Effects of preservation conditions on enrichment and high yield of shale gas in Sichuan Basin and its periphery

The main controlling factors of shale gas preservation conditions and evaluation of the Niutitang and Wufeng-Longmaxi formations in the Sichuan Basin and its periphery, were considered in view of typical shale gas wells in different regions with different gas contents and production. The preservation conditions of shale gas were controlled by several factors such as the roof and bottom rocks, the sealing properties of shale itself, the structural reformation degree, and the timing of when the shale reservoirs were reformed, etc. Adjacent rock conditions and its sealing effect was the basic factor determining whether the early formed hydrocarbons were preserved. Late tectonic activity controlled the effusion, degree and abundance of shale gas, and it was a key factor in shale gas preservation. Meanwhile, we found that preservation conditions had an obvious effect on gas content, porosity, electrical resistivity, water saturation and gas composition. With poor preservation conditions, the gas content, porosity and resistivity were low, but water saturation and N₂ content were high. All these parameters could be used as indirect indexes of shale gas preservation evaluation. A preservation evaluation system with 5 types and 28 parameters for the Lower Paleozoic marine shale gas was set up, illustrating the time and space combination of sealing conditions, the sealing properties of shale itself, and tectonic effects (structural reformation time and degree, fault). The parameters of gas-bearing capacity of shale layer and the pressure coefficients also guide the evaluation of preservation conditions.