The oil accumulation period in the Pinghu slope tectonic belt of the Xihu sag

The Xihu sag is located in the northeastern part of the East China Sea Shelf Basin. It is one of the petroliferous units with good prospects for offshore oil and gas exploration in China, and is dominated by natural gas resources. The geological conditions of the Xihu sag are complex because it is not a continuous subsidence basin, and has experienced multiple uplift-burial processes. Its complicated petroleum accumulation process makes it difficult to accurately determine the age and stage of oil charging. This paper comprehensively analyzed the oil accumulation history of the Pinghu slope tectonic belt in the west of the Xihu sag by combining fluid inclusion temperature measurement with burial history information and geological analogy. The research results show that there are two stages of hydrocarbon inclusions with different colors in the Pinghu Formation reservoir of the western slope belt. The fluorescence color of the first stage hydrocarbon inclusions is pale yellow to green-yellow, and the trapping temperatures are between 157 ℃ and 173 ℃. The other stage is blue-white fluorescent hydrocarbon inclusions with trapping temperatures of between 169 ℃ and 191 ℃. Combined with the simulation results of the burial history, it was determined that there were two stages of oil accumulation in the Pinghu slope belt. The first stage of oil accumulation is about 15 Ma to 13 Ma, and the second stage of oil accumulation is about 2 Ma to 0 Ma. The geological analogy method shows that the source rocks of the Pinghu Formation have entered the hydrocarbon generation threshold and began to supply hydrocarbons since the Miocene (17 Ma to 15 Ma). Although the Longjing movement caused a regional uplift, the burial depth of source rocks has always been below the hydrocarbon generation threshold, and hydrocarbons can be continuously supplied, which is consistent with the results of fluid inclusion studies. The study shows that the periods of hydrocarbons generation and expulsion and tectonic movements of the Pinghu slope belt in the Xihu sag jointly determined that these two stages of oil inclusions with different fluorescence color were preserved in reservoir minerals. The two periods of oil accumulation inferred by the fluid inclusions in the reservoir minerals correspond to the earliest era of crude oil entering the reservoir and the era of large-scale oil charging into the reservoir, respectively. The Longjing movement created favorable conditions for the late oil accumulation. The intensity of the early period oil charging was weak, and the late period was the critical period for the oil accumulation in this area.