Molecular organic geochemistry of the Tarim basin, northwest China

We conducted organic geochemical analyses on the largest suite of oils and source-rock extracts from the Tarim basin, northwest China, currently available. Statistical cluster analysis of the entire suite of Tarim oils distinguishes at least seven genetic groups of oils. The largest group of oils was collected from the Tazhong and Tabei uplifts and originated from marine Middle-Upper Ordovician anoxic marls that mark slope facies at the margins of structural uplifts. Two other genetic groups most likely originated from marine Middle-Upper Ordovician source rocks, but of distinct facies, with one an oxic shale-rich source west of the Bachu uplift and the other an anoxic shale source at Tazhong. Other genetic oil groups originated from various nonmarine source rocks. The largest of these groups consists of oils from the Luntai uplift, which best correlate with Jurassic lacustrine mudstones in the Kuqa depression, although Triassic lacustrine mudstones cannot be eliminated as a source for these oils. Two oils from southwest Tarim are highly mature. Despite uncertainty due to low biomarker concentrations, these oils probably originated from nonmarine shaly source rocks. The two remaining genetic groups consist of single oil samples: Yi603 (an oil likely derived from coal in the Kuqa depression) and Qu1 (derived from Carboniferous or Jurassic shaly source rock from the west flank of the Bachu uplift). Sample 63KLT (a seep sample from west of Kashi) has attributes of a lacustrine source rock and clusters with oils from Luntai. These results suggest that numerous source rocks occur in the basin, but they likely are areally restricted. Our results do not support previous published work that suggests that hypothesized euxinic source rocks might account for reserves of up to 350 billion bbl of oil.