Reservoir-forming analysis of tight gas in Weibei Sag, Bohai Bay Basin

There were good geological conditions for tight gas reservoirs in the northern subsag of the Weibei Sag, forming the combination of self-generation, self-storage and self-cover in the 2nd member of the Kongdian Formation. Tight gas reservoirs generated in the area deeper than 3 500 m. The natural gas migration patterns under different porosity conditions were analyzed, and the influencing factors for gas saturation were studied with experiments. The relationship between permeability and final gas saturation was defined, and the coefficient was 0.904. Experimental results showed that gas charged early and abnormal high pressure existed during gas accumulation stage. The homogenization temperature of fluid inclusions showed that tight gas reservoirs experienced 2 charging periods. The 1st period was the early Shahejie period (52-50.5 Ma) and the 2nd one was before the Shahejie structural movement (43.5-36 Ma), among which the 2nd period played a more important role. The Shahejie structural movement caused pressure reducing and tight gas reservoirs were destroyed. The deposition of Neogene made reservoirs further compact, which was favorable for tight gas reservoir preservation in the Kongdian Formation. The tight gas reservoirs of the Kongdian Formation in the Weibei Sag were residual gas reservoirs, and still had good exploration prospects.