Geochemical evidence for mudstone as the possible major oil source rock in the Jurassic Turpan Basin, Northwest China

Geologists and geochemists have debated whether hydrocarbons from Jurassic coal measures are derived from the mudstones or the coals themselves. This paper identifies mudstones as the possible major source rock of hydrocarbons in the Jurassic basins in Northwest China. The Turpan Basin is used as a representative model. Mudstones in the Middle-Lower Jurassic are very well developed in the basin and have an average genetic potential from 2 to 4 mg/g. The vitrinite reflectance of the source rocks ranges from 0.6 to 1.3%, exhibiting sufficient thermal maturity to generate oil and gas. Biomarkers in crude oils from the basin are similar to those in mudstones from the coal-bearing strata, with a low tricyclic terpane (cheilanthane) content, a relatively high content of low carbon number (less than C-22) tricyclic terpanes and a low content of high carbon number tricyclic terpanes, relatively high ratios of T-s/T-m, and C-29 T-s/17 alpha (H)-C-29 norhopane, and low ratios of T-m/17 alpha (H)-C-30 hopane and 17 alpha (H)-C-31 homohopane/17 alpha (H)-C-30 hopane. These characteristics and the distribution of steranes and terpanes in the crude oil and mudstone differ significantly from those of the Jurassic coals and carbonaceous shales of the basin, indicating mudstone is possibly the major source rock of the oils in the Turpan Basin. (C) 2001 Elsevier Science Ltd. All rights reserved.