DSpace at VNU: Formation Mechanism and Petroleum System of Tertiary Sedimentary Basins, Offshore Vietnam

This article was downloaded by: [Northeastern University] On: 07 October 2014, At: 06:13 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Energy Sources, Part A: Recovery, Utilization, and Environmental Effects Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/ueso20 Formation Mechanism and Petroleum System of Tertiary Sedimentary Basins, Offshore Vietnam ab bc a c H X Nguyen , N T San , W Bae & C M Hoang a Sejong University, Seoul, Korea b Ho Chi Minh City University of Technology, Ho Chi Minh City, Vietnam c PetroVietnam Exploration and Production Corporation, Hanoi, Vietnam Published online: 09 Jun 2014 To cite this article: H X Nguyen, N T San, W Bae & C M Hoang (2014) Formation Mechanism and Petroleum System of Tertiary Sedimentary Basins, Offshore Vietnam, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 36:15, 1634-1649, DOI: 10.1080/15567036.2010.551269 To link to this article: http://dx.doi.org/10.1080/15567036.2010.551269 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or 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Production Corporation, Hanoi, Vietnam The quantitative aspects of the petroleum system are important criteria in hydrocarbon exploration and production to prevent risk and rank prospects in Vietnamese Tertiary basins However, formation mechanism and heterogeneous structures are still questionable For increasing oil recovery, a thorough understanding of the petroleum system to discover new reservoirs in each basin is necessary The characteristics of petroliferous basins are reviewed, summarized, and updated according to the most recent studies In this article, Cenozoic basins formation and petroleum systems are discussed Keywords: petroleum system, pull-apart, reservoir, rift, sedimentary basins, source rock INTRODUCTION The Vietnamese continental shelf has gone through several tectonic deformation phases, forming several basins located in the direction of the main fault systems where hydrocarbon deposits accumulated in both clastics and fractured basement reservoirs Numerous depressions and subbasins were linked along the length of the rift system from northern to southern and deep-water part of South China Sea (Vietnam East Sea) Fault-bounded sedimentary basins are formed through repeated episodes of faulting, including the basins of Song Hong, Phu Khanh, Cuu Long, Nam Con Son, Malay-Tho Chu, Tu Chinh-Vung May, and basins group of Spratlys (Truong Sa) and Paracels (Hoang Sa) (Figure 1) Exploration activities have taken place in Hanoi trough and An Chau depression since the 1960s In 1975, the gas field of Tien Hai C was discovered in the Miocene sandstone of Ha Noi trough and has been producing since 1981 In the Southern continental shelf, oil and gas of White Tiger field (Bach Ho) had been found in the 1970s by BH-1 well of Mobil oil company Subsequently, the first oil flow has been producing from the Miocene reservoir since 1986 Two years later, oil production was depleted continuously from a pre-Cenozoic fractured basement reservoir White Tiger field contained a succession of oil reservoirs from Lower Miocene to a pre-Cenozoic basement The amount of produced oil from the fractured basement reservoir occupied more 85% of the total oil production Since then, several similar structures found oil Address correspondence to Prof Wisup Bae, Sejong University, 98 Gunja-dong, Gwangjin-ku, Seoul 143-747, Korea E-mail: wsbae@sejong.ar.kr Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ueso 1634 1635 Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS FIGURE Cenozoic basins distribution in offshore Vietnam and gas, such as Black Lion, Yellow Lion (15.1), South-Eastern Dragon, Eastern Dragon, Southern Dragon, Turtle, Ca Ngu Vang (09-2), Emerald, Pearl, Topaz, Ruby (01-02), Rang Dong, Phuong Dong (15-2), Ba Den, Tam Dao (16-2), Ba Vi (16-1), Soi (16-2), and Vai Thieu (17) in Cuu Long basin; Dai Hung (Big Bear, 05.1), Lan Tay-Lan Do (6.1), Rong Doi-Rong Doi Tay (11-2), Thanh Long (05-1B), Hai Thach (5.2), Moc Tinh (5.3) in Nam Con Son basin; Kim Long and Ac Quy (B, 52/97) in Malay-Tho Chu basin; 102-CQ-1X (102); VGP-112-BT-1X (112), CVX-1X (118), and CH-1X (119) in Song Hong basin Currently, petroleum activities opposite challenges in exploration and production problems are due to complex geological setting, abnormal pressures, Downloaded by [Northeastern University] at 06:13 07 October 2014 1636 H X NGUYEN ET AL FIGURE Effects of principal tectonically features on the formation of Cenozoic basins and heterogeneity reservoir This article provides a valuable document about petroleum systems in Vietnamese Tertiary basins where outlined reservoir distribution regulation, and suggests the best solutions for exploration and production in Vietnamese offshore as well as southeast Asia basins GEOLOGICAL SETTING AND THE FORMATION MECHANISM OF PETROLIFEROUS BASINS The Vietnamese continental shelf and its adjacent regions had experienced a long history of geological and geophysical surveys The contrasting attributes of geophysical anomalies interpreted that various territories were divided into a series of geological regions and its complicated fault systems by a vigorous energetic of tectonic geodynamic mechanism (Figure 2) The southeast Asia subcontinent and adjacent areas were bounded by the left-slip Ailao Shan–Red River shear zone (ASRRSZ) in the northeast, and the right-slip Gaoligong shear zone, the Mae Ping shear zone (MPSZ), and Three Pagodas shear zone (TPSZ) in the southwest part Peltzer and Tapponnier (1988) proposed that the Indochina microplate was extruded to the southeastward along ASRRSZ throughout the collision stage of Indian-Eurasian plate According to radiometrics, the left-lateral strike slip motion along ASRRSZ during Early Miocene was due to an extrusion process and Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS 1637 FIGURE Tectonic activity stages through seismic reflection profile SEAS 95-14-2AB from 09 Soi to 15-RD structure to East White Tiger trough © Ha Noi Science and Technics Publishing House Reproduced by permission of Ha Noi Science and Technics National University Publishing House Permission to reuse must be obtained from the rightsholder seafloor spreading the center of the Vietnam East Sea as a result of 20–30ı clockwise rotation with respect to the Indochina microplate after the collision of Indian and Eurasian plates Due to the subduction of the Indian plate beneath the southern edge of Sumatra-Java arch in Early Cenozoic, the eastern edge of the Eurasian plate was therefore strongly altered These mechanisms played an important role for setting up chain sedimentary basins in this region (Hall, 1996) The Song Hong pull-apart basin was formed in the Early Tertiary and elongated in ASRRSZ, which spread out the Vietnam East Sea (Clift and Sun, 2006) This basin is situated on connection with the Day Nui Con Voi metamorphic core complex on the Red River fault, northern Vietnam The marine seismic data interpreted a longitude 109ı meridian fault located on the central and central southern parts of the Vietnamese continental shelf (Que, 1994) It affected robustness on the formation process of Phu Khanh basin The NW–SE left-lateral strike-slip faulting extended the motion from the southern part of Hainan Island, passed the central and southern Vietnam, and the west coast of the Kalimantan Island to reach on the Sunda Gulf of Indonesia (Gatinski et al., 1984) The collision of Indian and Eurasian plates drove the southern part of the Indochina block to move the left-lateral along the NW–SE trending of TPSZ and MPSZ in Eocene-Early Oligocene time It created a regular basin segmentation pattern along the TPSZ where it formed the Pattani, Khmer, Malay, and West Natuna basins due to thermal subsidence (Replumaz and Tapponnier, 2003) Further to the eastern part of Cuu Long and Sarawak basins are Paleogene pull-apart basins connected to the MPSZ The normal faults of NE–SW and E–W directions were represented in Cuu Long basin during the Early Paleogene rifting phase These normal faults splay toward the W and WNW beneath the modern Mekong River delta, which may reflect the culmination of the MPSZ or one of its fault strands into the basin Cenozoic tectonic evolution of sedimentary basins can be separated into three stages (Figure 3): pre-rift (before Middle Eocene), rifting (Middle Eocene-Early Miocene), and post-rift (Middle Miocene to present day) 1638 H X NGUYEN ET AL Downloaded by [Northeastern University] at 06:13 07 October 2014 2.1 Pre-rift Period (Late Cretaceous-Middle Eocene) Uplift in the Vietnam territory and Indochina block has resulted in considerable erosion and deposition of the erosion products on the shelf that constituted intrusive granitic, granodiorite batholiths and tuff, and volcanic rocks in Late Mesozoic Sediments from Cretaceous to Early Eocene ages dominated terrestrial redbeds and lacustrine mudstones Volcaniclastic rocks were deposited in intermountain troughs, where they were separated by faults Geological signatures provided that the outcropping volcaniclastic rocks accumulated in northeastern part of Da Lat magmatic intrusion zone in southern Vietnam The intrusive rocks encompass a biotile-granite, granodiorite, and diorite, which were presented in Late Triassic-Hon Khoai complex, Late JurassicDinh Quan complex, Cretaceous-Deo Ca complex, and Late Cretaceous Ankroet complex (Hung and Trinh, 2008) These rocks are recorded in both the fractures basement reservoirs of the basins and the outcrop of rocks exposed on Da Lat zone The orogenetic magmatic process affected a considerable uplift and eroded in this region where it caused the absence of Cretaceous, Palaeogene, and Eocene sediment successions in seismic cross-section Simultaneously, the processes of erosion and weathering happened continuously with a powerful intensity in the highlands This is a good condition to become a storage where fractures and vuggy were created in granitoid basement reservoirs 2.2 Rifting Period (Middle Eocene-Oligocene-Early Miocene) Owing to an Indian plate extruding into a Eurasian plate and drifting into a northward in the Palaeogene and Eocene time (50–45 ma), it caused the shortening of the earth’s crust and the subduction of the Indian oceanic plate underneath the southern margin of the Eurasian plate (Hall, 2009) It is called the main folding-orogenic phase of the Himalayan tectonic cycle At that time, existing deep faults reactively formed that impacted the southeast part of the Indochina subcontinent generating rift-basins along shear zones Reflection on seismic data interpreted that rift-basins were established from the Middle Eocene to Early Miocene; however, a local subsidence still occurred until it ended in the Late-middle Miocene (Lee and Lawver, 1994) The evidence showed that the signatures of alluvial and fluvial sandstone were encountered by the CL1 well in the southwest border of Cuu Long basin The superb exposures and thick sedimentary sequences were deposited in rift-basins during the Oligocene time A large amount of shale, siltstone, and sandstone filled up 2,000–3,000 m thick in half- or full-graben and deep troughs In Early Miocene, a widespread marine incursion flooded the whole basins, depositing the Rotalia mudstones sequences, a thick shale section that is considered to be a regional excellent seal At the same time, the volcanic eruption also transpired in the western and northwest regions Therefore, the stretching phases have been divided into two phases: the Middle Eocene-Oligocene period is the rifting (or syn-rift) phase, and Early Miocene periods The Middle Eocene-Oligocene period (50–25 ma): With the rapid growth and linkage of normal faults, basin drainage patterns are established This period is called as syn-rift subsidence for petroleum accommodation of the Tertiary basins The subsidence rate exceeded the quantity of supplied sediments from adjacent regions where the products of lacustrines dominated near shore swamp facies with rich organic content Shallow lake border relief developed a flourished floristic carpet and an abundance of lacustrine organic matters in humid weather These are favorable conditions for the hydrocarbon generation process In Late Oligocene, the relative plate motion between the subducting Indo-Australian plate and the overriding Sunda plate released out a compression stress field to establish different tectonic inversions moving along its normal faults This event marked the termination rifting process and the regional break-up unconformity at intra-cratonic basins of Indochina margin Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS 1639 The seafloor spreading and rift propagation in the Vietnam East Sea made the motion strike slip along the longitude 109ı East Meridian fault and liberated a volcanic eruption of basalt and andesite in the southwestern part (Averyanov et al., 2003) During the Early Miocene period (25–17 ma): Rifting subsidence nearly completed in backarcs and fore-arc basins of Sumatra, east Java, and south Borneo, but still active with minor intensity in episutural intra-cratonic basins of the Indochina subcontinent Marine environment covered broadly the whole basins to the eastern margin of the Indochina subcontinent and developed carbonate rock and reef build-up successions (Lee et al., 2001) Periodic deep transgression into the Song Hong, Cuu Long, and Malay basins grew great diversity sediments with swamp, delta fan, and near shore facies (Nielsen et al., 1999) The subsidence of Phu Khanh basin was compensated by supplied sediments from eroded Kontum massif and alluvial prograding system tracts on large near-shore swamps The prograding delta sequence continuously grew to the Middle Miocene and it was considered as a potential reservoir 2.3 Post-rift Period (Middle Miocene to Quaternary) The tectonic subsidence of syn-rift basins ceased when the grabens and half-grabens filled up sediments and followed by the subsequent thermal sagging In Late Miocene, convection currents in the mantle caused the movement of the lithosphere crust to move right lateral strike-slip along both ASRRSZ and TPSZ where inversion structures and regional unconformities were created (Liu and Xia, 2006) The inversion structures were clearly observed in the onshore Song Hong and seismic section in Nam Con Son basin Bach Long Vi Island (the tip of an inversion fold) in the Gulf of Tonkin was truncated by Mio-Pliocene regional unconformity and usually became hydrocarbon traps in Tertiary basins along the Sunda shelf A deep subsidence motivated maximum regional transgression into the southern margin of the Indochina block to constitute the successions of marine shales and carbonate reef in Middle-late Miocene In Pliocene time, deep marine shale, submarine fans, and turbidite systems developed widely in the whole basins Before the inversion phase occurred, the succession of transgression sandstone, carbonate build-up, turbidite fans, and shale diapir had formed and were considered to be potential hydrocarbon prospects, typically gas reservoirs in the Middle-upper Miocene formation and Lower Pliocene turbidity PETROLEUM SYSTEM IN TERTIARY BASIN 3.1 Source Rock Geochemistry analysis of oils and source rocks has been reviewed and there is a consensus that the most organic matter originated from algal lacustrine and land-plants source material Source rock types summarized in the major petroleum basins are as follows (Figure 4) Song Hong basin is a pull-apart and extensional rifting basin with heterogeneity geological structures extending southeastward from Hanoi trough to Vietnam East Sea It filled up Cenozoic sediments up to 14 km thick in the depocenter Three potential source rocks have been recognized First, the Eocene-Lower Oligocene source rocks are primary kerogen types I/II, belonging to freshwater algae and land plants with a total organic carbon of 7–18 wt% Hydrogen index (HI) value is widely discrete from 200 to 600 mgHC/g total organic content (TOC) Vitrinite reflectance (Ro) is about 0.45 and Tmax of 428–439ıC (Petersen and Andersen, 2001) Consequently, it predominantly generated oil and natural gas in maturity thermal conditions Second, TOC of the upper part of Oligocene-Lower Miocene source rocks range from moderate to good in the southern region, kerogen of type III, type II, and minor I kerogen with high gas generation potential (Hue Downloaded by [Northeastern University] at 06:13 07 October 2014 1640 H X NGUYEN ET AL FIGURE Source rock types and thermal maturity (Source: Tien, 2003.) © HCMC National University Publishing House Reproduced by permission of HCMC National University Publishing House Permission to reuse must be obtained from the rightsholder trough, PV–XT-1X) (Chinh et al., 2005) The Upper Oligocene coaly shales appeared in well 112 BT-1X, and an amount of oil and gas was encountered in the wells of 112 HQ-1X, 112 BT-1X, and 114 KT-1X; Nielsen et al (1999) mentioned that likely source from similar beds Third, TOC of the Middle-upper Miocene source rocks contained moderate type III kerogen, generating mainly natural gas Based on a thermal maturity model, Middle Miocene source rock of the central part matured and generated hydrocarbon since Late Miocene The marine mudstones dominated type III kerogen with 0:2 ˙ 3:0% TOC in the southeastern part Most source rocks matured and generated hydrocarbon products that migrated into traps since 10–20 million years ago Phu Khanh basin, located offshore central coastal shore, is a narrow north-south trending basin approximately 250 km long and 50–75 km wide It is bounded by the fault systems of the N–S and NW–SE directions and can be divided into many distinctive blocks The major tectonic features are classified as Da Nang shelf, Phan Rang shelf, Phu Khanh trough, and Tuy Hoa shear zone The thickness of Cenozoic sediments changes from a few hundred meters to 7,000–8,000 m at the eastern depocenter of the western margin with terrigenous sediments and carbonate build-up Two kinds of source rocks of Oligocene and lower Miocene are hydrocarbon generation potential The geochemical results from the seepage oils in the Dam Thi Nai lagoon, Song Ba river, and Kontum massif indicated that Neogene source rocks are fine grain sediments organic richness represented samples with TOC of 1.79–2.64% and S2 of 8.22 mg/g, PI < 0.3, HI of 242–866 Based on the thermal maturity modeling, heat flow measured in the Early Oligocene (1.22 HFU), Late Oligocene (1.35 HFU), and Early Miocene (1.07 HFU) determined that the oil window is about 3,000–5,000 m in depth, respectively (Nielsen et al., 2007) Cuu Long basin positioned an elongated WSW–ENE trending basin, less than 100 m above sea level, southeastern offshore The Cenozoic sediments filled up from a few hundred meters of the western margin to 6,000–7,000 m thick in the depocenter of White Tiger eastern and western troughs The stratigraphic lithology encompasses Eocene terrigenous sediments, Oligocene Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS 1641 lacustrine mudstones, and marine shales of Miocene to Quaternary formations Geochemical analysis resulted that the Upper Oligocene source rock (Tra Tan formation) dominated fine grain sediments Average TOC varies from 3.5 to 6.1% and higher S1 ranges about 4.0–12.0 mg/g, S2 of 16.7–21.0 mg/g Source rocks mixed a kerogen type II and type I of lacustrine freshwater algal that generated mainly oil Source rocks of Lower Oligocene and Eocene (Tra Cu formation) are plentiful with TOC of 0.97–2.5% Kerogen type II, the most abundant, was typically derived from plant debris, phytoplankton, and bacteria in marine sediments that typically generated oil Kerogen type III was derived seldom from woody terrestrial source material Most sedimentary organic matter took place in what is called “oil window,” normally equivalent to the vitrinite reflectance of R0 D 0.6–1.35% in a depth of 2,900–4,200 m Below 4,400 m deep of White Tiger eastern and western troughs generated gas and condensate zones Furthermore, the lower Miocene source rock (Bach Ho formation) estimated approximately 1% TOC as potential source rocks, but only matured in center trough The modeling of thermal maturity defined that hydrocarbon migrated into traps 22 million years ago Nam Con Son basin is located on the SE part of Vietnamese shelf where it is intersected by two major fault systems between ASRRSZ and TPSZ The basin axis orientation is controlled by the NE-SW of the northern part and the NS of the southern part The greatest thickness is about 12,000 m of which deposited since Oligocene time Sediments are comprised of primarily clastics, occasionally coal seams Miocene carbonates beds present in eastern and southern areas Geochemistry analyzed that source rocks of Eocene-Oligocene dominated kerogen type III from remains of higher plant debris found in coals, rarely type II Fine grain sediments of Miocene carbonate sequences accumulated in lacustrine mudstones, swamp, and coastal plain environments TOC ranges from 1.18 to 4.83%, S1 of 0.87–3.09 mg/g, S2 of 2.11–6.57 mg/g Hence, source rocks generated major gas, condensate, and little oil (Tien, 2003) Tu Chinh-Vung May basin is situated on the deep marine region, near eastward Nam Con Son basin The seismic profiles interpreted that filled sediments estimated about 7,000–8,000 m thick at the depocenter The terrigenous sediments of Eocene-Oligocene formation and carbonate build-up of Miocene formation were positioned on the top of the swell zones and dome structures Basin structure was separated into complex blocks by NE–SW and EW faults The samples of the PV-94-2X well resulted that two source rocks of Oligocene and Lower Miocene were potential hydrocarbon generation that derived from lacustrine shale and humid coal However, total organic content of Middle Miocene sediments is inadequate to generate hydrocarbon Source rocks are most likely kerogen type III and II, which were allocated in Phuc Nguyen, southwestern, and Vung May northwestern troughs (Bo, 2007) Malay-Tho Chu basin, which is positioned in the southeast of the Thailand Gulf along a NW– SE direction, is 300 km long and 100 km wide The Cenozoic sediments were deposited about 8,000–9,000 m thick in the deepest trough, mainly terrigenous clastics interbedded a few coals seams in some Miocene formations The basin architecture is governed by the NNW–SSE faults in the northern part and NW–SE faults in the southern part Complex geological structures were divided into several uplift and subsidence zones with local basins (Khmer shelf, Pattani basin, Khorat swell, Malay basin) and a structural dome in the northern and southern regions Based on geochemical results, there are two source rocks in the basin First, Oligocene source rock dominated shale from lacustrines with rich organic matter that dominated kerogen type I and II In thermal maturity modeling, source rocks produced to hydrocarbon in Middle Miocene and hydrocarbon migration process occurred 26 million years ago A large amount of oil and gas were released from Oligocene source rock in deep troughs of northern and southern parts Second, lower Miocene source rock is coaly shales with kerogen II/III type, good conditions for both oil and gas generation The lower part of Lower Miocene source rock buried deep in the Pattani basin and in the northern Malay basin matured at the favorable thermal conditions and entered in an oil window (Minh, 1997) 1642 H X NGUYEN ET AL In brief, three source rocks in Tertiary basins comprise the Eocene-Lower Oligocene, the Upper Oligocene, and the Lower Miocene shale successions The burial history and basin thermal modeling defined that hydrocarbon migration time from Oligocene source rock started approximately 18.2 million year ago If traps have not been destroyed by tectonic activities and secondary alteration, a large amount of hydrocarbon might be charged into them However, most amounts of hydrocarbon migrated into traps before the Middle Miocene Downloaded by [Northeastern University] at 06:13 07 October 2014 3.2 Seals Three regional seals are covered commonly in whole basins: Upper Oligocene, Miocene, and Pliocene shales Seismic reflection surfaces are shown in Figure Local seals with interbedded silt-shale sequence presented a small scale in the Oligocene-Miocene formations a Upper Oligocene seal is a succession of interbedded black claystone, coaly shale, and claysiltstone, which derived in a variety of environments, including near shore swamps, lacustrine and deltaic, and locally transgression shale The sediment thickness changed from 15–30 m up to several hundred meters with high clay content (70–80%) This is a quite stable regional seal for stratigraphic traps in Eocene-Lower Oligocene formation, especially covering dome structures of fractured basement reservoir The shale barrier zones caused an abnormal overpressure in this formation It is an important factor to protect the stability of primary porosity in reservoirs b Lower Miocene seal broadly distributed in Cuu Long, Malay-Tho Chu, Nam Con Son, and Song Hong basins, is called the Rotalia shale sequence It is considered as an excellent seal in respect to 180–200 m thick for underlying reservoirs Clay content occupies to 80–90% Sand-shale ratio increases gradually towards the basin margin c Upper Miocene-Pliocene marine shale is a significant seal for Middle-upper Miocene gas reservoirs in the Malay-Tho Chu, Nam Con Son, Phu Khanh, and Song Hong basins The tectonic inversion occurred in Late Miocene to establish stratigraphic traps in Early Pliocene Late Miocene shale seal is deeply eroded on inversion structures to be thinner in margin, and its effect on the hydrocarbon accumulation 3.3 Reservoir Most reservoirs are pre-Tertiary fractured basement, Eocene-Oligocene-Miocene sandstones, Upper Oligocene-Lower Miocene fractured extrusive rock, Miocene carbonate build-up, and lower Pliocene sandstones with various environment facies a Pre-Tertiary fractured basement reservoir is described as granitoid rocks and Middle-upper Paleozoic carbonate Granitoid basement reservoir has a fracture and vuggy (secondary porosity), which play a significant role in both storage and channel path (Figure 5) This is exploration and production target in Cuu Long basin (Areshev et al., 1992) In Nam Con Son basin, an amount of oil was also produced from Thanh Long, Dai Hung, and Thien Ung structures In addition, a low oil rate was recorded in fractured carbonate reservoirs of Carboniferous-Permian and Middle-upper Devonian formations in the Hanoi trough, so these are considered as potential reservoirs in Song Hong basin (Figure 6) Potential reservoirs of fractured and weathered basement are represented in Phu Khanh basin, especially anticline structures (Figure 7) b Oligocene-Middle Miocene clastic reservoir is formed by fractured extrusive rocks characterized by basalts, andesite, and diabase porphyrite interbedded overlying the Mesozoic Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS 1643 FIGURE Fractured basement reservoir in White Tiger field (Source: Hung and Hung, 2004.) © HCMC National University Publishing House Reproduced by permission of HCMC National University Publishing House Permission to reuse must be obtained from the rightsholder fractured basement Its distribution is limited in some local regions, poor reservoir quality with low oil recovery However, there is a large amount of oil deposited at clastic reservoirs of the Oligocene-Middle Miocene formation in Cuu Long basin c Eocene-Oligocene-Miocene and Lower Pliocene sandstone reservoirs: Oil and gas are seldom encountered in the Upper Miocene part in the Nam Con Son and Song Hong basins FIGURE Reservoir types in Song Hong basin Downloaded by [Northeastern University] at 06:13 07 October 2014 1644 H X NGUYEN ET AL FIGURE Main play types in Phu Khanh basin (Huyen and Hoai, 2007) Gas reservoirs may be discovered in turbidity fans of Upper Miocene-Lower Pliocene formation (Figure 6) d Middle Miocene carbonate reservoir is spread out in the southern part of Song Hong basin (Figure 6), Nam Con Son basin, and Phu Khanh basin (Figure 7) Gas reservoirs were discovered in the Nam Con Son basin Carbonate reservoirs constituted platform and reef buildup with high porosity of 40–55%, including intergranular and vuggy-fractured Carbonic gas reservoirs were commonly distributed irregularly in the southern part of the Song Hong basin This is a big challenge to discover in a deeper reservoir 3.4 Hydrocarbon Plays and Traps Hydrocarbon plays and traps are formed at different ages (Figure 8) Well-known plays are classified into five categories: pre-Tertiary fractured basement (play 1); Eocene-Oligocene sandstone (play 2); Miocene-Early Pliocene sandstone (play 3); Miocene carbonate (play 4); and extrusive rock (play 5) a Hydrocarbon play of the pre-Tertiary fractured basement (play 1) consists of Mesozoic fractured granitoid, Middle-late Paleozoic fractures, and vuggy carbonates Fractured granitoid basement contained a large hydrocarbon reserve with high oil recovery in both Nam Con Son and Cuu Long basins (Figures and 9) Weathered and fractured-vuggy carbonate basement was considered as a production target in Song Hong basin Anticlinal traps of Middle-late Paleozoic carbonates are potential hydrocarbon plays and sealed by the regional Upper Oligocene black shale b Hydrocarbon play of the Eocene-Oligocene sandstone (play 2) has two types: the EoceneLower Oligocene and the Upper Oligocene reservoirs Petroleum is charged in secondary fractures and dissolution porosity and permeability Eocene-Oligocene play is the main oil production target in White Tiger, Dragon, Su Tu Den, and Rang Dong structures Gas reservoirs are produced in the Su Tu Trang, Dragon, and Emerald fields in Cuu Long basin and Malay-Tho Chu basin (Figure 10) c Hydrocarbon plays of the Miocene-Lower Pliocene sandstone (play 3) comprises of Lower Miocene, Middle Miocene, and Upper Miocene-Pliocene reservoirs Lower Miocene sand- Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS FIGURE 1645 Main play types in Tertiary sedimentary basins stone reservoir contained the principal oil and gas, and the Middle-upper Miocene sandstone accumulated gas deposits that distributed in Song Hong, Nam Con Son, and those basins adjacent d Hydrocarbon play of Miocene carbonate (play 4) embraces the carbonate platform and Earlymiddle Miocene reef build-up Carbonate reservoir is predominantly developed on the eastern margin of Nam Con Son, Phu Khanh, Song Hong basins, and Spratly-Paracel basin groups (Figures and 11) Reef build-up has a major role for containing hydrocarbon deposits in Lan Tay-Lan Do, Thanh Long, Hai Thach-Moc Tinh, Dai Bang, and Bac structures in FIGURE Seismic section across dome structure of White Tiger in Cuu Long basin H X NGUYEN ET AL Downloaded by [Northeastern University] at 06:13 07 October 2014 1646 FIGURE 10 Main play types in Malay–Tho Chu basin © HCMC National University Publishing House Reproduced by permission of HCMC National University Publishing House Permission to reuse must be obtained from the rightsholder Nam Con Son basin Porosity value is large up to 40% and permeability goes up to several darcy e Hydrocarbon play of fractured-vuggy extrusive rock (play 5) is represented in the Upper Oligocene-Lower Miocene sediments in Cuu Long and Phu Khanh basins Extrusive rocks characterize basalt-diabase, andesite, and volcanic tuff, which formed interbeds within clastic sections These rocks directly covered weathered-fractured basement in the Bach Ho, Ruby, and Southeast Dragon structures 1647 Downloaded by [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS FIGURE 11 Petroleum systems in Tertiary sedimentary basins, offshore Vietnam CONCLUSIONS The Vietnamese sedimentary basins are mainly rift and pull-apart types, strongly affected by tectonic geodynamic during the Tertiary period The formation mechanisms are divided into three main stages: pre-rift, rifting stage, and post-rift Three source rocks for petroleum generation are comprised of Eocene-Lower Oligocene, Upper Oligocene, and Lower Miocene sediments Local and regional shale barriers present in whole basins Three seal types include local seal on the Upper Oligocene formation, regional seal placed on Lower Miocene formation 1648 H X NGUYEN ET AL Downloaded by [Northeastern University] at 06:13 07 October 2014 (Rotalia shale sequence), and Upper Miocene-Pliocene regional seal, which characterize a thick marine shale Reservoirs dominated pre-Tertiary fractured basement, Upper Oligocene-Lower Miocene fractured extrusive rock, Eocene-Oligocene-Miocene and Lower Pliocene sandstones, and Middle Miocene carbonate reservoirs The fractures and vuggy in Mesozoic granitoid basement is the main production target in the Cuu Long and Nam Con Son basins Hydrocarbon plays are classified into five categories: Pre-Tertiary fractured basement, EoceneOligocene sandstone, Miocene-Early Pliocene sandstone, Miocene carbonate, and extrusive rock (play 5) ACKNOWLEDGMENT The authors wish to thank PetroVietnam Exploration and Production Corporation, Schlumberger K.K for the encouragement of writing this article FUNDING This work was supported by the Energy Resources R&D program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No 2012T100201728) REFERENCES Areshev, E G., Dong, T L., San, N T., and Snip, O A 1992 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Formation Mechanism and Petroleum System of Tertiary Sedimentary Basins, Offshore Vietnam Downloaded by [Northeastern University] at 06:13 07 October 2014 H X Nguyen,1;2 N T San,2;3 W Bae,1 and. .. [Northeastern University] at 06:13 07 October 2014 TERTIARY SEDIMENTARY BASINS FIGURE 11 Petroleum systems in Tertiary sedimentary basins, offshore Vietnam CONCLUSIONS The Vietnamese sedimentary basins... distribution regulation, and suggests the best solutions for exploration and production in Vietnamese offshore as well as southeast Asia basins GEOLOGICAL SETTING AND THE FORMATION MECHANISM OF PETROLIFEROUS