Microstructures and Pore Structures of Naturally Deformed Shales: Significance for Shale Gas Exploration in the Complex Tectonic Areas of the Margin of the Sichuan Basin, South China

Tectonic deformation is a significantmodification processin themultiscale structure of shale. Understanding the naturally deformedshale gas reservoir properties plays a significant role in visualizingand quantifying gas migration pathways. Here, microstructure and porestructure investigations are performed using two types of marine shale(weakly deformed shale and strongly deformed shale) by a combinationof thin section, scanning electron microscopy (SEM) imaging, mercuryintrusion porosimetry, and low-pressure gas adsorption. The resultsshow the following: (1) The tectonic deformation has a significanteffect on the evolution of shale microstructures and pore structures.(2) The weakly deformed shales are representative of slightly deformedfine-grained shales and develop significant laminated structures withoutapparent tectonic deformation. (3) Original sedimentary fabrics andpore networks have been largely destroyed and changed in the stronglydeformed shales, and the thin section and SEM images present a varietyof deformation microstructures, such as microfold, microfault, andcataclastic flow, showing evidence for ductile, brittle, and brittle-ductiledeformation, respectively. (4) Organic pores are existent and developed,but they are not the main pore type in both weakly and strongly deformedshale. Mineral pores and fracture pores are more developed in naturallydeformed shales, and they can contribute to gas migration. (5) A combinationof low-pressure gas adsorption and mercury intrusion porosimetry suggeststhat with increasing tectonic deformation, macropore volumes and fracturepores relatively increase. These results will provide a basis forunderstanding microstructural evolution and pore structure preservationin the complex tectonic areas at the margin of the Sichuan Basin.