Leaves in turbidite sands: The main source of oil and gas in the deep-water Kutei Basin, Indonesia

Abstract

More than 6 tcf of gas and 200 million bbl of oil and condensate have been discovered in upper Miocene sandstones in the deep-water Kutei Basin, offshore East Kalimantan, Indonesia. Chemical analyses of oil, gas, and condensate indicate derivation from land-plant source material. Conventional cores and cuttings of deep-water strata show that the best source rocks are sandstones, not shales. Organic material in deep-water sandstones includes laminar coaly fragments, pieces of wood, resinite, and other coaly debris. Laminar coaly fragments are dominant, and their size, shape, and cuticle structure indicate that they were leaf fragments. Fossil leaf fragments occur in all deep-water sandstone packages from the upper slope to the basin floor. Fossil leaf fragments were apparently carried into deep water by turbidity currents during lowstands of sea level. Within turbidite sandstones, fossil leaf fragments are concentrated in layers with up to 50 wt.% total organic carbon (TOC). Kerogen that was separated from these layers has hydrogen index values of 200–500 mg hydrocarbon/g TOC. Kutei Basin deep-water shales contain mainly silt-size vitrinite grains with poor generative qualities. Microscale sealed vessel pyrolysis and kinetic and generation modeling indicate that land-plant kerogens occurring in quantities similar to the upper Miocene sandstones are capable of generating the gas and oil found in deep-water fields. Hence, leaf fragments in turbidite sandstones are interpreted as the main source for deep-water oil and gas. Liquids derived from leaves have caused production problems that include high pour points because of high wax contents and emulsions formed when fatty acids combined with water.