Modelling of Hydrocarbon Generation in the Cenozoic Song Hong Basin, Vietnam: a Highly Prospective Basin

PERGAMON Journal of Asian Earth Sciences 17 (1999) 269±294

Modelling of hydrocarbon generation in the Cenozoic Song Hong Basin, Vietnam: a highly prospective basin

L.H. Nielsen a, *, A. Mathiesen a, T. Bidstrup a, O.V. Vejbñk a, P.T. Dien b, P.V. Tiem b

aGeological Survey of Denmark and Greenland, GEUS, Thoravej 8, DK-2400, Denmark bVietnam Petroleum Institute, VPI, Yen Hoa, Cau Giay, Hanoi, Vietnam Received 11 August 1998; accepted 2 November 1998

Abstract

The Cenozoic Song Hong Basin, situated on the northern part of the Vietnamese shelf, has been only sporadically explored for hydrocarbons. A review of the results of the exploration eorts so far shows that the distribution of potential source rocks and their time of hydrocarbon generation are the critical risks for ®nding commercial amounts of hydrocarbons. In the Song Hong Basin, including the Hanoi Trough, the rocks most likely to have source potential are: (1) oil-prone Eocene±Lower Oligocene lacustrine mudstones and coals, (2) oil- and gas-prone Middle Miocene coal beds, (3) gas-prone Upper Oligocene± Lower Miocene coals, and (4) gas- and oil-prone Miocene marine mudstones. To assess the time of hydrocarbon generation from these units, relative to the formation of traps, the generation history was modelled at 32 well and pseudo-well locations. The modelling demonstrates that the two ®rst-mentioned source rock units are especially important. In the northern and northeastern part of the basin and along its western margin traps may have been charged by Eocene±Lower Oligocene source rocks. In the Hanoi Trough, the excellent Middle Miocene coal beds have probably generated hydrocarbons within the last few million years. Thus the huge and still underexplored Song Hong Basin provides attractive areas for further exploration. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction stream soon. Similarly, exploration activities in the huge Song Hong Basin (Bac Bo/Yinggehai/Red River The Cenozoic basins on the Vietnamese shelf are Basin), located on the northern part of the Vietnam regarded as highly prospective, comparable to many continental shelf, have shown promising results. The other petroliferous basins in Asia. There has been a aim of this paper is to discuss the exploration potential remarkable increase during the last decade in the num- of the Song Hong Basin by reviewing the exploration ber of signed production sharing contracts, exploration history and presenting results from the modelling of blocks awarded, acquisition of 2D and 3D seismic hydrocarbon generation. data, and number of wells drilled. The yearly production of crude oil has shown a signi®cant rise from 0.4 million tons in 1986 to more than 8 million tons in 2. Exploration activities in the Song Hong Basin 1996, and is expected to increase considerably in the near future. The discovery rate on the Vietnam shelf The Song Hong Basin is one of the Cenozoic basins has been more than 20%Ðamong the world's high- located along the western margin of the East Vietnam estÐand recent discoveries of oil and gas in the Cuu Sea (South China Sea; Fig. 1). Exploration for hydro- Long and Nam Con Son basins, located in the carbons began more than 30 years ago, when the ®rst southern shelf area, are expected to be brought on deep onshore well was drilled in 1965 in the Hanoi Trough. In 1975 the Tien Hai gas ®eld was discovered in the Miocene section, approximately 90 km southeast * Corresponding author. e-mail: [email protected]. of Hanoi (Fig. 2). Several wells have tested oil, con-

Fig. 1. Present-day tectonic map of southeast Asia (modi®ed from Lee and Lawver, 1994). BBB = Beibu Wan Basin, BKB = Bangkok Basin, CB = Chuxiong Basin, EVBF = East Vietnam Boundary Fault, GTB = Gulf of Thailand Basin, LSB = Lanping±Simao Basin, MLB = Malay Basin, MT = Manila Trench, QB = Qiongdongnan Basin (South Hainan Basin), PI = Paracel Islands, RRF = Red River Fault (Song Hong Fault), SHB = Song Hong Basin.

densate and gas from siltstones and sandstones in the Likewise, oshore activities have shown encouraging Middle and Upper Miocene Phu Cu and Tien Hung results. These activities began with acquisition of seis- formations (Fig. 3). Recently, in mid-1996 Anzoil is mic data in the northern shelf area by the former reported to have tested heavy oil and gas from carbon- General Department for Oil and Gas, with assistance ates, probably of Devonian age, in their wells B10 from the USSR. Since then, more detailed seismic ac- STB-1x and D14-1x (Long, 1998) (Fig. 2). Other direct quisition has been undertaken by the various licence evidence of hydrocarbons is provided by natural as- holders, and in 1993 Geco-Prakla acquired a regional phalt in outcrops of Upper Devonian±Lower data set. Drilling activities began with the Con Den Carboniferous fractured carbonates on Cat Ba Island 110 well (LK 110) drilled by PetroVietnam in very and at Yen Bai, and by oil and gas seepages along the shallow water in the northwestern part of block 102 coasts of Vietnam and Hainan (Chen et al., 1993, (Fig. 4). The French company Total drilled three wells 1998; Traynor and Sladen, 1997) (Fig. 2). on inversion structures in the northern part of the L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 271

Fig. 2. Map showing principal structural elements, exploration wells and selected localities in the Song Hong Basin and adjacent areas. 272 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

Fig. 3. Stratigraphic scheme of northern Song Hong Basin (modi®ed after Dien, 1997). HD = Hai Duong, KX = Kien Xuong. basin in blocks 103 and 107, the 103 TH-1x, 103 TG- Song Lo Fault zone, but encountered only traces of 1x and 107 TPA-1x wells in 1990±1991. The 103 TH- gas. Farther to the south, Shell drilled two wells in 1x tested oil, condensate and gas from Miocene sand- block 114 and block 112 in 1990. The well 114 KT-1x stones, but was considered non-commercial by Total. was drilled on the rotated Kim Tuoc fault block Well 107 TPA-1x penetrated Eocene±Oligocene syn- (Fig. 2). The second well, 112 BT-1x was drilled to a rift sediments in a major inversion structure east of the total depth of 4114 m with oil and gas encountered in L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 273

close to Hainan Island, a number of wells on the Dong Fang structures have encountered signi®cant 106 102 amounts of gas, though mostly CO2. Similarly, gas or oil shows were found in several of the Ledong and 107 Ying wells in the southern and eastern part of the 103 basin (Fig. 2). Producing oil ®elds are present in the 108 Beibu Wan Basin north of Hainan, and a gas ®eld Hainan occurs in the Qiongdongnan Basin south of Hainan. (China) 104 109

3. Potential source rocks 105 110 Eocene±Miocene rift-lake claystones are widely dis- 111 tributed in many Asian basins, and constitute good to excellent sources for oil generation (e.g. Wu Jinmin, 112 113 1988; Sladen, 1993; 1997; Hao et al., 1995; Williams et al., 1995; Katz and Xingcai, 1998). In the Song Hong 114 115 Basin, organic-rich lacustrine claystones of Eocene to Oligocene age were probably deposited in grabens and half-grabens, when sedimentation was outpaced by 116 117 rift-induced subsidence, allowing strati®ed, oxygen- poor water columns to be established in tectonically Vietnam 118 controlled lakes. The formation of adequate source rocks in the rift-lakes may be related to the distri- 119 bution of Devonian±Permian carbonates, which in- itially prevented a large input of clastic material to the Fig. 4. Block delineation and oshore exploration wells in the Song early rift-lakes. Several seismic sections show distinct, Hong Basin and adjacent areas. high-amplitude re¯ectors in the lower part of the syn- rift sequences that are interpreted as lacustrine or- Oligocene sandstones and Devonian carbonates. Shell ganic-rich shales and coal beds. Outcrops at Dong Ho drilled well 112 HO-1x in the same year, with oil expose immature organic-rich Oligocene mudstones shows and a gas test in Lower Miocene carbonates with fresh-water algae and land plants interbedded overlying Palaeozoic rocks. Subsequently, Shell drilled with thin coals and asphaltic sandstones from a small 112 AV-1x at a more basinward position on the inverted half-graben on the northern margin of the Hoang Oanh structure, but both the Palaeozoic rocks Hanoi Trough (Fig. 2). These rocks show TOC values and the Lower Miocene mixed carbonate-siliciclastic of 6±42%, S2 values of 30±94 kg/tonne and HI values section were water-bearing. In 1990 and 1991 the Tri 520±670, indicating that they are strongly oil prone Ton Horst structure was tested by the IPL well 115 A- (Traynor and Sladen, 1997). These facies are similar to 1x, and by the BP wells 118 CVX-1x and 119 CH-1x. the prime sources for oil in the basins north of Hainan All three wells encountered signi®cant amounts of gas Island and the Pearl River Basin, and are probably in Miocene carbonates developed over the horst rather widespread (e.g. Allen et al., 1988; Zu Jiaqi, (Morris, 1993). In 1993, BP drilled 118 BT-1x in the 1985; Zhang Qiming and Kou Caixiu, 1989; Jishu et Quang Ngai Graben and gas was indicated. Later, in al., 1994, Sladen, 1997). In the southwestern part of 1995 BP drilled 117 STB-1x in which gas was detected. the Song Hong Basin, Oligocene sediments with type In the northern part of the basin, Idemitsu drilled III/II kerogen and TOC varying between 1±7% occur two wells in block 102, well 102 CQ-1x and 102 HD- in well 112 BT-1x. In the western part of the 1x in 1993 and 1994, of which the ®rst-mentioned Qiongdongnan Basin at the margin to the Song Hong encountered gas shows. Idemitsu later relinquished the Basin, gas is produced from Oligocene and Lower block. In Block 104 on the shallow footwall of the Miocene sandstones in the Yacheng 13-1 ®eld contain- Song Chay Fault bounding the central depocentre of ing estimated reserves of 3.5 TCF of gas (Asian Oil & the basin, OÈ MV in 1995 and 1996 drilled two wells, Gas, 1996). The ®eld has been charged by Oligocene 104 QN-1x and 104 QV-1x, to test a Miocene pinnacle gas-prone type III kerogen from both the reef and a buried basement hill (Dien et al., 1998; Qiongdongnan Basin and the Song Hong Basin (Chen Andersen et al., 1998). Within the disputed area et al., 1998; Hao et al., 1998). between Vietnam and China, many wells have encoun- Another excellent source of hydrocarbons is the tered hydrocarbons. In the eastern part of the basin, Miocene coal beds that occur abundantly in the Hanoi 274 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

Trough. They show both high TOC and HI values in nated by type III kerogen are a likely source of hydro- many wellsections. Several coal beds found in the o- carbons in the Ledong and Dong Fang structures shore wells 102 CQ-1x and 102 HD-1x show HI values (Hao et al., 1995). of more than 400, and are thus highly oil-prone (Fig. 5) Thus, potential source rocks in the Song Hong (e.g. Mukhopadhyay et al., 1991; Scott and Fleet, Basin may include: (1) oil-prone Eocene±Lower 1994). In addition, Upper Oligocene coaly shales were Oligocene lacustrine mudstones; (2) oil- and gas-prone drilled in well 112 BT-1x, and the oil and gas encoun- Middle Miocene coal beds; (3) gas-prone Upper tered in the wells 112 HQ-1x, 112 BT-1x and 114 KT- Oligocene±Lower Miocene coal beds; and (4) gas- and 1x was probably sourced from similar beds. In the oil-prone Miocene oshore marine mudstones. southeastern part of the basin, Middle±Upper Miocene A review of the exploration history and available in- oshore marine mudstones with 0.2±3.0% TOC domi- formation from neighbouring basins shows that the

Fig. 5. Geochemical plots of TOC, S2, HI and %Ro from the wells 102 CQ-1 and 102HD-1 (partly after Ha, 1998). L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 275 main geological risks associated with ®nding commer- 4.2. Regional extension phases cial amounts of hydrocarbons in the Song Hong Basin are: (1) distribution of source rocks; (2) timing of hy- Late Cretaceous to Paleocene±Early Eocene NW± drocarbon generation from the source rocks relative to SE regional extension caused the formation of a NE± formation of potential structures; and (3) presence of SW trending proto-East Vietnam Sea, as well as re- sealing shales in the sandstone-dominated sections. In gional uplift and a series of rift basins along the an attempt to assess the timing of hydrocarbon gener- southern margin of China. Cretaceous and/or ation relative to formation of potential traps, the hy- Paleocene±Lower Eocene sediments, predominantly drocarbon generation history of the main source rock terrestrial redbeds and lacustrine mudstones, were units was evaluated using the YuÈ kler 1D model deposited in these basins (Ru and Pigott, 1986; Sun (YuÈ kler et al., 1978). Shu et al., 1989; Pigott and Ru, 1994; Zhou et al., 1995). However, the ages of the oldest Cenozoic deposits in northern Vietnam, and their genetic relation to Mesozoic and younger Cenozoic rocks, are uncertain. According to Dien and Dzung (1994) and Dien (1997), molasse-type deposits, dominated by alluvial and ¯u- 4. Geological setting and development of the Song Hong vial conglomerates and sandstones, were deposited in Basin: framework for the modelling Paleocene±Early Eocene time in residual depressions formed during earlier continental convergence (Fig. 3). The Song Hong Basin is an elongated geological Other authors attribute the formation of the oldest structure, approximately 500 km long and 50±60 km Cenozoic sediments to initial rifting in Eocene time wide, striking NW±SE. It consists of an onshore part, (e.g. Dinh and Truong, 1995; Dinh, 1998). the Hanoi Trough, that reaches into the Hanoi area in A second phase of regional N±S extension, initiated the Song Hong river valley, and a much larger oshore by collision of the Indian Plate with the Eurasian part (Figs. 1 and 2). The development of the Song Plate, lasted from Mid±Late Eocene to Early Miocene Hong Basin is related to the large-scale evolution of (Tapponnier et al., 1986, 1990; Huchon et al., 1994; the East Vietnam Sea basin. This great basin is Lee and Lawver, 1994). This collision caused tectonic bounded by the continental margins of South China to escape of the Indochina Block toward the southeast, the north, Vietnam to the west, Borneo to the south subsequent clockwise rotation of 18±308 (of and the Manila Trench to the east. Its development Indochina) and opening of the East Vietnam Sea. Left- since the Early Cretaceous has been governed mainly lateral movements along the Song Hong Fault system by two or three stages of extension, rifting and sea- (Red River Fault) initiated the formation of the Song ¯oor spreading (Taylor and Hayes, 1983; Pigott and Hong Basin. The net displacement may amount to Ru, 1994; Lee and Lawver, 1994). more than 200 km, and possibly up to 500 km (Peltzer and Tapponnier, 1988; Briais et al., 1993; Leloup et 4.1. Basement of the Song Hong Basin and thickness of al., 1995). However, the amount of displacement and the Cenozoic basin-®ll the relationship of the Song Hong Fault to the major shear zone oshore Vietnam and the opening of the The basement of the Song Hong Basin is complex. East Vietnam Sea is still debatable (e.g. Roques et al., The onshore shallow part comprises Proterozoic schists 1997; Thi and Giang, 1998). The strike-slip movements and gneiss, and Palaeozoic and Mesozoic clastics, car- occurred mainly along the Song Chay and Song Lo bonates, and volcanics (Tien et al., 1991; Bao et al., fault zones (Fig. 2). During the rifting phase, grabens 1994; Dien, 1996, 1997). The deep oshore part is and half-grabens were formed in the Song Hong Basin poorly known, but presumably consists of similar and adjacent basins (Figs. 6±8). Within these grabens rocks. In some areas seismic pro®les indicate well-stra- thick rift-sequences of dominantly ¯uvial and lacus- ti®ed sections that are interpreted as Mesozoic low- trine deposits are preserved (e.g. Wu Jinmin, 1994). grade metamorphic sedimentary rocks. The depth to These deposits are included in the Dinh Cao the base of the Cenozoic basin-®ll in the central part Formation in the Hanoi Trough (Fig. 3) and in the of the basin is well below the base of conventional Hue Group in the southern Song Hong Basin (Dien, seismic, in excess of 8 sec TWT, which corresponds 1997). to more than 15 km. Estimates, based on gravity inversion and isostatic models, suggest a maximum 4.3. Drifting and sagging phase depth of 14 km (Vu and Rabitnowitz, 1996), whereas other estimates suggest as much as 15±20 km Continued regional N±S extension led to the estab- (Hirayama, 1991; Dinh and Troung, 1995; Hao et al., lishment of a spreading axis in the central East 1995, 1998). Vietnam Sea and formation of oceanic crust in the 276 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

Fig. 6. Part of seismic line 89-1-36A from SP 100 to 1600. The line is located at the northeastern margin of the Song Hong Basin, and shows Eocene±Early Oligocene half-grabens overlain by Upper Oligocene-Recent post-rift deposits. The top syn-rift unconformity, Re¯ector B, is correlated to the break-up unconformity associated with the opening of the East Vietnam Sea in late Early Oligocene time (32 Ma). Re¯ector A is

Early Oligocene (Fig. 9) (Taylor and Hayes, 1980, (Figs. 6±8). The age of the top syn-rift unconformity 1983; Briais et al., 1993; Lee and Lawver, 1994, 1995; may be variable, however, along the South China mar- Pigott and Ru, 1994). A major break-up unconformity gin, indicating that the rifting phase ended at dierent separating the syn-rift and post-rift sections was time in the various basins (Zhou et al., 1995; Chen et formed in late Early Oligocene time in many of the al., 1998). basins along the margin of the South China Block, The left-lateral transtension along the Song Hong and a similar unconformity is also evident in the Song Fault system continued during the Early Miocene, Hong Basin (Rangin et al., 1995; Vejbñk et al., 1996) causing rapid subsidence of the Song Hong Basin. The

Fig. 7. Part of seismic line 90-1-065 from SP 3030 to 4080. This line follows the strike of the Song Hong Basin along the northeastern margin. The break-up unconformity associated with the opening of the East Vietnam Sea (Re¯ector B) is only moderately aected by the NW±SE oriented strike-slip movements associated with the Song Hong Fault zone. For location see Fig. 10. L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 277

Fig. 8. Part of seismic line 89-1-62 from SP 1580 to 3080. The line shows position of 103 TH-1x and 103 TG-1x drilled by Total centrally in the northwestern part of the Song Hong Basin. The conspicuous reverse faults in the axis of the Song Hong Basin are related to strike-slip movements. Re¯ector A is near top basement, Re¯ector B is the late Early Oligocene break-up unconformity (32 Ma), Re¯ector C is near base Middle Miocene, Re¯ector D is intra-Upper Miocene, and Re¯ector E is base Pliocene. At the well locations Re¯ector E corresponds to a major erosional hiatus due to inversion in Late Miocene times. For location see Fig. 10. depositional environments during Late Oligocene± al., 1989, cited from Lee and Lawver, 1994; Pinglu and Early Miocene times vary widely, from ¯uvial, estuar- Chuntao, 1994). However, other workers have ine, and deltaic to oshore marine, with the deposition suggested that the change from left- to right-lateral of sandstones and mudstones. The deposits are displacement occurred during the Late Miocene to ear- included in the Thuy Anh and Phong Chau formations liest Pliocene time (Phach, 1994; Pigott and Ru, 1994; in the northern part (Fig. 3), which correspond to the Rangin et al., 1995). Thick prograding deltaic units of Da Nang Group in the southern part of the basin sandstones, siltstones, mudstones, and brown coals (Morris, 1993; Wu Jinmin, 1994; Dien, 1997). were deposited in Middle±Late Miocene times in the By Middle Miocene time sea-¯oor spreading in the northern Song Hong Basin. These deposits are East Vietnam Sea, and left-lateral movements on the grouped into the Phu Cu and Tien Hung formations Song Hong Fault ceased (Briais et al., 1993; Huchon (Fig. 3) (Tien et al., 1991; Wu Jinmin, 1994; Dien, et al., 1994; Lee and Lawver, 1994, 1995; Leloup et al., 1997). The corresponding deposits in the southern part 1993, 1995; Hall, 1996). The relative movements along of the basin are dominated by shallow marine clastics, grouped into the Song Huong and Quang Ngai for- the Song Hong Fault changed to right-lateral as the mations, or the Bac Bo Group, which partly inter®n- southeastward drift of Indochina was blocked by the gers with various carbonates, including platform Sundaland Plate, while the China Block continued its carbonates, barrier and pinnacle reefs of the Middle drift to the east, as the Indian Plate continued its Miocene Tri Ton Group (Morris, 1993; Dien, 1997). northward penetration (Fig. 9). This change in relative displacement along the fault probably occurred in the mid-Miocene (Lee and Lawver, 1994, 1995; Hall, 4.4. Late Miocene basin inversion and renewed sagging 1996), leading to the current right-lateral displacement (Allen et al., 1984). The change of displacement direc- The strike-slip activity, causing reversal of faults and tion is expressed in the Song Hong Basin by the for- the formation of signi®cant inversion structures and mation of a distinct unconformity near the base of the several minor unconformities in the northern part of Middle Miocene (Re¯ector C in Figs. 6±8), which in the basin, culminated in Late Miocene time, with the places shows deep channel incision, and a conspicuous formation of a signi®cant and widespread unconfor- lateral shift of depocenters (Vejbñk et al., 1996). Weak mity that deeply truncates the inversion structures compression and tectonic inversion in the Middle (Fig. 8). Renewed and increased subsidence of the Miocene of the Pearl River Basin, probably relate to Song Hong Basin is witnessed by the thick, draping, this change in movement along the fault (X. Wang et and virtually undisturbed latest Miocene to Pliocene± Fig. 9. Oligocene reconstruction (30 Ma, above) and Middle to Late Miocene reconstruction (10 Ma, below) according to Lee and Lawver (1994). Prior to Late Oligocene times, the area was dominated by N±S to NW±SE tension. Note the reversion to dextral movements along the Song Hong Fault Zone. L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 279

Quaternary section that overlies the Upper Miocene and pseudo-activation energies used in these equations unconformity (Figs. 6±8). South of Hainan, the thick- have later been modi®ed by YuÈ kler (unpublished ness of the Pliocene shelfal to bathyal mudstones is up data). The amount of generated hydrocarbons is calcu- to 5 km, indicating an extremely rapid subsidence rate lated for each model event and presented as empirical of up to 1400 m/Ma (Wu Jinmin, 1994; Hao et al., modi®ed transformation ratios in grams of hydro- 1995). The deposits in the northern part of the basin carbon per gram of original total organic carbon are dominated by oshore marine to shallow-marine (TOC). Values are calculated as if the original organic mudstones, siltstones, and sandstones, with minor pro- matter type was either type I, II or III kerogen. An portions of lagoonal and possibly ¯uvial deposits. empirical approach divides the hydrocarbon generation These deposits are included in the Dong Hoan, Vinh into zones, each of which is de®ned as a percentage or Bao, Hai Duong and Kien Xuong formations (Fig. 3). degree of alteration. These zones describe the main The dominant marine mudstones of the southern part types of hydrocarbon generated. of the basin are grouped into the Bien Dong Formation (Dien, 1997). 6. Event de®nition

5. Model concept The geological information for the modelling was compiled from sources at the Vietnam Petroleum The exploration potential of the Song Hong Basin Institute in Hanoi, supplemented by interpretation of has been assessed by stratigraphic analysis of well data seismic pro®les and nine onshore and ®ve oshore well and seismic data, and the use of the YuÈ kler 1D basin sections (Fig. 10). The well sections were extrapolated model (YuÈ kler et al., 1978). The YuÈ kler model is a for- down to the basement using seismic data. The data ward deterministic model that quanti®es the geological have been combined to model 18 pseudo-wells located evolution of a sedimentary basin by calculating com- at strategic positions in the basin (Fig. 10). The term paction, pressure, temperature, thermal maturity and pseudo-well is used as a point of compiled information hydrocarbon generation. within a given area, and is therefore not a real well lo- Geological information and input data for the cation. The thicknesses of the model events in the model (thickness, age, lithology, porosity, palaeotem- pseudo-wells are estimated from seismic sections using perature, heat ¯ow and palaeo water depth) are syn- a modi®ed velocity function based on information thesised into model events in such a way that the from well 103 TH-1x and assuming a maximum depth model can handle deposition, nondeposition, and ero- to the base of the basin of approximately 17 km. This sion in the basin. assumption is based on the likelihood of overpressure Computed values representing present time can be in the deeper part of the basin due to rapid loading, as compared with measured values of thickness, porosity, indicated by mud-diapirism and pressures measured in temperature and thermal maturity obtained in wells wells (e.g. Hao et al., 1995, 1998; Chen et al., 1998). from the basin. The geological model and input par- The history of the basin is subdivided into model ameters are optimised by minimising the dierences events, each of which represents deposition, non-depo- between computed and measured values. The lithologi- sition, or erosion (Table 1). Lithostratigraphic units cal properties such as compressibility, thermal conduc- such as formations or informal members de®ned in tivity etc. are assigned according to the lithology wellsections in the Hanoi Trough form the basis for speci®ed for each model event. Thickness is optimised the event de®nitions. The approach has been to use automatically by the programme, by changing the por- subdivisions not shorter than 0.5 Ma. The events are osity of the depositional unit. In some cases it may be extrapolated from the Hanoi Trough into the deeper necessary to change the assigned lithology to give a oshore part of the basin by correlating the principal reasonable match. seismic sequence boundaries to known unconformities The thermal history of each sedimentary unit is in the Hanoi Trough. The known, or inferred, strati- determined from an equation that describes the heat graphic ages have been transferred into absolute ages movement as a function of heat ¯ow into the basin, (Ma) using the time scale of Harland et al. (1989). and surface temperature at time of deposition (sea bottom for marine, or surface for continental sediments). 6.1. Base of the model and the syn-rift sequenceÐmodel Heat is transported by conduction and by compaction events 1 and 2±7 ¯uids moving up through the sediments as they com- pact. Based on palynomorphs, the age of the ®rst The hydrocarbon generation history is determined Cenozoic basin-®lling deposits seems to be Eocene from kinetic equations based on the work of Tissot (Tien et al., 1991; Bat et al., 1993; Trung et al., 1997), and EspitalieÂ (1975). The original frequency factors possibly Middle±Late Eocene (Bao et al., 1994). The Fig. 10. Position of modelled wells and pseudo-wells; seismic lines (Figs. 6±8) and cross-sections (Figs. 14 and 15). L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 281

Table 1 Model events of the development of the Song Hong Basin

No. Type of model event Duration (Ma) Age (Ma)

27 Pleistocene, deposition (``Kien Xuong Fm'') 0.5 0.5 26 Pleistocene, deposition (``Hai Duong Fm'') 1.0 1.5 25 Late Pliocene erosion/non-deposition 0.5 2.0 24 Late Pliocene erosion/non-deposition 0.5 2.5 23 Pliocene deposition (Vinh Bao Fm, upper part) 1.0 3.5 22 Pliocene deposition (Vinh Bao Fm, lower part) 1.5 5.0 21 Late Miocene (to earliest Pliocene) erosion on highs, deposition in depressions and 1.5 6.5 in the main part of the basin (Dong Hoang Fm) 20 Late Miocene erosion on highs, (Re¯ector D). Deposition in depressions and in 1.5 8.0 the main part of the basin (Dong Hoang Fm) 19 Late Miocene deposition (Tien Hung Fm, upper part) 1.0 9.0 18 Late Miocene deposition (Tien Hung Fm, middle part) 1.0 10.0 17 Late Mid-Late Miocene deposition (Tien Hung Fm, lower part) 1.5 11.5 16 Late Mid Miocene, minor erosion 0.5 12.0 15 Mid Miocene deposition (Phu Cu Fm, upper part) 1.0 13.0 14 Mid Miocene deposition (Phu Cu Fm, middle part) 1.0 14.0 13 Mid Miocene deposition (Phu Cu Fm, lower part) 1.0 15.0 12 Early Mid Miocene erosion (Re¯ector C). Stop of sea-¯oor spreading 1.0 16.0 11 Early Miocene deposition (Phong Chau Fm, upper part) 7.5 23.5 10 Late Oligocene minor erosion, ridge jump and change of spreading axis in the East 0.5 24.0 Vietnam Sea 9 Late Oligocene deposition (Thuy Anh/Phong Chau Fm, lower part) 6.0 30.0 8 Early Oligocene erosion (Re¯ector B). Break-up unconformity, onset of sea-¯oor 2.0 32.0 spreading in East Vietnam Sea 7 Late Eocene-Early Oligocene syn-rift deposition (Dinh Cao Fm, 2) 2.5 34.5 6 Late Eocene-Early Oligocene syn-rift deposition (Dinh Cao Fm, 1) 2.5 37.0 5 Late Eocene erosion 1.0 38.0 4 Late Eocene (``Phu Tien Fm, upper part 2'') 1.5 39.5 3 Late Eocene (``Phu Tien Fm, upper part 1'') 1.5 41.0 2 Mid Eocene, pre to early rift deposition (``Phu Tien Fm, lower part''). Mid Eocene 3.0 44.0 onset of sinistral movement on the Song Hong Fault, initiation of the basin 1 ``Pre-Cenozoic'' seismic acoustic basement (Re¯ector A) (Palaeozoic-Mesozoic) 1.0 45.0 age of the base of the Cenozoic basin ®ll is thus arbi- stones, siltstones, mudstones and thin coal beds depos- trarily set at 44 Ma (Table 1), assuming that depo- ited in a mixed alluvial and lacustrine environment. sition began during the initial tectonic phases of the left-lateral rifting which probably started in Mid- Eocene time (44 Ma; Lee and Lawver, 1994). 6.2. End of rifting and start of post-rift sagging phaseÐ The dominantly Upper Eocene±Lower Oligocene model events 8 and 9±19 syn-rift sequence was subdivided into six model events (2±7, Table 1), consisting mainly of the deposition of The rifting phase is interpreted to have ended in the late Early Oligocene, based on the correlation of the conglomerates, sandstones, and shales in alluvial, ¯u- signi®cant unconformity topping the rift-sequence, to vial, lacustrine, and paralic environments. the regional unconformity present in many other Occasionally, seismic data from the lower part of the basins along the margin of South China. A similar in- syn-rift prisms show a distinct unit that subparallels terpretation is presented by Lee and Watkins (1998) the acoustic basement and is unconformably overlain from the Phu Khanh Basin oshore central Vietnam. by onlapping deposits. This unit, which often shows a An age of 32 Ma, corresponding to magnetic anomaly chaotic re¯ection pattern, is interpreted as composed 11, is assigned to this break-up unconformity (Taylor of alluvial deposits. At other places high-amplitude, and Hayes, 1980, 1983; Briais et al., 1993; Lee and parallel and continuous re¯ectors indicate a uniform Lawver, 1994). This age conforms well with biostrati- depositional environment, such as a lake. The syn-rift graphic data from a marine horizon overlying the prisms are occasionally more than 5 km thick. The unconformity in the Song Hong Basin. That horizon is well 107 TPA drilled more than 2.5 km of Eocene± penetrated by the wells 103 TH and 103 TG and is Oligocene syn-rift sediments comprising conglomerates correlated to the Upper Oligocene Chattian NP 25 and sandstones overlain by an unit of alternating sand- zone, corresponding to approximately 27±28 Ma 282 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

(Rangin et al., 1995). In well 112 BT in the southwes- Inversion occurred along some of the half-graben tern part of the basin, biostratigraphic data likewise faults, and signi®cant structures were formed. suggest an Oligocene age for the unconformity (Trung, Concurrently, deposition occurring between the grow- 1997). Although the time represented by the unconfor- ing structures caused a complicated development. mity may be highly variable, even on a local scale, for Toward the central and southern part of the basin, the the modelling it is arbitrarily set at 2 million years prograding units show a pronounced downlapping pat- (Table 1, event 8). tern, and parts of the Middle±Upper Miocene section The unconformity is overlain by a thick post-rift seem to be thinly developed. succession composed of several seismic sequences that mainly contain paralic or deltaic to marginal marine 6.3. Truncation of inversion structures and renewed sandstones, mudstones, and coals along the basin mar- saggingÐmodel events 20±21 and 22±27 gins (Thuy Anh and Phong Chau formations in the Hanoi Trough; Fig. 3), grading into fully marine mud- A very pronounced Late Miocene unconformity deeply stones toward the central and southern part of the truncates palaeo-highs developed over inversion struc- basin (the Da Nang Group). A marked seismic uncon- tures (Figs. 7 and 8; Table 1, events 20 and 21). In the formity in the lower part of the post-rift succession areas around wells 103 TG, 103 TH and 107 TPA above the 27±28 Ma marine horizon probably corre- the unconformity corresponds to the base of the sponds to the unconformity recognised from the Pliocene. Ages of approximately 5.5 Ma and 6 Ma Hanoi Trough between the Thuy Anh Formation/ have been assigned to the surface by Rangin et al. lower part of Phong Chau Formation and the upper (1995) and Morris (1993), respectively. Lateral tracing Phong Chau Formation (Table 1, event 10). The of the unconformity into more conformable succes- unconformity may be related to the reorientation of sions indicates that it occurs within the Upper the spreading in the East Vietnam Sea, which seems to Miocene. However, the surface is clearly of composite have occurred in the Late Oligocene as a result of a nature and re¯ects a long period of erosion over ridge jump (24 Ma, Briais et al., 1993; or 23 Ma, palaeo-highs, while contemporaneous sedimentation Huchon et al., 1994), possibly as a consequence of the (the Dong Hoang Formation) occurred in lows. The collision of the Australian Plate with the Philippine overlying thick and virtually undisturbed Late Sea Plate arc (25 Ma, Hall, 1996). Miocene±Recent sequence comprises interbedded o- The most pronounced unconformity within the post- shore marine to shallow marine and lagoonal mud- rift succession, showing large incised valleys, is dated stones, siltstones and sandstones belonging to the Vinh to the Middle Miocene by biostratigraphic evidence Bao, Hai Duong and Kien Xuong formations (Fig. 3; from the oshore wells and by correlation to onshore Table 1 events 22±27). sections, where it is overlain by the Phu Cu Formation. The unconformity (Table 1, event 12) may be related to the cessation of the spreading activity in 7. Strategy for the modelling of the Song Hong Basin the East Vietnam Sea, which happened at approximately 16 Ma (Briais et al., 1993; Lee and Lawver, The strategy for the modelling is based on the 1994). Morris (1993) assigned an age of 17 Ma to the assumption that basal heat ¯ow development has been correlative surface in the southern part of the Song similar all over the basin, with a gradual increase at Hong Basin, and from the Phu Khanh Basin, Lee and the onset of the rifting phase (Mid Eocene), reaching a Watkins (1998) likewise related their SB 4 to the 16 maximum of 1.2 heat ¯ow units (HFU) during the Ma cessation of the sea-¯oor spreading. Rangin et al. active rifting phase (Early Oligocene). During the sub- (1995) assigned an age of either 16.5 Ma or 15.5 Ma sequent post-rift phases (Late Oligocene±Late to the unconformity, based on the assumption that it Miocene) of relatively uniform subsidence, lower and can be correlated to regressive events of Haq et al. gradually decreasing heat ¯ow values reaching a mini- (1988). However, the unconformity clearly has a tec- mum of 1.0 were used. In order to match present-day tonic component, and precise correlation to regressive temperatures and vitrinite re¯ectance data, a gradual events on an assumed eustatic chart is uncertain, with- increase in heat ¯ow during the latest Miocene± out strong biostratigraphic control. Pleistocene was used (Figs. 11 and 12). The increase is During Middle and Late Miocene times thick pro- supported by temperatures up to 568C at the surface grading deltaic units were formed. In the northern part in several onshore wells, presence of hot springs along of the basin, including the Hanoi Trough, large vari- basin margin faults, and warm water from shallow ations in both thickness and lithology occur due to wells used for ®sh-breeding farms southeast of Hanoi. strong structural in¯uence. The deposits are grouped Dao and Huyen (1995) also suggest high heat ¯ows, into the Phu Cu and Tien Hung formations separated based on calculations using thermal conductivities by a minor unconformity (Table 1, event 16). measured on core samples and temperature gradients L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 283

Fig. 11. Plots displaying the results of the optimisation process for well position LK 200 (onshore) and 103 TG (oshore). Left plots compare measured vitrinite re¯ectance (crosses) with the results of the modelling (®lled dots). Right plots compare modelled and measured temperatures. The temperature plot for LK 200 and the vitrinite plot for 103 TG shows a good ®t between measured and modelled results, whereas the other plots show some discrepancies. Note that the hydrocarbon generation zones on the vitrinite re¯ectance plots only refer to general guidelines, whereas the zonations shown on the subsidence plots (Fig. 12) are those used by the modelling programme. Emphasised layers are the model events 6, 11, 14 and 15. 284 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

Fig. 12. Plots showing subsidence history and development with time of hydrocarbon generation zones at the position of well LK 200 and 103 TG. Model event 1 began to subside at 45 Ma and is at present day at approximately 5000 m and 6400 m depths, respectively. The subsidence history of the four selected model events (6, 11, 14 and 15 corresponding to Dinh Cao Fm, 1; Phong Chau Fm, upper; Phu Cu Fm, middle; and Phu Cu Fm, upper) with possible source rocks are marked with thick stippled lines. The hydrocarbon generation zones are indicated according to the legend to the left. The used heat ¯ow history is annotated (relative to 1 heat ¯ow unit) in the lower part of the ®gures. L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 285 from wells. Likewise, Chen et al. (1998) suggested a umn, from the basement to the base of the drilled high heat ¯ow in the last stage of the basin develop- sections, is incorporated in the modelling by using the ment. information from neighbouring well sections and by The late Cenozoic±Recent high heat ¯ows may be the interpretation of the general basin history. Depth related to the volcanic activity in the region and the to basement is estimated from the top basement map suggested presence of a mantle plume beneath of Dung et al. (1995). The thicknesses of oshore units Indochina (Ru and Pigott, 1986; Bat et al., 1994; are based on interpretations of well-logs and seismic Flower and Hoang, 1994). Thick basaltic volcanic pro®les. rocks and intrusives of Late Miocene±Pliocene age In general, the amount of erosion decreases from occur in the Quang Ngai Graben at the southernmost the northwestern Hanoi Trough toward the southeast tip of the Song Hong Basin (Morris, 1993) and are along the length of the basin, and the section shows a clearly shown by magnetic anomaly data. Young oli- general increase in thickness. This trend is ac- vine basalts crop out on Con Co Island in the south- companied by a general change of lithology toward western part of block 112, and Pliocene±Pleistocene larger proportions of marine mudstones and fewer basalts are known from outcrops close to the 17th sandstones towards the southeast. Coal beds are com- Parallel (in Phy Quy District, Khe Sanh, and Cua mon in the central Hanoi Trough and in the 102 CQ Tung) and from the west Nghe An province approxi- and 102 HD wells, whereas coals disappear toward the mately 200 km south of Hanoi (Fig. 2) (Ru and margins of the Hanoi Trough and toward the central Pigott, 1986; Tien et al., 1991; Bao et al., 1994). part of the basin. Likewise, 5±13 Ma basalts are present along the Song Hong Fault northwest of Hanoi (Phan Truong Thi, personal communication, 1996). 8. Discussion of the modelling results Quantitative data on palaeo-surface temperatures such as oxygen isotopes, are not available from the The 1D forward model used in this study calculates Song Hong Basin. Thus, the estimates on palaeo-sur- amounts of hydrocarbons available for expulsion face temperatures are based on general palaeoclimatic through time. The model calculates values in the past models and palaeolatitude provided by Habicht (1979) as a function of the input parameters. Thus, optimis- and Scotese et al. (1988), and the interpreted palaeo- ation is of fundamental importance for calculation of bathymetry. For model events 1±3 that represent a the hydrocarbon generation history, especially of those continental pre- to early-rift phase, a surface tempera- parameters that aect the thermal maturity calcu- ture of 258C is used. For the remaining events, that lations. The single most critical parameter for the mostly represent periods with marine to deltaic sedi- modelling of the hydrocarbon generation has been the mentation, an average surface temperature of 208Cis heat ¯ow history. However, as a given layer has used. However, in order to match the present-day high reached its highest temperature in recent times in most temperatures that are measured at very shallow depths cases, temperature log data and bottom hole tempera- in the onshore area, the surface temperature for the tures provide good constraints for the recent part of youngest model event is typically given a value of the temperature history. In order to optimise the mod- 308C. In some cases, a higher or lower temperature is elling, all available measurements of vitrinite re¯ec- chosen to ®t the actual measurements in wells, where tance and subsurface temperatures were used. they are considered to represent true formation tem- Relatively few vitrinite re¯ectance data were available peratures. For the youngest model event in oshore from the wells in the Hanoi Trough, but detailed tem- and pseudo-wells, a value of 28.58C is used as based perature pro®les, measured long after (up to two on measurements from 102 CQ and 102 HD. years) the termination of the drilling activities, sup- Calculated sea-bottom temperatures based on oxygen plemented the optimisation (e.g. LK 200, Fig. 11, isotope ratios from the Qiongdongnan Basin indicate a upper part). From the oshore wells, vitrinite re¯ec- change from 208C in the Oligocene to 308C in the tance data were more numerous and reliable, while Quaternary (Chen et al., 1998). temperature data were more scarce (Fig. 11, lower The lithologies assigned to the model events are part). Vitrinite re¯ectance mainly records maximum based on in-house descriptions from onshore wells by temperatures and therefore does not constrain the the Vietnam Petroleum Institute, evaluation of well- palaeo-heat ¯ow very well. However, vitrinite re¯ec- logs and interpretations of lateral variation of the tance data from Late Miocene inversion structures depositional facies. Thicknesses and amounts of ero- gives some constraints on the preinversion heat ¯ow sion of the units in the Hanoi Trough are primarily history. Suppression of vitrinite re¯ectance in the based on lithostratigraphic tops picked by the Vietnam southern part of the basin has been suggested by e.g. Petroleum Institute, and on interpretation of cross-sec- Hao et al. (1995) due to overpressuring. However, tions prepared by Hoai (1985). The stratigraphic col- overpressure seems not to be signi®cant in wells from 286 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 the northern part of the basin, but may increase in (Fig. 16, left). In the shallow part of the trough, event importance toward the deep southern part of the 6 is in the condensate to oil window. In the oshore basin. areas at the well locations 103 TG, 103 TH, 102 CQ The modelling was conducted for type II and III kero- and 102 HD, the top of event 6 passed out of the oil gen (Nielsen and Dien, 1997). Comparison of the model- window between 15±17 Ma and is currently gas gener- ling of type II and III for the Hanoi Trough shows that ating, to overmature. Thus oil generation stopped the main dierence is the time of onset of early oil gener- before the Late Miocene inversion structures were ation. This time dierence is, however, negligible, as developed and ready to trap hydrocarbons. The lack compared to the overall uncertainty in onset time, re- of proven commercial amounts of hydrocarbons by lated to uncertainties of the other input parameters. these four exploration wells drilled on inversion struc- Thus, emphasis was placed on type III kerogen as this is tures is therefore in accordance with the results of the the dominating organic matter in the basin (Hao et al., modelling. Farther to the south and centrally in the 1995, 1998; Chen et al., 1998; Ha, 1998). basin, model event 6 reached the main oil window at Scenarios with a variety of heat ¯ow histories were approximately 34 Ma and was already overmature at calculated to investigate which heat ¯ow history gave approximately 25 Ma. Towards the northeast, model the best ®t. It appears that a scenario with a relatively event 6 is currently ranging from immature to the low heat ¯ow of 1.2 HFU during the Late Eocene± beginning of gas generation (Fig. 16, left). Early Oligocene rifting phase and a stable low heat Lower Miocene coal beds with high HI and S2 ¯ow of 1 HFU during the main part of the post-rifting (model event 11) occur in well sections in the Hanoi phase, followed by a slight increase in late Cenozoic Trough and in well 103 TG. The coal beds disappear times, shows the best ®t with the available control toward the intermediate and central parts of the Song points. The values in this heat ¯ow history are rela- Hong Basin. However, in the deeper part of the basin tively low as compared to typical rift-basins. The contemporaneous marine sediments with a signi®cant reason for this is that the early development of the amount of kerogen type III may occur (e.g., Hao et Song Hong Basin was caused mainly by strike-slip tec- al., 1995). In the Hanoi Trough, event 11 is in a very tonics rather that extensional rifting over a thermal favourable position. It ranges from immature in the dome. In addition, the rapid deposition caused con- western and northern part of the trough (well LK 104 siderable blanketing eects (e.g. Lucazeau and Le and LK 100, Fig. 14), through the main oil to conden- Douaran, 1985). Using this heat ¯ow history, the sub- sate and gas window in the eastern part of the trough sidence history and the development of the hydro- (well LK 102). Event 11 is immature in the northeast- carbon generation zones with time was modelled for ern part of the Song Hong Basin (PW 2, 3, 4, 10; each well and pseudo-well location (Fig. 12). In ad- Fig. 2), and is missing in 107 TPA, PW 5 and PW 6. dition, the present-day positions of the hydrocarbon In the oshore areas at wells 102 CQ, 102 HD, 103 zones were modelled (Fig. 13, left part). Examples of TH, 103 TG and PW 1, event 11 ranges from imma- the generation history of model event 6 are shown in ture over main oil to dry gas. In 102 CQ and 102 HD Fig. 13 (right part). A higher heat ¯ow during the rift- hydrocarbons were generated prior to the formation of ing phase would have accelerated the maturation and the Late Miocene inversion structures, conforming caused an earlier generation of hydrocarbons. Early with the lack of signi®cant amounts of hydrocarbon in generation of hydrocarbons would reduce the potential those wells. In the central part of the basin, the base in some areas due to erosion subsequent to the char- of model event 11 reached the main oil window ging of potential traps, such as the inversion struc- between 15±20 Ma, and the top of event 11 passed out tures. of the oil window at 3 Ma. A number of cross-sections combining the hydro- Middle Miocene coal beds (Phu Cu Fm., middle carbon generation zones modelled at well and pseudo- part, model event 14) occur abundantly in the Hanoi well locations were constructed (e.g. Figs. 14 and 15). Trough, but are immature in the main part of the In addition, hydrocarbon generation maps of the three trough (Fig. 16, right). In well LK 101, the base of most important source rocks, the Upper Eocene± model event 14 entered the early oil window at 1 Ma. Lower Oligocene syn-rift lacustrine shales and coals, In well LK 102 and LK 110, the base of model event the Lower Miocene coal beds, and the Middle 14 passed into the main oil window at 2 Ma, and is Miocene coal beds, were constructed to illustrate the currently near to the condensate-oil boundary. The situation at present day and at 5 Ma (Nielsen and main part of the coal beds in these two wells is now Dien, 1997). situated in the main oil window, thus being in a very The modelling results demonstrate that source rocks favourable and actively producing position (Fig. 14). in model event 6 are in a favourable position for hy- In the oshore area at well 102 CQ, the coal beds are drocarbon generation, as they are presently within the immature. In well 102 HD, the base of the model gas window in large parts of the Hanoi Trough event entered the early oil window at 13 Ma, and the L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 287

Fig. 13. Plots showing hydrocarbon generation zones at well positions LK 200 and 103 TG. Left plots display the depths of the generation zones at present day. The positions of the four selected model events, with possible source rocks, are shown along the depth scale to the right for direct comparison with the generation zones. The plots to the right show the generation history of model event 6 (geological time versus degree of organic matter alteration). The time when the base of the event entered the various hydrocarbon zones is indicated by numbers. The black line indicates peak generation. 288 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

Fig. 14. Two pro®les illustrating the results of the modelling in the present-day situation. The upper pro®le approximates a dip section, and the lower pro®le approximates a strike section (for position refer to Fig. 10). Each pro®le shows the distribution of the four selected model events, with possible source rocks, and the distribution of the hydrocarbon generation zones modelled at well and pseudo-well locations (same code as in Figs. 12 and 13). main oil and condensate windows at 8 Ma and 1 Ma, TH, model event 14 is immature. It is missing in 107 respectively. The top of the coals is immature at pre- TPA and seems to be absent or very thinly developed sent time (Fig. 14). After 11 Ma, hydrocarbon gener- in most of the pseudo-wells. ation did not occur, because of the lack of further The coal beds in the upper part of the Phu Cu signi®cant subsidence. In the wells 103 TG and 103 Formation, model event 15, are immature in all the L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 289

Fig. 15. The same two pro®les as shown in Fig. 14, backstripped at 5 Ma to illustrate the situation closely after the formation of the major structural traps formed by Late Miocene inversion tectonics.

onshore wells and the oshore wells 102 CQ, 102 HD 8.1. Possible plays and 103 TH (Fig. 14). Model event 15 is absent in most of the pseudo-wells, and seems to be absent or Late Miocene inversion structures are common in very thin in the central and southern part of the basin the northern part of the Song Hong Basin and to the (Fig. 14, lower part). northeast. In drilled sections these structures contain 290 ..Nesne l ora fAinErhSine 7(99 269±294 (1999) 17 Sciences Earth Asian of Journal / al. et Nielsen L.H.

Fig. 16. Map showing the hydrocarbon generation zones (same code as in Figs. 12 and 13) for top and base of the Dinh Cao Fm, model event 6 (left) and Phu Cu Fm, upper, model event 14 (right) at present day. L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 291 various Oligocene±Miocene reservoir sandstones. The started already in the Miocene. Marine sediments, with structures are sealed by overlying Pliocene transgres- organic matter dominated by kerogen type III, are pre- sive marine claystones, and form good structural traps. sent in the model event in the deep and central part of However, the timing of the hydrocarbon generation is basin (e.g. Hao et al., 1995). In this area, peak oil gen- critical, because migration and trapping should occur eration began after the formation of stratigraphic after the deeply truncating Upper Miocene unconfor- traps, such as lowstand deltas and incised ¯uvial-estu- mity was draped by a suciently thick and sealing arine sandstones, overlying the pronounced near-the- layer of Pliocene claystones. Due to the relatively slow base Middle Miocene unconformity. In addition, the subsidence in the northeastern part of the study area hydrocarbon generation occurred contemporaneously since Late Oligocene time, the precise time of hydro- with the formation of traps with Upper Miocene± carbon generation is highly dependent on the heat ¯ow Lower Pliocene mud-draped submarine fans and basin history, and the position of the potential source rocks ¯oor turbidites. Generated hydrocarbons may further- within the tilted, syn-rift sequences. more have migrated updip and into reservoirs at shal- The modelling indicates that potential source rocks in lower depths along the basin margin. model event 6 may have generated hydrocarbons after The excellent Middle Miocene coal beds in model Late Miocene time at locations PW 2, PW 3, PW 4, event 14 may have produced signi®cant amounts of PW 5 and PW 10 (Fig. 10), although the generation oil, gas, and condensate within the last few million began earlier in the Miocene. This means that structural years in the deepest part of the Hanoi Trough. It is traps may have been in a favourable position in these thus likely that the coal beds are the main source of areas for trapping hydrocarbons produced after Late the hydrocarbons encountered in the onshore wells. Miocene time. In the deeper and central part of basin Hydrocarbons may have migrated updip and into (e.g. at PW 14±17), the source rocks were mature reservoirs at shallower depths, and may account for already at approximately 25 Ma, and potential traps the hydrocarbons in the B10-STB and D14 wells would have had to have been in place at that time, in recently drilled by Anzoil. In the main part of the order to accumulate hydrocarbons generated from Song Hong Basin coal beds are unlikely to occur at model event 6. However, oil in a reservoir from this time this stratigraphic level. would have cracked to condensate or gas at their present depths, and it seems unlikely that a gas accumulation in the deeper part of the basin would have remained eec- tively sealed until present day. Generated hydrocarbons 9. Conclusions may, however have migrated updip and into reservoirs at shallower depths along the basin margin. Potential Modelling of the Song Hong Basin has been carried reservoirs could be draped, fractured basement out using all available geological, geophysical, tem- rocks, alluvial and submarine fan sandstones, shallow perature and maturity data. The optimised results have marine sandstones, and carbonate build-ups. This been used to estimate the depth of hydrocarbon gener- play was in part tested with some success in block 112. ation zones, and to calculate the timing of hydro- In the southeastern part of the area, detailed geo- carbon generation for intervals that are expected to chemical analyses from the Yacheng gas ®eld indicate contain the principal source rocks. The modelling that Oligocene coal-bearing strata, with type III kero- results are in good agreement with the exploration his- gen in the Song Hong Basin (corresponding to model tory of the basin, and indicate several areas for further event 7 or 9), in part charged the ®eld (Chen et al., exploration activities. These are primarily the Hanoi 1998; Hao et al., 1998). Peak gas generation occurred Trough, the northern and northeastern part of the o- probably in Pliocene time, and the major basin bound- shore basin, and the western basin margin. The model- ing fault, the southern extension of the Song Lo Fault ling shows that plays with Eocene(?)±Oligocene, syn- (also called No. 1 fault) functioned as a conduit for rift lacustrine source rocks (shales and coals) are most gas migration into the ®eld. promising in the northeastern part of the basin, and The coal beds in the Lower Miocene section (model along the basin margins. Plays sourced from Lower event 11) in the central part of the Hanoi Trough may Miocene coals are most promising in the central part have generated both oil and gas after the formation of of the Hanoi Trough and the area southeast of the the Late Miocene inversion structures. The gas, con- Hanoi Trough. Contemporary marine deposits in the densate and oil encountered in wells from this area central and southern part of the Song Hong Basin may thus have been produced from these coal beds. may be possible sources for several play types in these Model event 11 does not have potential in the areas of areas. Coals from the middle part of the Phu Cu the oshore wells 103 TG and 103 TH, whereas some Formation are very interesting source rocks, for both potential for gas should be present at the wells 102 CQ oil and gas, in the central and south-eastern part of and 102 HD, although gas generation may have the Hanoi Trough. 292 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294

Based on the occurrence of potential source rock Conference on Vietnam Petroleum Institute 20 years Development units, reservoirs and structures in the Song Hong and Prospects, Hanoi, pp. 273±284. Basin, and occurrences of hydrocarbon accumulations Asian, Oil, Gas, 1996. Yacheng 13-1: The making of a gas business. Asian Oil & Gas, 8±12. in the adjacent basins, a large number of plays may be Bao, N.X., Luong, T.D., Trung, H., 1994. Explanatory note to the proposed. The plays include among others: fractured geological map of Vietnam on 1:500,000 scale. Geological Survey basement highs of Devonian±Permian carbonates; of Vietnam. Late Miocene inversion structures with Upper Bat, D., Quynh, P.H., Que, P.H., Dong, T.L., 1993. Tertiary stratigra- Oligocene and Miocene ¯uvial, coastal and marine phy of the continental shelf of Vietnam. In: Proceedings of the First International Seminar on the Stratigraphy of the Southern Shelf of sandstones; stratigraphic pinch-out of various Upper Vietnam, Ho Chi Minh City, Dalat. Oligocene±Miocene sandstones; Miocene carbonate Bat, D.V., Tan, M.T., Toat, D.D., 1994. Cenozoic volcanic activities in build-ups; Pliocene turbidite sand over mud-diapirs; Vietnam. In: International Symposium/Workshop on Geology, and mudstone-draped basin-¯oor fans. In addition, Exploration and Development Potential of Energy and Mineral various types of lowstand depositional units with sand- Resources of Vietnam and Adjoining Regions. Abstracts, Hanoi, stones, such as mud-encased lowstand deltas and Vietnam, p. 59. Briais, A., Patriat, P., Tapponnier, P., 1993. Updated interpretation of incised valley-®lls, are likely to form stratigraphic magnetic anomalies and sea ¯oor spreading stages in the East traps, especially within the Miocene section. 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