Bach Ho Field, a Fractured Granitic Basement Reservoir, Cuu Long Basin, Offshore Se Vietnam: a “Buried-Hill” Play

Journal of Petroleum Geology, Vol. 32(2), April 2009, pp 129-156 129

BACH HO FIELD, A FRACTURED GRANITIC BASEMENT RESERVOIR, CUU LONG BASIN, OFFSHORE SE VIETNAM: A “BURIED-HILL” PLAY

Trinh Xuan Cuong* and J. K. Warren**

A combination of seismic, wireline, FMI and core data shows that Bach Ho field in the Cuu Long Basin, offshore SE Vietnam, is an unusual “buried hill” reservoir. There is little or no production from associated siliciclastic “grus” or granite wash, and the fractured reservoir matrix is largely made up of unaltered acid igneous lithologies (mostly granites and granodiorites). A major NE-SW late Oligocene reverse fault system cross-cuts the field, with about 2000 m of lateral displacement in the highly productive Central Block. The associated fracture meshwork greatly enhances reservoir quality. Transpressional wrench faulting in the late Oligocene in this part of the field emplaced a block of brittle granitic rock on top of organic-rich Eocene – Oligocene mudstones, and facilitated the early migration of hydrocarbons into the fracture network. Structure, not erosion, set up the 1000 m column of liquids in the fractured granodiorites which form the reservoir at Bach Ho. Faulted intervals with associated damage zones create an enhanced secondary porosity system in the granodiorite; effective porosities range from 3-5% and occasionally up to 20%. Some associated fractures are partially blocked by authigenic calcite and kaolinite. Features that degrade reservoir quality at Bach Ho include: (i) a thin, low-permeability clay- plugged “rind” created by surface-related (meteoric) Eocene – Oligocene weathering — this rind variably overprints the uppermost 10-40 m of exposed basement throughout the Cuu Long Basin; and (ii) widespread hydrothermal cements which largely predate late Oligocene wrench faulting; cementation mostly took place during post-magmatic cooling and precipitated zeolites, carbonates and silica in fractures which cut across both the igneous and the country rocks. Porosity-occluding hydrothermal and authigenic precipitates developed in pre-existing fractures in the Bach Ho granodiorite. These pre– late Oligocene mineral-filled fractures acted as zones of structural weakness during and after subsequent late Oligocene structural deformation. Together with new fractures formed during thrusting, the older fractures may have reopened during thrust emplacement, and subsequent gravitational settling of, the Central Block. INTRODUCTION estimated resources, both on- and offshore, total some 6.5-8.5 billion brls of oil and 75-100 trillion cu. ft of Vietnam has produced almost 1 billion brls of crude gas (AAPG Explorer, February 2005). Most of the oil and 300 billion cu. ft of natural gas. The country’s production in the Cuu Long Basin, offshore SE Vietnam, has comes from fractured and weathered *Exploration and Production Dept, Vietnam National granitic basement reservoirs at fields including Bach Oil Group, Petrovietnam, 22 Nyo Quyem St, Hanoi, Vietnam. Ho (White Tiger) (the largest field), Rang Dong ** Shell Chair, Sultan Qaboos University, Muscat, (Aurora), Rong Tay (Dragon West) and Hong Ngoc Sultanate of Oman. previously: Dept. Petroleum (Ruby). Use of the term “basement” in this paper Geoscience, Universiti Brunei. Present address: Chulalongkorn University, Bangkok, Thailand. author for Key words: Bach Ho, Cuu Long Basin, Vietnam, fractured correspondence: [email protected] basement, buried hill, basement high, Vung Tau, granite.

Red River Faul

N t

Song Hong

Basi

Basin boundary n

Major fault reading ridge

Extensional crustal margin

Dong Nai Fault 0 200 Mae Ping Faul km Cuu Long Basin t Ho Chi Minh City

3 Pagodas Faul Vung Tau h Nam Con Son Basin Con Son Hig

Fig. 1. Generalized map of the major Cenozoic sedimentary basins and current tectonic elements in the South China (East Vietnam) Sea. follows that of Landes (1960) who defined it as any fractured granodiorite in the Central Block of the field combination of metamorphic or igneous rocks produce as much as 15,000 b/d, although rates of (regardless of age), which are unconformably overlain 2000-5000 b/d are more typical (Cuong, 2000; Dien by a sedimentary sequence. North (1990; p. 216) gave et al., 1998). Matrix storage in the reservoir is low to a broader definition of “basement” that includes rocks non-existent, yet near-constant production rates have with a sedimentary origin, providing they have been maintained for more than five years, implying essentially little or no matrix porosity. significant recharge to the fracture network. To date, The discovery well for Bach Ho was drilled in 1975 the fine-grained Tertiary fluvio-deltaic sedimentary by Mobil Oil Co. but the company left Vietnam in the rocks which encase the productive basement block same year. The Bach Ho “buried-hill” was not have not produced sufficient volumes of hydrocarbons developed until the mid-1980s by Vietsovpetro. Today, to make them a target. the field’s average daily production (about 140,000 Not all basement highs in the Cuu Long Basin have b/d) is declining; in 2008 it was about 25,000 b/d less such outstanding reservoir and production than in the previous year (www.upstreamonline.com/ characteristics. A contrast to the success of Bach Ho live/article169696.ece). This rate of decline is is given by the development and production history somewhat higher than that predicted by Vietsovpetro, of Dai Hung (Big Bear) field, where hydrocarbons which envisaged an annual decline of about 20,000 are hosted in a similar granite/granodiorite matrix. In b/d for the period 2010-2014, but it can perhaps be 1993, BHP led a consortium that won the bid to expected in a field producing from an open fracture develop Dai Hung and predicted that the field could system with little matrix storage. produce up to 250,000 b/d. By the mid-1990s, some At Bach Ho, matrix between hydrocarbon-filled 25,000 b/d were being produced but then output fractures is largely composed of unaltered igneous and declined rapidly and BHP left the consortium in 1997 metamorphic rocks. Some production wells in announcing that the field was not profitable. Petronas Trinh Xuan Cuong and J. K. Warren 131

107° 20’ 107° 40’ 108° 00’ 108° 20’ Bach Ho 2 A. Basement 1.5 2 2 1.2 Isodepth (km) 1 1.5 3 Fault 15G-1x 0.5 4 1.5 Well 10° 20’ 2 3 4 6 5 4 3.5 4 2 5 107° 00’ 3 6 0.5 1 3.2 5 3.5 1.5 3 2 7.5 2 3.5 8 3 1 4 6 4 1.5 3.2 5 2 1.5 5 15B-1x 15A-1x 7 4 3.5 10° 00’ 3 5.5 3.2 6 6 7 15C-1x 1 4 2 2.5 3 4.5 3 3 5 6 7 5 4 2 2 2.5 4.5 BH-10 3.5 5.5 4 BV BH-12 1.5 3 4 5 6 6 3 2 2 8 BH-4 2.5 6 6.5 6 2 6 BH-1 5.5 7 8 3 4 6.3 4 9° 40’ 2.5 3 5 5.5 7.2 2 1.5 2.5 4 BD 2 3 R-3 3 3 6.2 6 1 1.2 0.5 4 TD R-4 3 1 5 4.5 4.5 2.5 3 5 3 2.5 4 4 2 0.5 3 2.5 4 2 4.3 DD 3 3.2 3 3 9° 20’ 2 0 km 25 1.5

DD-1X TD-1X BD-1X BH-4 15C-1X 15G-1X B. 0 Middle-Late Miocene 2000 Early Miocene 4000

Eocene 6000 Depth (m)

Fig. 2. Basement distribution in the Bach Ho region, Cuu Long Basin. (A) Top of basement structure map; rectangle indicates position of the Bach Ho oilfield and the various shaded areas indicate basement highs. (B) Regional geological section showing variation in basement depth along line indicated by dashed line in A. (after Que, 1994). then took over as operator but failed to raise output we attempt to integrate available data in order to beyond 10,000-15,000 b/d and left in 1999. In 2000, compile a predictive exploration model for the Bach Vietsovpetro (the operator of Bach Ho and Rong Ho type of “buried-hill” play. We attempt to evaluate fields) took over Dai Hung but was only able to fracture distribution in the basement reservoir and to produce some 5400 b/d. assess factors influencing reservoir quality. There is therefore a need to investigate the geology of Bach Ho field and to understand why successful REGIONAL STRUCTURE AND economic production comes from this unusual STRATIGRAPHY OF BACH HO FIELD reservoir. Previous studies of the granitic basement reservoirs in the Cuu Long Basin include papers by The Cuu Long Basin is filled with 6 to 8 km of Areshev et al. (1992), Dmitriyevskiy et al. (1993), sedimentary rocks which overlie pre-Tertiary Tran et al. (1994), Koen (1995), Tuan et al. (1996), basement. The sedimentary fill rests on an undulating Dong (1996), Dong et al. (1996, 1997, 1999), Dong basement surface which divides the basin into a series and Kireev (1998), Quy et al. (1997), Tandom et al. of sub-basins trending parallel to the main basin axis (1997), Tjiia et al. (1998), and Vinh (1999). However, (Fig. 2) (Hall and Morley, 2004; Lee et al., 2001). these studies are based on relatively old data or The pre-Tertiary basement is mostly composed of focused on particular areas of research. In this paper, Upper Triassic – Cretaceous plutonic rocks whose 132 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Hydrocarbon Geologic Age Stratigraphy Palaeobathymetric Change Associations Thick Seis. Open Structure Tectonics Source Reser- Period Ma Formation Description Lithology Lacustrine Seal (m) Hor. marine rock voir

QUAT. Fine-grained marine sands interbedded with siltstones 1.64 BIEN 400 - DONG B4 PLIOCENE 700 5.2 Continent. shelf cover REACTIVE Coastal environment to the west, river-mouth and shallow marine sedimentation in the basin DONG 600 - center and to the east. Mostly fine to B3 LATE NAI medium-grained sandstones, intercalated with grey siltstones and mudstones. 900 10.4 Coastal setting with fluvial dominance. Mostly in the basin centre and in east where coarse CON 500 - MIOCENE and medium-gained sandstones alternate with SON Postrift trough

grey mudstones and several interbeds of B2 Thermal sag

MIDDLE 600 lignite. 16.3 Mainly shallow marine with a strong fluvial influence in its lower part. Mostly sandstones, interbedded with grey, green or brown 800 - BACH mudstones. It is absent in the west of the HO basin. Its upper part (the Rotalia Bed) is 1400 B1

EARLY composed of green mudstones.

23.3 DIVERGENCE Predominantly lacustrine sediments, mostly in the centre and the east of the basin. Sediments are mostly black mudstones C TRA intercalated with grey-brown, fine to BASIN RIFTED SEDIMENTARY LATE TAN medium-grained sandstones. In the west, 200 - where fluvial deposition dominated, Rifting pull apart

2700 Synrift graben 29.3 sediments show a higher percentage of sand

OLIGOCENE and coarser grain sizes. Volcaniclastics are also common in this unit.

EARLY 35.4 TRA Only in the western part,dominated by inter- 100 - EOCENE CU bedded conglomerates and medium-grained sandstones, minor mudstones. 300 56.5 Mostly hydrothermally-altered Late Cretaceous granites, lesser Late Triassic & Late Jurassic intrusives (basic and weakly acidic). Together with the generation of the Late Cretaceous granite, some andesitic and

LATE dacitic dykes were emplaced with patterns PRERIFT COMPLEX BASEMENT

MESOZOIC controlled by conjugate faults and fractures. CONVERGENCE Country rocks were metamorphosed by both BASEMENT OF CONTINENTS CONTINENTAL mechanical and thermal processes especially ENVIRONMENT during Late Cretaceous. HETEROGENOUS

Fig. 3. Cuu Long Basin stratigraphy (after Dien et al., 1998). composition ranges form basic (diorite) to acidic were locally higher in the Mesozoic and early Tertiary (granite) (Table 1; Dong and Kireev, 1998). during magmatism. The shallowest part of the highly fractured “buried- hill” at Bach Ho lies at a depth of 3050 m (Fig. 2), Tectonic evolution while the effective base for liquids production in the The structural evolution of the extensional Cuu Long field is typically shallower than 4650 m. However, the Basin can be explained in terms of (Figs 1, 2): (i) the deepest producing interval in the field is a fractured propagation of the South China (East Vietnam) Sea zone at 5013 m, with a flow rate around 100 b/d at a rift around 17-32 Ma (Taylor and Hayes, 1983; temperature of 162oC. Productive intervals in Bach Ho Hutchison, 2004), and (ii) the collision of India and wells are invariably fractured zones. Eurasia beginning in the Palaeogene, which drove Production data indicates that Bach Ho field is made the extrusion of Indochina along the Mae Ping and up of a number of separate basement blocks with Red River Faults (Tapponier et al., 1982, 1986), differing hydrodynamic properties. For production although more recent studies have shown that purposes, the field is divided into the Northern, Central collision-driven strike-slip extrusion, and its and Southern regions. Communication between influence on basement tectonics throughout fractured basement and onlapping sandstones is not Indochina (including offshore Vietnam) has probably seen in current production and pressure data. been over-emphasized (Morley, 2004; Hall and The current geothermal gradient at Bach Ho field Morley, 2004; Searle, 2006). Local variations in increases with formation age. It is 2.2-2.5oC/100m in tectonic activity have had a significant influence on Quaternary – middle Miocene sediments; 3.4oC/100m basement structure in the Cuu Long and Nam Con in the lower Miocene and Oligocene; and 3.9oC/100m Son Basins, resulting in subsidence and uplift with in the basement. Temperatures in the basement at different intensities and styles at various times since depths of 3-4 km range from 120 to 165oC. It has been the Cretaceous. Hall and Morley (2204.) concluded observed that 90°C formation waters from Bach Ho that the basement “grain” in northern Sundaland contain thermophilic sulphate-reducing archaea/ (including offshore Vietnam) is not related to the bacteria (Rozanova et al., 2001). Geothermal gradients Himalayan collision but is due to localised collisional Trinh Xuan Cuong and J. K. Warren 133

Fig. 4. Character of Bach Ho basement. (A, above). Classification of acid igneous rocks in the basement of well BH-12 (see Fig. 6 for well location). Open circles indicate core samples, solid circles indicates side-wall core samples. (B, right). Cores of typical fractured granodiorite basement. Sub-vertical tectonic fractures (0.1-1.5 cm wide) are filled with zeolites, silica and calcite. There is residual oil in some of the fractures (appears brown and is indicated by red arrow). Drilling-induced fractures (light blue arrow) are also present. Scale bars are 5 cm long. These cores come from deeper intervals with few open fractures, hence the higher recovery efficiency.

events which preceded the main India–Asia collision. 3). Sands in the mud-dominated Eocene and lower These smaller-scale events included left-lateral Oligocene succession are mostly immature arkoses, movements on the Mae Ping and Three Pagodas with lesser lithic-arkoses and some feldspathic Faults. Initial movement on some of the major faults litharenites deposited in alluvial/fluvial and lacustrine which are still active at the present day therefore took environments. Upper Oligocene sediments are fine- place in the Cretaceous, and not all movements were grained lacustrine muds, while the overlying lower driven by the mid- to late Tertiary uplift of the Tibetan Miocene succession passes from muddy delta-front Plateau. deposits up into marine shales (Que, 1994). The Cuu Long Basin is located along-strike from Sandstones in the lower Oligocene and lower Miocene the youngest spreading centre in the South China (East successions are possible secondary reservoir targets Vietnam) Sea. Since spreading began at around 32 at Bach Ho. Late Oligocene lacustrine and lower Ma, its dominantly SW propagation has influenced Miocene marine shales constitute regional seals. Oils structural grain in the Cuu Long Basin basement (Fig. throughout the Cuu Long Basin are sourced from 2; Hutchison 2004). The regional structural grain is lacustrine muds (ten Haven, 1996; Reid, 1997). Source NW–SE to north-south (Hall and Morley, 2004; rocks in Bach Ho are present in the Eocene and Morley, 2004). Extension-derived stresses from this Oligocene intervals (Canh et al., 1994; Areshev et al., spreading centre may still result in fault movements 1992), with the upper Oligocene being the most in the Cuu Long Basin (Shi et al., 2003). prolific (Hung et al., 2003). None of the hydrocarbons offshore Vietnam are derived from a deep mantle Stratigraphic evolution of the Bach Ho cover source, contrary to a suggestion which was made The sedimentary succession at Bach Ho comprises during early field development studies (Dmitriyevskiy Eocene, Oligocene and lower Miocene deposits (Fig. et al., 1993). 134 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Table 1. Primary minerals, composition and K-Ar age ranges in igneous rocks that constitute the reservoir in Bach Ho field, offshore Vietnam (After Dong and Kireev, 1998). See Fig. 6 for the location of the Central and Northern blocks.

Central Eastern part of the Centre of Northern Block Block Northern Block Ca-Na complex Dinh Quan complex Hon Khoai complex (108-115 Ma) (135-154 Ma) (194-261 Ma) Biotite Quartz Quartz Biotite quartz Granite Granodiorite quartz biotite biotite monzonite monzonite monzonite monzonite Plagioclase 25-40 40-54 47-58 30-40 43-52 62-68

K-feldspar 20-40 14-28 17-28 30-45 Oct-27 03-Dec

Quartz 25-35 18-24 Dec-17 May-18 May-17 <5

Biotite 02-Oct 06-Oct 05-Oct Oct-13 May-13 13-15

Muscovite 0-3 - - - - -

Hornblende - 0-3 0-2 0-2 0-2 -

Hastingsite - - - Jun-18 Jun-18 05-Oct

Dominant type Oligoclase Oligoclase Oligoclase Oligoclase Oligoclase Andesine of plagioclase andesine andesine andesine andesine

Igneous and hydrothermal evolution hydrothermally-cemented and fractured zones tended Widespread Mesozoic plutonism on- and offshore SE to be weaker than adjacent zones dominated by Vietnam was dominated by Jurassic – Late Cretaceous unaltered and unfractured granites. granites and granodiorites (Hall and Morley, 2004). At a regional scale, plutonism was driven by the Bach Ho Basement – seismic character NWward subduction of the Proto-Pacific Plate beneath Seismic responses can be used to divide the Bach Ho East Asia. Lateral forces associated with the northward basement into three depth-related seismic zones (Fig. movement of India gave rise to stress changes within 5, line positions shown in Fig. 6; see also the Indo-china which resulted in crustal thickening and schematic diagram in Fig. 16). Interval velocities in pluton emplacement in Vietnam and Thailand (Morley, the upper, middle and lower zones typically range 2004; Hutchinson, 2004). from 3500 to 3800 m/s, 3900 to 5200 m/s and 5000 At Bach Ho, three phases of igneous intrusion are to 5500m/s, respectively. recognised (Late Triassic, Late Jurassic and Late The upper zone is characterized by high continuity Cretaceous) and correspond to the Hon Khoai, Dinh of the basement-top reflector immediately below the Quan and Ca Na complexes, respectively (Table 1) purple reflector in all three lines in Fig. 5. Its character (Areshev et al. 1992; Dong and Kireev, 1998; Morley, is created by the sharp acoustic impedance contrast 2004). Late Triassic and Late Jurassic intrusives consist between the weathered top of the basement, the of basic and weakly acidic rocks. These are a minor overlying fine-grained sediments and the underlying component of the Bach Ho basement and occur mainly fractured igneous basement. This upper zone in the northern part of the field. Elsewhere, the signature is best developed at the highest parts of the basement is composed of Late Cretaceous acidic rocks Bach Ho structure and loses thickness and definition (mostly granitic) which are characterised by their brittle away from the crest. behaviour under stress (Fig. 4a). Andesitic and dacitic Below is a zone characterized by groups of lower dykes were emplaced with orientations controlled by frequency, higher amplitude and lower continuity conjugate faults and fractures. Thermal haloes signatures within a chaotic reflection background (the associated with each phase of magma emplacement approximate vertical extent of this zone is indicated altered country rocks into greenschist meta-sediments. by the yellow arrows in Fig. 5a-c). Its presence in all Cements precipitated during associated hydrothermal the Bach Ho seismic lines implies that there are circulation tended to occlude any porosity in the laterally-developed velocity contrasts between the granites and adjacent Mesozoic sediments (Fig. 4b). relatively brecciated and altered zones (or dykes) in During subsequent episodes of tectonism, regions of the upper part of the basement high and the Trinh Xuan Cuong and J. K. Warren 135 e ee lines, Seismic Zone 2 field (see Fig. 6 for line locations). 6 for field (see Fig. Bach Ho this interval is referred to as Seismic Zone 1. Below this, Below to as Seismic Zone 1. this interval is referred Top of basement shows high continuity and amplitude in all thr of basement shows Top amplitude high and low frequency, is made up of low arrows) (yellow Seismic Zone 3 is a of chaotic reflectors. with a background reflections fault and its reverse Note the low-angle reflection. of free zone which continues to lines, in all three present associated alteration zone 6) Some high-angle basement faults ar both north and south (see Fig. intensity within the basement block in reflection Variations also visible. to variations in variations in acoustic impedance related in part, indicate, based on lines 1 A schematic interpretation the intensity of alteration. 16. in Fig. and 2 is shown Fig. 5. Seismic profiles across across Seismic profiles 5. Fig. 136 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Fig. 6. Main fault and fracture system in Bach Ho field (after Cuong, 2000). The highest basement, shown by yellow and green shading, is in the Central Block adjacent to the region with most obvious displacement on the reverse fault (approx. 2000 m; see line 1 in Fig. 5). The Central Block shows the most intense deformation and wells here have higher production capacities. surrounding granitic rocks. There is a suggestion of chaotic reflections and is thought to possess few open pervasive faulting in this interval (indicated by the fractures; it is not considered to be economic. red lines in Fig. 5). but individual faults cannot be Seismic, image logs and core data can be used to located accurately due to the relatively low resolution divide the Bach Ho basement into three blocks (North, of the seismic data. Central, and South), each bounded NE-SW fault Seismic attribute contrasts in lines 1 and 2, and segments (Fig. 6). The reverse fault to NW of the comparison with outcrop analogues at Vung Tau (see Central Block locally has a lateral displacement of up below), imply that the upper part of the Bach Ho high to 2000 m (Dong, 1996). The “chaotic” seismic zone, is more altered and brecciated compared to the described above as indicating a high degree of underlying zones. Cores and sidewall cores show the fracturing and deformation, is particularly associated lower part of the high are more weakly altered and, with this reverse fault (see Figs 5a, b). The reverse although fractures and breccias are still common, they fault dies out along strike and passes into a linked are typically occluded and cemented by hydrothermal normal fault to both NE and SW (Fig. 6). This minerals (Fig. 4b). An underlying third zone (beneath relationship implies that the reverse-faulted margin the yellow arrows in Fig. 5a-c) is characterized by of the Central Block is an inverted portion of a pre- Trinh Xuan Cuong and J. K. Warren 137 existing normal fault. FMI analyses (see below) show ranges from several centimetres to 3 m and increases that fracture intensity is greatest in the Central Block. progressively with depth. Production wells in the Central Block consistently Exfoliation fractures form in the shallow show production rates as high as 4000-6000 b/d. subsurface as a result of stress release as a pluton By contrast, the seismic profiles indicate the exhumes and associated overburden stresses first presence of normal faults along the SE flank of the decrease then disappear (e.g. Younes et al., 1998). field, dipping to the south and SE. Seismic character The fractures are formed due to changes in thermal of the basement in this part of the field is characterized stress during annual or daily ambient temperature by lower amplitudes, and reservoir quality is lower cycles in combination with meteoric-driven compared to the NW margin of the Central Block. weakening of matrix strength due to mineral Most fault sets at Bach Ho, as interpreted from transformations (Holzhausen, 1989). The lateral extent seismic profiles, are confined to the basement and do of individual sheet fractures could not be measured at not appear to continue into the overlying sedimentary the Vung Tau exposures. Nermat-Nasser and Horri cover; nor can they be resolved in deeper parts of the (1992) noted that open sheet fractures in granites can basement (Fig. 5a-c). The lower transitional zones of reach widths of 200m. Tandom et al. (1997) showed both the weathering profile and the faulted interval in that at depths of up to 200-250 m, exfoliation sheet the basement are difficult to define on seismic profiles. fractures in an igneous host can be 25-30 m or more Comparison with the Vung Tau outcrops (see below) apart. By analogy, sub-horizontal exfoliation fractures suggest that the lower transition of the basement may therefore be present at Bach Ho in the top 50- reservoir zone follows an irregular boundary, probably 100 m of basement rocks. controlled by variations in fracture intensity in the At the Vung Tau outcrops, exfoliation fractures basement. were observed to become blocked at depths of 10-30 Seismic analyses indicates a significant association m below the surface with fine clay soils and other between late Oligocene reverse faulting and the Bach weathering products including kaolin and Fe-oxides. Ho Central Block. NW displacement of up to 2000 m The contribution of these types of fracture to reservoir on this fault emplaced a block of igneous basement porosity at Bach Ho will therefore probably be low. on top of Tertiary mudstones. Subsequent gravitational Sub-horizontal features (“temperature reduction collapse of the block and stress relaxation served both fractures”) can also form as a magma body cools and to open uncemented fractures within the block and to contracts. The intensity of this type of fracturing varies reopen pre-existing zeolite- and kaolinite-filled with the relative proportions of quartz, K-feldspar and fractures. Intense fracturing of basement rocks in the plagioclase minerals. Contractional fractures are more Central Block and enhanced reservoir characteristics likely to occur in acidic rocks with elevated are reflected in the high well outputs. proportions of quartz (Osipove, 1974). Rocks with minor quartz but more feldspar (quartz-monzonites THE VUNG TAU GRANITES: or diorites) show less volume shrinkage and hence ANALOGUES FOR BACH HO BASEMENT fewer contractional fractures. The acidic lithologies which dominate the Central Block at Bach Ho will Triassic-Cretaceous granites and granodiorites outcrop be susceptible to contractional fracturing. However, on the coast near Vung Tau, about 100 km SE of Ho the contribution of early thermal reduction fractures Chi Minh City (Fig. 1). Weathered outliers can be to effective primary porosity will probably be low. At found up 0.5-1km away from the main outcrops which the Vung Tau outcrops, these fracture types were extend for several km along the coast. A number of observed to be filled by hydrothermal mineral cements granite quarries and road cuts were visited, mapped such as zeolites or coarse calcite spar. and photographed in order to better understand fracture style, timing and host lithologies. The outcrop Tectonic fractures studies defined a number of fracture styles and Tectonic fractures in the Vung Tau outcrops dip at exposure-related features which are discussed in the angles of 50-90o and are typically composed of cross- following paragraphs. cutting conjugate sets (Fig. 7a, b). Dominant orientations are NE-SW and north-south; the NE-SW Contractional and surface-related fractures trend is aligned with the trends of the regional Dong Stress-release sheet fractures (exfoliation fractures) Nai and Mae Ping faults, and parallels the regional in granodiorites at the Big Mountain location, Vung structural “grain” associated with opening of the South Tau, are sub-parallel to the topography of the basement China Sea (Morley, 2004; Hall and Morley, 2004; Tjiia block. Dip angles tend to be low – sub-horizontal to et al., 1998; Dong et al., 1999). 10-15° (Fig. 7a). Near the present-day land surface, At a quarry at Round Mountain near Vung Tau, the spacing between stress-release sheet fractures tectonically-induced fracture apertures are mostly mm- 138 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Fig. 7. Field photographs of basement outcrops at Vung Tau (see Fig. 1 for location) (A) A weathered zone, 2-5 m thick, overlies the top of basement rocks at Small Mountain, Vung Tau. Sub- vertical (red) and sub-horizontal (green) fractures with varying densities occur beneath the weathered zone. Sub-horizontal fractures are a combination of sheet and exfoliation fractures with fracture spacing varying from 0.05-0.50m (yellow arrows) and 0.5-3.0m (light blue arrows). (B) Basement rock (at Small Mountain, Vung Tau) is deformed by tectonic fractures showing high dip angles (red). A narrow igneous dyke (ca.0.8m wide) cuts across the basement. The dyke runs along a fault and separates hanging- and foot-walls. Near the dyke, fractures are mostly parallel to the dyke margin and there are vugs or aperture fractures at the contact with the country rocks. Rocks to the left of the dyke are more highly fractured than those to the right. (C) Plan view of basement (at Small Mountain, Vung Tau) showing conjugate, relatively steeply-inclined fractures. (D) A weathered zone (0.5 to 2.0 m thick) caps the basement rocks which are cross-cut by conjugate, steeply dipping fractures and faults (Small Mountain, Vung Tau). (E) Close-up of Fig. 7d showing deformation and brecciation in the fault damage zone, especially at the intersection of two faults where a cave has formed (indicated by white arrow). Numerous steeply inclined fractures are associated with the breccia. There are two fracture sets: a tectonic (sub-vertical) set and a release (sub-horizontal) set. (F) Close-up of breccia in Fig. 7e, showing composition of broken basement fragments (0.3 to 3.0 cm across) in a matrix dominated by the products of weathering and diagenesis. There are two sets of fractures: a steeply inclined tectonic set and a set of sub-horizontal release fractures. Trinh Xuan Cuong and J. K. Warren 139

Fig. 8. Fault-damage fracture map showing distribution of fracture density in a brecciated fault zone, based on measured fractured separations collected from the Vung Tau outcrop at sites shown in Figs 7e, f. The fault centre consists of a breccia (0.1 – 10m wide; Fig. 7f), while zones of high fracture density occur adjacent to the brecciated zone (10-35 m; Fig. 7e). Mineral precipitates, and less commonly sediments, fill or partially fill dissolution voids equivalent to those in the subsurface but more labile minerals are leached at outcrop. scale but range up to 1-2 cm wide, with rare apertures adjacent granitic rocks. Analogous features in the up to tens of cm across. About 30% of measured Bach Ho subsurface contained zeolites and other fracture apertures were between 0.2 cm and 1.0 cm. hydrothermal products. Oil staining in such fractures Most fractures were filled, or partially filled, by indicated later re-opening. The mineral-filled fracture varying combinations of modified magma and constituted zones of relative structural weakness hydrothermal precipitates and clays. They are compared to the adjacent granodiorite (Fig. 4b). interpreted as outcrop analogues of the mineral-filled fractures illustrated in Fig. 4b. Weathered zones Fault damage zone widths ranged from tens of A weathered and lateritized soil profile blankets the decimetres to several metres, depending on the Vung Tau outcrops, and soil-related muds infiltrate associated fault displacement (Fig. 7b, f). The intensity exfoliation, tectonic and contractional fractures (Fig. of fault-related fracturing varies from very high near 7a, d). Soil penetrations into open fractures, and to major faults (100-150 fractures/metre), and associated weathering haloes, constitute a zone up to decreases at greater distances (e.g. 35-50 fractures/ 40-50 m thick on the flanks of Vung Tau outcrop metre at a distance of 1.5 m from the fault in Fig. 8). blocks; the zone is 10-20 m thick over the crests of Major shear zones with high associated fracture the blocks. Inland from the coast, in areas where soils intensities are up to 35 m or more wide at Vung Tau are less well developed, weathered and bleached (Tjiia et al., 1998). granite domes up to tens of metres across are exposed subaerially. Basment blocks are Bach Ho were Igneous dykes likewise subaerially exposed. By analogy with the Andesitic and rhyolitic dykes cut across the granites Vung Tau outcrops, weathering and the infiltration of and granodiorites exposed along the Vung Tau coast clay-sized fines will have occluded open fractures in (Fig. 7b). The dykes are in general oriented parallel the subsurface (Vinh, 1999). to nearby faults and fractures, with widths varying from 0.5m to several metres. The dykes are FRACTURE CHARACTER IN BACH HO characterized by higher densities of fractures (about BASED ON IMAGE LOGS 5-10 cm spacing) than the surrounding granites (10- 50 cm spacing). Fracturing in the dykes is typically Image log analysis at Bach Ho detected several oriented parallel to the dyke axis. Mineral-filled (clay fracture types and their interpretation was aided by and zeolite) mm-cm scale vugs and contractional calibration against outcrop analogues (Fig. 7) and fractures occurred at contacts between dykes and core. An interpreted image suite from well BH-12 is 140 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir shown in Fig. 9 (fracture identification follows the Natural fractures classification scheme of Schlumberger, 1992, 1997a, Continuous fractures b). This interval illustrates the utility of running a These fractures show a continuous conductive combination of imaging tools (DSI, FMI, UBI, ARI) response in the electrical image (e.g. 3705 m in well when defining the nature of fractures intersecting a BH-12: Fig. 9). They also have a strong corresponding borehole. Stoneley wave reflection coefficient. The amount of The FMI (Fullbore Formation MicroImager) tool energy lost in the fracture is thought to correspond to produces a high-resolution microresistivity image of how open the fracture is (Horby and Luthi, 1992). the borehole wall, covering 80% of the wall in an 8.5- However, zones with extremely rugose or washed- inch borehole. Each microconductivity button can read out profiles can generate similar acoustic loss up to 1 cm into the formation and in combination with anomalies (above 3590 m). adjacent buttons is capable of distinguishing conductivity anomalies smaller than the tool’s Discontinuous fractures effective resolution of 0.2 cm. Of currently available Partly-filled mineralized fractures generate imaging tools, the FMI tool provides the most detail discontinuities which in the image log appear as only for analysis of the Bach Ho reservoir. It sees most partly conductive intervals (e.g. 3715m in Fig. 9). This natural and induced open fractures intersecting the style of fracture development is not laterally extensive borehole wall together with matrix variations in bulk and does not drain as effectively as a grouping of formation textures, but is has problems seeing finer continuous fractures intersecting the well bore. open fractures and in separating open from closed mineral-lined fractures at the sub-millimetre scale. Boundary fractures The DSI (Dipole Shear Sonic Imager or Stoneley These fractures are normally associated with changes wave) tool, in contrast to the FMI tool, has a 15 cm in the lithology and/or texture of the rock (e.g. 3696 (6-inch) depth of investigation and a vertical resolution m, 3719.5 m, 3722 m in Fig. 9). of 1 m. The presence of open fractures strongly affects the waveform, so this tool is better suited to locating Brecciated fractures broader-scale intervals (m-scale) characterized by These are commonly associated with major faults, numerous open fractures. creating brecciated zones with polygonal textures on The ARI (Azimuthal Resistivity Imager) tool the electrical image and more conductive signatures records lateral resistivity up to 60 cm (24 inches) into in the deep resistivity log (e.g. 3690-3696m in Fig. the formation. The increased depth of investigation 9). The thickness of the brecciated zones can vary attained when using a combination of DSI and ARI from several decimetres to tens of metres. In the images allows for intervals with numerous open Stoneley waveform image, these zones show obvious fractures to be distinguished from intervals with intersections of downgoing and upgoing wave fields numerous but perhaps smaller, probably cemented and (e.g. 3719.5-3722m in Fig. 9). Ties to resistivity logs unproductive fractures. As with the FMI images, it is and production data show that these brecciated zones possible to orient fractures picked from ARI images within the basement reservoir act as significant (Fig. 10). hydrocarbon storage systems and have high The UBI (Ultrasonic Borehole Imager) tool is an permeability. In well BH-12, major breccia intervals acoustic imaging log providing a 360° scan of the occur at 3597-3604 m, 3617-3625 m and 3704-3723 borehole wall. Although it has a lower resolution than m. the FMI tool, the full-bore coverage of the UBI tool provides a detailed caliper of the borehole, and can Vuggy fractures separate larger mineral-lined and closed from open Strong Stoneley wave and corresponding low fractures. This is invaluable in confirming fractured resistivity responses can be used to indicate the likely intervals seen in FMI, and is also useful for stress presence of vugs in the reservoir. They can occur in analysis and in confirming good hole conditions for unaltered granite but are more common within setting MDT packers. fractured and altered zones (e.g. 3705m in Fig. 9). The following discussion, together with the results Typically they form the large cm-scale “cavities,” as shown in Fig. 10 and the individual well fracture roses seen in the FMI and UBI, but these logs indicate illustrated in Fig. 6, are based on the application of resistivity or acoustic contrast and so are indirect these imaging tools to the Bach Ho reservoir. Much indicators of fracture style. The logs may be of the image-to-rock calibration was based on a responding to mineral fills, not fluid content. Even if detailed core study of well BH 12 (Fig. 11), this was the vugs are fluid-filled, they may be isolated and such also the well where a complete multi-image log suite a vuggy zone may not be capable of generating was run across the reservoir (Fig. 9). substantial fluid interconnection to the well bore. Trinh Xuan Cuong and J. K. Warren 141

Fig. 9. Interpreted image log suite from 3690 – 3730 m in well BH-12, showing naturally induced fractures and breakouts, togetgher with dykes and brecciated zones (see Fig. 6 for well location).

Faults are typically parallel to the least principal horizontal Faults are often indicated by associated rapid increases stress. Long, straight drill-induced fractures are in fracture intensity, truncation features or brecciated usually perpendicular to the direction of borehole zones on the image log, as can be expected by enlargement, as seen in the image logs (e.g. 3725- anaology with observations from Vung Tau (Fig. 7). 3730m in Fig. 9). Recognizing induced fractures and borehole breakout is invaluable in determining the Drilling induced fractures: current orientation of the principal stress field. Sudden Borehole enlargements are a common feature along rotations of the image tool travelling across basement the axis in well BH-12 and other wells at Bach Ho sections indicates that principal stress directions vary and are obvious in the caliper response. They are with reservoir depth and reflect the complex tectonic related to stress release failure, and FMI shows they history of the basement. 142 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Fig. 10. Fracture pattern and distribution from FMI/FMS interpretations of wells in Bach Ho field. Dip angles vary from 20 to 45o in Group 1 fractures (release and exfoliation) and 45 to 75o in Group 2 fractures (tectonic). Dominant strike angles vary from block to block across the field (see Fig. 6 for fracture roses for this data).

Enhanced fractures Fracture identification using conventional logs In addition to natural fractures, pre-existing fractures Fracture identification using conventional logs is which are extended or reopened in the borehole by important in Bach Ho wells where there are no image drilling are classified as “enhanced fractures” logs. Many authors have discussed the use of (Standen, 1991). It is difficult to differentiate enhanced conventional logs for fracture detection (e.g. Stearns from other fractures using image and conventional and Friedman, 1972; Aguilera, 1993; Ershaghi, 1995; logs. For example, possible enhanced fractures may Schlumberger, 1997b). In well BH-12, image logs occur below a depth of 3750m in FMI images in well could be used for fracture calibration and verification BH-12, but production test data shows they do not of the interpretations arising on conventional log affect the volume or rate of hydrocarbon production approaches (Figs 11, 12, 13). The vertical resolution in this interval. of conventional logs is typically lower than that of Trinh Xuan Cuong and J. K. Warren 143

Struct. Crystal size Porosity Crystal size Porosity Struct. e y t le p ra

Depth se rate d Description Depth sive olog e o stose Description ry fine i s arse ample ithology th Sam or h i Fine L S Massive Ve c Coa r o Go L Medium Poor Mode Fine Schistose Ma Very fine Co Medium Good Mod S 3597.0 3830.0 P Schist: Dark grey to light Granite: White and very grey, strong schistose light grey with dark layering, very fine to finely grayish olive speckles, crystalline, hard. Bulk of the very hard, medium rock is crosscut by fracture grained, slightly coarser meshworks, usually filled by locally at 3830.80 m, massive structure. zeolites, quartz, and calcite. Abundance of quartz and White to white pinkish grey, feldspar crystals, minor friable. Locally, there are black mica, trace of somecavernsof1mmto3 yellowish pyrite, trace of mm width, which are partially fractures. filled by zeolites and other vein minerals. Abundant Poor fracture porosity whitish feldspar, black biotite. Very finely crystalline 3598.0 3831.0 yellowish pyrite is common. Granite: similar to above Intense fracturing. Size of but is medium to coarse fractures is variable, ranging grained. Bulk of rock is from 0.05 mm to 1-2 mm in crosscut by a fine width and commonly > 1-5 fracture meshwork mm in length. Moderate to generally filled with zeolite(?) and quartz Core 1

fair fracture porosity. Core 2 (white grayish). Some caverns 5 mm wide with grey white euhedral crystals of 1 mm to 2 mm in diameter growing in some cavern pores.

Poor to moderate fracture porosity. 3599.0 3832.0

Schist with the same features as above, intensely fractured. Common zeolite veining (0.5-2.5 mm wide, locally 2.5 cm). Some open fractures and caverns more than 1 mm in width. Fair to good fracture porosity. Pore Permeability connectivity is good. Depth (Kair md) (m)

Schist, similar to above, with (gm/cc)

medium sized crystals (3600 to Porosity

Max. 90° Vert. (helium %) 3600.0 3600.1 m) and whitish grey Grain density patches (>1 cm diameter) of possible relict lithology. 3598.49 0.68 0.09 0.13 1.37 2.73 3598.55 1.70 0.16 0.37 1.71 2.72

Schist: dark grey, friable, soft 3599.83 0.06 0.01 0.01 0.91 2.75 thin schistose layering. 3599.70 to base core highly fragmented Abundant black biotite, common pyrite, trace whitish zeolite veins (>1 mm in width) 3830.25 0.25 0.09 0.01 0.72 2.65 developed parallel to 3830.65 0.25 0.01 0.02 1.22 2.66 schistosity. The rock has been strongly deformed, compacted, 2831.92 0.35 0.19 5.10 0.77 2.63 fractured and locally weathered. 3601.0

Fig. 11. Core descriptions of cores 1 and 2 from well BH-12. Core 1 sampled a strongly altered rock (SAR electrofacies), while core 2 sampled the unaltered granite (FR electrofacies) from 3830-3830.8m and the moderately altered rock (MAR electrofacies) from 3830.8-3832m. Inset at lower right shows standard core- plug derived permeability, porosity and grain density. The listed plug permeabilities are maximum (plug drilled parallel to mineral-filled microfracture), at 90° to microfracture and vertical (with respect to core axis). Definitions of the various electrofacies were given by Cuong (2000). 144 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

voir interval, the caliper is one log of interval, voir

ty. Flow rates indicate Zones Flow III and ty.

summarizing reservoir zones based on wireline logs, oil flow rates and core analysis data. Wireline logs indicate logs lithological Wireline ratesanalysis and data. core oil flow summarizing basedlogs, reservoir on wireline zones

BH-12

Fig. 12. Composite log of well 12. Fig. properties in each zone. Note the rotation of the FMI tool in fractured intervals, this is a zone of this secondaryprominent is a zone Note the of rotation the FMI porosi intervals, tool in fractured properties in each zone. perhaps IV are effective reservoir. Pores in Zones I and II are poorly connected and are sub-economic. Within the Bach Ho reser in connectedZones poorly Isub-economic. and and II are are Pores reservoir. effective perhaps IV are the best direct indicators of reservoir quality. Definitions of the various electrofacies are given by Cuong (2000). by given Definitions of the various are electrofacies indicatorsthe best of direct reservoir quality. Trinh Xuan Cuong and J. K. Warren 145 image logs and the depth of investigations is broader, PROBLEMS WITH OBTAINING so that conventional logs define fractured intervals REPRESENTATIVE POROSITY rather than individual fractures. MEASUREMENTS IN CORES A higher gamma signal is typical of fractured intervals in Bach Ho and reflects precipitation of During core-based petrophysical studies, it was not radiogenic hydrothermal minerals along fractures possible to quantify porosity distribution in the (although these fractures only contribute to reservoir fractured basement at Bach Ho, even using whole core porosity when re-opened as a result of later determinations. Core and plugs of suitable quality for deformation) Electric logs, especially the core analysis tended to come from relatively microlaterlog and the proximal log, can indicate unfractured intervals, while fractured zones tended fractures in Bach Ho basement. However, conductive to be recovered as rubble without little coherency. minerals, such as pyrite and other metal sulphides, Studies showed that neither water saturation nor and drilling-induced fractures, reduce the accuracy permeability could reliably be estimated from either of natural fracture determination using these logs. logs or core (Fig. 14). This reflects both the inherent Porosity-logging tools, neutron, density and sonic, limits of conventional wireline log interpretation in are the most useful conventional logs when defining the high resistivity environment of the Bach Ho fractured zones. Our studies show the sonic log is the igneous and metamorphic basement, and also the most useful porosity log in Bach Ho field as it not limited and biased core recovery. only indicates fractures by elevated values on shear Total porosities in Bach Ho basement rocks are and compressional responses, but also differentiates low, typically between 0 and 3%, this is less than the secondary and primary porosity. Caliper logs reliably standard deviation or error range of tool-based define fractured intervals by enlargements of the porosity estimates (e.g. inset in Fig. 11). Therefore, wellbore (e.g. 3600m, 3626m, 3705m, 3720m, and calculated values of porosity from conventional 3721m in well BH 12: Fig. 12), while associated hole wireline logs, even in fractured intervals, are at best rugosity tends to create anomalous values in most semi-quantitative at Bach Ho. Simple equations of conventional logs, especially pad-mounted porosity wireline porosity calculation were applied and logs. This is an important factor in deriving reliable appeared to give consistent results, but are influenced poroperm core-to-wireline transforms (see below). by poorly controlled variations in matrix mineralogy In general, individual wireline logs do not provide and fracture intensity. An assumption was made that reliable definitions of fractured intervals at Bach Ho. unaltered rocks have zero effective porosity. Therefore Logs in combination, such as cross-plots, give more porosities calculated by different logging tools had to reliable results but still show variable quality outputs be corrected to zero for intervals of unaltered matrix, that are subject to error. Based on conventional log even where apparent porosity is listed in log outputs and cross-plot approaches, we attempted to define due to lithology effects within these intervals. Multiple fractured zones in well BH-12. We then compared mineral models can remove some of these lithological these outputs to image-log based fracture effects, but are only accurate when volumetric inputs interpretations (compare Figs 12 and 13). The results for each mineral are known. Wireline porosities were variable and only the M-N, M-PEF and DT4S- illustrated in Fig. 12 were calculated as best-estimates DT4P cross-plots could be considered partially based on combinations of neutron, density and sonic reliable fracture indicators. The M-N, M-PEF cross- logs, and verified against core data and image logs. plots detect fracture zones through the presence of In many fractured reservoirs, especially in hydrothermal minerals in the fractures. They do not carbonates, the difference between porosities obtained clearly distinguish between zones with open versus from neutron/density logs (i.e. total porosity) and sonic closed fracture networks. Comparison of the fractured porosity can be used to indicate intervals of secondary zones defined by the three cross-plots is shown in Fig. porosity. But such wireline-determined intervals of 13. These zones are generally coincident and given secondary porosity in Bach Ho basement typically did the resolution problems of conventional logs to image not correspond to open fractures or vugs. The intervals logs, the outputs are considered to reasonably indicate also encompassed zones of non-effective porosity due the intensity of fracturing in Bach Ho basement. to hydrogenation by authigenic minerals (read as Fracture presence or absence can be estimated from porosity by the neutron log) or microporosity in the wireline determined compressional and shear authigenic clays and zeolites. These features were velocities (Sibbit, 1996), but again the result is not created during hydrothermal alteration and episodes quantifiable in Bach Ho due to washout in almost all of pore occlusion, and were not tied to effective the fracture zones. This also strongly influences the porosity in the reservoir. reliability of sonic readings (e.g. 3617-3628m, 3704- The matrix in Bach Ho has extremely high 3723m in well BH-12). resistivities as it is composed of non-porous igneous 146 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Fig. 13. Fracture interpretation using conventional logs in well BH-12. Likely fracture zones interpreted from conventional cross-plots are indicated by red highlights in the column to the left of each of the three conventional log tracks. (A) is based on an M-N cross-plot, (B) uses M-PEF, while (C) is based on a DT4S-DT4P cross-plot. All three show only moderate agreement and matches to fracture interpretation based on FMI (see Fig. 12).

rock. This means a formation factor exponent “m” orientation of the seismically-defined faults (Fig. 6). could not be measured accurately in the laboratory, The lack of a general trend in fracture strike is most making water saturation calculations (via Archies marked in wells in the Central Block compared with Equation) tentative. the surrounding regions (Fig. 6). Regions to the north, south and east of the central FRACTURE DISTRIBUTION AT BACH HO high show a reasonably consistent tie between seismic and image-log data, whereby FMI fracture strikes are Fracture analysis shows that there is no consistent oriented sub-perpendicular to seismically-defined field-wide relationship between the faults and fault trends. This signature is consistent with the fractures mapped by seismic methods and the natural regional-scale early-mid Tertiary extension tied to the fracture orientation defined by image logs (Figs 6 and opening of the proto-South China Sea (see Hall and 15). In the Southern Block (well BH8), the strike of Morley, 2004). The region with the poorest tie between the FMI-defined fractures are sub-perpendicular or at FMI- and seismically-defined fault orientation is the a high angle to strikes of seismically-mapped faults Central Block. This is the structurally highest block (Fig. 6). FMI-defined fractures in the Central Block in the field and unlike the adjacent blocks, was (wells BH2, BH3, BH4) show a less consistent set of effected by late Oligocene thrusting. Blocks to the strike orientations. In wells to the north of the Central north, south and east of the Central Block show Block (wells BH1, BH5 and BH6), and wells to the evidence of extensional block faulting but no reverse east but downdip of the Central Block (well BH9), faulting (Fig. 6). As discussed above, seismic analysis the strike of the FMI-defined fractures is less well shows the Central Block was translated by late oriented than in wells in the Southern Block, but again Oligocene reverse faulting, which laterally displaced is sub-perpendicular to the general NNE-SSW it by up to 2 km to the WNW. Trinh Xuan Cuong and J. K. Warren 147

70 Permeability from different diameter plugs 60 and whole core in Bach Ho basement 50 40 30

Percentage (%) 20 10 0 30 mm 0-1 1-10 10-100 100-1000 >1000 50 mm Gas permeability (md) Full core

Fig. 14. Fragment size controls the scale of measurement volume in a fractured basement sample in Bach Ho and so influence measured permeability (after Tuan et al., 1996). Standard plug values for porosity and permeability in well BH-12 are listed in Fig. 11.

The less ordered orientation of fracture strike in intersects an open fracture system tied to a deeper- the Central Block is perhaps related to a combination seated; as study of the Vung Tau outcrops showed, of buckling and subsequent stress release. fracture density and porosity increase significantly in Compression-driven fracturing has partially the damage zone (Figs 7, 15b). overprinted pre-existing extensional fractures in the In summary, open fractures tend to be most block but also opened new set of compression-related significant in the Central Block of Bach Ho, where fractures. During the Miocene, the thrust-emplaced thrust faulting emplaced a brittle basement block on Central Block was subject to gravity-induced top of Eocene-Oligocene mudstones. Thrusting, extension, a process that opened pre-existing fractures. together with subsequent gravity-driven extension, Fracture sets in Bach Ho exhibit structurally served to open or re-open tectonic fracture networks. consistent variations in dip angle; dominant dips of Cross-linkage of such fractures was probably aided fractures in all wells are 50-75° (Fig. 10). This is by the formation of exfoliation fractures. consistent with stress patterns created both by extension or compression, and the associated fractures RESERVOIR ZONATION TIED TO cannot be differentiated in the FMI data (see GEOLOGIC HISTORY discussion in Aydin, 2000). A subordinate set of fractures have moderate dips of less than 40°. By Combining all the available geological, petrophysical analogy with the Vung Tau outcrops, these fractures and seismic data, the Bach Ho basement can be are interpreted as exfoliation fractures. They are most divided into three reservoir zones, A-C (Fig. 16). Zone obvious in wells located at some distance from the boundaries are not horizontal, but are probably most highly tectonised part of the field (the Central irregular and fracture-controlled. Block); compare, for example, the dip angles of well Zone A is strongly altered and fractured due to a BH 3 with those of other wells listed in Fig. 10. combination of tectonism and fluid flushing. It is Across Bach Ho field the fracture density recorded capped by a relatively thin, intensely weathered and in FMI and core data shows two trends: (i) porosity clay-plugged interval, similar to that observed at tends to decline with depth (Fig. 15a). and (ii) porosity outcrop at Vung Tau, which varies in thickness and tends to increase in zones of faulting and fracture (Fig. can be unconsolidated, even at depths of 4000 m. This 15b). These trends outline the most highly deformed interval can be considered as a regional-scale seal upper portion of the basement. Alhough the basement capping the underlying basement reservoirs. reservoir is a granodiorite, it can be modelled as a The remainder of Zone A is characterized by the fracture system with the greatest flow potential in the presence of open to partially-open conjugate fractures uppermost interval. High flow will also occur if a well and brecciated zones, and has good but variable 148 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

(fault zone) ed zone is similar to ed zone

(Fractures/m)

10 40

Fracture density

Breccia from logs n deep wells. Layering (1-10) is based Layering n deep wells. he fault zone itself. The figure is based The figure itself. he fault zone 6.

wells changes with depth below top-basement. with depth below changes wells

Core porosity (%)

12345 6

Bach Ho

ditra 9. %) (97.6 interval ed r Co

Well BH-420

X415

X393.5 en. This trend of fracture increasing toward a brecciat toward increasing of fracture This trend en.

NPHI (%)

(m)

X410

X400

X420

Depth field. (A) Fracture density in (A) Fracture field. Bach Ho ed damage zone near a fault. There was no core recovery in t recovery was no core There near a fault. zone ed damage

(μs/m)

200 240 e not given, but all wells are located in the region shown in Fig. shown located in the region are but all wells e not given,

7 t of the basement block (Zone A) but also increase in the vicinity of faults i A) but also increase (Zone t of the basement block

This well penetrated the main fault

Layer5:100m Layer6:100m Layer7:125m Layer8:125m Layer9:250m Layer 10: 500m (unaltered rock)

3456 but for reasons of confidentiality the location of this well is not giv the location of this well of confidentiality but for reasons

FMI fracture Density (#/m)

12 BH-420

Layer 1: 0-60m (Weathered zone) Layer 2: 25m Layer 3: 50m Layer 4: 75m

1 2 3

300

600

900 et norsror(m) reservoir into Depth 1200

1500

A. on FMI. For reasons of confidentiality the well locations ar of confidentiality the well reasons For on FMI. Fractures tend to be most common in the upper par Fractures (B) Fracture density and porosity distribution increases in the cor distribution increases density and porosity (B) Fracture in well on core 7). (see Fig. Tau Vung near seen in outcrop the trend Fig. 15. Fracture distribution variations with depth and faulting at Fracture 15. Fig. Trinh Xuan Cuong and J. K. Warren 149

Weathered rind (zone IV in BH-12 basement, and from outcrop, seismic data) Early Oligocene shales (source rock)

Late Cretaceous granite (from core analyses, Eocene lacustrine shales (source rock) BH-12 wireline logs and seismic data)

Eocene and Oligocene sands (immature and cemented - zeolites and calcite) Triassic - Jurassic intrusions (from core analyses, BH-12 wireline logs and seismic data) Release, exfoliation and contractional sheet fractures (from core analyses, BH-12 wireline logs and seismic data) Conjugate faults, associated fractures and breccias (from FMI data, Andesite - Dacite dykes (from core analyses, seismic data and outcrop analogues) BH-12 wireline logs and seismic data) Late Oligocene reverse fault in Central Block, associated fractures and breccias (from seismic data) Metasediments (Pre Late Triassic) Zone A-B boundary (coincides with boundary of wireline -defined zones II and III in BH-12 and the lower part of the upper second seismic zone and is around 3800-3900m in Central Block) in BH-12 and the top of the third seismic zone)

Not to scale

Fig. 16. Reservoir model for the Central Block in Bach Ho field based on the integration of outcrop, seismic, wireline and core data. Reservoir zones A, B and C are discussed in the text. reservoir qualities. Zone A forms the principal effective production. Zone B reservoir quality is reservoir interval at Bach Ho. Pores and fractures may variable and patchy. The zone corresponds to the have been locally enlarged by meteoric dissolution. wireline-defined Zone II in well BH-12, and the lower However, most of the effective porosity is associated part of the second seismic zone in Fig. 5. with fractures of tectonic origin. New fractures were Zone C is weakly altered and fractured, and almost formed, and pre-existing zeolite-filled fractures were all pores and fractures are filled by zeolites and other re-opened, during late Oligocene thrust-related hydrothermal minerals. Reservoir quality is very poor emplacement of the Central Block. This combination and the zone is not therefore considered an exploration of fracture origins explains the wide spread of fracture target. Zone C is equivalent to wireline-defined Zone orientations seen in FMI data (Fig. 6). I in well BH-12 and to the third seismic zone located Regionally, the base of the fractured interval in below the yellow arrows in Fig. 5. Zone A at Bach Ho is at a depth of around 3800-3900 m. This depth is approximately equivalent to the top DISCUSSION: BACH HO AND OTHER of the Eocene sedimentary interval where it onlaps “BURIED HILL” PLAYS the basement block. The top-Eocene also corresponds regionally to the base of the upper part of the second In the oil-industry the term “buried hill” is widely used seismic zone (roughly at the level of the blue reflector to describe a reservoir associated with a in Fig. 5 and part way into the zone defined by yellow palaeotopographic high, where onlap by subsequent arrows). In well BH-12, the boundary between Zones sedimentary layers indicate that the feature was once A and B is coincident with the wireline-defined exposed at the landsurface (e.g. Luo et al., 2005). boundary separating Zones II and III. Reservoirs are in general located in the uppermost Zone B is less intensely fractured and altered than (fractured and weathered) parts of the “hill” or in the Zone A, and many fractures are closed or filled by immediately surrounding sediment apron. Table 2 and authigenic or hydrothermal precipitates. The base of Fig. 17 summarise how the reservoir characteristics the zone lies at a depth of around 4000-4600m and of the Bach Ho structure compare with those at other probably coincides with the lowermost extent of “buried-hill” type traps. Table 2 does not list discovery 150 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

S Panhandle Field Granite wash play N A Palo Duro Anadarko Basin Basin AmarilloArch, Texas gas oil

After Sorenson, 2005 fractured basement grus wash Oil 3 km

with minor contribution from H2O mud 80 km granite (fresh) 0 Raservoir is mostly in reworked “grus”

NW SE S N B Sea level Oil & gas Oil & gas Cenozoic Paleogene congl.(wash) Classic “Buried Hill” Analogs Cretaceous Tertiary lake muds (seal)

Palaeozoic Breccia Basement (Proterozoic granite) Basement reworked “grus” reworked Clair Field, Xinlungtai Field, 02Offshore Shetland After Guangming & Combined fracture & North China Basin km After Ridd, 1981 Quanheng, 1982

Sea level C 0.0 Reservoir Top Eocene Top Eocene (fractured Misoa Fm. carbonate) Top Paleo. La Luna Fm.Guasera Fm. La Luna Fm. 1.0 ce rock) (sour Top Socuy BASEMENT Reservoir Two-way travel time Two-way La Paz-Mara Field La Paz (fractured basement) Venezuela Field After Nelson et al., 2000 2.0 influence on quality Reservoir is tectonically “minor sedimentological induced set of fractures,

Fig. 17. “Buried-hill” plays (see text and Table 2 for detail): (A) Cross-section through the Amarillo-Kansas Arch, USA, which hosts giant oilfields (Panhandle-Hugoton) in reefal carbonates which developed on the basement high and which are sealed by subsequent platform evaporites. “Granite wash” derived from the eroded basement block (see inset) hosts local accumulations. (B) Cross-sections through two “buried-hill” fields, Clair and Xinlungtai. At Clair, the reservoir is dominated by sediments reworked from the exposed granite high; at Xinlingtai, the reservoir is a combination of a fractured basement block, a weathered sediment “halo” and an apron of reworked grus. (C) Cross section through La Paz Field, Venezuela, where the reservoir is hosted in a fractured thrust-block. Hydrocarbons are sourced from the underlying La Luna Formation. wells in fractured basement which did not pass into Oklahoma (Sneider et al., 1977). Fracture-associated production (e.g. the Lago Mercedes No. 1 well in porosity tends to be tied to structurally-induced Chile: Wilson et al., 1993; see also wells listed by permeability fairways, e.g at La Paz field, Venezuela, Schutter 2003a, b); nor does it list wells and fields where production comes from a combination of producing from fractured volcaniclastics or shallow fractured Mesozoic carbonates and granitic basement gabbroic and serpentinite intrusives which were (Nelson et al., 2000). synchronous with nearby sedimentation (e.g. Kalan A common characteristic of all the fractured et al., 1994; Harrelson, 1989). basement fields listed in Table 2 is that the original In most “buried-hill” type reservoirs within igneous reservoir matrix was tight and had high resistivities, or metamorphic matrix, two main processes contribute as in Bach Ho. Effective porosity in such a reservoir to effective porosity: (a) weathering and mechanical is secondary, and is related to tectonic deformation or reworking, which can produces a “granite wash”; and fluid flow. Tectonically-produced porosity is related (b) open fracture and joint networks (Powers, 1932). to stress release during faulting and uplift/unroofing, A classic “granite wash” play occurs at Elk City field, and comprises open faults, fractures, microfractures Trinh Xuan Cuong and J. K. Warren 151 and joints produced at a variety of scales ranging from Bach Ho, and there is little evidence that surface- kilometres to millimetres, and with variable degrees driven meteoric alteration was responsible for of matrix damage ranging across the same scales. generation of secondary porosity. Nor can the Secondary porosity produced by fluid circulation hydrothermal circulation, as inferred by Areshev et includes dissolution via meteoric waters in weathering al. (1992), explain the porosity. Both meteoric zones, and deeper-seated alteration and leaching leaching and hydrothermal alteration tend to degrade produced by hydrothermal circulation. The two types effective porosity at Bach Ho, not enhance it. Rather, of secondary porosity development can be interrelated, porosity at Bach Ho is largely tied to zones of brittle where, for example, pre-existing tectonic fractures act fracturing associated with late Oligocene thrusting and as conduits for meteoric waters or hydrothermal fluids. subsequent gravity-driven extension. A more A combination of deeply-circulating meteoric appropriate structural analogy for Bach Ho would waters moving along tectonic fractures, and a therefore be the La Paz field. reworked sedimentary “halo”, characterizes rift- associated basement plays in China and the Gulf of CONCLUSIONS Suez, as well as at the Clair field, offshore Shetlands (Fig. 17b). However, weathering or “granite wash” Compared to many other “buried hill’ plays associations do not occur in the Cuu Long Basin. The worldwide, Bach Ho field is unusual in that the limited dataset in Table 2 suggest that meteoric- fractured reservoir matrix is largely made up of enhanced basement plays tend to occur at depths of unaltered acid igneous lithologies (mostly granites and less than 3000 m, suggesting that diagenesis associated granodiorites). Unlike classic “buried-hill’ plays, a with deeper burial causes a reduction in porosity. This “halo” or “granite wash” of reworked basement may reflect the dominance of immature first-cycle material is absent. A large-scale NE-SW trending, late arkoses and litharenites which weather from uplifted Oligocene reverse fault system cross-cuts the field, basement blocks in rift settings. with approximately 2000m of lateral displacement in An uplifted basement block in a rift setting will the Central Block, where a block of basement rock subsequently undergo thermally-induced subsidence, was emplaced on top of organic-rich Eocene- and can commonly be overlain by rapidly-deposited Oligocene mudstones. lacustrine or marine mudstones, which can be over- Using a combination of core-calibrated wireline pressured. The mudstones may however include and seismic data, the igneous matrix at Bach Ho can source rock intervals, and a build-up of overpressure be divided vertically into three zones (A-C): the two will cause hydrocarbons, once generated, to migrate lower zones (below 3800-3900m) are relatively into the sediment “halo” or into the fractured basement weakly deformed and are in general sub-economic; (e.g. Fig. 17a). the upper zone, which constitutes the main reservoir A third group of “fractured basement” plays, in the Central Block, is strongly fractured. Fractures including that at Bach Ho, is characterized by a have a variety of origins, and include pre-existing dominance of tectonically-induced fractures with little structures reopened during late Oligocene deformation or no overprint by meteoric dissolution, and an and more recent syntectonic fractures. absence of sediment “haloes” associated with surface- Future exploration offshore Vietnam should not driven weathering. These plays tend to occur in just define “buried hills”, but should attempt to systems characterized by active (Neogene) tectonics identify basement highs where fractured basement in thrust-wrench fault settings, and include the La Paz/ blocks have been thrust onto mudstone successions. . Mara fields of Venezuela and perhaps the Sarkadkeresztúr field, Hungary (Fig. 17c; Table 2). Postscript At La Paz, thrusting emplaced a block of fractured This study, based on the results of more than 30 years granitic basement and associated limestone cover on of exploration and development by Vietnamese and top of the prolific La Luna source rock (Nelson et al., other earth scientists, underlines a number of applied 2000).These plays may have effective porosity at geological principles known to explorationists for depths below 3000 m, related to brittle fracturing of many years. the recently deformed basement matrix. (1) You will not fully understand a complex Many basement highs have been drilled in the Cuu reservoir before you must produce from it. Long Basin; none, however, have the production (2) Seismic collected across possible fractured capacity of Bach Ho. Historically, geologists working basement reservoirs gives a reliable indicator of in the Cuu Long Basin have used the weathered structural culminations in a basin and so defines the “buried hill” plays of China as appropriate analogues. first-order drilling targets. Subsequent higher seismic However, the weathered crust at Bach Ho is less than resolution obtained from a 3D survey may better 30 m thick. Also, sediment “haloes” are absent from define additional targets, and have regional 152 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

Table 2. Summary of oil- and gasfields known to have produced economic volumes of hydrocarbons from fractured and variably weathered igneous and metamorphic basement highs, where the unaltered host rock had little or no permeability.

Field and Age and General reservoir character Seal References Setting depth

Utikuma & Red Middle to Granite wash is organized into two systems of Devonian Cant, 1988;

ded Earth fields, Upper Devo- partially superimposed clastic wedges (alluvial fan Muskeg Fm Dec et al., Peace River nian clastics deltas & estuaries) that interfinger with, and are (marine-fed 1996 Arch, Alberta, ≈ 1500-2500 onlapped by, the evaporitic Keg River and Muskeg basinwide Canada m Formations. Lower and upper Granite Wash clastic evaporite) wedges reach a combined maximum thickness of 70 m. In the Devonian the Peace River Arch (Pre-

rphic high. cambrian granitic/cratonic core) formed an active series of parallel horsts (with exposed crests) and graben depressions (basins).

Elk City, Hall- Precambrian Minor, deeper reservoir in fractured, variably- Permian Landes, 1960; Gurney, Gor- basement, weathered pink granites and quartzites in some salts and Sneider et al., ham, Panhan- Pennsylva- fields, but clastic production is mostly within a mudstones, 1977; dle and other nian clastics drape made up of a mechanically reworked (marine-fed Walters, 1991; fields along the (wash) and Paleozoic grus and arkose “halo” and in syn-high platform Sorenson, Kansas Uplift Permian car- reefal build-ups. The high/arch formed in a pre- saltern* and 2005 and the bonates (re- Pennsylvanian collision event. There is some 120 mudflat) Amarillo Arch, gional reser- m of fault displacement at the top of the USA voir) Precambrian granite on the SW flank on the ≈ 1000 -2700 regional (10 km long) Gorham anticline. m “Granite wash” play - most of the reservoir resides in a clastic apron derived from weathering and mechanical reworking of material ero from an adjacent or underlying igneous-metamo

h- Clair field, Precambrian Reservoir is elongate NE-SW trending, normally Cretaceous Coney et al., offshore (Lewisian) faulted horst made up of a combination of mudstone 1993 Shetlands, UK basement fractured Devonian-Carboniferous redbeds and (marine) 1850 m Lewisian granites. Most storage is in clastics but basement fractures are higher permeability conduits

Xinglongtai Precambrian Highly fractured and weathered Archean granites Tertiary Luo et al., field, ≈ 2700 m and Mesozoic volcanics with a clastic halo in and mudstone 2005; Western de- about a horst block bound by normal faults. Relief (lacustrine) P’An, 1982 pression of the on the Archaean-cored “hill” is 1000 m and the oil Liaohe Basin, column in the field is 700 m high. China

Dongshenpu Precambrian Produces from fractured and weathered Archean Tertiary Xiaoguang & field, Da Ming ≈ 2800 m metagranites and Proterozoic metasediments. mudstone Zuan, 1991 Tun Depres- Structure is the higher part of normally faulted set (lacustrine) sion, China of basement blocks (edge of half graben) and is exposed and supplying clastics and supplying exposed sediment to the adjacent

store and flow hydrocarbons is due to exposure and weat onlapped by Tertiary sediments (source and seal).

ination of fractured weathered basement and a variably devel-

Yaerxia field, Silurian Highly fractured and weathered Palaeozoic meta- Tertiary Guangming & Western part of (Caledonian) morphic host making up the crestal zone the mudstone Jianguo, 1992; the Liuxi Basin, basement normal-faulted (wrench-faulted?) Yaerxia horst (lacustrine) P’An, 1982 China ≈ 2200 me- block. Situated along trend from the thrusted tres asymmetric anticline of Laojunmiao field, which is broken up by imbricated minor thrust faults on forelimb. Intense thrusting during Miocene.

Zeit Bay & Precambrian Highly fractured and weathered basement granitic Miocene Salah & Al- Geisum fields, basement basement horst forming a fault-defined high within evaporites sharhan, 1998 Gulf of Suez, ≈ 1200 me- Gulf of Suez rift. The 250 m thick oil column in Zeit (basinwide* Younes et al., Egypt tres Bay field extends from Mesozoic sandstones and salt & 1998 granite wash down into the fractured and weath- evaporitic ered basement, which contains one-third of oil in mudstone) the field.

Classic “Buried-Hill” fields where the reservoir is a comb apron oped clastic apron. Much of the ability of the basement to Much of the ability oped clastic apron. ering of the fractured basement when it was subaerially Trinh Xuan Cuong and J. K. Warren 153

Table 2 (continued).

Augila-Nafoora Late Precam- Rotated and exposed rift blocks highs with some Late Creta- Williams, 1972 field, Sirte brian or early production from fractured and weathered granites ceous Basin, Cambrian beneath the main Cretaceous Lower Rakb car- mudstone Libya basement, ≈ bonate reservoir. (marine) 2400 m

La Paz-Mara Palaeozoic Inverted (thrusted) wrench-fault block with frac- Late Creta- Nelson et al., fields, ≈ 4000 m tured granite/granodiorites and schists beneath ceous 2000

rusting Venezuela fractured Mesozoic carbonate (initial anticlinal mudstone Smith, 1956 reservoir prior to deepening in early 1950s). (marine) Thrusting/inversion began prior to Eocene but was most intense in Miocene-Pliocene. The total hydrocarbon column is ≈ 915 m high, 305 m of ted brittle basement. which is in a basement reservoir made up of fractured basement with little evidence of matrix weather.

Zdanice- Precambrian Fractured granites and granodiorites subjected to Eocene - Krejci, 1993 Krystalinkum ≈ 900 m exposure and weathering prior to Miocene when Oligocene field, the Upper Eocene to Oligocene pelitic seal was mud- Czech Repub- thrust into place by Zdanice-Subsilesian Nappe stone/pelite lic event (structure is not well documented). (marine)

Sarkadkeresz- Precambrian Majority of oil is stored in a narrow horst block Tertiary Dank, 1988 túr field, Pan- & Tertiary? composed of fractured Precambrian mudstone nonian Basin, ≈ 2700 m metamorphics. Oil column is 370 m thick (structure (Lacus- Hungary is not well documented) trine?)

Little or no contribution from a sediment halo to oil/gas storage. Th places reservoir in proximity to source rock. Reservoir is in a thrust-associated, fracture-domina Bach Ho field Cretaceous Inverted (thrusted) rift block of fractured gran- Upper Oli- This paper Offshore Viet- (mostly) ite/granodiorite. Thrusting occurred mostly in the gocene nam, Cuu Long ≈ 3500 m Late Oligocene. Oil column height is around 1000 muds Basin m. (lacustrine?)

significance for future exploration (e.g. the importance access to the Bach Ho core. XRD analytical services of the cantilevered blocks in controlling reservoir were kindly donated by Core Laboratories, Malaysia quality at Bach Ho), but still may not be capable of and the various software modules used to manipulate reliably defining variation in quality in the known the seismic and wireline data were kindly donated by reservoir blocks within the survey area. Schlumberger. Comments by J. Gutmanis (Geoscience (3) Often it is the simplest wireline tools, which Ltd, UK) and G. H. Lee (Pukyong National make direct physical measurements through a complex University) on a previous version of the MS are reservoir, which are the most reliable in terms of acknowledged with thanks. indicating variations in reservoir quality. Thus caliper logs are the most reliable indicators of fracture density REFERENCES and reservoir quality in wells at Bach Ho. AGUILERA, R., 1993. Formation evaluation of naturally fractured reservoirs. In: M. D. Diana and A. M. Woods (Eds), “Experience is a hard teacher because she gives the Development geology reference manual. AAPG, Tulsa, test first, the lesson afterwards” Oklahoma, pp 192-193. ARESHEV, E. G., T. L. DONG, N. T. SAN and O. A. SNIP, 1992. ACKNOWLEDGEMENTS Reservoirs in fractured basement on the continental shelf of southern Vietnam. Journal of Petroleum Geology, 15, 451- 464. The wireline, rock property, and seismic data from AYDIN, A., 2000. Fractures, faults, and hydrocarbon entrapment, Bach Ho field was provided by Petrovietnam, as was migration and flow. Marine and Petroleum Geology, 17, 797- 154 Bach Ho field, offshore SE Vietnam: A fractured basement reservoir

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