Abstract

Wells located on the margins of the Bengkalis Trough depocentre have encountered Eocene-Oligocene Pematang Group lacustrine mudstones ('Brown Shale') in a shallow, thermally immature sub-basin. The Brown Shale stratigraphic interval has yet to be penetrated in a mature setting within the Bengkalis Trough but oil-source correlation strongly suggests that similar lacustrine sediments in the Bengkalis Trough arc the main source of the oil found in the area.

Cores have provided new data on the variations in the depositional setting, palynofacies and geochemical characteristics of the Brown Shale. Relative 'deep' and 'shallow' lake and marginal lake sediments have been encountered and all have characteristic palyno- and organo- facies. Deposition is envisaged to have occurred within a closed basin in which predominantly freshwater shallow lakes with anoxic bottom conditions were established. Repeated periods of exposure, soil development and weathering, and possible minor salinity fluctuations are interpreted to be associated with periods of lake contraction.

The anomalous low velocity and density of the Brown Shale formation interval causes a distinct seismic response which has been used to map the extent (but not the facies or richness) of the source interval away from the wells.

Both the 'deep' and 'shallow' lacustrine lithofacies have good to excellent oil-prone source potential whereas the lake margin' lithofacies has little or none. The oils in the area are best correlated with the algal-rich 'shallow' lake sediments.

The data demonstrate that small shallow Pematang lake systems have the potential to accumulate rich oil-prone source rocks and consequently the distribution of source rocks in the basin is not restricted to the major graben areas in which deeper, more extensive, lake systems were established.