Gas Field Cenozoic Basin

Fig. 17 Major tectonic features and basins in mainland Southeast Asia.

6.2 Petroleum systems in the Khorat Plateau Basin

6.2.1 Petroleum systems

Magoon and Dow (1994) described a petroleum system as follows (Fig. 18). A petroleum system is defined as a natural system that encompasses a pod of active source rock and all related oil and gas. It includes all the geologic elements and processes that are essential if a hydrocarbon accumulation is to exist. The systems account for independence of the essential elements (source rocks, reservoir rock, seal rock, and overburden rock) and processes (trap formation and generation-migration-accumulation of petroleum).

I introduced the papers regarding the aspects of the petroleum geology of the Khorat Plateau

Basin in section 1. However, most of the papers related to whole petroleum geology have not

been published recently. There is a gas-prone hydrocarbon system in the Khorat Plateau even

if oil shows are confirmed in a couple of wells (Racey, 2011). The fingerprinting of

hydrocarbon source is very difficult as source rocks have been over-cooked. In the Khorat

Plateau Basin, two commercial gas fields were discovered from the Permian carbonates under

the Indosinian I unconformity, in addition, the significant gas discoveries have been found in

Permian carbonates in some wells (eg. Dong Mun-1, Si That-1). Gas shows in several wells

were found in sandstones of Mesozoic age above the Indosinian I Unconformity, however, it

was not accumulated certain volume of gases above the unconformity as far as confirmed at

the present results. Although it is assumed that excellent source rocks deposited in the Khorat

Plateau basin in late Paleozoic, it must be noted that only two commercial gas fields were found

in the vast sedimentary basin.

In the following, I describe the exploration history and results of the Khorat Plateau Basin, and study the gas reservoirs, generation of source rocks and trap mechanism, and summarize the overall petroleum systems.

Fig. 18 Plan map (a) and geological cross section (b) showing the geographic extent for the petroleum system. Modified from Magoon and Dow (1994)

6.2.2 Exploration and producing gas fields in the Khorat Plateau

The exploration history in the Khorat Plateau has been summarized by Booth (1998),

Sattayarak (2005), Racey (2011) . Union Oil was granted exploration license in early-1960s for the Khorat Plateau basin, and they drilled one exploration well, Kuchinarai-1 with only minor gas shows. The first gas discovery well in the Khorat Plateau was Nam Phong-1A which was drilled by Esso in 1981. At present, two gas fields, Nam Phong and Phu Horm are producing commercial gas from the Permian carbonates. UNOCAL (former Union Oil) and Mitsui Oil

Exploration Co., Ltd. explored in northeastern Khorat region nearby Laos, and drilled four wells with some gas shows from Triassic sandstones in early 1990s. The oil companies carried out seismic survey for whole Khorat Plateau region and drilled more than 40 exploration wells to date. The operators of oil companies in the area have included Total, Esso (Exxon), Unocal,

Shell, Texaco, Phillips, Amerada Hess, PTTEP, etc. In Laos, Pakse-1 and Naxay-1 were both dry with lack of Permian carbonate, but, Bang Nouan-1 was penetrated the Permian carbonates and encountered gas show in the Triassic section. Table 1 is the list of exploration wells, and

Fig. 19 shows the representative cross sections of producing gas fields for Phu Horm and Nam

Phong. The field outline is described below.

Phu Horm gas field: The field is located about 80km north of Khon Kaen city. The structure is the N-S trending, and the reservoirs are Permian Pak Nok Khao fractured carbonates. The initial proved reserves are reported at about 500 BCF (middle size) of gas 1.5 million barrels of condensate (Sattayarak, 2005). Booth (1998) interpreted several deformation episodes for gas fields below. During Indosinian I Orogeny the carbonate reservoirs were cut by thin- skinned thrusting and then cut by a transpressional strike-slip fault, before deeply eroded. A paleo-high structures that existed after the Indosinian II Orogeny were draped by the Lower

Nam Phong and Upper Nam Phong formations clastics, which form the top seals.

Nam Phong gas field: The field is located about 30 km north of Khon Kaen city. It forms a large N-S trending anticline of the Khorat Group which showed the inversion structure. The

reservoirs consist of Permian carbonates, and the gases are composed of dry and the proved

reserves of the field is about 450 BCF. Booth (1998) described complex tectonic history for the

gas field as below. The Saraburi Group, including the carbonate reservoir of the Pak Nok Khao

Formation, was cut by normal active faults at least in the later stages of its deposition. It was then thrusted and deeply eroded during the Indosinian I Orogeny. A paleo-high structure was formed during the Indosinian II Orogeny and draped by Lower Nam Phong Formation clastics, which form the top seal. In the Nam Phong 1 well, it was recorded at 500 ppm of H2S, which

may imply a marine source.

Indosinian II Unconformity Upper Triassic (Kuchinarai Group) Permian carbonate platform Indosinian I Unconformity

0 5 km

Fig. 19 (a) The structure of the Phu Horm gas field is the N-S trending, and the reservoirs are Permian Pak Nok Khao fractured carbonates. During the Indosinian I Orogeny the carbonate reservoirs was cut by thin-skinned thrusting and then cut by a transpressional strike-slip fault, before deeply eroded. A paleo-high structures that existed after the Indosinian II Orogeny was draped by the Lower and Upper Nam Phong formations clastics, which form the top seals. (b) The structure of the Nam Phong gas field forms a large N-S trending anticline of the Khorat Group which showed the inversion structure, and the reservoirs are Permian Pak Nok Khao fractured carbonates. The Permian carbonates are cut by normal faults after deposition, and then thrusted and deeply eroded during the Indosinian Orogeny. 61

6.2.3 Reservoirs

The main reservoir for gas exploration in the Khorat Plateau Basin is found in Lower to

Middle Permian carbonates (Pak Nok Khao Formation) as described in section 6.2.2. The

Permian carbonate reservoirs in the basin are composed of massive to thick-bedded limestone

and fractured dolomite deposited in the shallow marine environments. The Permian carbonates

as hydrocarbon reservoirs are present worldwide in the Middle East (Kendall et al., 2014),

North America, China, and also southern Thailand (Racey, 2014).

A study based on seismic and petrographic examination for the Permian carbonates in the

Khorat Plateau by Kozar et al. (1992) recognized eight 3rd -order depositional sequences ranging in age from the Late Cisuralian to Guadalupian (approximately 271–255Ma). Their study by core and seismic interpretation suggested that carbonate deposition occurred on isolated and apparently attached platforms as linear platform margin carbonate buildups and prograding platform-edge sands; high energy platform interior sands and low energy interior lagoonal muds and tidal flat deposits. These reservoirs normally contain low porosities and permeabilities. However, it was enhanced by early dolomitization, fracturing and karstification

in some parts. Karstification of Permian carbonates are considered to occur throughout the

Mesozoic in response to repeated uplift and weathering.

The secondary reservoirs are the sandstones of the Mesozoic Khorat Group. Racey et al.

(1996) and Canham et al. (1996) studied the sedimentology, sandstone petrography, petroleum reservoir quality, and diagenesis of Khorat Group rocks. Based on their studies, porosity vary from 4.9% in the lowest part of the Khorat Group to 11% in the uppermost part of it, and the ratio of secondary grain-dissolution porosity to primary porosity increases with age (and depth).

They also suggested on the basis of burial and history modeling that the basal part of the Khorat

Group reached a maximum depth of 8000 m and paleotemperatures of 150–170°C. The

sandstone reservoirs are generally tight as shown by the porosity range. It is considered that

the fracturing by tectonics and diagenesis is the condition for improving the reservoir quality.

However, based on the gentle structures with only a few faults and the limited porosity

enhancement by diagenesis in the Khorat Group, it is still high risk for reservoir quality.

Khorat Group sediments were deposited and deeply buried across the entire Khorat Plateau,

and they were considerably uplifted during the Paleocene (Mouret et al., 1993, Mouret, 1994).

The deep burial of Khorat Group rocks contributed to hydrocarbon generation in the underlying

source rocks of the Kuchinarai Group.

6.2.4 Source rocks and maturation

Before discussing the geochemical study for the source rocks and maturation in the basin, I

introduce the principal theory and methods for the petroleum geochemistry by quoting

Bordenave and Durand (1993) as below. Oil and most of gas are produced as the result of the

gentle “cooking” of organic matter contained in organically rich sediments: increasing

temperatures that accompany burial results in a progressive cracking, i.e. a thermal breakdown

of sedimentary organic matter. This cracking produces both mobile, migratable products

enriched in hydrogen, essentially made up hydrocarbons, and insoluble, hydrogen-depleted,

carbonaceous solid residuum (i.e. kerogen and pyrobitumen). As for the source rock evaluation,

three basic components for the quantity (TOC), type of organic matter (kerogen), and

maturation will be measured. The TOC of a rock sample is measured by combustion of the

organic matter under air or oxygen atmosphere at a certain temperature. The type of organic

matter was typically measured by the Rock-Eval Pyrolysis method which is the decomposition of organic matter by heating in the absence of oxygen. The maturation was measured by several

methods, typically Vitrinite Reflectance (Ro value) method which increases almost linearly

63 with depth in continuously subsiding basins.

In previous geochemistry studies for the Khorat Plateau Basin, Chantong and Booth (2007),

Thongboonruang (2008), Koysamran and Comtrie-Smith (2011), and Racey (2011) described the source rocks abilities, hydrocarbon maturation, and timing. The source rocks are assumed to be multiple horizons, but there are mainly two favorable source rocks for each sedimentary environments in the area, shallow marine behind carbonate reefs and lacustrine in continental areas. Satyana et al. (2000) concluded about the Miocene carbonate reservoirs in the eastern

Indonesia by a geochemical study that the proven source rocks of the Salawati Basin are shales, marls, and carbonate of the Kais and Klasfet formations deposited in a lagoonal area behind carbonate reefs. Considering about paleo-geothermal gradient, the present-day geothermal gradients are fairly low and they are in the range of 1.72 - 2.49 ℃/100m (Racey, 2011).

However, much higher paleo-temperature was estimated by geochemical basin modeling.

I regard as important candidates of source rocks are as follows (Fig. 5).

・Si That formation (Upper Carboniferous)

The Si That Formation consists mainly of calcareous shales and carbonates and interbedded sandstones in the Non Sung-1 well, and coal, chert, and evaporite of outcrop in the Loei area

(Ueno and Charoentitirat, 2011). The formation was deposited in a shallow-marine to littoral– fluvial environment. There was no report on the organic contents as source rock ability, but it is assumed as a candidate of source rocks considering the depositional environments.

・Pha Nok Khao Formation (Permian)

During the early to middle Permian, the deposition was controlled by horst features with carbonate reef and platforms of the Pha Nok Khao Formation. The source rocks for the formations are thought to be lagoonal organic rich shales behind reef buildups or deeper shales between carbonate platforms. As for total organic carbon (TOC), it showed 0.5% to 30.05% in

the Phu Lop-1 well and 0.9% to 18.19% in the Phu Wiang-1 well, which are interpreted to be fair to excellent organic richness and late to over-mature and to have kerogen type III/II

(Thongboonruang, 2008).

・Kuchinarai Group (Upper Triassic)

The Kuchinarai Group contains coarse, fluvial, alluvial-fans deposits and organic-rich lacustrine shales interbedded with volcaniclastic rocks. Chantong and Booth (2007) described the potential source rocks of Kuchinarai group as follows. The geochemical analysis shows that the dark gray shale in the middle part of the Kuchinarai Group are rich in organic content ranging from 1.0 to 3.7 % of TOC and are potentially good sources of oil and gas hydrocarbons.

As for the vitrinite reflectance value (Ro) in the Tao area of Laos and at the Chum Phae in the western part of Khorat, it showed higher value exceeding 2.5% (over-mature), which was probably a higher TOC value prior to maturation and destruction of kerogen (Koysamran and

Comtrie-Smith, 2011). As for other source rocks data, Triassic rock samples from the outcrop contain TOC between 1.06% to 5.76%, which shows fair to excellent organic richness and kerogen type III/II in the stage of late to over-mature.

There is a discussion for the source rocks of gases in the Permian carbonates, whether it comes from shallow marine shales of late Paleozoic or lacustrine shales of Triassic. Considering the volume of source rocks, the shallow marine source rocks of the late Paleozoic are much larger than Triassic lacustrine shales.

The basin modelling has been carried out to understand the petroleum system in the basin, and it was run a simulation by the software of “BasinMod 1D” developed by Platt River company. The analysis has been integrated with well data and tectonic history to determine the burial history, maturity, and timing for hydrocarbon generation in the basin. The modeling points include actual wells (Non Sung-1, Phu Phra-1 and Si That-1) and in the deep Kuchinarai

graben (set pseudo well). The burial history diagrams for the representative wells are shown in

Fig. 20. The well calibration of modeling in which three wells for lithology, vitrinite reflectance

and temperature as input data, suggests that the paleo-heat flow was higher in the early Triassic

(120 mW/m2) and lower at present (60 mW/m2) (Koysamran and Comrie-Smith, 2011). The results suggested that most of Permian and Upper Carboniferous source rocks generated and expelled gases during the late Permian and Early Triassic, and Triassic source rocks generated and expelled in the late Jurassic and Cretaceous. Inversion in the Middle Cretaceous and

Paleocene is likely to have terminated generation. Source rock maturity in the basin is difficult to reconstruct due to uncertainties about the existence and geometry of source rocks by subsequent uplift and erosion.

Fig. 20 Burial history diagram by geochemical basin modeling. The diagrams show the actual well of Non Sung-1, Phu Phra-1 on this page (a), Si That-1and pseudo well in deep Kuchinarai graben on next page (b). The location of well is shown on following page (c). The well calibration of modeling which used 3 wells for lithology, vitrinite reflectance and temperature as input data, suggests that the paleo heat flow was h igher in early Triassic (approximately 120mW/m2). It was applied the paleo heat flow value of 40 mW/m2 in late Carboniferous and 60 mW/m2 in recent (Koysamran and Comrie-Smith, 2011).

Fig. 20 (Continued)

6.2.5 Migration

Hydrocarbon from the Permian source rocks was directly updip migration into the Permian

carbonate reservoirs as described in section 6.2.2 for the two gas fields. For the Triassic

lacustrine source rocks as secondary candidate in the half graben, the hydrocarbon migration

is considered to be vertical through Triassic extensional faults into the Permian carbonates on

up-thrown blocks.

It can be considered as a possibility that the hydrocarbon remigration to the reservoirs of

Khorat Group occurred from older accumulations. The Middle Cretaceous and Paleogene

faulting provide viable pathways, but based on the gentle structures with only a few faults in

the Khorat Group, the migration pathways from the late Paleozoic and Triassic sources are not

generally well-developed.

6.2.6 Trap

There is a wide variety of hydrocarbon trap types during the Permian and Mesozoic due to

the complex tectonic history in the Khorat Plateau Basin (Fig. 21). The combination trap of

“structural-unconformity” for the Pha Nok Khao Carbonates is the primary target for the

exploration which is proven as two gas fields (Type ① in Fig. 21). The reservoirs consist of

the Pha Nok Khao carbonates which are sealed by four-way dip closure on the Indosinian I

Unconformity. Another type of the Pha Nok Khao carbonates is Type ② which typically includes the carbonate reef buildups as an example of the Dong Mun structure. The sandstones for the Kuchinarai Group, and the Lower and Upper Nam Phong formations from the Triassic to Jurassic are assumed to form the trap of “structure” or “combination of structure and unconformity”(Type ④ in Fig. 21). There encountered gas shows in Cretaceous Khorat

Group sandstones, which form the “structural trap” (Type ④ in Fig. 21).

As the Pha Nok Khao carbonates are proved as the trap play type, it is important to recognize the Permian carbonate reservoir features by improving the seismic resolution by acquiring 3D seismic survey.

6.2.7 Seal and Preservation

The main seal rocks in the basin are shales and claystones of the Kuchinarai Group, Lower

and Upper Nam Phong formations for Permian carbonates reservoirs. These sediments are

present across most of the basin (Fig. 6). The seal for the sandstone of the Khorat Group is

provided by interbeded shale and claystones. Preservation for the traps is a critical issue in the

basin because of the complex tectonic history since the Triassic.

. n i s a b e h t n i s u o e c a t e C r d n a n a i m r e P e h ) , t 1 g 1 n 0 i r 2 u y d e s c l l e R a w d f n o a g 8 n 9 i t 9 t 1 e s h t c o i h p B o r a m g i t f r o r a t d s e d f i i n d a o l r a ( m u t m c a u r r t g s a i f d o t y p t e e c r i n a o v c e y d a i l w p e n h o t i t g r a n i o l w p o x h E s 1 2 . g i F

6.3 Summary of petroleum geology

There is a gas-prone petroleum system in the Khorat Plateau Basin. It was summarized as the petroleum systems event chart in Fig. 22. The Permian and Upper carboniferous source rocks generated gases during late Permian and Early Triassic, and Triassic source rocks generated in late Jurassic and Cretaceous (Fig. 22). Most of the structural traps under Indosinian I unconformity were formed by Indosinian Orogeny (latest Permian to Late Triassic). The structural traps above Indosinian I unconformity were formed by multiple tectonic events.

Although it is assumed that excellent source rocks deposited in the Khorat Plateau Basin in the late Paleozoic and Late Triassic, as mentioned the issue previously, only two commercial gas fields as the evidence of proven petroleum system were found in the vast sedimentary basin up to present. I can here point out several reasons on these issues, from the viewpoints of the source rocks maturity and timing, trap preservation, and effective reservoir characteristics as below. As for the issue of maturation timing by the basin modeling, the hydrocarbon generation for the late Paleozoic source rocks was presumably quicker by high paleo-heat flow and deep burial in the late Permian. Therefore, it is considered to occur the destruction of traps and leakage of hydrocarbon to the surface because of many times of structural deformations in a long time during Mesozoic to recent. In addition, the Permian carbonates are necessary to form the condition for enhancing the porosity and permeability by karstification and fracturing.

Therefore, it was focused on the areas where the carbonate platforms were truncated by the

Indosinian I unconformity. The Mesozoic sandstones show generally poor reservoir characteristics indicating low porosity and permeability.

There are many uncertainties regarding the conclusion of petroleum systems of the Khorat

Plateau Basin. These are ultimately the reflections of differing tectonic models, identification of source rocks horizons, and estimation of paleo-heat flows etc.

e n e

g c o i ) . c e s c i i s N s o r a s a u z i e J r o n e T t n e r a l e e g ) . p n o c i p i C 0 e s d l s 5 ( U e a t a s r i a r a P T e r e n e a t e l a g a L t s n k o e t c n i n r o t a i s n e o u r c c r m o u 0 e n o i e 0 P s t c e 1 c s n a i e t s t r i s t a e s l a r ( u r i e c y C c T a r n l u e d o d n g s a n c r o a i , s c ) O i s c s a i i n s r o a a 0 T i z c r i n 5 e o t i i T e 1 s a l o L o y a r d z r l P c o i a r F I n o s e E s t y s p s d n b p e a n e d r a v e ( U e M u n a c J f s r m i i e n e f o o r r m t e c e r e t 0 P e e t a 0 e n w t e 2 l o a y l p t i i r b t g e l a c m n c u r r c i r i u d m o u f o s n s n d i y s c o b h i s a c e o e i d z n s b r e o a u e e T l g I n a r m d n r i P e a f o a r t 0 i e n m 5 r a p r e i p e s e 2 w U n o w o e r d l o g l y n n t F I a s i a h r i k s e c r m r d o m r o n r f o f u e n e s o s c r c k P c i p c u n r a o o u t r o s z I l 0 e s n o u 0 r a r c a u s i 3 e u t r o l n o c u i f e s a s i o r u . e o n t r l P r t s d o e b a n f e I h r b i ) r a h c a t e o a n c t v f v o r n o o M e e f a ( b b t v p s a r o e p o s a w s U t p M C m d y . r a l e t s n t n a l u s i k o y a n r a s e k c a u m c i / n t k c a o c m t n o n r e c u o r m r u R a o b e e o t l i o i r s R t C r e P r r e p s a r o i e R a e e t l d c e a r h e h h n o u r a e o T P p c T T v p r r T n a r x d 2 e u e C E y 2 G o s . H g e S i F R

7. Conclusions

The main conclusions of my study are summarized below.

1) The Mesozoic sequence of the Khorat Plateau Basin comprises thick, gently folded non-

marine sediments. In the deeper subsurface, the thick Upper Paleozoic sequence below the

Indosinian I unconformity is intensely deformed and geologically complex in some area.

Late Carboniferous and Permian shallow-marine carbonates and shales have a maximum

thickness of about 6000 m in a series of distinct horst and graben systems that formed

during Upper Paleozoic rifting. On the basis of structural trends in the basin, the Upper

Paleozoic sequence can be subdivided two distinct tectonic regions: dominantly NW–SE

to E–W trends in the northeastern, central and southern parts of the basin and NNE–SSW

trends in its westernmost part.

2) Seismic profiles provide clear evidence of intense thrust faulting and folding in the Upper

Paleozoic section below the Indosinian I unconformity in the area of the Phu Phan Uplift

in the northeastern part of the Khorat Plateau Basin and in the Dao Ruang Fold Belt in the

westernmost part of the basin. Major erosion during the Indosinian I event removed

approximately 2000 m - thick Upper Paleozoic rocks in some areas of the Khorat Plateau

Basin; this erosion has previously been attributed to uplift associated with subduction of

the Paleo-Tethys Ocean. On the basis of the dominant NW–SE to E–W structural trends

over most of the basin, I conclude that the Indosinian I event was caused mainly by uplift

associated with the collision of the South China and Indochina blocks.

3) Metamorphic rocks in the Song Ma Suture zone provide evidence of a continental collision

during the Late Permian and Middle Triassic. The robust amalgamated continental block

produced by the collision presumably enhanced subduction of the Paleo-Tethys Ocean,

which intensified volcanic activity in the Sukhothai Arc. The Nan back-arc basin closed

during the early Late Triassic before the Indochina Block collided with the Sibumasu Block

during the latest Triassic and closed the Paleo-Tethys Ocean. Thus, I propose that the

Indosinian Orogeny was initiated by the collision of the South China and Indochina blocks

and that paleo-arc systems then developed in response to subduction of the Paleo-Tethys

Ocean.

4) At present there are two commercial gas fields producing gas from Permian carbonates in

the basin. It was considered in the evaluation of petroleum geology from viewpoints of

reservoirs, source rocks, seals, migration, and traps. There are two potential source rocks

of gases; the shallow marine shales behind carbonate reefs of the late Paleozoic as primary

source rocks, and the lacustrine shales in continental areas of the Triassic as secondary

source rocks. The hydrocarbon maturation by the basin modeling suggested that the late

Paleozoic source rocks generated gases during the late Permian and Early Triassic, and

Triassic source rocks generated in the late Jurassic and Cretaceous. It is considered that the

structural deformations occurred many times during Mesozoic to recent, which made the

destruction of traps and leakage of hydrocarbon to the surface. The preservation for gas

reservoirs is a critical issue for petroleum exploration in the Khorat Plateau Basin. As an

actual method for further exploration success, it is important to recognize the Permian

carbonate reservoirs accurately by improving the seismic resolution with acquiring 3D

seismic survey.

Acknowledgements

I thank the Department of Mineral Fuels of Thailand and Chevron Thailand for their

permission to disclose the data and maps. I also thank Dr. Hidetoshi Hara of the Geological

Survey of Japan, AIST, Dr. Punya Charusiri of Chulalongkorn University and Professor Dr.

Owada at Yamaguchi University for valuable discussions. Support for the field excursion of

this study from the Department of Geology at Chulalongkorn University and the Department of Geotechnology at Khon Kaen University is gratefully acknowledged. I also thank staff of the University of Tsukuba and Mitsui Oil Exploration Co., Ltd. for their support and discussions about this study. For the satellite image basemap, the GIS system named as

“GRIAS” based on ASTER GDEM was supplied by J-spacesystems founded by Japanese

Ministry of Economy, Trade and Industry.

References

Arboit, F., Collins, A.S., King, R.C., Morley, C.K., Hansberry, R.L., 2014. Structure of the

Sibumasu-Indochina Collision, Central Thailand: A section through the Khao Khwang Fold and Thrust Belt. Journal of Asian Earth Science 95., 182-191.

Barber, A.J., Ridd, M.F., Crow, M.J., 2011. The origin, movement and assembly of the pre-

Tertiary tectonic units of Thailand. In: Ridd, M.F., Barber, A.J., Crow, M.J. (Eds.), The

Geology of Thailand. Geological Society, London, pp. 507–537.

Booth, J., 1998. The Khorat Plateau of NE Thailand – exploration history and hydrocarbon potential. Proceedings of the 1998 SEAPEX Exploration Conference, Singapore, 169-202.

Booth, J., Comrie-Smith, N., Koysamran, S., Sattayarak, N., 2014. SEAPEX Feld Trip Guide;

Permian carbonate reservoirs of the Khorat Basin Complex NE Thailand, SEAPEX.

Booth, J., Sattayarak, N., 2011. Subsurface Carboniferous – Cretaceous geology of Northeast

Thailand. In: Ridd, M.F., Barber, A.J., Crow, M.J. (Eds.), The Geology of Thailand. Geological

Society, London, pp. 185–222.

Bordenave, M.L., 1993. Screening techniques for source rock evaluation. In: Bordenave, M.L.,

(Eds.), Applied Petroleum Geochemistry, pp.217-278.

Burnhill, T.J., 2010. Oil and Gas Exploration in the Lao PDR. Proceedings of Thai-Lao

Technical Conference on Geology and Mineral Resources, September 7-8. 2010, pp. 239-251.

Burrett, C., Khin Zaw, Meffer, S., Lai, C.K., Khositanont, S., Chaodumrong, P., Udchachon,

M., Ekins, S., Halpin, J., 2014. The configuration of Greater Gondwana evidence from LA

ICPMS, U-Pb geochronology of detrital zircons from the Palaeozoic and Mesozoic of

Southeast Asia and China. Gondwana Research. 26, 31–51.

Canham, M.A., Love, M.A., Racey, A. and Polachan 1996. Stratigraphy and reservoir

potential of Mesozoic Khorat Group, NE Thaiand: Part 2 Diagenesis and Reservoir Quality.

Journal of Petroleum Geology, 19(3), pp. 321-338.

Chantong, W., Booth J., 2007. Is the Kuchinarai Group of Khorat Plateau a good source of

hydrocarbons ? Proceedings of GEOTHAI 07 International Conference of Thailand: Towards

Sustainable Development and Sufficiency Economy, p. 132.

Chantong, W., Srisuwon, P., Kaewkor, C., Praipipan, C., Ponsri, S., 2013. Distributions of the

Permo-Carboniferous rocks in the Khorat Plateau Basin. Proceedings of the 2nd Lao-Thai

Technical Conference on Geology and Mineral Resources, January 17-18, 2013, pp. 73-80.

Chaodumrong, P., Burrett, C., 2014. Upper Paleozoic. In: Nuchanong, T., et al. (Eds.), Geology of Thailand. Department of Mineral Resources of Thailand, pp. 85-111.

Charusiri, P., Daorerk, V., Archibald, D., Hisada, K.-I., Ampaiwan, T., 2002. Geotectonic evolution of Thailand: a new synthesis. Journal of the Geological Society of Thailand 1, 1–20.

Chonglakmani, C., 2011. Triassic. In: Ridd, M.F., Barber, A.J., Crow, M.J. (Eds.), The

Geology of Thailand. Geological Society, London, pp. 137–150.

Chuaviroj S., 1997. Deformation in Khorat Plateau, Thailand. Proceedings of International

Conference on Stratigraphy and Tectonic Evolution of Southeast Asia and the South Pacific,

Bangkok, Thailand, 19-24 August 1997, p. 321-325.

Cromie, P., Makoundi, C., Khin Zaw, Cooke, D.R., White, N., Ryan, C., 2018. Geochemistry

of Au-bearing pyrite from the Sepon Mineral District, Laos DPR, Southeast Asia: Implications for ore genesis. Journal of Asian Earth Science 164, 194-218.

Cullen, P. J., Birch, P. L., Wright, S. C., Keamey, C. J., Pink, A. T., 1997. Exploration in the

Savannakhet basin, Peoples Democratic Republic of Laos. In: Howes, J. V. C. and Noble, R.

A. (Eds.), Proceedings of Petroleum Systems of SE Asia and Australasia Conference, Indonesia

Petroleum Association, May 1997, Jakarta, p.425-447

Faure, M., Lepvrier, C., Nguyen, V.V., Vu, T.V., Lin, W., Chen, Z., 2014. The South China

block–Indochina collision: where, when, and how? Journal of Asian Earth Sciences, 79 (Part

A), 260–274.

Fyhn, M.B.W., Pedersen, S.A.S., Boldreel, L.O., Nielsen, L.H., Green, P.F., Dien, P.T., Huyen,

L.T., Frei, D., 2010. Paleocene-early Eocene inversion of the Phuquoc-Kampot Som Basin: SE

Asian deformation associated with the suturing of Luconia. Journal of the Geological Society

80 of London, Vol. 167, pp. 281–295.

Gardiner, N.J., Searle, M.P., Morley, C.K., Whitehouse, M.P., Spencer, C.J., Robb, L.J.,

2016. The closure of Palaeo-Tethys in Eastern Myanmar and Northern Thailand: new insights from zircon U-Pb and Hf isotope data. Gondwana Research 39, 401–422.

Gardner, N.J., Graham, I.T., Belousova, E., Booth, G.W., Greg, A., 2017. Evidence for

Ordovician subduction-related magmatism in the Truong Son terrane, SE Laos: Implications for Gondwana evolution and porphyry Cu exploration potential in SE Asia. Gondwana

Research 44, 139–156.

Hada, S., Bunopas, S., Ishii, K., Yoshikura, S., 1999. Rift-drift history and the amalgamation of Shan-Tai and Indochina/East Malaya Blocks. In: Metcalfe, I. (Ed.), Gondwana Dispersion and Asian Accretion. A. A. Balkema, Rotterdam, pp. 67–87.

Halpin, J.A., Tran, H.T, Lai, C.-K., Meffre, S., Crawford, A.J., Zaw, K., 2016. U–Pb zircon geochronology and geochemistry from NE Vietnam: A ‘tectonically disputed’ territory between the Indochina and South China blocks. Gondwana Research 34, 254–273.

Hara, H., Kunii, M., Hisada, K., Ueno, K., Kamata, Y., Srichan, W., Charusiri, P.,

Charoentitirat, T., Watarai, M., Adachi, Y., Kurihara, T., 2012. Petrography and geochemistry of clastic rocks within the Inthanon zone, northern Thailand: implications for Paleo-Tethys subduction and convergence. Journal of Asian Earth Sciences 61, 2–15.

Hara, H., Kunii, M., Miyake, Y., Hisada, K., Kamata, Y., Ueno, K., Kon. Y., Kurihara, T.,

Ueda, H., Assavapatchara. S., Treerotchananon. A., Charoentitirat, T., Charusiri, P., 2017.

Sandstone provenance and U-Pb ages of detrial zircons from Permian-Triassic forearc sediments within the Sukhothai Arc, northern Thailand: Record of volcanic evolution in response to Paleo-Tethys subduction. Journal of Asian Earth Sciences 146, 30–55.

Hara, H., Wakita, K., Ueno, K., Kamata, Y., Hisada, K., Charusiri, P., Charoentitirat, T.,

Chaodumrong, P., 2009. Nature of accretion related to Paleo-Tethys subduction recorded in northern Thailand: constraints from mélange kinematics and illite crystallinity. Gondwana

Research 16, 310–320.

Hisada, K., Charusiri, P., Ueno, K., Kamata, Y., Hara, H., Okuzawa, K., Charoentitirat, T.,

Chutakositkanon, V., Ampaiwan, T., 2005. Suture line in collision zone: a case of northern

Thailand. In: International Conference on Geology, Geotechnology and Mineral Resources of

Indochina (GEOINDO 2005), 28–30 November 2005, Khon Kaen, Thailand, pp. 466–467.

Hoang, C.M., Du, M.V., Hoan, P.K., Hung, T.D., 2013. Hydrocarbon Potential of Champasak

& Saravan area, Southern Lao PDR. Proceeding of the 2nd Thai-Lao Technical Conference on

Geology and Mineral Resources, January 17-18, 2013, pp. 226-235.

Horiuchi, Yu., Charusiri, P., Hisada, K., 2012. Identification of an anastomosing river system in the Early Cretaceous Khorat Basin, northeastern Thailand, using stratigraphy and paleosols.

Journal of Asian Earth Sciences 62, 62–77.

Hu, L., Cawood, P.A., Du, Y., Xu, Y., Wang, C., Wang, Z., Ma, Q., Xu, X., 2017. Permo-

Triassic detrital records of South China and implications for the Indosinian events in East Asia.

Palaeogeography, Palaeoclimatology, Palaeoecology 485, 84–100.

Kamata, Y., Ueno, K., Hara, H., Ichise, M., Charoentitirat, T., Charusiri, P., Sardsud, A.,

Hisada, K., 2009. Classification of the Sibumasu–Palaeotethys tectonic division in Thailand using chert lithofacies. Island Arc 18, 21–31.

Kamvong, T., Zaw, K., Meffre, S., Maas, R., Stein, H., Lai, C.-K., 2014. Adakites in the Truong

Son and Loei fold belts, Thailand and Laos: genesis and implications for geodynamics and

metallogeny. Gondwana Research 26, 165–184.

Kendall, C., G., C., Alsharhan, A., S., Marlow, L., 2014. Stratigraphy and depositional systems

of the Southern Tethyan Region. In: Marlow, L., Kendall, C., Yose, L., (Eds.), Petroleum

systems of the Tethyan region: AAPG Memoir 106, p. 29-57.

Khin Zaw, M., Meffre, S., Lai, C.-K, B., Burrett, C., Santosh, M., Graham, I., Manaka, T.,

Salam, A., Kamvong, T., Cromie, P., 2014. Tectonics and metallogeny of mainland Southeast

Asia — A review and contribution. Gondwana Research 26, 5–30.

Koysamran, S., Comrie-Smith, N., 2011. Basin Modeling of Block L26/50, Eastern Khorat

Plateau, Northeast Thailand. Proceeding of the 4th Petroleum Forum, May 26-27, 2011, pp. 5-

10.

Kozar, M.G., Crandall, G.F., Hall, S.E., 1992. Integrated structural and stratigraphic study of

the Khorat Basin, Rat Buri Limestone (Permian), Thailand. In: Piancharoen, C. (Ed.),

Proceedings of a National Conference on Geologic Resources of Thailand: Potential for Future

Development, Bangkok, 17–24 November 1992, pp. 692–736.

Kuroda, J., Hara, H., Ueno, K., Charoentitirat, T., Maruoka T., Miyazaki, T., Miyahigashi, A.,

Lugli, S., 2017. Characterization of sulfate mineral deposits in central Thailand. Island Arc 26

(2), 1-13.

Lai, C.-K., Meffre, S., Crawford, A.J., Zaw, K., Xue, C.-D., Halpin, J.A., 2014. The Western

Ailaoshan Volcanic Belts and their SE Asia connection: a new tectonic model for the Eastern

Indochina Block. Gondwana Research 26 (1), 52–74.

Lepvrier, C., Maluski, H., Nyugen, V.V., Roques, D., Axente, V., Rangin, G., 1997. Indochina

NW-trending shear zones within the Truong Son belt (Vietnam). Tectonophysics 283, 105–

127

Lepvrier, C., Maluski, H., Tich, V.V,, Leyreloup, A., Thi, P.T., Vuong, N.V., 2004. The Early

Triassic Indosinian orogeny in Vietnam (Truong Son Belt and KontumMassif); implications for the geodynamic evolution of Indochina. Tectonophysics 393, 87–118.

Lepvrier, C., Vuong, N.V, Maluski, H., Thi, P.T., Vu, T.V, 2008. Indosinian tectonics in

Vietnam. Comptes Rendus Geoscience 340, 94–111.

Liu, J., Tran,M., Tang, Y., Nguyen, Q.L., Tran, T.H.,Wu,W., Chen, J., Zhang, Z., Zhao, Z.,

2012. Permo-Triassic granitoids in the northern part of the Truong Son belt, NW Vietnam: geochronology, geochemistry and tectonic implications. Gondwana Research 122, 628–644.

Lovatt-Smith, P.F., Stokes, R.B., 1997. Geology and petroleum potential of the Khorat Plateau basin in the Vientiane area of Lao P.D.R.. Journal of Petroleum Geology, vol. 20 (1), pp 27-

50.

Lovatt-Smith, P.F., Stokes, R.B., Bristow, C., and Carter, A. 1996. Mid-Cretaceous inversion in the northern Khorat Plateau of Lao PDR and Thailand. In: Hall, R. & Blundell, D. J. (Eds.)

Tectonic Evolution of SE Asia. Geological Society, London, Special Publications, 106, 233-

247.

Magoon, L.B., Dow, W.G., 1994. The Petroleum System. In: Magoon, L.B, and Dow, W.G.

(Eds.), The Petroleum System - from source to trap, AAPG Memoir 60, pp. 3–24.

Maluski, H., Lepvrier, C., Leyreloup, A., Tich, V.V., Thi, P.T., 2005. 40Ar40–39Ar

geochronology of the charnockites and granulites of the Kan Nack complex, Kon Tum Massif,

Vietnam. Journal of Asian Earth Sciences 25, 653–677.

Manaka, T., Khin Zaw, Meffre, S., Vasconcelos, P., Golding, S., 2014. The Ban Houayxai

epithermal Au–Ag deposit in the Northern Lao PDR: mineralizationrelated to the Early

Permian arc magmatism of the Truong Son Fold Belt. Gondwana Research 26, 185–197.

Meesook, A., 2011. Cretaceous. In: Ridd, M.F., Barber, A.J., Crow, M.J. (Eds.), The Geology

of Thailand. Geological Society, London, pp. 169–184.

Metcalfe, I., 2011. Palaeozoic–Mesozoic history of SE Asia. In: Hall, R., Cottam, M. A.,

Wilson, M. E. J. (Eds.), The SE Asian Gateway: History and Tectonics of the Australia-Asia

Collision. Geological Society, London, Special Publications, 355, pp. 7–35.

Metcalfe, I., 2013. Gondwana dispersion and Asian accretion: tectonic and palaeogeographic evolution of eastern Tethys. Journal of Asian Earth Sciences 66, 1–33.

Minezaki, T., Moriyama, K., 2002. The origin of hydrocarbon and carbon dioxide in the gas fields of the Pattani Trough, the Gulf of Thailand. Journal of the Japanese association for petroleum technology, 67, No.1, 16–29.

Morley, C.K., Ampaiwan, P., Thanudamrong, S., Kuenphan, N., Warren, J., 2013.

Development of the Khao Khwang Fold and Thrust Belt: Implications for the geodynamic setting of Thailand and Cambodia during the Indosinian Orogeny. Journal of Asian Earth

Sciences 62, 705–719.

Mouret, C., 1994. Geological history of northeastern Thailand since the Carboniferous.

Relations with Indochina and Carboniferous to Early Cenozoic evolution model. In:

Angsuwathana, P., Wongwanich, T., Tansathien, W., Wongsomsak, S., Tulyatid, J. (Eds.),

Proceeding of the International Symposium on Stratigraphic Correlation of Southeast Asia

(IGCP306), Bangkok, 15-20 November 1994, 132-158.

Mouret, C., Heggemann, H., Gouadain, J., Krisadasima, S. 1993, Geological history of the

siliciclastic Mesozoic strata of the Khorat Group in the Phu Phan Range area, Northeastern

Thailand; Proceeding of the International Symposium on Biostratigraphy of Mainland

Southeast Asia: Facies & Paleontology, 31 January – 5 February 1993, Chiang Mai, Thailand, v. 1, 23-49.

Nakano, N., Osanai, Y., Owada, M., Tran, N.N., Charusiri, P., Khamphavong, K., 2013.

Tectonic evolution of high-grade metamorphic terranes in central Vietnam: constraints from large-scale monazite geochronology. Journal of Asian Earth Science 73, 520–539.

Nakano, N., Osanai, Y., Sajeev, K., Hayasaka, Y., Miyamoto, T., Minh, N.T., Owada, M.,

Windley, B., 2010. Triassic eclogite from northern Vietnam: inferences and geological significance. Journal of Metamorphic Geology 28, 59–76.

Nildam, S., Veeravinantanakul, A., Kumrangwat, S., Salami, A., Charusiri, P., 2016.

Petrochemistry of volcanic rocks of Sisophon Area in Northwestern Cambodia: Implication for tectonic setting. Proceeding of the 5th Symposium of International Geoscience Programme

(IGCP 589), 27–28 October 2016, Yangon, Myanmar, pp. 13.

Osanai, Y., Nakano, N., Owada, M., Nam, T.N., Miyamoto, T., Minh N.T., Nam, N.V., Tri,

T.V., 2008. Collision zone metamorphism in Vietnam and adjacent South-eastern Asia:

Proposition for Trans Vietnam Orogenic Belt. Journal of Mineralogical and Petrological

Sciences 103, 226–241.

Owada, M., Osanai, Y., Nakano, N., Adachi, T., Kitano, I., Tri, T., V., Kagami, H., 2016. Late

Permian plume-related magmatism and tectonothermal events in the Kontum Massif, central

Vietnam. Journal of Mineralogical and Petrological Sciences 111, 181–195.

Polachan, S., Chantong, W., Srisuwon, P., Kaewkor., C., Kaewkor C., Praipiban, C., 2010.

Petroleum Potential of the Khorat Plateau, Thailand. Proceeding of the Thai-Lao Technical

Conference on Geology and Mineral Resources, September 7-8, 2010, pp. 42-63.

Qian, X., Feng, Q., Yang, W., Wang, Y., Chonglakmani, C., Monjai, D., 2015. Arc-like volcanic rocks in NW Laos: Geochronological and geochemical constraints and their tectonic implications. Journal of Asian Earth Science 98, 342–357.

Qiu, L., Yan, D.-P., Yang, W.-X., Wang, J., Tang, X. and Ariser, S., 2017. Early to Middle

Triassic sedimentary records in the Youjiang Basin, South China: implications for Indosinian orogenesis. Journal of Asian Earth Science 141, 125–139.

Racey, A., 2011. Petroleum geology. In: Ridd, M.F., Barber, A.J., Crow, M.J. (Eds.), The

Geology of Thailand. Geological Society, London, pp. 351–392.

Ridd, M.F., 2012. The role of strike-slip faults in the displacement of the Palaeotethys suture zone in Southeast Thailand. Journal of Asian Earth Sciences 51, 63–84.

Rossignol, C., Bourquina, S., Hallota, E., Poujola M., Dabarda, M.-P., Martinic, R.,

Villeneuved, M., Cornéee, J.=J., Brayardg, A., Rogere, F., 2018. The Indosinian orogeny: A perspective from sedimentary archives of north Vietnam. Journal of Asian Earth Sciences

158, 352–380.

Sattayarak, N. 2005. Petroleum Potential of Northeast, Thailand. Proceedings of International

Conference on Geology, Geotechnology and Mineral Resources of Indochina (GEOINDO

2005), 28-30 November 2005, Khon Kaen, Thailand, pp. 21-30.

Satyana, A.H., Salim, Y., Demarest, J.,M., 1999. Significance of focused hydrocarbon migration in the Salawati basin: Controls of faulted and structural noses. Proceeding, Indonesia

Petroleum Association, 27th Annual Convention, p.513-529.

Sone, M., Metcalfe, I., 2008. Parallel Tethyan sutures in mainland Southeast Asia: new insights for Palaeo-Tethys closure and implications for the Indosinian orogeny. Compte Rendus

Geosciences 340, 166–179.

Surakotra, N., Pisutha‐Arnond, V., Warren, J. K. ,2005. Some characteristics of gypsum‐ anhydrite deposit in the Loei‐Wang Saphung, Northeastern Thailand. Proceedings of the

International Conference on Geology, Geotechnology and Mineral Resources of Indochina

(GEOINDO 2005) 28-30 November 2005, Khon Kaen, Thailand, pp. 421–430.

Thongboonruang, C., 2008. Petroleum Source Rock Potential of NE Thailand. Proceedings of the 2nd Petroleum Forum: Blooming Era of Northeastern Thailand, 15-16 September, 2008,

Department of Mineral Fuels, Bangkok, Thailand. 33-50.

Udchachon, M., Burrett, C., Thassanapak, H., Chonglakmani, C., Campbell, H., Qinglai, F.,

2014. Depositional setting and paleoenvironment of an alatoconchid-bearing Middle Permian carbonate ramp sequence in the Indochina Terrane. Journal of Asian Earth Sciences 87, pp.

37–55.

Ueno, K., Charoentitirat, T. 2011. Carboniferous and Permian. In: Ridd, M.F., Barber, A.J.

Crow, M.J. (eds.), The Geology of Thailand, Geological Society of London, pp. 71–136.

Ueno, K., Hisada, K., 2001. The Nan-Uttaradit-Sra Kaeo Suture as a Main Paleo-Tethyan

Suture in Thailand: Is it Real? Gondwana Research 4, 804–806.

Vysotsky, V.I., Rodnikova, R.D., Li, M.N., 1994. The geology of Cambodia. Journal of

Petroleum Geology 17 (2), pp. 195–210.

Wongwanich, T., Boucot, A., J., 2011. Devonian. In: Ridd, M.F., Barber, A.J., Crow, M.J.

(Eds.), The Geology of Thailand. Geological Society, London, pp. 53–70.

Yang, W., Qian, X., Feng, Q., Shen, S., Chonglakmani, C., 2016. Zircon U-Pb geochronological evidence for the evolution of the Nan-Uttaradit Suture in northern Thailand.

Journal of Earth Science 27, 378–390.

Zhang, R.Y., Lo, C.H., Chung, S.L., Grove, M., Omori, S., Iizuka, Y., Liou, J.G., Tri, T.V.,

2013. Origin and tectonic implication of ophiolite and eclogite in the Song Ma Suture Zone

90 between the South China and Indochina Blocks. Journal of Metamorphic Geology 31, 49–62.

Appendix

Geological description of the outcrops in the Khorat Plateau Basin

A geological excursion was carried out in the Khorat Plateau area in December 2016. The purposes of the trip were to examine the actual rocks and gain better knowledge of the sedimentary facies, and the actual scale the of structures and basin. It was focused on the factors of the development for effective reservoir facies in the Permian carbonate platforms. The Department of Geology at Chulalongkorn University and the Department of Geotechnology at Khon Kaen University supported its excursion. The outcrop location and description were used by the SEAPEX field trip guide as a reference (Booth et al., 2014). Page 1. Upper Carboniferous / Si That (Wang Saphum) Formation……………… 4 Loc. 1, 2, 3

2. Permian / Pha Nok Khao Formation……………………………………… 6 Loc. 4, 5, 6, 7

3. Permian / Hua Na Kham Formation………………………………………. 9 Loc. 8, 9

4. Upper Triassic / Kuchinarai Group (Huai Hi Lat Formation)…………….. 10 Loc. 10, 11

5. Lower Cretaceous / Phra Wihan Formation (Khorat Group)……………… 11 Loc. 12

6. Lower Cretaceous / Sao Khua Formation (Khorat Group)………………. 12 Loc. 13, 14

Outcrop location on geological map (Fig.1)

From Geologic Map of Thailand. Department of Mineral Resource, 1983.

Permian Cretaceous

Carboniferous Jurassic

Triassic

Stratigraphy in the Khorat Plateau Basin (Fig. 2)

The Khorat Plateau Basin represents a complex sequence of basins formed during five periods of deposition from the Late Carboniferous to the end of the Cretaceous (Booth & Sattayarak, 2011). The five sequences are separated by unconformities related to major orogenic events. The Indosinian I unconformity is the most regionally extensive and oldest of four Mesozoic unconformities recognized in the basin and was followed by the Indosinian II unconformity in the Late Triassic. Within the basin, the Upper Carboniferous and Permian sequence is predominantly shallow-marine carbonates and shales, however, the Mesozoic section consists of terrigenous sediments. Between the Indosinian I and II unconformities, half-graben-filling sediments deposited during an extensional period are preserved (Kuchinarai Group). Jurassic to Early Cretaceous thick continental deposits (Khorat Group), which form gentle long-period folds, overlie the Indosinian III unconformity. The Khorat Plateau Basin began to form during Late Carboniferous rifting, developed into a distinct horst-and-graben system during the Permian, and was uplifted during the Early to Middle Triassic. 3

1. Upper Carboniferous / Si That Formation The Si That Formation consists mainly of calcareous shales and carbonates and interbedded sandstones in the Non Sung-1 well, and coal, chert, and evaporite in outcrop in the Loei area (Ueno & Charoentitirat, 2011). The formation was deposited in a shallow-marine to littoral– fluvial environment. There was not reported the organic contents as source rock ability, however, it is assumed the candidate of source rocks considering the depositional environments.

Loc. 1: Carboniferous Wang Saphum Formation (eastward from Loei, pass through Ban Na Din Dam to the base of Phu Khi Bao) ・Limestone breccia, from a few mm to 30cm. Matrix-supported ・Interpreted as slope breccia and talus breccia (Booth et al., 2014)

Loc. 2: Carboniferous Wang Saphum Formation near Permian (Nam Mahoran Fm.) boundary (eastward from Loei, pass through Ban Na Din Dam to the base of Phu Khi Bao) ・Thinly interbeded yellowish brown fine grained sandstones and shales ・Fossiliferous Permian carbonates nearby, and the section possibly include the boundary of Carboniferous and Permian (Booth et al., 2014) ・The formations are folded and faulted.

Loc. 3: Carboniferous Wang Saphum Formation / Gypsum (Loei-Wang Saphung area) ・It crop out gypsum in the pit before, however, currently it could not be observed because it buried in a pond. Right photo is the gypsum as a bounding stone. ・Evaporite sediments by cores composed of an up to 50m thick gypsum - anhydrite beds (Surakotra et al., 2005)

2. Permian / Pha Nok Khao Formation The Pha Nok Khao Formation consists predominantly of shallow-marine massive and thick- bedded limestones, fractured dolomites, and thin-bedded shales. The formation is more than 1000 m thick in several wells. Seismic facies analysis and the distribution of wells that penetrated carbonates indicate that the carbonate platforms and reefs of the Pha Nok Khao Formation likely developed on a horst and their lateral equivalents were deposited in moderately deep water in a neighboring graben. Many thrust faults were observed at the outcrop. The karst topographic features provided us the image for the enhancement of reservoir quality. The source rocks for the formations are thought to be lagoonal organic rich shales behind reef buildups or deeper shales between carbonate platforms. As for total organic carbon (TOC), it showed 0.5% to 30.05% of Phu Lop-1 well and 0.9% to 18.19% of Phu Wiang-1 well, which interpreted fair to excellent organic richness, and late to over-mature and has kerogen type III/II (Thongboonruang, 2008).

Loc. 4: Pha Nok Khao Carbonates (westward from Chum Phae, quarry on north side of road) ・Light gray, thick bedded to massive wackestone including fusulinids with mudstone. ・Some fractures develop, mostly calcite filled. Thrust duplex observed at left side of quarry with open fractures system. No matrix porosity and very low permeability except fracture system.

Loc. 5: Pha Nok Khao Carbonates (westward from Chum Phae, highway 12 deep road cut) ・Limestone breccia consisting of poorly sorted fragments ・Duplex thrust fault, SE dipping thrust planes with open fractures ・Interpreted as a foreslope breccia (Booth et al., 2014)

Loc. 6: Pha Nok Khao Carbonates (westward from Chum Phae, quarry near Thung Na Lao sub-district office) ・Massive thick grey wackestone with fusulinids and large corals (right). ・At top of outcrop, it looked obvious (transgressive) surface with bedded carbonates (Booth et al., 2014) ・Thrusted faults crossing the outcrop with fractures filled calcite.

Loc. 7: Pha Nok Khao Carbonates (Pha Ngam rock garden) ・Large karst topographic features ・Thick bedded to massive limestones

3. Permian / Hua Na Kham Formation The formation consists of mixed siliciclastics and carbonates found above the massive Lower and Middle Permian platform carbonates. The lower portion of the formation in Non Sung-1 is considered to be fully marine, probably deposited in a shallow-water shelf environment. Upward the condition became more continental, with the coals probably deposited in deltaic to coastal marsh environments (Booth and Sattayarak, 2011).

Loc. 8: Hua Na Kham (Chulaporn Dam road cut, NW of Ban Pa Wan side road) ・Brownish shales and siltstones intercalated fine to coarse sandstones ・It is close to transition of the Pha Nok Khao carbonates. Significantly deformed formation in clstics (apparently not seen in the limestones) with isoclinal folding and associated shear (Booth et al., 2014)

Loc. 9: Hua Na Kham (eastward from Loei, quarry near Ban Nam Suai) ・Various coloured poorly sorted sandstones interbeded siltstones ・Interpreted to be terrestrial deposits (Booth et al., 2014)

4. Upper Triassic / Kuchinarai Group (Huai Hin Lat Formation) The Kuchinarai Group contains coarse, fluvial, alluvial-fans deposits and organic-rich lacustrine shales interbedded with volcaniclastic rocks. The Kuchinarai Group is from 100 m to more than 5000 m thick, and the sedimentary facies within it are extremely variable because of the variety of depositional environments. The geochemical analysis shows that the dark grey shale of middle part of Kuchinarai Group are rich in organic content ranging from 1.0 to 3.7 percent of TOC and are potentially good sources of oil and gas hydrocarbons. As for the vitrinite reflectance value (Ro) in the Tao area of Laos and in the Chum Pae in the western part of the Khorat, it showed higher value in excess 2.5% (over-mature), which was probably higher TOC prior to maturation and destruction of kerogen (Koysamran and Comtrie-Smith, 2011)

Loc.10: Kuchinarai lacustrine shale (westward from Chum Phae, Highway 2216) ・Dark grey shales and coals including sulphur contents ・High TOC (to 40%) but high maturity (Booth et al. 2014) ・Interpreted as a lacustrine environments (Booth et al. 2014)

Loc. 11: Kuchinarai Group (westward from Chum Phae, Highway 2216) ・Thinly interbeded calcareous grey mudstones and siltstones with mud cracks ・Interpreted deposited in a shallow water lacustrine environments (Booth et al., 2014)

5. Khorat Group The Khorat Group consists of extensive and thick continental deposits that cover much of the Khorat Plateau. It is 2000–3000 m thick and extends from the Indosinian III unconformity (which separates the Lower and Upper Nan Phong Formations) to the Aptian–Albian mid- Cretaceous unconformity. The sediments are characterized by fluvial deposits, including red beds, which consist of conglomerates, sandstones, and mudstones. In ascending order the sedimentary succession comprises the Upper Nan Phong, Phu Kradung, Phra Wihan, Sao Khua, Phu Phan, and Khok Kruat Formations.

Loc. 12: Lower Cretaceous / Phra Wihan Formation (Highway 210)

・1 – 3m thickness of white sandstone and including wood fragments ・Ranging from arkosic to orthoquartzitic ・Interpreted as an anastomosing river system (Horiniuchi et al., 2012), a stacked series of braided rivers (Booth & Sattayarak, 2011)

Loc. 13: Lower Cretaceous Sao Khua Formation (Phu Wiang Dinosaur Park) ・The park locates in the large syncline structure ・Dinosaur footprint was recognized in the formation ・The world new genus called Phuwiangosaurus sirindhornae

Loc. 14: Lower Cretaceous Sao Khua Formation (Dinosaur Museum) ・Greenish coarse – fine grained massive thick sandstones ・Pebbly layer indicating high energy ・Interpreted in a fluvial flood plain environments, generally with meandering-river systems (Booth& Sattayarak, 2011)