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 pro- duction 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- 1367-9120/99 $ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0743-9547(98)00063-4 270 L.H. Nielsen et al. / Journal of Asian Earth Sciences 17 (1999) 269±294 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.