Upper Oligocene lacustrine source rocks and petroleum systems of the northern Qaidam basin, northwest China

摘要

Our organic geochemical study of oils from the northern Qaidam basin defines a family of genetically related oils that contain bio- markers indicative of source rocks deposited in Tertiary hypersaline, anoxic lacustrine settings. Although Cenozoic outcrop samples from northern Qaidam are too organic lean to be of source quality I dark laminated upper Oligocene mudstones containing gypsum crystals and pyrite from the Shi 28 well yield total organic carbon ('rOC) and Rock-Eval data indicative of fair to good source rocks. Organic matter is derived from algae and bacteria and there appar- ently was little contribution from terrestrial material. Biomarker data provide a good correlation between the produced oils and the upper Oligocene Shi 28 core samples. Hydrocarbons derived from these source rocks are contained in upper Oligocene, Miocene, and Pliocene reservoirs. Although eight of the oil samples are from the northwest corner of the basin, one sample in this genetic family of hypersaline oils comes from northeast Qaidam, an area previously believed to only produce oils derived from Jurassic freshwater la- custrine source rocks. This sample thus indicates the presence of an unidentified and undocumented source rock in the northeast part of the basin. Hypersaline oils and the associated source rocks have low biomarker maturity parameters. Thermal modeling indicates that hydrocarbon generation probably occurred in northwestern Qaidam within the last 3 million years.