A quantitative model and controlling factors of secondary pore development for tight sandstone reservoirs in the carboniferous Benxi Formation, Ordos Basin, China

Abstract

As the Ordos Basin is the largest gas-producing area in China, its central and eastern Paleozoic Carboniferous Benxi Formation is poised to become a significant source for increasing reserves and production within the basin. However, the extreme diagenesis, complex pore structure, and strong heterogeneity of this formation hinder the exploration of its “sweet spots”. The reasons are usually considered to be its large burial depth (>3000 m), differential uplift, highly acidic environment and strong hydrocarbon supply in the coal-bearing strata of this area. In this study, we performed qualitative and quantitative analyses of thin sections of Benxi Formation rocks, combined with scanning electron microscopy, X-ray diffraction, and fluid inclusion analyses to determine the development, type, and evolution sequence of secondary porosity from the diagenesis of the Benxi Formation. The main reservoir type of the Benxi Formation is characterized by secondarily dissolved pores (>0.5 μm) and micropores (<0.5 μm) within clay aggregates; the low porosity and permeability of <5.8% and <1 × 10−3 μm2 indicating a tight sandstone reservoir. Throughout the reservoir formation, its porosity has decreased by strong compaction and cementation, and also increased primarily by dissolution. Quantitative analyses of the thin-section porosity of the rocks based on the Scherer initial porosity recovery model and pore evolution parameters defined by Paxton and Ehrenberg showed 39.8% recovery of original porosity and a porosity increase due to dissolution of 1.0–5.4% (mean, 4.1%); this constituted 40–250% of the porosity prior to dissolution. The model-simulated results were closely correlated with the experimental measurements. The lithology, diagenetic fluid, and organic acids of the source rocks jointly controlled the development of secondary pores in the Benxi Formation. These findings indicate that the evolution and controlling factors of secondary pore development must be clarified to identify favorable areas for gas exploration. The results will provide a basis for predicting favorable reservoirs in the tight sandstones of the Benxi Formation in the central and eastern Ordos Basin.