Comparative study on pore structure characteristics of marine and transitional facies shales: A case study of the Upper Permian Longtan Formation and Dalong Formation in the Lower Yangtze area, south China

Abstract

The Permian shale reservoir in the Lower Yangtze area of South China is regarded as a target with a high potential for shale gas exploration. To investigate the pore structure characteristics and evaluate reservoir quality, 32 shale samples from the Upper Permian Longtan Formation and Dalong Formation in the Lower Yangtze area were studied using a suite of techniques including total organic carbon (TOC) content, X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), nitrogen (N2) and carbon dioxide (CO2) gas physisorption, mercury injection capillary pressure (MICP), petrographic observations, and fluid inclusion analysis. Petrographic observations reveal that the Longtan Formation is rich in vitrinite and inertinite, while the Dalong Formation contains a large amount of solid bitumen. Primary organic pores (POPs) are the original cellular pores that develop in inertinite. Vitrinite particles did not show visible pores. Solid bitumen infills fractures and occupies the interparticle (interp) space between the mineral grains in the Dalong Formation. A small number of speckled secondary organic pores (SOPs) were observed in the solid bitumen with diameters less than 10 nm. Both the TOC and quartz contents exhibit negative relationships with the pore volumes (PVs) for the Longtan and Dalong Formations, owing to the poor development of pores in the organic matter (OM) and occlusion of most mineral pores by migrated bitumen. There is a positive relationship between clay minerals and PVs owing to the sheet-like meso-pores and macro-pores in the clay minerals. The evolution of organic porosity is related to the differences in maceral composition and thermal maturity. Thermal cracking of residual oil and bitumen and the expulsion of gaseous hydrocarbons contributed to the formation of SOPs in marine shales. In contrast, vitrinite and inertinite in gas-prone shales do not display noticeable morphological changes across the entire thermal evolution process, owing to their low hydrocarbon generation potential.