Investigation of pore evolution and variation with magma intrusion on Permian Gufeng shale formation and their implications on gas enrichment

Abstract

Magmatism can significantly alter the pore structure of shale formation due to the thermal and pressure effects, thus the gas enrichment mechanism is naturally affected. The uncertainty of shale gas development has been raised in the petrogeology community due to the magma intrusion. This study investigates pore evolution, pore alteration, and gas enrichment of gas shale in the Permian Gufeng Formation (Fm.) in the Lower Yangtze, Liqiao district. In this work, multiple techniques were used for shale characterization including total organic carbon (TOC) content quantification, reflectivity microscope tests, X-ray diffraction, high-pressure mercury injection, low-pressure nitrogen adsorption, and carbon dioxide adsorption measurements. The results show that the closer the shale approaches the intrusion, the higher the vitrinite reflectance (Ro) and quartz content, the lower the TOC and clay mineral content, and metamorphic minerals for instance augite and anatase appear in the affected shale formation. As shale is closer to the intrusion, the type of pores changes from small pores (10～100 nm) to super-large pores (1000～100,000 nm), and the super-small pores (6～10 nm) almost disappeared. In addition, a decrease in the fractal dimension of the pores was observed indicating that the heterogeneity and complexity of the pore structure are reduced. Within the contact metamorphism halo, the maximum adsorption capacity, pore volume, and specific surface area decrease with increasing proximity to the intrusion, which reduces the potential for shale gas enrichment. In basin-scale, a huge contact metamorphic range and plugging effect of the intrusion in sill intrusion generate enormous amounts of hydrocarbon and block the escape of the shale gas. Due to these factors, the intrusion can be conducive for shale gas enrichment. However, the dike that penetrates the shale reservoir is not conductive to shale gas enrichment because the fracture channels created in the contact metamorphism halo accelerate the escape of the shale gas. This study proposed critical theories of shale gas enrichment and could increase the success rate of shale gas exploration.