Abstract

High-resolution geochemistry shows genetic relationships among 27 crude oils from eastern Indonesia and suggests the ages and paleoenvironments of their source rocks. Oils inferred to originate from Tertiary and Triassic-Jurassic source rocks in the study occur north and south of 2 degrees S latitude, respectively. Twenty oils mainly from Irian Jaya and Sulawesi originated from Tertiary marine marlstone source rocks that contained type II/III kerogen deposited under suboxic conditions, probably the upper Miocene Klasafet Formation. These low-sulfur oils show high oleanane, C 26 24-nordiacholestane, and pristane/phytane ratios, and 13 C-rich carbon isotope compositions. High oleanane and 24-nordiacholestane ratios are diagnostic of Tertiary oils and source rocks. These oils account for about 16% of the estimated ultimate recoverable reserves in eastern Indonesia. Five oils from Seram originated from Triassic-Jurassic marine carbonate source rock that contained type II kerogen deposited under anoxic conditions. These high-sulfur oils lack oleanane and generally show low C 26 24-nordiacholestane and pristane/phytane ratios. Low-sulfur Aliambata seep oil from Timor originated from type II/III kerogen in a more oxic, terrigenous-influenced marine clastic equivalent of this carbonate source rock. These six oils account for only about 2% of the estimated ultimate recovery in the area. Low-sulfur shallow oil from Miocene Kais reservoirs in the Wiriagar field in Irian Jaya lacks oleanane, is highly mature, and has undergone extensive migration fractionation. This oil probably originated from synrift-postrift Lower-Middle Jurassic Kembelangan Formation clay-rich shales that contained mixed type II/III kerogen deposited under oxic conditions. The huge gas reserves from the deep Wiriagar field were not analyzed but probably originated from Paleozoic source rocks. The shallow Wiriagar accumulation accounts for about 5% and the deep Wiriagar accumulation accounts for about 77% of the estimated ultimate recovery in eastern Indonesia.

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