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

Our organic geochemical study of oils from the northern Qaidam basin define s a family of genetically related oils that contain biomarkers indicative o f source rocks deposited in Tertiary hypersaline, anoxic lacustrine setting s. Although Cenozoic outcrop samples from northern Qaidam are too organic l ean to be of source quality, dark laminated upper Oligocene mudstones conta ining gypsum crystals and pyrite from the Shi 28 well yield total organic c arbon (TOC] and Rock-Eval data indicative of fair to good source rocks. Org anic matter is derived from algae and bacteria and there apparently was lit tle contribution from terrestrial material. Biomarker data provide a good c orrelation between the produced oils and the upper Oligocene Shi 28 core sa mples. Hydrocarbons derived from these source rocks are contained in upper Oligocene, Miocene, and Pliocene reservoirs. Although eight of the oil samp les 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 lacustrine s ource rocks. This sample thus indicates the presence of an unidentified and undocumented source rock in the northeast part of the basin. Hypersaline o ils and the associated source rocks have low biomarker maturity parameters. Thermal modeling indicates that hydrocarbon generation probably occurred i n northwestern Qaidam within the last 3 million years.