Petroleum Prospectivity of the Northeastern Malay Basin, Offshore SW Vietnam

Henrik I. Petersen¹, Lars H. Nielsen¹, Neil Sherwood², Anders Mathiesen¹, Michael B. W. Fyhn³, Nigel Russell², Nguyen T. Dau⁴, and Jørgen A. Bojesen-Koefoed^1
1Geological Survey of Denmark and Greenland (GEUS), Copenhagen K, Denmark
²CSIRO Petroleum, North Ryde, NSW, Australia
³Geological Institute, University of Copenhagen, Copenhagen K, Denmark
⁴Vietnam Petroleum Institute (VPI), Hanoi, Viet Nam

Danish and Vietnamese geoscientists are currently evaluating the hydrocarbon potential of the Vietnamese part of the Malay Basin as a part of the ongoing ENRECA-project. In block 46 in the northeastern Malay Basin several exploration wells have with success tested seismically identified direct hydrocarbon indicators (DHIs). Most oils were probably generated from mature terrestrially-influenced lacustrine source rocks. The DHIs are mainly developed in the post-rift succession overlying narrow grabens. Seismic facies analyses indicate that the syn-rift successions contain fluvial sandstones and lacustrine mudstones. A well-constrained onshore analogue to narrow offshore grabens suggests that such small grabens may contain highly oil-prone lacustrine source rocks. The petroleum discovery well 46-CN-1x penetrated a Cenozoic succession, including Upper Oligocene immature alginite-bearing lacustrine mudstones with HI values up to >400. Despite the presence of cavings and bitumens in most samples reliable maturity trends were established by integrated application of vitrinite reflectance (VR) measurements and FAMM analyses. Suppressed VR values were identified by the FAMM technique and omitted from the trends. Based on the established depth-maturity trends the top of the oil window is located between 2750 and 2825 m depth, and modelled temperature histories indicate onset of oil generation from the Upper Oligocene mudstones between ~2 Ma and present-day. However, a source kitchen with well-developed lacustrine facies within the oil window is located in the local basin-depocentre to the NW, and the DHI distributions in block 46 are related to the kitchen areas and to anticlinal structures.

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