Making sense of the nature of shale pore surface at the nanoscale

Pore surface properties of shale control hydrocarbons content in place. This study presents a visual and quantitative analysis of shale pore surfaces using high-resolution electron microscopy. At least five distinct pore types are distinguished: organic-matter pore， clay interparticle pore， carbonate dissolution pore， pyrite interparticle pore， and quartz interparticle pore. Organic-matter pores include three subtypes: depositional organic-matter pore， migrated organic-matter pore， and predepositional pore. Using global literature images and fractal theory， we classified seven nanopore types into three tiers by surface complexit， revealing how their microstructural heterogeneity affects hydrocarbon occurrence. An analysis of 1，067 global shale samples identifies TOC content as the primary control on adsorbed methane capacity. The organic fraction greatly enhances methane adsorption， not only by correlating with TOC but also through the coarse， irregular pore walls of thermogenic depositional organic-matter pore and migrated organic-matter pore that provide abundant adsorption sites. No relations exist between adsorbed gas and total clays， carbonates， or quartz. Their hosted pores have large pore sizes and smooth to slightly irregular surfaces， which may be available for free gas. The results visually highlight the shale pore structure complexity and strong heterogeneity at the nanoscale.