Source controls on geochemical characteristics of crude oils from the Qionghai Uplift in the western Pearl River Mouth Basin, offshore South China Sea

The Qionghai Uplift is a sub-uplift tectonic unit in the western Pearl River Mouth Basin. Though lacking source rocks itself, it adjoins the Wenchang A sag, the Wenchang B sag and the Qionghai sag which contain significant source rocks. For the special geological background, the Qionghai Uplift has become an important accumulation area for oil and gas, and the oils in the uplift show significant variation in important geochemical parameters. In this study, the geochemistry of oils in the Qionghai Uplift oil pools has been analyzed systematically, revealing geographic differences. Three groups of oils were identified in the northern part of the Qionghai Uplift: Group I oils, represented by the WC8-3E oil pool located in the northeast; group II oils in the middle represented by the WC13-1 oil pool, and group III oils in the southwest represented by the QH18-1 oil pool. According to oil-source correlations, the oils of the three groups have mixed geochemical characteristics between typical oils sourced from the Enping Formation in the Wenchang A sag, and those sourced from the Wenchang Formation in the Wenchang B sag. We show that based on knowledge of hydrocarbon generation and evolution of source rocks around the Qionghai Uplift, as well as the formative features of traps within the uplift, the geochemical characteristics of the oils are mainly controlled by the distribution of mature hydrocarbon kitchens surrounding the Uplift. Oils in the northeast of the northern Qionghai Uplift were mainly sourced from the Enping Formation in the Wenchang A sag with a secondary contribution from the Wenchang Formation in the Qionghai sag. Oils in the middle section are of mixed origin from the Enping Formation in the Wenchang A sag and the Wenchang Formation in the Wenchang B sag. Oils in the southwest part of the northern Qionghai Uplift were mainly generated from the Wenchang Formation in the Wenchang B sag. This research provides guidance for future exploration of the Qionghai Uplift.

► Geochemistry of oils in the uplift presents regional differences from northeast to southwest.
► Three groups of oils were identified and all have mixed geochemical characteristics.
► Source rocks in three sags surrounding the uplift have different thermal maturation.
► Mature hydrocarbon kitchens and migration pathways control formation of oil pools.