Pore structure and fractal characteristics of marine-continental transitional shales in the southern North China Basin

Abstract

Research on marine-continental transitional shale in China is significant because this type of shale contains rich gas resources. The shale pore development, which is the key factor in determining the reservoir gas-bearing capacity, is complex. The organic matter (OM) content of the Shanxi and Taiyuan Formations shale in the southern North China Basin is medium to high, the mineral composition differs from that of other transitional shales, and the quartz content is higher than the clay content. The OM content and mineral composition exhibit a strong longitudinal heterogeneity. The pore types are mainly inorganic pores, and OM pores are less developed. The pore morphology is mainly parallel plate-like fissure pores, and there are a few ink-bottle-like pores composed of micro- and macropores. The specific surface area and total pore volume (TPV) of mesoporous-macropores are mainly controlled by mesoporous. The micropore volume is mainly controlled by micropores with sizes below 1.2 nm. Pore structure parameters and fractal dimension find that Shanxi and Taiyuan Formations shale has complex pore structure and rough pore surface. The fractal dimension D2 more accurately characterizes the roughness of the micropore surface. Compared with other shales, the micropore surface roughness of Shanxi and Taiyuan Formations shale is lower. The fractal dimensions and TPV do not correlate. The pore morphology and mineral characteristics of Shanxi and Taiyuan Formations shale leads to relatively low gas adsorption capacity and strong gas storage capacity, and is favorable for oil and gas migration. Due to the effects of compaction, macropores formed by quartz break and are filled with clay, resulting in a low daily gas production of the transitional shale in well MY1 after fracturing. The results provide references for the optimization of favorable reservoirs and the prediction of enrichment areas during transitional shale gas exploration.