2023 Volume 88 Issue 1 Pages 14-28
The Cenozoic rift basins in Southeast Asia have high petroleum potential and the petroleum system of each basin has been analyzed and characterized. Many studies revealed the presence of two types of source rocks （lacustrine shale and fluvio-deltaic coal）in the petroleum basins. In the Cenozoic basins in Southeast Asia, it has been recognized such a "dual petroleum systems" that the source rock distribution and kitchen area vary and the timing of oil and gas generation differ which make migration and accumulation processes more complex. Therefore, it is important to identify the effective source rock of oil and gas and well understand the petroleum system working in the area in exploration phase. Geochemical inversion utilizing biomarkers and isotopes is one of the commonly used methods to identify the effective source rock. Thermal maturity of each source rock and timing of oil and gas generation can be evaluated by basin modeling.
We performed geochemical inversion on fluid samples taken from reservoirs of a gas field associated with oil and condensate recently discovered offshore Vietnam. Two oil families（mixed oil derived from both lacustrine and fluvio- deltaic source rocks and purely fluvio-deltaic oil）were identified by biomarkers and diamondoids. Carbon isotope compositions suggested that gas generated from both lacustrine and fluvio-deltaic source rocks mixed in the reservoirs. Fluid inclusion analysis revealed the presence of paleo-oil column in a current gas-condensate reservoir which could be flushed by late-charged gas. Multi-dimensional basin modeling confirmed that mixing of different type of fluids during trapping and the possibility that oil initially charged into the field spilled-out to adjacent structures by late-charged gas after Pleistocene.
The well-understanding of this kind of complex dual petroleum systems will give us new insights on petroleum potential and useful information to develop exploration strategies targeting gaseous hydrocarbon in basins in Southeast Asia.
