Distribution and geological significance of 17α(H)-diahopanes from different hydrocarbon source rocks of Yanchang Formation in Ordos Basin

Based on GC-MS testing data of many saturated hydrocarbon samples, 17α(H)-C 30 diahopanes (C 30 * ) are extensively distributed in the lacustrine hydrocarbon source rocks of the Yanchang Formation in Ordos Basin, but show remarkable differences in relative abundance among various source rocks. Generally, Chang 7 high-quality source rock (oil shale) developed in deep lake anoxic environment shows lower C 30 * content, whereas Chang 6–9 dark mudstone developed in shallow to semi-deep lake, sub-oxidizing environment shows relatively high to high C 30 * value. Particularly, Chang 7 and Chang 9 black mudstones in Zhidan region in the northeast of the lake basin show extremely high C 30 * value. A comparative analysis was made based on lithology, organic types and various geochemical parameters indicative of redox environment, and the results indicate that environmental factors such as redox settings and lithology are key factors that control the C 30 * relative abundance, while organic types and maturity may be minor factors. High to extremely high C 30 * values are indicative of sub-oxidizing environment of fresh-brackish water and shallow to semi-deep lake. Therefore, research on C 30 * relative content and distribution in lacustrine hydrocarbon source rocks in the Yanchang Formation, especially on the difference in C 30 * between Chang 7 high-quality source rocks (oil shale) and Chang 6–9 1 source rocks (dark mudstone), will provide an important approach for classification of Mesozoic lacustrine crudes and detailed oil-source correlation in the basin.