Abstract
•Resistivity curve is one of the methods to identify shale maturity.•The Emeishan basalt leads to the shale reservoir deterioration of Longmaxi formation in northern Yunnan and Guizhou.•The Eemeishan basalt brought forward the generation time of source rocks to the end of Permian.
The eruption or invasion of the Emeishan basalt has a systematic impact on the maturity of shale organic matters, hydrocarbon-generation evolution, reservoir physical properties and gas-bearing properties of shale, which is directly related to the preservation and accumulation of shale gas. By taking four typical shale gas wells in northern Yunnan and Guizhou provinces of China as examples, the influence of the basalt eruption on shale gas enrichment of Wufeng-Longmaxi formations is analyzed. The results show that: The shale near the basalt eruption area reached the late stage of over-mature ahead of time, with the equivalent Ro greater than 3.0%. Close to the basalt eruption zone, the porosity and permeability are significantly reduced. The low shale gas content and high N2 component are related to the abnormal thermal effect of the basalt and the acceleration of hydrocarbon-generation process. N2 content indicates the shale gas preservation conditions. Near the basalt eruption area, the formation temperature increases, the liquid hydrocarbon wet gas is formed early, the peak-period of gas-generation moves forward greatly, the shale gas experiences more tectonic movements and prolonged preservation periods. In conclusion, the eruption or invasion of the Emeishan basalt is unfavorable for the shale gas enrichment of Wufeng-Longmaxi formations, while the large amount of discharged shale oil and gas in the early stage is favorable for the conventional natural gas accumulation of overlying strata. It is suggested that the exploration should be focused on conventional gas and supplemented by shale gas in area with large basalt distribution.