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 i 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 ii 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 v 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 vi 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