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Deepwater Petroleum Systems of the Southern Basin, North Lombok Sea, Indonesia
The Southern Basin is an inverted Early Tertiary half-graben located in deep water of the North Lombok Sea, 125 km northeast the island of Bali. This east to west trending depocentre 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, one drilled by BP in 1998, and one drilled by GULF RESOURCES in 2000.
Well samples indicate that the Southern Basin has excellent source rock potential. Source rocks are principally Middle Eocene lacustrine shale, coals and coaly shales of the Lower Ngimbang Formation. 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 around 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, 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 that 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 these closures, indicating small associated gas caps.
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