Geochemical characteristics and origin of petroleum in the southwest complicated fault subsag, Dongpu Depression of China's Bohai Bay Basin

Abstract

The Dongpu Depression is one of the major petroliferous structures in the Bohai Bay Basin of eastern China. The Shahejie Formation is the key target unit for petroleum exploration in the Dongpu Depression. The current studies on this fossiliferous structure mainly focus on the northern depression, and the detailed study on the southern part of the depression is still scarce. To further study on the southeast subsag located in the southern part of this depression, we examined the geochemical characteristics and origin of petroleum using stable carbon isotopes, components of crude oil and natural gas, micro Fourier transform infrared spectroscopy (FT‐IS), gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). Fluid inclusions and 1D model were employed to reconstruct the burial and thermal history and indicate the timing of hydrocarbon generation. Finally, the petroleum exploration in this region is guided based on the combination of geochemical results and geological setting. The result of geochemical characteristics of crude oils indicates the oil was from the combination of algal and terrestrial plants, deposited in reducing to sub‐oxic and fresh to marginally brackish environment. Furthermore, the oil in the northeastern part could be traced to the Haitongji subsag and has relatively lower thermal maturity than that in the central part. Stable carbon isotope data suggest that natural gas is a mixed oil‐type and coal‐type gas. The equation estimated indicates that natural gas in the northeast is mainly oil‐type gas, with an average ratio of 80.0%, whereas coal‐type gas is dominant in the central part (average percentage: 74.3%). Geochemical characteristics of hydrocarbon indicate that the Changyuan Fault was a migration pathway for the northeastern petroleum, whereas it gradually changed to be sealed towards the central part.