Publications

Migration of oil and gas from a source to a reservoir is a process which can be assessed quantitatively only with great difficulties. Geochemical methods allow to measure properties in source rocks, oils and gases which change with maturation of organic matter. Migration distances can therefore be estimated if maturity dependent properties in oils and gases are compared to those of their source.The Mahakam Delta provides an excellent geologic setting to apply this concept. Detailed studies in Field Nilam show humic organic matter of type I11 to uniformly increase in carbonization with depth from immature at 2,000 m to mature at 4,000 m depth.Biomarker and isotope properties of extracts from cuttings also show systematic changes with depth. Comparison of reservoired oils and mature rock extracts indicates that oils were derived from humic organic matter at a depth of approximately 3,500 - 4,000 m.Isotope analyses on cutting gases in this well indicate that gases collected in the immature section around 2,000 m depth have the signature of mature to overmature gases, which must have formed at considerably greater depth. Extrapolation based on uniform increase of maturity with depth predicts that these gases were formed at depths of between 5.000 and 6.000 m.The study reveals that gases and liquid hydrocarbons are generated at various depths. It is conceivable that migrating gases from greater depth were instrumental to migration of oils, This is in support of the general hypothesis that methane plays a key role in migration of petroleum in Cenozoic delta systems.