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

Keywords: petroleum system, pull-apart, reservoir, rift, sedimentary basins, source rock 1.. Before the inversion phase occurred, the succession of transgression sandstone, carbonate bui

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Formation Mechanism and Petroleum System of Tertiary Sedimentary Basins, Offshore Vietnam

H X Nguyenab, N T Sanbc, W Baea & C M Hoangc 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

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ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567036.2010.551269

Formation Mechanism and Petroleum System of Tertiary

Sedimentary Basins, Offshore Vietnam

H X Nguyen,1;2N T San,2;3 W Bae,1 and C M Hoang3

1Sejong University, Seoul, Korea

2Ho Chi Minh City University of Technology, Ho Chi Minh City, Vietnam

3PetroVietnam Exploration and 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

1 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 sub-basins 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

FIGURE 1 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,

FIGURE 2 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

2 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

FIGURE 3 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)

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 Jurassic-Dinh 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, Palaeo-gene, 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

CL-1 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

1 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 sub-sidence for petroleum accommodation of the Tertiary basins The subsub-sidence 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

and andesite in the southwestern part (Averyanov et al., 2003).

2 During the Early Miocene period (25–17 ma): Rifting subsidence nearly completed in back-arcs 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

3 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 fresh-water 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

FIGURE 4 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

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)