Making sense of micro-fractures to the Longmaxi shale reservoir quality in the Jiaoshiba area, Sichuan Basin, China: Implications for the accumulation of shale gas

Abstract

The processes of shale gas transport and enrichment are closely related to the micro-fractures structure. Here, porosity, permeability test, gas adsorption, scanning electron microscopy (SEM), fluid spontaneous imbibition, Nano-CT, Micro-CT, and core observations were used to determine the types and characteristics of micro-fractures in the Longmaxi shales, Jiaoshiba area. The effects of micro-fractures on shale reservoir properties and shale gas accumulation were also analyzed. Diverse sizes of intra-particle (intraP), particle-edge, and transparticle micro-fractures were widely developed in shales. The difference of micro-fracture characteristics was mainly controlled by the mineral composition. The well-developed micro-fractures in the Longmaxi shales can act as a “bridge” to link nano-pores and macro-fractures. In such cases, a large-scale “escape network” composed of “nano-pores, micro-fractures, and macro-fractures” was formed. The escape of hydrocarbons along this network results in lower pore pressure. Most pores were dramatically compressed due to the compaction of overlying strata. As a result, the pore space and seepage capacity of organic-rich shales were disappointed, which is not benefit to the enrichment and production of shale gas. However, the limited development of micro-fractures will reduce the probability of forming “escape network”. Thus, some of the pores could be remained and act as storage space along with micro-fractures for shale gas even within the tectonically deformed area.