Effects of different types of fractures on shale gas preservation in Lower Cambrian shale of northern Sichuan Basin: Evidence from macro-fracture characteristics and microchemical analysis

Abstract

The Lower Cambrian Qiongzhusi shale is the main producing layer of shale gas in the northern Sichuan Basin. Natural fractures are considered an important factor that influences the exploration and development of shale gas. In this study, outcrops, drill cores, thin sections, fluid inclusions and rare earth element (REE) were analyzed to determine the characteristics, changes in fluid chemistry, and fluid source of different types of fractures and then reveal their effects on the accumulation and preservation of shale gas. Natural fractures of Qiongzhusi shale are mainly composed of non-strata-bounded fractures (NSBFs), strata-bounded fractures (SBFs), and bed-parallel fractures (BPFs). NSBFs span multiple layers, which can significantly enhance the vertical connectivity of shale reservoirs. There are three dominant orientations of NSBFs, as N-S, NNE-SSW, and E-W trends. The E-W trending fractures, which intersect with the maximum principal stress (NWW-SEE) at a small angle, have larger permeability and control the vertical leakage of shale gas. SBFs are confined to individual layers. Homogenization temperatures of aqueous inclusions in SBF veins vary from 149.7 to 172.3 °C. Calcite veins exhibit positive Eu anomalies (δEu = 11.71, 27.9). Analysis of fluid inclusions and rare earth elements indicated that SBF vein-precipitating fluids were derived from the surrounding wall rock. The development of SBFs contributes to shale gas enrichment. BPFs are parallel to bedding planes or intersect at a low angle (<20°). Microstructural analysis suggests that cemented BPFs form from the multiple crack-seal processes. The temperature of fluid during mineral precipitation exceeds 200 °C. REE concentrations vary significantly across the BPF veins (δEu = 2.31–119.37), donating that fluid characteristics episodically changed during vein growth. The methane inclusions trapped in cements and mixing externally sourced fluids indicate that BPFs are the preferential fluid-flow pathway of shale gas and controls the lateral migration of shale gas.