Study on the Coupling Mechanism of TOC and Porosity and Its Controlling Effect on the Gas-Bearing Property of Shale Reservoirs: A Case Study of the Wufeng Formation–Long‑11 Submember in the Z3 Well Area of West Chongqing–Central Sichuan

Abstract

To address the poorly understood controlling mechanisms of gas-bearing property in shale reservoirs and the low accuracy of sweet spot prediction, this study investigates the shale reservoirs of the Wufeng Formation–Long-11 Submember in the Z3 well area, western Chongqing–central Sichuan. Based on the logging curve data and the analysis and test data of various rock samples, the coupling relationship between the key parameters under the constraint of lithofacies is clarified by using the scatter plot correlation analysis method. The controlling effect of the coupling mechanism on gas-bearing property was further analyzed. The results show that Porosity and TOC content exhibit a nonlinear, unimodal “inverted U-shaped” relationship, first increasing and then decreasing. Free gas content is most significantly controlled by porosity (R 2 = 0.518), while adsorbed gas content is most significantly controlled by TOC content (R 2 = 0.6369). Based on the coupling control relationship of gas-bearing properties in the study area, organic matter is newly classified. It is clarified that shales with medium-to-high organic matter (TOC = 2–5%) and high porosity have the best gas-bearing property; shales with ultrahigh organic matter (TOC > 5%) and medium porosity rank second; and shales with organic-poor (TOC < 2%) and low porosity (<3.5%) have the poorest gas-bearing property. The results of this study provide an important theoretical basis for shale gas sweet spot prediction.