Abstract

The Khmer Trough is located in the eastern part of the Gulf of Thailand. Oil and gas have been discovered by exploration wells drilled in this area. The understanding of the petroleum system has to be a key for future exploration and development.

Oils in the Khmer Trough are characterized by high wax content, low Pr/Ph ratio, negative canonical value of carbon isotope, low content of oleanane and gammacerane. We also found that a plot of Ts/Tm vs oleanane-index is useful to distinguish algal lacustrine oils from terrestrial fluvio-deltaic oils in Southeast Asia. The results of this plot and the aforementioned parameters are interpreted to indicate that these oils were generated from a lacustrine source rock.

Geochemical analyses on rocks indicated that good lacustrine source rock exists in the upper part of Oligocene section and their potential appears to increase towards the trough center. This kind of distribution can be explained by the analogue of modern Lake Tanganyika. It is suggested that the rift activity was enhanced during the late Oligocene age, so that the rate of subsidence exceeded the rate of sediment supply, which resulted in deep basin development.

Vitrinite reflectance in the Khmer Trough demonstrated relatively lower values compared to the high geothermal gradient, as seen in other regions of Southeast Asia. But FAMM analysis revealed that these values are suppressed. The optimization with temperature data and corrected vitrinite reflectance suggested that the basement heat flow is constant and as high as 1.6 HFU.

Finally, two-dimensional basin modelling was conducted by JNOC's SIGMA-2D to understand generation, migration and accumulation of oil and gas. Oil generation from the upper Oligocene source rock has been initiated from late Miocene times. Gas generation occurred in the Pliocene. The direction of migration is mainly vertical due to disconnection of fluvial sands. Gas could migrate to near surface even without faults and fractures because of lower density and viscosity. The source rock distribution and the vertical nature of the migration resulted in oil and gas accumulation only within the trough area.