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ABSTRACT: Source and migration in the Makassar-Mahakam deep water petroleum system, East Kalimantan, Indonesia

Lin, Rui1, Hans F. Schwing2, and John Decker2
(1) Unocal Thailand Ltd, Bangkok, Thailand
(2) Unocal Indonesia Ltd, Balikpapan, Indonesia

The Mahakam delta area (Indonesia) represents a significant petroleum province where several giant oil and gas fields were discovered. Until recently, discoveries were limited to the onshore and shallow water, and the deep-water Makassar Strait remained largely unexplored. Early industry models were concerned that the deep-water area may lack organic source richness, sand and overburden (maturity) to form viable petroleum systems.

Recent oil/gas discoveries at the Merah Besar and Seno fields, however, testify to the existence of operating petroleum systems in deep water. Oil analyses suggest that these hydrocarbons were sourced largely from land-plant (type III/II) organic matter (which is prone to generating gas-rich volatile oils), though a gradual reduction in resin input outboard suggests enhanced marine influence. Source analyses of samples from Pliocene and Miocene detected abundant land-plant organic matter in coaly laminae, shales, hemipelagic mudstones and as disseminated particulates in turbidite sands (with TOCs ranging from 1 to >50%). It is believed that the rapid transport and settlement of turbidites from the low-stand delta and other coastal deposits allow the preservation of the allochthonous organic matter. Additional sources may be assumed in the underlying Oligocene and Eocene strata.

Geochemical analyses and thermal modeling support that hydrocarbons were generated from mature sources (most notably mid- and lower-Miocene) underlying the reservoirs. The fluids migrated vertically via faults and turbidite sands and become entrapped in the overlying upper Miocene and Pliocene reservoirs. Migration fractionation is responsible for segregating the gas zones from the oil zones, whereas gas leakage via faults further facilitated the relative enrichment of oils.

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