Origin and significance of organic-matter pores in Upper Ordovician Wufeng-Lower Silurian Longmaxi mudstones, Sichuan Basin

Abstract

The types of organic-matter (OM) pores in shale gas reservoirs were comprehensively investigated from the Wufeng-Longmaxi (W-L) mudstones in the Changning gas field, southern Sichuan Basin, China. The aim of this research is to provide the origin and significance of OM pores development in the nanometer-scaled pore structures within shale rocks. To understand the contribution of OM pores development to the W-L mudstones, the thin-section, OM extract from bulk cores, mineralogy, pore size distribution, and image analyses by scanning electron microscopy (SEM) and atomic force microscopy (AFM) were comprehensively carried out. Combining SEM photographs and AFM observations, two types of OM pores were identified, including structural OM pores and non-structural spongy OM pores. Through organic petrology analyses, the structural OM pores evolve from telalginites mainly sourced from phytoplankton and acritarch, while the non-structural spongy OM pores are derived from bacteria, benthic algae and certain graptolites. Three basic lithofacies were observed in the first member of the W-L shales, based on the results of the low-pressure N2 adsorption. We did not find any complex interaction between mineralogy and OM pores evolution. However, siliceous shales generate the most amount of bitumen which is the main OM pores producer, and the rigid siliceous particles can make the shelter of bigger OM pores. OM pores connectivity therefore increases within the siliceous shales. OM pores development, especially structural OM pores, has a crucial effect on shale gas exploitation.