Formation and occurrence of organic matter-hosted porosity in shales

Abstract

Porosity within organic matter (OM) is considered to be the main site for gas storage (via fluid phase saturation of the pores and sorption on the pore walls) and thus its presence or absence is very critical for in-place gas assessment for organic-rich rocks. Numerous workers have suggested that OM-hosted porosity increases with thermal maturity mainly related to the process of bituminized organic matter cracking to gas. Comprehensive reviews of published literature enable us to conclude that organic porosity dominantly develops within bituminized organic matter (i.e., that portion that is petrographically identified, mainly based on its morphology, as solid bitumen (Mastalerz et al., 2018)) and primary (i.e, structured or amorphous) organic matter (kerogen) is mostly deficient in porosity. We show that in the same shale sample, structured kerogen shows no porosity whereas solid bitumen contains abundant porosity. It has been previously reported that sample preparation for SEM by ion milling may alter the organic matter. In this study, some adverse effects of ion milling have been observed by comparing SEM-visible pores of the same sample prepared by mechanical grind polish (MGP) and ion milling (IM) methods. Since solid bitumen is more labile, and probably more prone to chemical/physical alterations, we recommend that SEM observations of ion milled samples to be conducted with more attentiveness.We show that 1) MGP is a suitable sample preparation method for SEM observations, 2) the overwhelming majority of SEM-visible organic matter-hosted pores develop within solid bitumen and not in structured organic matter (kerogen), and 3) we reaffirm that shale mineral content is a key factor in preservation or destruction of solid bitumen-hosted porosity.Finally, we propose a conceptual model for the formation, preservation and destruction of OM-hosted porosity for shales. Accordingly, we suggest that, all other factors being equal, a quartz-rich brittle shale will preserve more SEM-visible solid bitumen-hosted (meso and micro) porosity compared to that of a clay-rich ductile shale. Therefore, gas storage capacity of a quartz-rich shale will be relatively high compared to that of a clay-rich shale.