Hydrocarbon charging and accumulation in the Permian reservoir of Wangguantun Buried Hill in Huanghua Depression, Bohai Bay Basin, China

The Permian reservoirs of Wangguantun Buried Hill are deep buried reservoirs in the Bohai Bay Basin, with a depth of more than 4000 m. The superposition of multi-stage tectonic movements also increases the complexity and unclear nature of the hydrocarbon charging and accumulation history. This study aims to address these issues by considering the structural evolution and burial–thermal history of the strata and studying the hydrocarbon expulsion process of source rocks by means of rock pyrolysis experiments. In addition, petrography, fluorescence spectroscopy, GC-MS (gas chromatography–mass spectrometry) of the oil from inclusions, laser Raman spectroscopy, and microthermometry of fluid inclusions were conducted and these analyses were coupled with cathodoluminescence tests of minerals and typical geological profiles to reveal the hydrocarbon charging process of the Permian reservoir in Wangguanshan Buried Hill. The results show that the oil of the Permian reservoirs in Wangguantun Buried Hill is generated from both upper Palaeozoic coal-measure and Paleogene lacustrine mudstone. The corresponding vitrinite reflectance (Ro) values at the expulsion thresholds of the Palaeozoic coal, Palaeozoic carbonaceous mudstone, Palaeozoic mudstone, and Paleogene mudstone are 0.68%, 0.72%, 0.72% and 0.73%, respectively. The Permian reservoirs have experienced three oil and gas charging periods. In the first period, during the late Jurassic to early Cretaceous (135–100 Ma), the reservoirs were charged primarily with less mature oil. The subsequent uplift of the strata in the late Cretaceous exposed the ancient reservoir, and most of the oil transformed into carbonaceous bitumen. In the second period, the middle and late Paleogene (45–33 Ma), the reservoirs were charged primarily with less mature oil from the Paleogene source rocks and mature to highly mature oil and gas from the coal-measure source rocks. In the third period, from the late Neogene to the present (12–0 Ma), the reservoirs were primarily charged with highly mature oil from the Paleogene mudstone and highly mature and overmature oil and gas from the coal-measure. Fractures provide the main storage space for hydrocarbons, while some were stored in intergranular and dissolved pores. Consequently, fractured reservoirs should be important targets for gas and petroleum exploration in such deep reservoirs. In addition, the regional structure evolution controlled the hydrocarbon expulsion and charging processes, which guided the formulation of the hydrocarbon accumulation model of the Permian reservoir.