Comparison of Depositional Environments and Reservoir Characterization of “O-sands” in Pailin and “Thick stacked sands” in Benchamas Fields, Pattani Basin, Gulf of Thailand

Abstract

Conventional cores from the “Massive O-sands” in Pailin-07 and the “thick stacked sands” of Benchamas Field in BE-01, BE-03 and B-08 were observed to study the depositional environment and reservoir characteristics. The Benchamas cores are fining-upward channel-fill sandstones and floodplain/overbank mudstones deposited on an alluvial plain by a meandering river. All the sands in the Pailin-07 core are single-storey channel sands that fine upward but they have highly variable thicknesses over short stratigraphic intervals and tidal sediments form the transition to overlying mudstones, indicating increasing marine influence upward. The Pailin sands are interpreted as lowstand fluvial channels that were back-filled during a subsequent relative sea level rise. Despite the different depositional environments, wireline log signatures through the cored intervals in the two areas are virtually identical, with sands in both areas generating blocky to bell-shaped gamma ray curves. This suggests that log signatures are not always reliable indicators of depositional environment, which has important implications for prediction of sand body geometry and reservoir properties. In the Gulf of Thailand post-rift, there are many other areas and stratigraphic intervals that may have back-filled channels. It is possible in any area near a paleoshoreline or stratigraphically near the transition to or from a marine stratigraphic unit.