ABSTRACT: Deep-water petroleum system of the Southern Basin, East Java Sea, Indonesia

Gadjah E. Pireno and Rachmat Mudjiono , Gulf Indonesia Resources Ltd, Jakarta, Indonesia, [email protected]

The Southern Basin in the East Java Sea is an inverted Early Tertiary half-graben located 125 km northeast the island of Bali. This east-to-west trending depocenter formed early in the Tertiary and filled with a thick accumulation of Paleogene sediments. Neogene compression uplifted basin deeps, followed by regional subsidence in the Quaternary to current water depths of 400 to 1,000 meters. Well control in these waters consists of three wildcats drilled by AMOCO in 1984-1985 and one drilled by BP in 1998.

Source rocks are principally Middle Eocene lacustrine shale, coals and coaly shales of the Lower Ngimbang Formation. Well samples indicate that the Southern Basin has excellent source rock potential. Coals and coaly shales (type II/III) have 8 to 76% TOC, and the lacustrine shale (type I/II) has 1 to 4% TOC. Geohistory modeling of source areas using the present day Geothermal Gradient shows that the onset of oil generation (Ro 0.7) generally occurs at 11,500 feet depth.

Maturation modeling in the deepest part of the basin depocentre indicates that the entire section of Lower Ngimbang source rocks continues generating hydrocarbons at present. Peak oil generation occurred during the Early Oligocene (33 mybp), and around 1,000 feet of the basal sediments above basement have now entered the gas window.

The basement configuration in the Southern Basin defines short and direct routes for hydrocarbon migration from the mature kitchen to the structural highs. Wells drilled on young inverted structures found thick sequences of sand-poor basinal facies. Early structures in the basin formed during the Late Eocene remained continuously high until the present day. These structures have potential for large, liquid hydrocarbon accumulations. Several DHI’s are recognized near the top of the closures, indicating small associated gas caps.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia