Natural fractures and their influence on shale gas enrichment in Sichuan Basin, China

Abstract

Natural fractures, which serve as the main flow channels and important storage spaces, have significant effects on the formation, distribution and development of shale gas. We found three types of natural fractures developed in the Paleozoic marine shales of the southeastern Sichuan Basin, including tectonic fractures, diagenetic fractures and abnormal high-pressure-related fractures. Tectonic fractures can be further divided into intraformational open fractures, transformational shear fractures and bed-parallel shear fractures, whereas diagenetic fractures can be divided into bed-parallel lamellated fractures and shrinkage fractures. We determined that the formation of shale fractures is determined by the brittle-mineral content, shale texture, strata attitude, fluid pressure and geological structure. Brittle minerals, e.g., quartz, feldspar and dolomite, are necessary for the formation of shale fractures. Lamellation could change the mechanical property of fractures, which caused the formation of bed-parallel shear fractures. Geological structure is a key factor determining the maturity of shale fractures. The distribution and development of shale gas is barely influenced by shrinkage fractures and abnormal high-pressure-related fractures due to the low transport effectiveness. Intraformational open fractures, transformational shear fractures, bed-parallel shear fractures and lamellation fractures are significant to shale gas. The denser intraformational open fractures and lamellation fractures can enrich the degree of shale gas. We determined that the scale of shear fractures is the key factor influencing gas preservation rather than the density of natural fractures. Transformational shear fractures determine the gas preservation in areas with low dip-angle strata, whereas bed-parallel shear fractures determine the gas preservation in areas with high dip-angle strata.