Origins of Jurassic oil reserves in the Turpan-Hami Basin, northwest China: Evidence of admixture from source and thermal maturity

Middle-Lower Jurassic coaly shales were considered to be the source of Jurassic oil reserves in the Turpan-Hami Basin. In this study, we investigated the molecular and stable isotopic compositions of 35 gas samples and the geochemical parameters for 28 oil samples to reexamine the origin of these hydrocarbons. We show that the Jurassic natural gas reserves are derived mainly from coaly source rocks in the Middle Jurassic Xishanyao and Lower Jurassic Badaowan formations with maturities varying from 0.6% to 0.9%Ro. However, analysis of the low molecular weight hydrocarbons and diamondoid hydrocarbons shows that the maturities of the oils are clearly higher than those of the Jurassic coal measures and the natural gas generated from them. Their molecular and carbon isotopic compositions indicate that most of the Jurassic oil reserves are derived from pre-Jurassic lacustrine mudstones. The Hongtai gas/condensate field is an exception where the Jurassic coal measures are major contributors of light hydrocarbons. In contrast, the biomarker fingerprints of the Jurassic oils reveal a predominantly humic origin, which is most probably resulted from migration contamination. As such, the Jurassic petroleum system of the Turpan-Hami Basin is a mixed oil system derived from pre-Jurassic lacustrine mudstones and the Middle-Lower Jurassic coaly source rocks. This study opens pre-Jurassic structures in the Turpan-Hami Basin as favorable exploration prospects and illustrates the value of multiple correlation techniques to determine the sources of associated light and heavy hydrocarbons.