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

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 microfractures 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 microfractures 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.

Automated summary

The study analyzed the types and characteristics of microfractures in Longmaxi shales and their impact on shale gas enrichment, showing that microfracture characteristics are mainly controlled by mineral composition.