Geochemistry of crude oils from eastern Indonesia

High-resolution geochemistry shows genetic relationships among 27 crude oil s from eastern Indonesia and suggests the ages and paleoenvironments of the ir source rocks. Oils inferred to originate from Tertiary and Triassic-Jura ssic source rocks in the study occur north and south of 2 degrees S latitud e, respectively. Twenty oils mainly from Irian Jaya and Sulawesi originated from Tertiary marine marlstone source rocks that contained type II/III ker ogen deposited under suboxic conditions, probably the upper Miocene Klasafe t Formation. These low-sulfur oils show high oleanane, C-26, 24-nordiachole stane and pristane/phytane ratios, and C-13-rich carbon isotope composition s. High oleanane and 24-nordiacholestane ratios are diagnostic of Tertiary oils and source rocks. These oils account for about 16% of the estimated ul timate recoverable reserves in eastern Indonesia. Five oils from Seram originated from Triassic-Jurassic marine carbonate sou rce rock that contained type II kerogen deposited under anoxic conditions. These high-sulfur oils lack oleanane and generally show low C-26 24-nordiac holestane and pristane/phytane ratios. Low-sulfur Aliambata seep oil from T imer originated from type II/III kerogen in a more oxic, terrigenous-influe nced marine das tic equivalent of this carbonate source rock. These six oil s 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 i n 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 containe d mixed type II/III kerogen deposited under oxic conditions, The huge gas r eserves from the deep Wiriagar field were not analyzed but probably origina ted 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.