Petroleum generation and migration from Talang Akar coals and shales offshore N.W. Java, Indonesia

The Oligocene Talang Akar formation is the major source for petroleum (oil and gas) in the Ardjuna sub-basin. Source rock quality varies within the deltaic to marginal marine complex, with coals deposited on the lower delta plain being distinctly oil-prone, whereas the surrounding delta plain shales are largely gas prone. Shales and “drifted” coals laid down in marine-influenced interdistributary bay environments show increasingly oil-prone characteristics, possibly due to preferential transport of hydrogen-rich plant components to more distal setting. Molecular characteristics of the coals and shales are similar, with higher plant (angiosperm) biomarkers and their aromatic derivatives occuring widely. Kinetic studies of hydrocarbon generation, and stimulated maturation experiments using hydrous pyrolysis, indicate that the delta plain coals are the major source for crude oils in the basin, with a secondary, but minor contribution from marine-influenced interdistributary bay shales. Delta plain shales are interpreted to expel gaseous products only. The stratigraphic and spatial location of oil and gas fields were examined with respect to their position relative to oil and gas generating source rocks. The field distribution could not be explained in terms of source rock geochemistry alone, and phase behavior of petroleum during secondary migration was shown to exert an important influence on the distribution of oil and gas deposits in the basin.