Clay mineral content, type, and their effects on pore throat structure and reservoir properties: Insight from the Permian tight sandstones in the Hangjinqi area, north Ordos Basin, China

Abstract

Clay mineral content and pore throat structure, including type, morphology, size distribution, and porosity and permeability, are very important in factors that control gas storage in tight sandstone reservoirs. In this study, integrated thin section and field emission scanning electron microscope (FE-SEM) observation, X-ray diffraction (XRD), nuclear magnetic resonance (NMR) and rate-controlled mercury porosimetry (RCP) analysis were used to investigate pore throat structure and the effects of clay minerals on pore throat connectivity, size distribution and reservoir properties of tight sandstones in the middle Permian successions of the Hangjinqi area, north Ordos Basin, China. Results show that the tight sandstones have very low porosity and permeability (averaged 9.76% and 0.968 mD) with diverse clay mineral compositions and grain sizes. The main pore types are intergranular and intragranular dissolution pores, intercrystalline and interstitial micropores. Most pores and throats are infilled with authigenic clay minerals, such as acicular chlorite, booklet kaolin, and scaly and filamentous illite with minor silica and calcareous cements. The pore and throat sizes in tight sandstones exhibit large variation, with 147.648 μm and 1.051 μm in sandy conglomerates, 160.998 μm and 1.285 μm in pebbly coarse-grained sandstones, 155.016 μm and 0.868 μm in coarse-grained sandstones, 158.134 μm and 1.178 μm in medium-grained sandstones, and 154 μm and 0.379 μm in fine-grained sandstones, respectively. The results indicate that clay minerals affected the pore throat connectivity, size distribution and reservoir properties of different lithofacies. The properties of tight sandstone reservoirs are heavily influenced by illite content, and throat size played a more important role influencing porosity and permeability. This study helps to clarify the relationship of clay minerals, pore throat structure and reservoir properties of the Permian tight sandstones in the Hangjinqi area and can guide associated work on tight sandstone reservoirs in the Ordos Basin and other basins worldwide. • The pore throat structure of the Permian tight sandstones is characterized. • Lithofacies are associated with pore and throat size distribution. • Throat size plays important role on pore throat connectivity and reservoir permeability. • Clay minerals content especially illite has negative impacts on reservoir properties.