Hydrocarbon generation and expulsion quantification and contribution of multiple source rocks to hydrocarbon accumulation in Yanchang Formation, Ordos Basin, China

Abstract

There are several sets of source rocks in the Yanchang Formation of Mesozoic in the Ordos Basin. At present, Chang 7 source rock is considered to be the main source rock of the Yanchang Formation, although there are no evaluation methods for hydrocarbon generation and expulsion of other source rocks, as well as the hydrocarbon accumulation contribution of source rocks. Utilizing basin simulation and based on a large number of basic geological data and exploration results, the geological body model and thermal history model are established to carry out simulation research on hydrocarbon generation, expulsion, and accumulation of multi-source layers in the Yanchang Formation under geological constraints. The results show that the conversion rate of hydrocarbon generation of each source rock in the Yanchang Formation is mainly distributed in the range of 45%–75%, indicating that the formation still has significant hydrocarbon generation potential and has the geological conditions for in-situ exploitation of shale oil. At present, accumulative hydrocarbon generation is 123.3 × 10⁹ t while hydrocarbon expulsion is 90 × 10⁹ t, with heavy hydrocarbons of C₁₄₊ dominating. The Early Cretaceous had the highest concentration of hydrocarbon generation and expulsion, accounting for 68%–82% of total hydrocarbon generation and expulsion. The simulation of hydrocarbon accumulation revealed that before the deposition of the Cretaceous, only a limited amount of hydrocarbon charging occurred in each layer of the Yanchang Formation, with the early Cretaceous being the key period of hydrocarbon charging and accumulation. The “near-source accumulation” characterizes the upper and lower hydrocarbon source layers, and the basin simulation results are in good agreement with proven oil reservoirs and predicted oil reservoir distribution ranges. The study proposed for the first time that the “Chang 7 source rock make major contribution to multi-layer reservoir” mode could be modified to “proximal hydrocarbon accumulation from diverse source rock” mode to stimulate shale oil production and potential layer exploration in the Yanchang Formation of Mesozoic in the Ordos Basin.