Behavior of organic matter-hosted pores within shale gas reservoirs in response to differential tectonic deformation: Potential mechanisms and innovative conceptual models

Abstract

Remarkable breakthroughs have been achieved in exploration of marine shale gas, acting as important cleaner energy resources in complex tectonic regions, encouraging us to accurately estimate different reservoir capacity of tectonically deformed gas shales. In particular, organic matter (OM) pore system, acting as the most important storage space for high-over-mature marine gas-shale reservoirs in South China has not yet been specifically targeted and comparatively studied in a regional and differential tectonic deformation regime. A set of shale drilling core samples from the Upper Silurian to Lower Ordovician Wufeng-Longmaxi Formations in the Sichuan Basin and its periphery were targeted by a multi-methodical approach utilizing organic geochemistry and mineralogical investigations, FE-SEM observation and digital image processing and extraction, and combined fluid intrusion (N2 and CO2 adsorption analysis). From undeformed shales (UDS) through slightly deformed shales (SDS) to intensely deformed shales (IDS), an obviously and progressively “Triple Jump” reduction in multi-scale pore volume and specific surface areas, and characteristic parameters for OM pores including the plane porosity (Phi) (the average dropped from 23.14% to 10.33%), the equivalent circle diameter (ECD) (the average dropped from 28.38 nm to 10.09 nm), the perimeter over area (PoA) (the average rose from 0.1126 to 0.2718), and the dominant pore diameter (DOM size) (the average dropped from 113.80 nm to 20.78 nm). The reactivation of seepage channels and intrusion of brittle minerals are proposed as two main microscopic forcing mechanisms, updated and innovative conceptual models are proposed to reconstruct an OM pore response processes under a differential tectonic deformation regime in panoramic view for Lower Paleozoic marine gas shale reservoirs of China. Thus, an evolution of spatial resolution spanning a total of three different scales respectively as structural styles, shale reservoir architectures, and microscopic petrological compositions are together portrayed.