Geological evolution, regional perspectives and hydrocarbon potential of the northwest Phu Khanh Basin, offshore Central Vietnam

ARTICLE IN PRESS

Marine and Petroleum Geology 26 (2009) 1–24 www.elsevier.com/locate/marpetgeo

Geological evolution, regional perspectives and hydrocarbon potential of the northwest Phu Khanh Basin, offshore Central Vietnam Michael B.W. Fyhna,, Lars H. Nielsenb, Lars O. Boldreela, Le D. Thangc, Jørgen Bojesen-Koefoedb, Henrik I. Petersenb, Nguyen T. Huyenc, Nguyen A. Ducc, Nguyen T. Dauc, Anders Mathiesenb, Ian Reida, Dang T. Huongc, Hoang A. Tuanc, Le V. Hienc, Hans P. Nytoftb, Ioannis Abatzisb a

Department of Geography and Geology, University of Copenhagen, Geocenter Copenhagen, Øster Voldgade 10, 1350K, Denmark b Geological Survey of Denmark and Greenland, GEUS, Geocenter Copenhagen, Øster Voldgade 10, 1350K, Denmark c Vietnam Petroleum Institute (VPI), Trung Kinh Street, Yen Hoa, Cau Giay, Hanoi, Vietnam Received 20 February 2007; received in revised form 12 June 2007; accepted 5 July 2007

Abstract Seismic stratigraphic and structural analyses of the northwest Phu Khanh Basin, offshore Central Vietnam, based on 2-D seismic data, indicate that the initial rifting began during the latest Cretaceous? or Palaeogene controlled by left-lateral transtension along the East Vietnam Boundary Fault Zone (EVBFZ) and northwest–southeast directed extension east of the EVBFZ. Rifting stopped due to transpression during middle Oligocene times but resumed by left-lateral transtension during the Late Oligocene. Thick sequences of lacustrine and alluvial sediments were deposited during the Palaeogene rift periods. The Late Oligocene rifting ended due to inversion, triggered by right-lateral wrenching near the Palaeogene–Neogene boundary. Following the onset of this inversion regional uplift and volcanism took place in the southern half of the study area and contemporaneous subsidence and transgression took place farther north, leading to widespread carbonate deposition. As the right-lateral wrenching decreased during the early Neogene, thermal subsidence and siliciclastic sedimentation became dominant, resulting in the buildup and southward propagation of the shelf slope. Sediment accumulation and subsidence rates increased after the Middle Miocene times due to eastward tilting of Central Vietnam and the adjacent offshore area. Potential direct hydrocarbon indicators (DHIs) in the Phu Khanh Basin include common amplitude anomalies, gas chimney-like features and seafloor gas seeps. In addition, oil seeps are found at Dam Thi Nai, immediately landward of the basin. Geochemical analyses of the oil seeps indicate the existence of at least two early to peak mature source rocks. Maturation modelling, combined with the seismic analysis, suggests the likely presence of oil kitchens 40–50 km downdip in the basin, and a fairly simple oil migration route. The basin is probably charged from lacustrine syn-rift mudstones, humic coals and fore-reef marls. Potential reservoirs are turbidite, lowstand delta, shelf and coastal sandstones, as well as platform and reef carbonates and fractured basement. Various structural and stratigraphic traps formed during the Palaeogene and early Neogene, prior to the main stage of petroleum generation. r 2007 Elsevier Ltd. All rights reserved. Keywords: Phu Khanh Basin; Extrusion tectonics; Basin analysis; Seismic facies analysis; Hydrocarbon assessment; Vietnam; South China Sea

1. Introduction The undrilled Phu Khanh Basin, offshore southern Central Vietnam, is the least explored basin on the Vietnamese margin. The basin is situated along the narrowest part of the South China Sea’s shelf and is Corresponding author. Tel.: +45 38142718; fax: +45 38142050.

E-mail address: [email protected] (M.B.W. Fyhn). 0264-8172/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2007.07.014

characterised by a water column ranging from a few tens of metres in the west to abyssal depths towards the east. The Phu Khanh Basin borders oceanic crust to the east and is situated south of the Song Hong, Qui Nhon and Qiongdongnan basins and north of the oil and gasproducing Cuu Long and Nam Con Son basins (Fig. 1). The Phu Khanh Basin has previously been interpreted as a classic rifted continental margin formed by latest Cretaceous (?) and Palaeogene rifting and subsequent Late

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90°E

100°E

110°E

Nepal Ai

an Sh lao

25°N

China

R

ed

India

Myanmar

Ri

ve

rS he ar

Dong Ho Beibuwan Basin

Zo ne

er rl Riv

asin

th B

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Bengal Basin

ng

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n Ho gB

ng ul

gB

n Lo

ram

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u Cu Nam Con Son Basin

in

as

t

e

Lin

Malay Basin

a in

Phu Khanh Basin

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Fa

Pattani Basin

Oceanic crust

Malaysia

Pa

la w

an

gh ou Tr

W. Natuna Basin

ra

at

m

Su

Penyu Basin

n t

ul

Fa

500 km

h th C S ou

Song Ba Trough

Pi

Mergui Basin

Metamorphic core complex Approximate area of basin

Main Cenozoic strik-slip direction

TR

Cambodia

ae M

Wells 1: 119-CH-1X 2: 120-CS-1X 3: 121-CM-1X Andaman East Vietnam Sea EVBFZ Boundary Fault Zone

Se

EVBFZ Qui Nhon Basin

Three Pagodas Fault

15°N

a

in as

Thailand

Indonesia

Fig. 1. Location of major Cainozoic basins and areas underlain by oceanic crust as well as a simplified structural outline of the region. The study area outlined in the box is shown in detail in Fig. 2. Locations of three wells used for seismic ties are included. ‘‘Song Ba Trough’’ and ‘‘Dong Ho’’ marks the position from where lacustrine mudstones analogue to offshore source rocks were sampled. TR ¼ Triton Ridge. Structural features and basin outlines modified after Morley (2002); South China Sea Oceanic Crust after Briais et al. (1993).

Oligocene to recent post-rift subsidence, on the basis of the interpretation of a limited amount of seismic reflection data (Canh et al., 1994; Lee and Watkins, 1998; Than et al., 2003). The overall hydrocarbon potential of the basin was summarised by Lee and Watkins (1998). We present a revised structural and stratigraphic interpretation of the Phu Khanh Basin related to the complex Cainozoic Indochinese evolution. In addition, a general evaluation of the hydrocarbon potential of the basin is presented based on the integrated analysis of available seismic, gravity and regional source rock data, as well as oil seeps. The present geological analysis is based on an improved open-gridded 2-D seismic database relative to previous studies. The seismic data were acquired during the period between 1974 and 1993 and form an open grid (Fig. 2). Direct age estimates of the seismic sequences are not possible in the Phu Khanh Basin; the seismic units have, therefore, been put into a depositional framework, which has been correlated to a number of known regional geological events. In addition, ages of the inferred Neogene

sequences are supported by seismic ties to three wells north of the basin (Fig. 1). This work is the result of a multidisciplinary research project, carried out during 2002–2005 by cooperation between the Vietnam Petroleum Institute (VPI), the Geological Survey of Denmark and Greenland (GEUS), University of Copenhagen and Hanoi University of Mining and Geology [first phase of the ‘‘Enhancement of Research Capacity in Developing Countries project’’ (ENRECA project)] (Nielsen et al., 2003; Nielsen and Abatzis, 2004). The data used in this study include ca. 2000 km of multichannel seismic profiles covering the study area, 3 wells and well ties from neighbouring basins and gravimetric data from Sandwell and Smith (1997), as well as onshore oil seeps sampled by the group at Dam Thi Nai, immediately west of the Phu Khanh Basin. Furthermore, a fully cored ca. 500 m deep stratigraphic research well through the Neogene Song Ba Trough onshore was carried out as part of the project in order to analyse an analogue to the syn-rift succession of the Phu Khanh Basin (Tuan et al., 2004; Bojesen-Koefoed et al., 2005; Nielsen et al., 2005, 2007).

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109°

3

N

110°

Dam Thi Nai Lagoon 14°

Fig. 3, 12, 13

Fig. 8 Fig. 7

Fig. 5a

2000

Fig. 9

1500

1000

200

500

Vietnam

Fig. 5b

Fig. 6

Mesozoic igneous basement

13°

Fig. 5c

Fig. 11

Neogene–Recent sediments Late Miocene– Quarternary basalt Mesozoic sediments

Fig. 10

40 km

Fig. 2. Location of seismic reflection data with bathymetry indicated in metres (dashed lines). Heavy lines and respective figure numbers indicate the location of illustrated seismic profiles shown in other figures. Simplified onshore geology compiled after Tien (1991) and Tinh (1998).

2. Regional geological setting The geological evolution of the marginal basins bordering the South China Sea is commonly related to one of two genetic models: the ‘‘slab pull’’ and the ‘‘extrusion’’ models. The slab-pull model postulates rifting and opening of the South China Sea as a consequence of a slab pull related to southward subduction of a proto-South China Sea beneath Borneo. The slab pull model predicts a rightlateral, north–south trending transform zone along the Central and South Vietnamese coast (Taylor and Hayes, 1980, 1983; Holloway, 1982). Tapponnier et al. (1982) suggested the alternative extrusion model, in which rifting and seafloor spreading occurred as a result of southeastward extrusion of Indochina caused by the continental collision between India and Eurasia (Leloup et al., 1995,

2001). The extrusion resulted in left-lateral shearing along the Ailao Shan–Red River Shear Zone and its seaward pendent, the EVBFZ. Later studies have integrated the two proposed models into a single model in which subduction of a proto-South China Sea and extrusion of Indochina acted together, causing the present structural outline of the region (e.g. Lee and Lawver, 1994, 1995; Hall, 1996, 1997, 2002; Morley, 2002). Early Neogene counterclockwise rotation of the Malaysian and Indonesian region is also inferred from paleomagnetic studies of the region, although the exact effect of the rotation to the evolution of the Vietnamese basins remains unknown (Hall, 1996, 1997, 2002; Fuller et al., 1999; Richter et al., 1999). Seafloor spreading in the South China Sea took place in the period between ca. 32 and 15 Ma (Taylor and Hayes, 1980, 1983; Briais et al., 1993). Symmetric Palaeogene

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spreading occurred across an approximately east–west trending spreading ridge in the northern part of the sea (Fig. 1). During the latest Oligocene the spreading axis jumped to the south and propagated southwestward throughout the Early Miocene (Briais et al., 1993; Barckhausen and Roeser, 2004). Rifting in the Vietnam offshore basins started between the latest Cretaceous and the Eocene, although the lack of deep well control hinders exact age estimates (Chen et al., 1993; Rangin et al., 1995b; Matthews et al., 1997; Roques et al., 1997a, b; Lee and Watkins, 1998; Nielsen et al., 1999; Lee et al., 2001; Andersen et al., 2005; Pubellier et al., 2005). The Song Hong and Qui Nhon basins located north of the Phu Khanh Basin formed through left-lateral strikeslip movement across the EVBFZ (Rangin et al., 1995b; Nielsen et al., 1999; Sun et al., 2003, 2004; Andersen et al., 2005; Clift and Sun, 2006). A distinct regional midOligocene unconformity separates an initial rift phase from a later rift phase in these basins. The initial rift succession in the Song Hong Basin is composed of nonmarine sediments, but a gradual marine transgression during the Late Oligocene caused marine deposition to dominate during Neogene time (Nielsen et al., 1999). Starting in the earliest Miocene, a multi-phase basin inversion occurred, attributed to right-lateral transpression after Middle Miocene time (Nielsen et al., 1999; Andersen et al., 2005; Clift and Sun, 2006). Right-lateral transpression was followed by latest Miocene–Holocene post-rift subsidence. In the Cuu Long and Nam Con Son basins, non-marine and restricted marine syn-rift deposition dominated until earliest Neogene when uplift followed by a major flooding occurred (Matthews et al., 1997; Lee et al., 2001). The Triton Ridge east of the Qui Nhon Basin was, like the basins further south, transgressed during the Early Miocene, which marked the halt of rifting and the start of widespread carbonate growth (Holland et al., 1992; Roques et al., 1997a). The stratigraphy of the Phu Khanh Basin has previously been divided into a non-marine Palaeogene syn-rift period and a mainly marine post-rift period separated by a middle Oligocene breakup unconformity. The offshore rifting was interpreted to have occurred across a coast-parallel fault system and locally to have resumed after middle Oligocene (Lee and Watkins, 1998). Structural analysis of the region immediately onshore of the Phu Khanh Basin indicates three regional Cainozoic rift generations (Rangin et al., 1995a). Left-lateral strikeslip faulting was suggested to have acted along northwest to north–northwest oriented faults which cut the Mesozoic igneous basement underlying upper Neogene sediments (Fig. 2). This rift phase was related to the extrusion of Indochina and was followed by a right-lateral strike-slip phase that inverted older faults and caused north–south trending right-lateral faults to form (Rangin et al., 1995a). The right-lateral strike-slip faulting has been related to changes in the regional stress pattern induced by the northward indentation of India into Eurasia (Huchon

et al., 1994; Rangin et al., 1995a) or to rifting offshore Central Vietnam following the slap-pull model (Marquis et al., 1997; Roques et al., 1997a, b; Pubellier et al., 2005). The two strike-slip phases were succeeded by an extensional phase related to late Neogene—Recent volcanism in Central and South Indochina (Rangin et al., 1995a). The volcanism led to extensive uplift and denudation of the region causing accelerated depositional rates in the offshore basins (Carter et al., 2000). 3. Unconformities and megasequences: data analysis and interpretations Four distinct regional unconformities forming the boundaries of two Palaeogene and two Neogene megasequences were mapped. The megasequence boundaries are referred to as MB1, MB2, MB3 and MB4, in decreasing age and the megasequences resting on the corresponding megasequence boundary are respectively referred to as MS1, MS2, MS3 and MS4 (Fig. 3). The Neogene megasequences, MS3 and MS4, were further divided into four (S3.1–S3.4, from oldest to youngest) and two (S4.1–S4.2, from oldest to youngest) submegasequence, respectively by four less distinct regional internal unconformities to emphasise the depositional development. The seismic facies of the megasequences and sequences were analysed using the methods proposed by Bubb and Hatlelid (1977), Mitchum et al. (1977) and Sangree and Widmier (1977). The seismic facies analysis was used to construct eight facies maps outlining the depositional history of the basin (Figs. 4a–h). The spatial and temporal distributions of facies, combined with thicknesses of the sequences, were analysed in order to construct and support a tectonic evolutionary model of the basin. Unless otherwise stated depth and thicknesses below are given as two-way traveltimes (TWT). 3.1. Regional unconformities 3.1.1. MB1 (top pre-Cainozoic) Description: MB1 forms the deepest continuous reflection surface in the western part of the basin, topping the reflection-free acoustic basement (Fig. 3). It represents a faulted surface with an overall inclination towards the east, showing a relief of more than 6.5 s within the study area (Figs. 4a and 5). MB1 is onlapped by gradually younger megasequences as it shallows shoreward. In the eastern half of the study area MB1 is imaged as a distinct angular unconformity that fades towards the deepest parts of the basin. MB1 is offset mainly by extensional faults, although a few local compressional fault zones exist. A major coastparallel fault zone transects the study area, dividing the basin in two: a deep eastern half and a shallower western half. The fault zone forms a structure up to 20 km wide and over 1.5 s deep in places. The area west of the fault zone is cut by less dominant northwest to north–northwesttrending extensional faults that show a right-stepping

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W

E

0

Troughs MB4 ~ 10 Ma MB3 ~ 24 Ma MB2 ~ 30 Ma MB1 ~ ? Ma

Figure 8

1 MB4

Multiples

MS4-I

Two-way traveltime (sec.)

2 MS3-II

MS3-IV MS3-III

Figure 7

MS4-II

3 MS3-I

Inversion structure

MS2

4 MS1

MB3

Pre-Cainozoic

MB2 Bas

em

5

ent

MB1

6

East Vietnam Boundary Fault Zone

10 km

7 Fig. 3. Seismic section located immediately east of the Dam Thi Nai lagoon illustrating unconformities and depositional units of the Phu Khanh Basin. The profile illustrates the structural style of the EVBFZ as well as the overall seawards dipping basement. Boxed areas outline Figs. 8 and 9. See Fig. 2 for location.

trend parallel to the oldest onshore faults (Fig. 4a). Younger, more northerly trending faults also offsets MB1 west of the through-going fault zone, with strikes parallel to the second Cainozoic generation of faults onshore. East of the through-going fault zone, northeast-trending faults were mapped although the great burial depth of MB1 limits the structural interpretation in this part of the basin. In addition, a few northeasterly striking faults have been interpreted in the southwestern part of the study area (Fig. 4a). Interpretation: The seismic expression of the acoustic basement and the proximity of MB1 to the exposed or shallow buried crystalline basement onshore (Tinh, 1998) suggest a similar hard rock interpretation of the acoustic basement in the western half of the basin. The angular unconformity characterising MB1 east of the throughgoing fault zone is interpreted as the rift-onset unconformity marking the boundary between pre-Tertiary and younger deposits. The strikes of the faults in the western and central part of the study area correspond to the structural trends observed onshore. The major throughgoing fault zone is interpreted as the southward continuation of the EVBFZ, on the basis of its size, structural style and its northward continuation into the Qui Nhon Basin. This strongly suggests left-lateral transtension, as major contemporary left-lateral strike-slip occurred across the EVBFZ farther north. Left-lateral offset is further supported by the presence of coeval northwest to north–

northwest-trending right-stepping extensional synthetic faults in the study area, similar in strike to faults onshore that were formed by left-lateral movements (Rangin et al., 1995a). The juxtaposition of different pre-rift lithologies across the EVBFZ indicates the very large offset across the fault zone. The pre-rift sequence east of the EVBFZ is interpreted as pre-Tertiary since pre-Tertiary deposits form the only pre-Neogene sediments onshore. The Mesozoic igneous basement onshore is overlain by genetically related Upper Cretaceous extrusives (Tinh, 1998), which does not favour onshore erosional removal of a pendent to the prerift sequence east of the EVBFZ. The northeast-trending extensional faults east of the EVBFZ appear to be the southward continuation of a similarly striking fault system northeast of the study area (Pubellier et al., 2005; Roques et al., 1997a, b), suggesting regional northwest–southeast extension. This could be compatible with a coeval slab pull related to subduction of the proto-South China Sea beneath Borneo (e.g. Holloway, 1982). 3.1.2. MB2 (middle Oligocene) Description: MB2 forms an angular unconformity that seems most pronounced in the northern half of the basin and continues beyond the northern boundary of the basin. Truncation appears most intense towards the EVBFZ and near inverted graben-structures, as with the mid-Oligocene unconformity reported in the Song Hong Basin farther north (Figs. 5a and b) (Rangin et al., 1995b; Nielsen et al.,

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1999; Andersen et al., 2005). The unconformity shows a relief of more than 5 s and is offset by a series of north- to northwest-trending faults (Fig. 4b), as well as a few northeast-trending faults farther south. The offset of MB2 across the northwest-trending faults is less than that of MB1. Most faults cutting MB2 show signs of extension, although a limited number of younger compressional faults are mapped. Interpretation: MB2 is suggested to correspond to the middle Oligocene unconformity reported from the Song Hong Basin, owing to the pronounced truncation in the northern half of the basin and the structural similarities. The increased truncation towards the EVBFZ and the inversion of older extensional structures indicate a temporary change in the tectonic regime from extension to compression around middle Oligocene times, probably as a

Alluvial fan/fan delta facies

Deposits removed by erosion

Alluvial plain facies

Volcanic extrusives

Lacustrine facies

Exstinct volcano

Mixed alluvial and lacustrine facies

Palaeo-shoreline

result of the kinematics of the EVBFZ. A return to leftlateral transtension took place after middle Oligocene times, as interpreted from the extensional offset of MB2 along the EVBFZ and part of the northwest trending synthetic faults, which is in agreement with the coeval slip on the EVBFZ north of the basin (Leloup et al., 1995, 2001; Rangin et al., 1995b; Nielsen et al., 1999; Gilley et al., 2003; Andersen et al., 2005). 3.1.3. MB3 (near base of Miocene) Description: MB3 is an eastward-dipping onlap surface correlated to the Palaeogene–Neogene boundary in the 119-CM-1X well located north of the basin (Fig. 1). It has a relief of more than 4 s and forms a distinct truncational unconformity in the southern and western parts of the study area. MB3 further marks a change from extension to

109° N

Facies map of MS1 110° pre-Late Oligocene

Mixed lacustrine and lagoonal siliciclastic facies

Strike-slip fault Fault with reverse component

Siliciclastic shelf facies

Fault with normal component

Siliciclastic shelf-slope facies

Fault formed during subsequent depositional periods with >250 ms heave

Siliciclastic basin-floor facies Siliciclastic and marly basin-floor facies Fore-reef marly facies

14°

East Vietnam Boundary Fault Zone Area characterised by submarine gullies (<200 ms deep) and related turidite facies

Lagoonal and shoreface carbonate facies

Submarine canyon (<500 ms deep)

Carbonate platform facies

Large submarine canyon (>1000 ms deep)

Vietnam

Carbonate reef facies Area dominated by turbidite facies Mixed fore-reef and pinnacle reef facies Carbonate platform buried by prograding siliciclastic shelf-slope during depositional period Area raised above base-level Mixed volcanogenic and shelf facies thermally alterated by magmatic activity

13°

Area characterised by channels formed by submarine currents Depth contour of lower sequence boundary Tentative depth contour of lower sequence boundary Track of seismic line

40 km

Fig. 4. (a–h) Inferred depositional facies superposed onto TWT-isochor maps of the base of the units illustrating dominating fault trends. Contours are depth of lower boundaries with intervals of 0.5 s TWT. The facies distributions on the maps are the inferred dominant facies for the given periods (a) MS1 (pre-middle Oligocene). MS1 represents the initial syn-rift megasequence interpreted as mainly lacustrine and alluvial deposits. (b) MS2 (Upper Oligocene). MS2 was deposited during the second rift phase and is mainly composed of lacustrine and alluvial deposits. (c) MS3-I (lowest Miocene). Carbonate deposition dominated during the initial marine transgression after the onset of right-lateral wrenching along the EVBFZ. Carbonate platforms and buildups formed adjacent to a narrow trough that existed along the EVBFZ. Volcanism took place in the southern part of the area. (d) MS3-II (upper Lower Miocene). As transgression continued, siliciclastic sedimentation became more dominant with alluvial fans radiating seaward from faults and inclined surfaces in the western part of the basin. Volcanism continued in the southern part of the area. (e) MS3-III (lower Middle Miocene). Continued transgression resulted in a deeper depositional environment in the eastern central part of the area. Furthermore, a shelf slope began to build in the northern part of the basin during this phase. Volcanism took place in the southern part of the area. (f) MS3-IV (upper Middle Miocene). An almost full marine setting was established. The shelf slope propagated further south and the deep marine area expanded. Carbonate buildups formed locally along the boundary between deep and shallow waters. (g) MS4-I (Upper Miocene). Major transgression followed a significant fall in relative sea level. This resulted in a full marine setting with a significant north to south verging shelf slope. (h) MS4-II (Pliocene–Recent). As transgression continued, the depositional environment approached the environment presently observed in the area. In the southern part of the study area, deep incisions in the shelf-slope and turbidite facies on the basin floor witness enhanced subsidence rates east of the EVBFZ. Further north, submarine canyons related to a more gentle sediment-laden current system formed.

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109° N

Facies map of MS2

109° N

110°

Late Oligocene

7

Facies map of MS3-I 110° earliest Miocene

Marine

pathw

ay

14°

14°

Vietnam

Vietnam

13°

13°

40 km

109° N

Facies map of MS3-II 110° e

14°

Vietnam

109° N

late Early Miocene Marin

40 km

ay pathw

Facies map of MS3-III 110° early Middle Miocene

Marine

pathw

ay

14°

Marine pathway?

Vietnam

Main marine pathway

13°

13°

40 km

Fig. 4. (Continued)

40 km

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109° N

Facies Map of MS3-IV 110°

Facies map of MS4-I 110°

109° N

late Middle Miocene

14°

Late Miocene

14°

Vietnam

Vietnam

13°

13°

40 km

40 km

109° N

Facies map of MS4-II 110° Pliocene–recent

14°

3000

Vietnam

2000

1000

3000

13°

40 km

Fig. 4. (Continued)

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a W

E

Two-way traveltime (sec.)

0

MB4 ~ 10 Ma MB3 ~ 24 Ma MB2 ~ 30 Ma MB1 ~ ? Ma

1 2 3 4 5 6 7

b W

E

Two-way traveltime (sec.)

0 1 2 3 4 5 6 7

c W

E

Two-way traveltime (sec.)

0 1 2 3 4 5 6 7 Fig. 5. Stratigraphic and structural cross sections illustrating the two separate Palaeogene rift phases (MS1 and MS2) followed by slowed early Neogene rifting (MS3) and increased late Neogene subsidence- and depositional rates (MS4). (a) Represents the northernmost of the three sections. The EVBFZ develops a more pronounced overall heave towards the south, significantly down-faulting MB2 seaward. See Fig. 2 for location.

compression in part of the through-going EVBFZ and in certain other Palaeogene structures (Fig. 3, compare Fig. 4b with 4c and d). Approximately north-trending faults cut MB3 in the northwestern part of the basin and locally control the thickness of the overlying megasequence, apparently without affecting the underlying stratal thicknesses. Most of these faults have extensional components, but with offsets an order of magnitude smaller than the Palaeogene structures described in section MB1 and MB2. Interpretation: The truncation at MB3 reflects erosion caused by a widespread uplift event which marks the

9

termination of the Late Oligocene rifting and regional subsidence. The inversion structures in the EVBFZ are interpreted to reflect a change from left- to right-lateral movements in the fault zone around the Palaeogene–Neogene boundary. This interpretation is supported by the coeval formation of the approximately north-trending faults parallel to the onshore right-lateral fault system. 3.1.4. MB4 (near base of Upper Miocene) Description: MB4 marks a distinct seaward-dipping onlap surface with a relief of more than 3.5 s and is tied to slightly above the base of Upper Miocene in the 121CM-1X and 120-CS-1X wells. It shows truncation on the western flank of the basin and coincides with MB3 in the southernmost part of the study area. A few extensional faults in the southern part of the study area cut MB4. Farther north, a few faults die out above MB4 and moderate flexural deformation of the strata above MB4 is seen in relation to a few older structures (Fig. 3). The overall deformation above MB4 related to these structures appears very modest compared to older deformation. In the southwestern part of the study area, a zone characterised by very high amplitude and wavy to chaotic reflections makes up the seismic package underlying MB4 (Figs. 4f and 6). The reflection pattern resembles that observed near volcanoes and above subsurface igneous activity in other parts of Vietnam. The zone borders a particularly active volcanic area onshore characterised by abundant Late Miocene basalts with a radiometric age of ca. 7.5 Ma (Figs. 2 and 4f) (Rangin et al., 1995a). Interpretation: The truncation observed in the western half of the study area indicates a significant drop in relative sea level caused by a landward regional uplift. The modest faulting above MB4 reflects a calm tectonic phase after the formation of MB4. The wavy and chaotic reflection pattern observed in the southwestern part of the study area may be linked to the Late Miocene volcanism immediately onshore, in support of an earliest late Miocene age of MB4 (Figs. 2 and 4f) (Barr and Macdonald, 1981; Rangin et al., 1995a; Hoang and Flowers, 1998; Carter et al., 2000; Wang et al., 2001). The strongly reflected MB4 and the diffuse seismic expression of the subjacent strata reflect the presence of basaltic flows and particularly strong diagenesis driven by high thermal activity and related subarial exposure. Minor igneous intrusions of the deposits below MB4 may have added to the diffuse reflection pattern beneath MB4 in this part of the study area. 3.2. Megasequences 3.2.1. MS1 (pre-Cainozoic–middle Oligocene) Description: MS1 is present across much of the study area east of the EVBFZ, with seismic thicknesses approaching 2 s in places (Fig. 4a). The largest thicknesses are observed along the EVBFZ as well as in grabens and half-grabens, where MS1 is characterised by diverging reflection pattern towards the local depocentres. Two

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10

0

W

E

Two-way traveltime (sec.)

Thermally altered facies

MS4-II

0.5 MS4-I MS3-IV

MB1

MB4~

10 Ma

2 km

1 Fig. 6. A strong reflection marks MB4 in an area that borders a Late Miocene volcanic region onshore. This is interpreted stacked volcanic flows and to reflect magmatic driven diagenetic alterations of underlying sedimentary rocks. Diffuse smaller intrusions deteriorate the seismic resolution below MB4. See Fig. 2 for location.

MB4~10

3 MB3

~24

Ma

Lacustrine/ lagoonal facies

Ma

Shingled clinoforms

MS

Two-way traveltime (sec.)

3-I

MS3-III

Pre-Cainozoic

4

MS3-II

MS2

Alluvial fans/ fan deltas

MB2~24 Ma

Ba

se

5

me

nt

Lacustrine mudstone

MS1

MB1

6

10 km

Fig. 7. Example of MS2 deposited syn-tectonically in a graben-like structure mainly filled by lacustrine facies with alluvial fans/fan deltas radiating from the adjacent horst blocks towards the depocentre. MS3-I–MS3-III consist dominantly of lagoonal and lacustrine sediments interpreted from lateral facies associations, internal and external geometries as well as amplitudes and reflector terminations. MB1 corresponds to the base of the Phu Khanh Basin, MB2E30 Ma, MB3E24 Ma, and MB4E10 Ma. See Figs. 2 and 3 for location.

general types of seismic pattern can be distinguished, although a detailed mapping of the patterns is hampered by the low seismic resolution in the deepest part of the basin and in structurally disturbed areas. Seismic packages found centrally in structural depressions generally have high amplitude, low-frequency reflections, with good continuity when not subsequently deformed. Furthermore, distinctly increased post-rift subsidence occurred above the central

parts of the structural depressions. Reflection packages found at the base or at the margins of these depocentres often have lower amplitudes and higher frequencies, or have internal wedge-forms that radiate from inclined riftmargins (Fig. 7). Interpretation: MS1 is interpreted as the oldest Cainozoic syn-rift unit in the Phu Khanh Basin, deposited in the period between the latest Cretaceous? or early Palaeogene

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W

E

Two-way traveltime (sec.)

1

Alluvial fans/ fan deltas

Mounded carbonate buildup 2

Carbonate platform

MS3-IV MS3-II

Carbonate platform

MS3-III

Marls 3

MS3-I

MB3

~24M a

Marls

MB2~30Ma MS2

4

10 km

Fig. 8. Carbonate platforms and alluvial fans/fan deltas bordering the central depocentre of MS3-I and MS3-II are seen. The lows off the platforms are separated by an inversion ridge topped by a reefal buildup. The relative position and the seismic expression of the deposits in the depocentre reflects a deeper marine depositional setting dominated by marly facies in MS3-I, which is considered as source rock for the ‘‘Dam Thi Nai Oil’’. MS3-II in the depocentre is interpreted to consist of terrigenous mudstone facies probably with a moderate marly content. MB2E30 Ma, MB3E24 Ma, and MB4E10 Ma. See Figs. 2 and 3 for location.

and the middle Oligocene. The continuous high amplitude, low-frequency reflections found in depocentres are interpreted as mainly lacustrine sediments. The increased postrift subsidence localised above syn-rift depocentres is a result of increased sedimentary compaction of the syn-rift sediments deposited in these depocentres, which could indicate a higher content of mud rocks in the central part of the rifts. The relatively low-amplitude, high-frequency reflections comprising the adjacent margins and lower parts of the rifts probably reflects coarser-grained fluvial and alluvial deposits. The wedges are interpreted as related alluvial fans/fan deltas, in accordance with regional analogues and the Cainozoic syn-rift depositional pattern of large parts of Southeast Asia (e.g. Sladen, 1997; Lee et al., 2001; Andersen et al., 2005; Nielsen et al., 2007).

frequency and good continuity when not deformed (Fig. 7). Wedge-shaped seismic units radiate sporadically away from bounding faults or steeply inclined graben floors and are onlapped by the main fill of MS2. Locally, a reflection package of lower continuity and moderate amplitude characterises MS2 outside the depocentres. Interpretation: MS2 is interpreted as a syn-rift unit deposited during the Late Oligocene. The main reflection configuration characterising the depocentres of MS2 is interpreted to be relatively fine-grained lacustrine deposits (Fig. 4b). The wedges that radiate towards the depocentres are interpreted as alluvial fans/fan deltas that are generally composed of coarser-grained sediments. The relatively lowamplitude, discontinuous reflections observed locally outside the depocentres, are interpreted as alluvial sediments.

3.2.2. MS2 (middle–latest Oligocene) Description: MS2, unconformably overlying MB2 is present across larger parts of the study area, filling a series of rift depressions and is often characterised by a diverging reflection pattern towards the depocentre of the depressions (Figs. 4b and 8). The main depocentre is seen along the EVBFZ, with thickness of nearly 2 s. The structural depressions are filled mainly with bidirectional onlapping deposits, seismically characterised by high amplitude, low

3.2.3. MS3 (Early–Late Miocene) MS3 forms an up to about 1.5 s thick unit. It is divided into four submegasequences, MS3-I–MS3-IV, characterised by an overall upward decrease in fault offsets. MS3 is associated with a gradual enlargement of the depositional area which followed the decrease that occurred across MB3 (Figs. 4b–f). MS3 is interpreted to be of Early to Middle Miocene age and shows a gradual decrease in rift activity.

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W 2.5

E MS3-IV

MS4-I

MB4~10 Ma

Two-way traveltime (sec.)

MS3-III

3.0

MS3-II

MB3~24 Ma

3.5 MS2

MS3-I

Mounded carbonate buildup

MB2~30 Ma

4.0

MS1 MB1

2 km

Fig. 9. An example of a Lower Miocene mounded carbonate buildup. The buildup was deposited on Palaeogene synrift deposits and borders marly facies to the west interpreted as potential source rocks. See Fig. 2 for location.

3.2.4. MS3-I Description: MS3-I occurs in the northern part of the basin as an up to ca. 0.3 s thick unit and in an isolated area toward the southeast as an up to ca. 0.5 s thick package (Fig. 4c). In the north, MS3-I is characterised by mounded, sub-parallel and divergent reflections with high reflection amplitude and low frequency, usually with an upper boundary marked by a strong positive reflection. Broad platform buildups with vertical aggradation of up to about 0.25 s covering up to several hundred km2 make up an important part of MS3-I. These features occur in relatively elevated areas, with convex lateral terminations towards a narrow depression along the trace of the EVBFZ, and differ from smaller symmetrically mounded buildups (Figs. 8 and 9). The narrow depression is filled by a prism, characterised by reflectors diverging towards the central depocentre (Figs. 4c and 8). The prism gradually thins northward and appears to wedge out toward the south. In the east, a thin seismic package defined by very continuous reflections drapes the local depocentre and the area to its south. The package passes into the large platform buildup in the central part of the area onlapping symmetrically along the margins of the depocentre (Fig. 7). The isolated MS3-I in the southeast exhibits a distinct relief and is characterised internally by kilometre-scale mounds up to 0.4 s high. In this area, MS3-I shows a diffuse reflection pattern, the mounds are internally characterised by chaotic reflections, surrounded by strong, wavy reflections with intermediate continuity, some of which show chaotic, internal baselaps. Interpretation: The reflection pattern of the northern part of MS3-I is similar to that of carbonate deposits in other

parts of the South China Sea and interpreted accordingly (e.g. Erlich et al., 1990; Mayall et al., 1997; Bachtel et al., 2004). The carbonate deposition in MS3-I indicates a marine transgression during the earliest Miocene, marking the end of the non-marine Palaeogene period. The broad platform buildups are interpreted as shallow-water carbonate platforms of varied lithologic composition. The smaller, symmetrical mounds are local carbonate buildups or reefs (e.g. Borgomano and Peters, 2004). The prismshaped sedimentary body is interpreted as relatively deepmarine detrital and pelagic carbonates deposited in a trough situated between the buildups. The northward onlap and thinning of MS3-I suggest a silled marine pathway toward the north. The seismic facies in the easternmost part of the area is interpreted to reflect lagoonal carbonates deposited between the palaeo-shore and the carbonate platform. The southeastern part of MS3-I is interpreted as volcanigenic and mass-transport volcaniclastic sediments related to the local paleo-topography, with the chaotic mounds reflecting volcanoes. The strong, semi-continuous reflections between the mounds are interpreted as volcaniclastic deposits, sills and stacked volcanic flows. 3.2.4.1. MS3-II. Description: MS3-II forms an up to 0.7 s thick sequence, onlapping onto MS3-I (Figs. 4c and d). The southwestern part of MS3-II consists mainly of wedgeshaped seismic bodies that onlap MS3-I and radiate from the inclined, faulted substratum towards the depocentre (Fig. 8). In the northernmost part of the area, the base of MS3-II forms a conformable surface within the large platform buildup that initially started to accrete during MS3-I (Figs. 4c, d and 8). This buildup passes eastward into a seismic unit above a Palaeogene graben. Here, MS3II is characterised by symmetrical onlap onto MS3-I as well as by eastward-decreasing reflection amplitude. Farthest east, shingled clinoforms prograde westward into the depocentre above the graben centre (Fig. 7). Smaller symmetrical mounds are seen on structural highs. In the northern half of the study area, such a buildup offlap toward a seismic package in the depocentre above the EVBFZ (Fig. 8). This package is characterised by continuous, divergent reflections and reflection sets with alternating intermediate and high amplitude. MS3-II, where isolated towards south, is similar to the underlying MS3-I, characterised by chaotic mounds separated by strong semi-continuous, wavy reflections. Interpretation: The wedges found along the western margin of MS3-II are interpreted as relatively coarsegrained alluvial fans and fan deltas sourced from the uplifted crystalline basement farther west. The continued accretion of the broad platform buildup into MS3-II indicates continued growth of the carbonate platform. The shingled clinoforms east of the carbonate platform suggest an eastward passing into another shallow-water facies, e.g. related to a larger lagoon. The low reflection amplitudes and the progradional pattern of the shingled clinoforms

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toward the carbonate platform may suggest siliciclastic sedimentation in the eastern part of the lagoon. Farther west, the high reflection amplitude and the closer proximity to the carbonate platform indicate more calcareous deposits. The offlap towards the depocentre above the EVBFZ of the symmetrical mounds, interpreted as reefs, indicate a relatively deep-marine depositional setting along the trace of the EVBFZ. The fairly strong amplitude reflections of this deeper marine package and the location adjacent to the reefs and the large carbonate platform could suggest a marly content in this part of MS3-II. The gradual southward decrease in reflection amplitude may indicate decreasing calcareous content as a result of the increased distance to the carbonate platform. The isolated parts of MS3-II to the southeast is, as is the underlying MS3-I, interpreted as volcanigenic and related mass-transported deposits. 3.2.4.2. MS3-III. Description: MS3-III forms an up to 0.5 s thick sequence onlapping MS3-II (Figs. 3 and 5). The westernmost part of MS3-III is composed of a set of wedge-shaped bodies, similar to those seen in MS3-II, radiating eastward from a series of faults and steeply inclined surfaces, as seen in MS3-II. The wedges grade northward into small, prograding clinoforms and grade seaward into relatively parallel reflections that offlap and thin seaward (Fig. 5b). The otherwise parallel reflections are to the west sporadically interrupted by less than 0.2 s high mounded buildups, and by erosional features less than 0.2 s deep. A gradual thinning of the eastern part of MS3III occurs as the sequence onlaps the underlying strata toward the south. Reflection amplitudes and continuities decrease in the same interval toward the south and reflections have a slightly wavy appearance. A lens-shaped depositional unit characterised by strong to very strong, symmetrically onlapping internal reflections generally of relatively good continuity constitutes MS3-III in the northeastern part of the study area (Fig. 7). In the southern half of the study area, a seismic facies characterised by chaotic internal mounds separated by strong semi-continuous, wavy reflections is observed in two isolated settings (Fig. 4e). A very strong reflection marks the upper boundary of this seismic facies. Interpretation: The wedges observed along the western margin of MS3-III are interpreted as alluvial fans/fan deltas (Fig. 4e). The clinoforms farther north are interpreted as the initial development of a narrow shelf and a moderate shelf slope, indicating a marine environment. The relatively parallel set of reflections east of the fans is also interpreted as marine deposits, with the internal mounded buildups interpreted as reefs. This interpretation is supported by the seaward offlap, reflecting a distal setting with increased depositional depths. The deep erosional features observed in the same area are interpreted as submarine slope canyons that probably acted as conduits for turbidity currents. The southward thinning of MS3-III

13

is interpreted to reflect southward shallowing with paralic deposition in the southernmost part of the study area. The lens-shaped unit occupying the northeastern part of the basin probably represents lacustrine or restricted marine deposits (Fig. 4e). The two internally chaotic mounds in the south are interpreted as volcanigenic material and related mass-transported deposits. 3.2.4.3. MS3-IV. Description: MS3-IV, up to 0.5 s thick, is the most widespread among the submegasequences of MS3 (Fig. 4f). The base of MS3-IV is a distinct onlap surface, characterised mainly by landward onlaps (Fig. 3). Distinct, seawards prograding clinoforms characterise the northwestern part of the study area and suggest a significant southward propagation relative to MS3-III. In addition, the clinoforms of MS3-IV are more prominent compared to MS3-III, with topsets generally more than 10 km wide, foresets up to ca. 0.3 s high and clearly defined bottomsets (Fig. 3). Mounded symmetrical buildups with strong external reflections and conformable internal reflections as well as underlying velocity pull-ups are also seen (Fig. 4f). The reflection package east of the foresets and the adjacent buildups is composed of wavy, semi-continuous reflections, bundled in less than 0.1 s thick lens-shaped bodies that onlap the sequence boundary of MS3-IV towards the west. MS3-IV thins northward where it is characterised by parallel reflections. The reflection pattern changes again furthest to the northeast, where MS3-IV fills a bidirectional onlapping lens-shaped body (Fig. 7). Relatively weak reflections with moderate continuity characterise MS3-IV in the southeasternmost part of the study area where the reflection amplitude and continuity increase upward and the reflections onlap small, symmetrically mounded buildups observed in the margins of the facies. Interpretation: The clinoform architecture of MS3-IV testifies to eastward progradation and a southward expansion of the shelf slope. Furthermore, the gradual landwards expansion of the topsets and the increasing foreset heights indicate a marine transgression that caused a landward expansion of the shelf and increasing depth off the shelf slope. The transgression further led to the drowning of the alluvial fans and fan deltas deposited during earlier periods of MS3-II and MS3-III (Figs. 4e and f). The mounded buildups are interpreted as reefs with high acoustic velocities relative to the surrounding strata, as indicated by the velocity pull-ups. The presence of reefal buildups to the south in the study area also indicates marine conditions. The stacked lens-formed reflection package observed seawards of the reefs and the shelf slope is interpreted as stacked turbidite fans. The seismic facies observed further north, characterised by parallel reflections, are interpreted as basin-floor sediments deposited in a lower energy regime. The lens-shaped seismic unit found farthest northeast is interpreted as sediments deposited in shallow water relative to the coeval deposits farther south

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because of its external shape and the lapping pattern, and due to the depositional evolution delineated by the underlying part of MS3-II and MS3-III. The seismic facies in the southeasternmost part of the study area are interpreted as paralic sediments that grade upward into deeper marine deposits and thus reflect a marine transgression throughout the depositional period. This accords with the presence of the small carbonate buildups, interpreted as drowned carbonate reefs. The gradual enlargement of the depositional area seen throughout the deposition of MS3 is interpreted as a result of the gradual decrease in volcanism in the southern half of the study area. In addition, termination of right-lateral faulting may have contributed to the enlargement of the study area. Thermal subsidence thus became the controlling tectonic factor throughout the rest of the Neogene. 3.2.5. MS4 (Late Miocene–Recent) MS4 forms the uppermost megasequence with a thickness of up to about 2.5 s. The megasequence is divided into two submegasequences, MS4-I and MS4-II, which characterise the Late Miocene to present evolution of the Phu Khanh Basin. The megasequence thus encompasses a shorter depositional period than that of MS3 (ca. 10–0 Ma compared to 24–10 Ma), but has a significantly greater thickness (Fig. 5). 3.2.5.1. MS4-I. Description: MS4-I, up to about 0.8 s thick, covers much of the study area (Figs. 3 and 4f). It is characterised by prograding sigmoidal and complex sigmoidal clinoforms that divide the study area into three zones dominated by topsets, foresets and bottomsets, respectively (Fig. 4g). The topset zone is characterised by sub-parallel, continuous, relatively high amplitude reflec-

Two-way traveltime (sec.)

0

tions that grade seaward into the foreset zone. The height of the foresets increases upward and approaches 0.7 s in the central part of the study area where the foreset are well developed (Fig. 5), and is cut by east-trending erosional features. The foreset zone grades seaward into the bottomset zone. The bottomsets are generally characterised by relatively continuous, intermediate to high amplitude reflections, although subintervals of more wavy, semicontinuous reflections are seen as well. A wedge-shaped body up to about 0.5 s thick underlies the clinoform foresets in the central part of the study area and makes up the lower part of MS4-I. The reflections of the wedgeshaped body onlap MB4 in the west and are characterised by a very wavy pattern with small internal wedges and channels. In the southern part of the study area, broad platform buildups show convex lateral terminations and strong internal and upper reflections (Figs. 4f and 10). The platforms have back-stepping reflection geometries or form pointy buildups and are in places down-lapped by foreset reflections. Interpretation: MS4-I marks a significant expansion of the depositional area relative to the older megasequences. The clinoforms pattern is interpreted as the progradation of the shelf and shelf slope. The topsets are interpreted as coastal plain and shelf deposits, dominated by paralic and shallow marine sediments, on the basis of the continuous, relatively high amplitude reflections. The shelf passed seawards into the slope represented by the foreset zone, and the shelf-slope transition appears to have been well developed in the central part of the study area, where the erosive features probably represents submarine canyons that acted as conduits for sediment gravity-flows. The increase in foreset height indicates a gradual increase in

S

Gas seeps

N

Carbonate platform Shelf-edge incision

1

Trough-like features

2

MB1

Fig. 10. Seismic section parallel to the shelf-slope transition showing a large erosional feature interpreted as a submarine shelf-slope incision and smaller trough-like features of MS4-II, interpreted as smaller-sized submarine canyons. A perpendicular transect of the large erosional feature is shown in Fig. 11. Also note the gas seeps in the seafloor and the carbonate platform. See Fig. 2 for location.

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depositional water depth. The reflection pattern of the bottomset zone is interpreted to reflect a deep-water complex, composed of mass-transport deposits and hemipelagic sediments. The wedge comprising the bottom part of MS4-I in the eastern part of the study area is interpreted as lowstand deposits. The wavy reflection pattern in the wedge, at the limit of seismic resolution, is interpreted as turbidites and the small internal troughs as feeder channels. The thickness (up to ca. 0.5 s) and the lateral extent of the wedge suggests a significant sediment source, and is compatible with a major, relatively long-lasting uplift toward the west during the formation of MB4, and with a genetic link between MB4 and the onset of the late Neogene uplift of Central Vietnam. The overall increased thickness of MS4, representing the upper Neogene deposits compared with the lower Neogene deposits testifies to an increased sediment accumulation rate that most likely also relates to the uplift and denudation of Central Vietnam. The platform buildups observed in the southernmost part of the study area are interpreted as carbonate platforms. The back-stepping reflection pattern and the pointy buildups topping the platforms in places are interpreted to reflect late stage growth due to a relatively fast relative sea level rise (e.g. Erlich et al., 1990). This

15

eventually led to platform drowning and deposition in certain places as observed from the shelf-slope downlaps onto the carbonate deposits. 3.2.5.2. MS4-II. Description: MS4-II forms the most recent sequence in the basin, with thickness up to about 2 s. The reflection architecture of MS4-II is characterised by onlap onto the lower boundary and distinct clinoforms with well developed topset, foreset, and bottomset zones (Figs. 4g and 5). The clinoforms prograde in a sigmoid and complex sigmoid fashion resulting in the present width of the shelf and height of the shelf slope approaching 1500 m in places. A large erosional feature more than 50 km wide and more than 1 s deep cuts into the foreset and topset reflections in the southern part of the study area (Figs. 10 and 11). This feature is filled by a package characterised by relatively diffuse internal reflections and minor internal trough-like features (up to ca. 0.3 s deep). The bottomset thickens seaward of the large erosive feature across the trace of the EVBFZ (Fig. 11). The bottomsets in this area are characterised by internal wedges with a chaotic reflection pattern and overlie relatively steeply inclined deposits predating MS4.

0 Shelf-edge incision MB4 ~ 10 Ma MB3 ~ 24 Ma MB2 ~ 30 Ma MB1 ~ ? Ma

Two-way traveltime (sec.)

1 Turbidite wedges

Amplitude anomaly and gas chimney

2

3

Fig. 11. Seismic section illustrating the large submarine incision of MS4-II and associated turbidite facies deposited seaward. Also note amplitude anomaly and the chimney like feature below. See Fig. 2 for location.

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In the northwestern part of the basin, migrating troughlike features resulted in up to about 0.3 s of relief in the eastern part of the topset package and up to about 0.5 s in the foreset package where similar features influence the bathymetry (Figs. 3 and 4h). The trough-like features radiate seaward across the offlap-break towards the deeper part of the basin and have both constructive and destructive lower boundaries. Interpretation: The clinoform pattern of MS4-II reflects shelf, shelf-slope and basin-floor deposits, representing a clastic sedimentary environment ranging from paralic to deep marine. The more than 50 km wide erosional feature is interpreted as a large submarine canyon cutting deep into the shelf slope and later filled by mass-transported deposits. The canyon is interpreted to have formed through shelf-slope erosion triggered by intense late Neogene subsidence east of the EVBFZ, speculated to be strongly influenced by the cessation of volcanism in this area. The seawards subsidence caused a strongly sloping seafloor as evidenced by onlap onto the strongly, seaward-dipping older deposits. This led to a depositional regime dominated by high-energy turbidity currents that passed through the canyon to the basin floor, as indicated by the lapping pattern and the internal foreset wedges characterised by the chaotic reflection patterns of the basin-floor deposits. The seaward radiating trough-like features observed in the northern part of the study area are interpreted as smaller submarine canyons related to more gentle sediment-laden currents. 4. Basin development and stratigraphic evolution Rifting in the northern Phu Khanh Basin mainly occurred during Palaeogene times in two phases separated by uplift and erosion, evidenced by the MB2 unconformity. Alluvial and lacustrine deposition dominated during the Palaeogene rift-phases, following the overall depositional trend of many Cainozoic Southeast Asian continental riftbasins in humid climates (Figs. 4a and b) (Sladen, 1997). Deposition during the earliest rift phase was dominated by subsidence along the left-lateral EVBFZ and related synthetic faults. This may suggest a close relation between the southeastward extrusion of Indochina, and the rifting in the western part of the Phu Khanh Basin. In the eastern part of the basin, on the other hand, the northeast-striking extensional faults may suggest that slab-pull forces, related to the subduction of the proto-South China Sea, controlled the extension during the initial rift phase. The initial rifting ended around the middle Oligocene due to a change to left-lateral transpression along the EVBFZ. Extension renewed during the Late Oligocene along the EVBFZ and some of its synthetic faults associated with left-lateral transtension, similar to the Song Hong Basin farther north (Rangin et al., 1995b; Andersen et al., 2005; Clift and Sun, 2006). The almost absence of contemporary extension along the northeasttrending faults in the eastern part of the basin may indicate

a decreased influence of the slab pull during the Late Oligocene, which could be relate to the onset of seafloor spreading further east. Estimation of the left-lateral offset distance across the EVBFZ is difficult due to the lack of directly correlatable markers offset by the fault movements. However, the juxtaposition of the pre-rift sequence and crystalline basement across the EVBFZ indicates a large lateral offset across the fault zone. A more precise estimate of the total lateral offset is only possible if the prerift sequence could be tied with pre-Tertiary deposits east of the EVBFZ. Onshore Mesozoic deposits, located more than 100 km south of the northernmost observed pre-rift deposits, form the only pre-Tertiary deposits in the region. However, a direct correlation of the two pre-Tertiary units remains speculative. The right-lateral inversion in the Phu Khanh Basin around the boundary between the Palaeogene and the Neogene was simultaneous with mild inversion in part of the Song Hong Basin and in the left-lateral Mae Ping Fault in southern Indochina as well as inversion in the West Natuna Basin and the southeastern Malay Basin (Clift and Sun, 2006; Morley, 2002; Morley et al., 2001). The apparent temporal correspondence of the inversion events observed in and around Indochina and the unconformities reported from the Cuu Long and the Nam Con Son basins and the Triton Ridge east of the Qui Nhon Basin strongly suggest a regional common causal mechanism. The Neogene counterclockwise rotation of the area south of Indochina has been previously linked with the inversion in the West Natuna and the southeast Malay basins (Hall, 2002) and may also relate to the early Neogene inversions further north. The early Neogene strike-slip movement interpreted in the EVBFZ in the Phu Khanh Basin and its onshore continuation the Ailao Shan–Red River Shear Zone appear to have been opposing. The right-lateral movement in the EVBFZ in the Phu Khanh Basin is therefore speculated to have been taken up in some of the onshore northwest striking fault zones between the Phu Khanh Basin and the southeasternmost part of the Ailao Shan–Red River Shear Zone. The marine transgression that took place after the regional uplift probably commenced around 21–22 Ma, as significant transgression occurred in the areas immediately north and south of the Phu Khanh Basin at that time (Holland et al., 1992; Matthews et al., 1997; Roques et al., 1997a, b; Lee et al., 2001). The lowest Neogene facies pattern indicates a southward transgression along the EVBFZ, suggesting a marine pathway between the Phu Khanh and Qui Nhon basins and the Triton Ridge further north (Fig. 4c). Rapid initial transgression facilitated carbonate deposition whereas clastic sediments were trapped at the transgressive shorelines. The decrease in the rate of transgression during the later Early–Middle Miocene, possibly combined with expansion of the catchment area or increased erosion rates in the hinterland, led to more widespread siliciclastic deposition (Figs. 4c and d). Reduced rifting and decreasing magmatism in the

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southeastern part of the study area gradually caused thermal subsidence to dominate through the later part of the Early and Middle Miocene. An open marine environment was established by merging the Phu Khanh Basin and the South China Sea and the initiation of a shelf-slope setting was facilitated due to continues subsidence in the area (Figs. 4d–f). Rifting in the study area appear to have ceased or almost ceased before the Late Miocene, as only a few minor faults cut MB4 above the EVBFZ. A few normal faults cut MB4 to the south, suggesting local extension, which may be related to extension onshore and south of the study area as well as coeval flexural movements across the EVBFZ. Progradation of the shelf slope above MB4 documents increased sediment accumulation rates after Middle Miocene, probably a result of the late Neogene uplift of Central Vietnam (Figs. 4g and h). The landward truncation at MB4 and magmatism document the influence of the onshore, late Neogene volcanism and uplift on contemporary basin development. The gradual transgression and the increased basin-floor water depths during the late Neogene indicate increased subsidence coeval with the onshore uplift. This configuration argues for a late Neogene, eastward tilting of the region, in contrast to simple uplift of Central Vietnam. 5. Hydrocarbon potential 5.1. Oil seeps Oil seeps in the Dam Thi Nai lagoon have long been known and led to the drilling of a number of shallow wells in the lagoon in the early part of the twentieth century without commercial amounts of oil (Traynor and Sladen, 1997; Bojesen-Koefoed et al., 2005). The shallow lagoon is situated in the landward extension of the seismic section shown in Fig. 3 and separated from the Phu Khanh Basin and the South China Sea by a 700 m wide fractured granite horst cut by small faults and chloritised dolorite dikes (for location see Fig. 2).

17

A number of oil seep samples were collected by the ENRECA group along the lagoonal coast in 2002 and 2005. The samples included bitumen from fissures in fractured weathered granite, tar mats from the beach, water with petroleum odour from holes dug in the beach, oil drops from the muddy sediments north of the lagoon and oil-stained leaves from intertidal vegetation. The samples provide evidence of crude oils, which despite varying states of biodegradation clearly point to an origin from Cainozoic source rocks at early to peak maturity. These crude oils were derived from: (1) a mixed source probably composed of marine marls with a minor contribution of terrigenous organic matter; (2) a marine marl; and (3) a lacustrine source rock very similar to oilproducing Oligocene lacustrine mudstones in the Song Hong Basin (e.g. Petersen et al., 2001, 2004, 2005). The mixed-source oil, referred to as the ‘‘Dam Thi Nai Oil’’ by Bojesen-Koefoed et al. (2005), is relatively widespread, found in granite fissures, in muds in the northern part of the lagoon, and in water recovered from pits dug in the beach sand on the east coast of the lagoon, while the marlyand the lacustrine-sourced oils were identified only in a few samples. The lacustrine-sourced oil is compositionally comparable to those oils from wells in the northern part of the Song Hong Basin and the Black Lion, Sun Rise and White Tiger fields and also, to some extent, the Dragon Field in the Cuu Long Basin. Oil characteristics are summarised in Table 1. As there is no local source of the oil seeps, they are speculated to have migrated from the adjacent Phu Khanh Basin through the fractured granite horst, possibly through minor faults and chloritised dolerite intrusions. 5.2. Source rocks Cainozoic lacustrine mudstones and coals/coaly mudstones are the principal source rocks in the Vietnamese and adjacent Chinese basins (Geochem Group Limited, 1994; Todd et al., 1997; Chen et al., 1998; Hao et al., 1998; Binh, 2000; Huang et al., 2003; Petersen et al., 2004;

Table 1 Qualitative discrimination of oil types in the Dam Thi Nai area—for details see Bojesen-Koefoed et al. (2005) Oil type

n-Alkane distribution

O/Ha

H29/H30b

30N/Hc

BC/Hd

Neoe

Diaf

Tric/Hg

S27/S29h

TPPi

Mixed marl Lacustrine Marine marl

Biodegraded Waxy Non-wax

++ +/ +/

71 o1 41

++ +/ ++

+++ ++ ++

+ ++ +

+ ++ +/

+++ + ++

o1 41 71

+ +++ +/

+++: high; ++: moderate; +low; +/: trace. a Oleanane/hopane. b Norhopane/hopane. c C30 30-norhopane/hopane, measured using m/z 412–191 transition. d Bicadinane ‘‘T’’/hopane. e Neohopanes/hopane. f Diahopane/hopane. g C21–C29 tricyclic triterpanes/hopane. h Regular steranes C27/C29, sum of four isomers. i Tetracyclic polyprenoid ratio.

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5.3. Hydrocarbon modelling 2-D hydrocarbon modelling was performed to give a first assessment of the maturation and the hydrocarbon generation history of the potential source rocks for the oil seeps in the Dam Thi Nai lagoon as well as to illuminate timing and control on hydrocarbon generation and migration in the study area. The modelling was performed along the seismic section displayed in Fig. 3 using PetroMods software [version 9.0; Software of Integrated Exploration Systems, GmbH (Aachen, Germany)] following the modelling concepts of Welte and Yu¨kler (1981), Sweeney and Burnham (1990), Poelchau et al. (1997), and

Yalcin et al. (1997). The seismic interpretation and gravimetric modelling along the line were used to constrain lithology, ages, structures, crustal thickness, and heat flow. Furthermore, pre-defined standard PetroMods physical rock parameters were assigned in the 2-D model due to the absence of well data. The kinetic model for Type II kerogen of Tissot et al. (1987) was used as analogue to the mixed Type I, II and III kerogen anticipated in the Phu Khanh Basin since authentic kinetic data for the potential source rocks are not available. The interpreted seismic profile was depth converted using stacking velocities. Velocities were further converted to densities (Ludwig et al., 1970), and used for the gravimetric modelling of the upper part of the section. The gravity curve along the seismic section is characterised by a relatively stable profile west of the EVBFZ and a prominent gravity high across the EVBFZ, east of which the gravity profile smoothens again and displays the lowest gravity range observed (Fig. 12). All consistent crustal models show a continental crust with MB1 and Moho generally mirroring each other and a ca. 24–29 km thick, unstretched, continental crust west of the EVBFZ. Further east an abrupt crustal thinning occurs to a minimum thickness of ca. 2–6 km underneath the EVBFZ, east of which the crust thickens moderately (Fig. 12). Heat flows are assumed to have been high during the Palaeogene across the stretched part of the profile, because of the extensive crustal stretching and the volcanism (Figs. 4a and b). During Palaeogene times the deeper parts of MS1 and MS2 entered the oil window, defined as vitrinite values between 0.5 and 1.3, and the deepest part of MS1 entered the main gas window, defined as vitrinite values between 1.3 and 2.0 (Fig. 13a). On the other hand,

50 Gravity mgal

Savinykh et al., 2001, 2003; Andersen et al., 2005; Bojesen-Koefoed et al., 2005; Huyen et al., 2005; Toan, 2005; Quan, 2005). Total Organic Carbon (TOC) and Hydrogen Index (HI) values of immature onshore Cainozoic lacustrine mudstone analogues [from the Song Ba Trough and Dong Ho (Fig. 1)] mainly range from 4 to 20 wt% and from 300 to 700 mg HC/g TOC, respectively, indicating that the organic matter is composed of algal-rich kerogen (Type I/II) to a large extent and is similar to the lacustrine source rocks encountered in offshore wells (Binh, 2000; Petersen et al., 2004, 2005; Quan, 2005; Nielsen et al., 2007). Onshore humic coals display HI values up to 350 mg HC/g TOC, comparable to those measured in offshore wells suggesting a potential for oil generation. Cainozoic lacustrine mudstones and coals/coaly mudstones therefore provide excellent potential source rocks for oil and gas generation in the region. These source rocks are interpreted to be abundant in the Palaeogene syn-rift megasequences of the Phu Khanh Basin, and sporadically present in the mid-Miocene MS3-III based on seismic interpretation (Figs. 4a, b and e). Oil from marly source rocks is the most common seep oil in the Dam Thi Nai area (Bojesen-Koefoed et al., 2005). Biological marker distribution of the ‘‘Dam Thi Nai Oil’’ exhibits characteristics resembling extracts of Miocene marly source rocks in the Nam Con Son Basin (Traynor and Sladen, 1997; Bojesen-koefoed et al., 2005). Marl in the Nam Con Son basin was deposited in a series of sedimentary settings, including reefal and intra-reefal settings as the Early Miocene MS3-I. It is speculated that the ‘‘Dam Thi Nai Oil’’ originated from carbonaceous forereef marls deposited in the narrow depression along the trace of the EVBFZ in the northern half of the Phu Khanh Basin (Figs. 4c and 8). Carbonaceous sediments are likely to have been deposited in a narrow, deep strait surrounded by shallow marine environments with the main marine pathway probably silled by a structural high (e.g. Demaison and Moore, 1980; Fulthorpe and Slanger, 1989). The small amount of organic matter derived from higher land plants in the source rock of the ‘‘Dam Thi Nai Oil’’, as indicated by the oil composition, probably corresponds to minor allochthonous, terrigenous organic matter derived from nearby coastal areas.

W

E

0

-50 0

Depth (km)

18

10 20 Moho 30

20 km East Vietnam Boundary Fault Zone Density 1000

1500

2000

2500

3000 Kg/m3

Fig. 12. Density model along the seismic profile shown in Fig. 3, with observed satellite altimetry (dots) and computed gravity (solid line). See Fig. 2 for location.

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19

b

a East Vietnam Boundary Fault Zone

W

E

East Vietnam Boundary Fault Zone

W

E

0

0 MS2

MS3

MS1

2

2

Depth (km)

Depth (km)

0.7

1

6

MS2

0.7

0.5

4

0.5

1.3

4

MS1

1 1.3 2

6

2

8

8

10 km

10 km 10

10

c

East Vietnam Boundary Fault Zone 0

2

W

E

0.5 MS4 0.7 1

Depth (km)

4

MS3

1.3 MS2 2

6

8

10

Immature Early oil Main oil Late oil Main gas Post mature Pre-Cenozoic Basement Mantle

MS1

10 km

Fig. 13. Hydrocarbon modelling of the seismic section shown in Fig. 3. Oil and gas windows are based on modelled vitrinite reflectance. Vitrinite values are marked by numbered isovitrinite curves. MS1 and MS2 are interpreted to contain the main source rock intervals. MS3-I deposited above the EVBFZ are assumed to contain source rock intervals as well. (a) Maturation level at a time close to the Palaeogene–Neogene boundary, restored to the end of the deposition of MS2. (b) Maturation levels close to the Middle–Late Miocene transition restored to a time prior to the deposition of MS4. (c) Present day maturation level; dashed lines with arrows indicate the interpreted sources and approximate migration routes of the Dam Thi Nai oil seeps. See Fig. 2 for location.

heat flow along the modelled section decreased as the main rifting ceased near the Palaeogene–Neogene boundary and burial depth became the controlling factor for organic maturation along the section. As a consequence, the oil window only climbed a few hundred metres during the early Neogene along much of the profile (Figs. 13a and b). An exception to this occurred in the depocentre above the EVBFZ, where the isovitrinite curves ascended significantly through the intervals of MS2 and part of MS3-I entered the early oil window (Figs. 13a and b). As the sediment accumulation rate increased during the late Neogene, the

thick wedge of shelf and shelf-slope deposits prograded across the depocentre along the EVBFZ, burying potential source rock intervals. The significantly increased sediment load forced the upper part of the EVBFZ-depocentre, including MS3-I, through the main oil window and caused the majority of the potential source rocks in MS1 and MS2 farther east to be situated in the oil window (Fig. 13c). The late Neogene is therefore interpreted as the single-most important period for oil generation in MS2 and MS3-I along the profile, and the Dam Thi Nai oil seeps probably formed during this period. The oil seeps most likely

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originate from marly (MS3-I) and lacustrine (MS2) potential source rocks in the westernmost part of the EVBFZ (Fig. 13c). This favours a fairly simple, 40–50 km, up-dip migration pathway, which suggests late Neogene migration as late Neogene sediments offshore the Dam Thi Nai lagoon form the only sediment cover of the shallow basement and provide a sealed migration pathway. The thermal evolution of the central part of the study area is probably similar to the northern part due to structural and gravitational similarities. The burial depth of the potential source rocks of MS2 and MS3-I is therefore interpreted as the most important factor for oil generation, although the early Neogene magmatism in the southeastern part of the study area probably influenced source rock maturation locally. The burial depths of the inferred potential source rocks in, and south of, the modelled profile appear to be comparable. South of the modelled profile, late Neogene sediments (MS4) make up the significantly thickest part of the depositional drape covering MS1, MS2 and MS3-I, suggesting a late Neogene main oil-generating stage. In the northern and central part of the study area, the increased late Neogene sediment accumulation rate promoted shelf-slope progradation across the axial depocentre of MS1 and MS2 along the EVBFZ, causing a sudden increase of sediment thickness (Figs. 5a and b). The progradation of the shelf slope is therefore inferred to have been one of the main controlling

W

Neogene coastal and shelf sands

factors in the generation of hydrocarbons in most of the north Phu Khanh Basin. 5.4. General hydrocarbon assessment The Dam Thi Nai oil seeps could indicate an active petroleum system in the Phu Khanh Basin. This is substantiated by common potential DHI’s, such as gas seeps, amplitude anomalies, and chimney-like features, mostly situated in various sand-prone intervals (Figs. 10 and 11). The analysis of the oil seeps, the seismic interpretation, and the hydrocarbon modelling, suggest two mature to post-mature potential source rock types in the area. Our study has shown that potential structural and stratigraphic traps are situated in favourable positions relative to potential source rocks and that they mainly formed before or during early Neogene time, preceding the late Neogene main oil generation (see for example Fig. 9). The seismic facies interpretation indicates that potential reservoir rocks are composed of Miocene carbonates, various sand-prone depositional facies ranging from non-marine fluvial deposits to deep marine turbidite sequences and fractured basement highs in the western half of the study area. Drowned carbonate sequences and transgressive shale units and lacustrine mudstones may be the main potential seals in the basin. A series of promising hydrocarbon plays thus exist in the basin,

Miocene–Pliocene turbidite sands

Miocene Pinnacle reefs

Miocene carbonate platforms

E

R

R R R

R

R

R

R

R R R

Sand-prone marine facies R

Mud-prone marine facies (potential seal rocks)

R R

Mud-prone potential source rock/seal facies

R

Sand-prone fluvial facies Carbonate buildup and platform facies Pre-Tertiary R Potential reservoirs

Upper Oligocene and lower Neogene fans

Palaeogene fluvia sands

~5 km

Fig. 14. Schematic diagram summarising potential hydrocarbon play types. The potential plays are based on source rocks primarily composed of Palaeogene lacustrine mudstones and Lower Miocene marly mudstones. Various reservoir types and structural and stratigraphic traps are outlined.

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as summarised in Fig. 14, many of which are located in shallow water (e.g. Fig. 9) Only minor faulting occurred during the main stage of the hydrocarbon generation, which increases the probability of maintaining trap integrity, although seal rupture due to late stage tectonics could be a risk for some potential structural traps. A further risk factor could be flushing of reservoirs by gas derived from deeply buried source rocks or cracking of oil previously filling the deepest buried reservoirs or by gas sourced from the mantle in conjunction with the observed magmatic activity or the thin crust along the EVBFZ. 6. Conclusions Initial rifting and non-marine deposition in the Phu Khanh Basin started during Late Cretaceous or Palaeogene time as a result of left-lateral transtension along the coastparallel EVBFZ, caused by the Indian and Eurasian collision and slab pull from subduction of the proto-South China Sea underneath Borneo. Rifting was terminated by middle Oligocene uplift and erosion associated with leftlateral transpression, but rifting resumed by left-lateral transtension during the Late Oligocene while non-marine deposition continued. A second phase of inversion occurred near the Palaeogene–Neogene boundary, caused by right-lateral wrenching along the EVBFZ. This late inversion is speculated to be genetically linked to the coeval tectonic events observed in the Mae Ping Fault and in the Cuu Long, Nam Con Son, West Natuna, and Malay basins. After the onset of right-lateral wrenching, a marine transgression began, resulting in widespread carbonate deposition during the Early Miocene that was contemporary with the onset of volcanism in the southeastern part of the study area. As the transgression continued, a shelf slope started to build in the northern part of the basin and siliciclastic deposition gradually became dominant, although limited carbonate platforms and reefs continued to exist until the Late Miocene. Subsidence and sediment accumulation rates increased significantly around the early Late Miocene, responding to seaward tilting of Central Vietnam caused by regional magmatism. This led to progradation of the shelf, as well as to increased water depth towards the basin centre in the east and a depositional regime dominated by mass flow deposits east of the shelf slope. Oil seeps, potential DHIs, and at least two widespread potential source rock types, indicate the presence of active petroleum systems in the Phu Khanh Basin. Maturation modelling suggests that Palaeogene lacustrine mudrocks and humic coals entered the early oil window during the Late Oligocene and that marly source rocks entered the oil window during the later part of the early Neogene. The modelling further indicates that the main petroleum generation occurred during the late Neogene after the deposition of reservoir and seal rocks and that a significant

21

part of the potential source rock remains in the oil and gas windows. Various types of structural and stratigraphic traps are predicted in the basin with potential reservoirs including Palaeogene fluvial sandstones, Neogene turbidite, shelf, lowstand delta and coastal sandstones as well as Miocene platform and reef carbonates and fractured basement. Acknowledgements This study is part of a Ph.D. project funded by the University of Copenhagen to the first author. The study of the other authors was funded by the Danish Ministry of Foreign Affairs through a Danish International Development Assistance (DANIDA) grant to an ongoing research capacity project with participation of Vietnamese and Danish research institutes and universities (the ENRECA project). J. Halskov and S. Sølberg assisted in the draft of the figures. We also thank PetroVietnam, VPI for providing the seismic reflection and well data and permission to publish them, and GEUS for providing facilities for data interpretation and analysis. The paper benefited significantly from the helpful and stimulating reviews of G.H. Lee and J. Lambiasse as well as from the kind suggestions of the editor D.G. Roberts. References Andersen, C., Mathiesen, A., Nielsen, L.H., Tiem, P.V., Petersen, H.I., Diem, P.T., 2005. Evaluation of petroleum systems in the northern part of the Cainozoic Song Hong Basin (Gulf of Tonkin), Vietnam. Journal of Petroleum Geology 28, 167–184. Bachtel, S.T., Kissling, R.D., Martono, D., Rahardjanto, S.P., Dunn, P.A., MacDonald, B.A., 2004. Seismic stratigraphic evolution of the Miocene–Pliocene Segitiga platform, East Natuna Sea, Indonesia: The origin growth and demise of an isolated carbonate platform. In: Eberli, G.P., Masaferro, J.L., Sarg, J.F.R. (Eds.), Seismic Imaging of Carbonate Reservoirs and Systems. AAPG Memoir, vol. 81, pp. 309–328. Barckhausen, U., Roeser, H.A., 2004. Seafloor spreading anomalies in the South China Sea revisited. In: Clift, P., Wang, P., Kuhnt, W., Hayes, D. (Eds.), Continent–Ocean Interactions Within East Asian Marginal Seas. Geophysical Monograph, vol. 149. AGU, Washington, DC, pp. 121–126. Barr, S.M., Macdonald, A.S., 1981. Geochemistry and geochronology of late Cainozoic basalts of Southeast Asia. Geological Society of America Bulletin 92, 1069–1142. Binh, N.T.T., 2000. Thermal and geochemical kinetic model for basin evolution and hydrocarbon generation—application to offshore southwestern Vietnam. In: Hiep, N., Chinh, T.D., Quy, N.H., Bao, N.V., Huy, P.V. (Eds.), Conference on The Oil and Gas Industry on the Eve of the 21st Century. Youth Publishing House, Hanoi, pp. 124–131. Bojesen-Koefoed, J.A., Nielsen, L.H., Nytoft, H.P., Petersen, H.I., Dau, N.T., Hien, L.V., Duc, N.A., Quy, N.H., 2005. Geochemical characteristics of oil seepages from Dam Thi Nai, Central Vietnam. implications for exploration in the offshore Phu Khanh Basin. Journal of Petroleum Geology 28, 3–18. Borgomano, J.R.F., Peters, J.M., 2004. Outcrop and seismic expression of coral reefs, carbonate platforms, and adjacent deposits in the Tertiary of the Salalah Basin, South Oman. In: Eberli, G.P., Masaferro, J.L.,

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