Geological Characteristics of Deep Shale Gas and Their Effects on Shale Fracability in the Wufeng-Longmaxi Formations of the Southern Sichuan Basin, China

Abstract

The deep shale gas reservoirs of the Wufeng-Longmaxi Formations in the southern Sichuan Basin have strong heterogeneity and complex geological characteristics, resulting in a poor fracturing effect and low vertical production. Based on whole-rock X-ray diffraction analysis, scanning electron microscopy, shale gas-bearing experiments, rock mechanical parameter tests and well logging and elemental logging data, the sedimentary environment, and geological characteristics of this deep shale gas are analyzed, and the shale fracability is evaluated. (1) The type of organic matter is affected by factors such as sea level change, hydrodynamics, reducing environmental conditions, algae bioenrichment, and tectonic stability, and the contents of Type I and Type II kerogens in the lowermost reservoir of the Wufeng-Longmaxi Formations are high. (2) The pores between the biogenic siliceous minerals (the framework) and numerous organic pores provide space for the occurrence of shale gas. High-quality reservoirs have a high brittle mineral content, a high Young’s modulus, a low Poisson’s ratio, an appropriate fracturing pressure, a small net stress difference, and a high shale fracability. (3) Multicluster perforation, temporary plugging near the wellbore, and multistage fracturing can be used in the Wufeng Formation-Longmaxi Formation, increasing the near-wellbore hydraulic fracture complexity and improving the hydraulic fracturing effect.