ABSTRACT – Barito Basin lies between two Mesozoic features, the Sunda shield to the west, and the Meratus range, a belt of melange and ophiolite, to the east. Sedimentation in the basin was completed in a transgressive- regressive cycle, interupted by local subcycles and variations. Hydrocarbons are generated from two types of source rocks: Tanjung and Lower Warukin Formations consisting of shales and coals. Tanjung Formation source rocks are characterized by organic-rich shales and carbonaceous clay (TOC: 0.6 – 5.4 wt%) and thin coal layers (TOC: 43.6 – 65.9 wt%). The maturity of this source rock type varies from early mature in the north to over mature towards the center of the basin, with kerogen types vitrinite(III) and amorphous (1111). Tanjung formation is deposited in a shallow lacustrine environment and generates oil and- gas. gas. The Warukin Formation source rocks consist of claystones and shales (TOC: 1.3 to 4.2 wt%) and coal (TOC: 44.8 – 59.3 wt%) and have reached mature stage, especially towards the basin center. Kerogen type is dominated by vitrinite (III), deposited in a deltaic environment, capable of generating gas and oil. The maturity approach using Lopatin Method shows that the initial hydrocarbons generation occured since Middle Miocene from Lower Tanjung source and during Plio-Pleistocene from Warukin source. Based on distribution of isotope ( 613 C) and hopanelsterane biomarker ratio, two types of oil classes are recognized: Tanjung oil type (lacustrine oil) and Warukin oil type (fluviodeltaic oil). Warukin oil. type has undergone biodegradation which is associated with aquifer water flushing at shallow depths and temperatures less than 70o C. Hydrocarbon migration starts since Middle Miocene from Lower Tanjung source rock. Migration becomes more effective when the embrionic thrust system formed during Mio-Pliocene, both horizontally (up dip) or vertically across the fault planes. Migration from Lower Warukin source rock to Warukin Formation traps is a short distance migration controlled by sandstone layers that are associated (associated beds).
Authors : Sumuyut Sardjono & Lindy Rotinsulu