Effect of organic matter, thermal maturity and clay minerals on pore formation and evolution in the Gulong Shale, Songliao Basin, China

Abstract

The successful exploration of shale oil in the clay-rich Gulong shale of Songliao Basin has got wide attention. While silica-rich and carbonate-rich shale reservoirs have been well documented, clay-rich shale reservoir characteristics and evolution are still ambiguous. In this study, a large number of Gulong shales from 34 wells across the basin were subjected to integrated analyses of total organic carbon (TOC), vitrinite reflectance (Ro), X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), low-pressure N2 gas adsorption and helium porometry. The coupling evolution of organic matter, clay minerals and pores with increasing thermal maturity was investigated. The results show that (1) The Gulong shale is mainly clay-rich and argillaceous mudstone. In contrast with other oil producing lacustrine shales, silt and carbonate interbeds are thin in thickness with the least oil potential. (2) The primary pore type is organo-clay complex hosted pore (OMC-hosted pore), which is defined as pores between clay platelets or aggregates but previously was occupied by labile kerogen or migrated oils. In contrast to OM-hosted pores, OMC-hosted pores have less solid bitumen relics due to clay catalysis effect. (3) Pore evolution is coupled with thermal maturation stage and significant porosity build-up occurs in the oil window (Ro ∼0.9–1.3%). While most pores are associated with clay minerals, organic carbon content is the primary factor controlling porosity at each thermal maturation stage. (4) Oil generation overpressure is suggested as the most important factor controlling pore preservation, while phyllosilicate fabrics resulting from clay mineral transformation may provide rigid flakes resisting pores from collapse. (5) In contrary to sandstone or carbonate reservoirs, the excess chlorite (>5.0%) was observed to has a positive effect on pore formation in clay-rich shales. One genesis of chlorite was identified in this study, that the chlorite was formed in dissolved ferrodolomite particle, which possibly has provided both Fe and Mg needed for smectite-chlorite conversion.