Abstract

The Barito Basin lies between two Mesozoic features, the Sunda shield to the west, and the Meratus range, a belt of melange and ophiolites, to the east. Sedimentation in the basin was completed in a transgressive-regressive cycle, interrupted by local subcycles and variations.

Hydrocarbons are generated from two types of source rocks: the Tanjung and Lower Warukin Formations consisting of shales and coals. The Tanjung Formation source rocks are characterized by organic-rich shales and claystones (TOC: 0.6 - 5.4 wt%) and thin coal layers (TOC: 43.6 - 65.9 wt%). Kerogen types alternate between vitrinite (III) and amorphous (I/II) organic matter. The maturity of this source rock varies from early mature in the north to over mature near the center of the basin. The Tanjung formation was deposited in a shallow lacustrine environment and generates oil and gas. The Warukin Formation source rocks also consist of claystones and shales (TOC: 1.3 to 4.2 wt%) and coal (TOC: 44.8 - 59.3 wt%) but with the kerogen type dominated by vitrinite III. These source rocks were deposited in a deltaic environment. They have reached maturity towards the basin deep, and are capable of generating gas and oil.

Maturity modelling, using a variety of geochemical data, shows that the initial hydrocarbon generation began in the Middle Miocene from Lower Tanjung source rocks and in the Plio-Pleistocene from Warukin source rocks. Based on the distribution of isotopes (k13C) and hopane/sterane biomarker ratios, two types of oils are recognized: Tanjung oils (lacustrine oil) and Warukin oils (fluviodeltaic oil). The Warukin oil type has undergone biodegradation which is associated with aquifer water flushing at shallow depths and temperatures less than 70°C.

Hydrocarbon migration started in the Late Miocene from Lower Tanjung source rocks, close to the basin center. Migration probably became more effective when the embrionic thrust system formed during Mio-Pliocene time, both horizontally (up dip) and vertically up the fault planes. Migration from Lower Warukin source rocks to Warukin traps is controlled by associated beds of sandstone.