Definition and sturcture characteristics of pores in mylonitized organic-rich shales

Abstract

Gas shales with a high gas content were drilled in the Lower Cambrian Lujiaping Formation in the northeastern Sichuan Basin, close to the Chengkou Fault in the Dabashan arc-like thrust fold belt. The equivalent vitrinite reflectance values of organic matters are over 4.0% Ro. The pore structures of the shales were investigated based on microscopy, field emission scanning electron microscopy (FESEM) observations, and low temperature N2 adsorption analysis. The study suggests that cleavages, comprising clay minerals mixed up with organic matter and other insoluble residues, were developed in the rock layers. The clay minerals are directionally arranged, displaying a mylonitized texture. Abundant nanometer-size organic matter and clay mineral particles are well mixed in the cleavage domains, which developed the mylonitized pore system that consists of nanometer-size intergranular pore spaces, aggregate pore spaces in clay mineral flakes and pore network. This mylonitized pore system has high specific surface area, high methane adsorption capacity, and high capillary pressure, which collectively contributes to the preservation of shale gas in such a complex tectonic area. The discovery of the mylonitized pore structure in organic-rich shales may reveal a new mechanism of shale gas enrichment in complex tectonic areas with over-mature organic matter in the northeastern part of Sichuan Basin.