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Reservoir quality is critical in tight gas exploration. A systematic study of the petrological, petrophysical, and diagenetic characteristics of the Shanxi Formation sandstones in the north‐eastern Ordos Basin (China) was undertaken to characterize the pore system of these tight sandstones and recognize the distribution of reservoirs favourable for hydrocarbon accumulation. The results suggest that the Shanxi Formation sandstone has low compositional and moderate textural maturity, with its porosity ranging from 0.7% to 11.8% (average 5.97%) and permeability ranging from 0.001 to 2.77 mD (average 0.36 mD). In addition, thin section and SEM observations suggest that the sandstone is dominated by secondary dissolution pores, primary intergranular pores as well as microfractures, and has undergone compaction, cementation, and dissolution. Diagenetic minerals, such as carbonate cements, authigenic quartz, clay minerals, and dissolved feldspar, are identified. Fluid inclusions are observed in healed microfractures of quartz grains and in quartz overgrowths, while the homogenization temperatures of fluid inclusions in healed microfractures and in quartz overgrowth are in the ranges of 92.8–179.1°C (average 134.8°C) and 104.7–169.1°C (average 145.7°C), respectively. Authigenic kaolinite is sourced from the process of K‐feldspar dissolution, authigenic illite is sourced from transformation of smectite and kaolinite, while sources for quartz cements include mineral alteration and pressure dissolution. Selective dissolution of K‐feldspar in the presence of carbonate minerals is observed in the Shanxi Formation tight sandstones due to the different equilibrium constants of carbonate minerals and K‐feldspar leaching, resulting in dissolution pores associated with K‐feldspar. Reservoir quality in the Shanxi Formation tight sandstones is greatly influenced by diagenesis, as compaction and cementation are responsible for the loss of porosity while dissolution accounts for the enhancement of secondary porosity. [ABSTRACT FROM AUTHOR]

The Lower Permian Taiyuan Formation is a lithic-abundant, tight gas sandstone reservoir in the Kangning field, northeastern Ordos Basin. To clarify the gas accumulation process in tight gas sands, the relationship between the densification of Taiyuan Formation sandstones and gas charging is analyzed. Core porosities of the Taiyuan Formation are mostly below 10%, and air permeabilities are below 1 mD. The sandstone reservoirs are characterized by strong heterogeneity. Dissolution pores and large amounts of micropores in clay minerals dominate the storage space. This porosity is the result of the impact of diagenesis. Porosity evolution modeling confirmed well the prediction of the period of sandstone densification (225-215 Ma) based on the basin analysis as well as diagenetic calibration. Strong compaction dominated the physical property (porosity and permeability) reduction with cementation as a subordinate factor. The integration of burial history and fluid inclusion data suggested that large quantities of gas migrated into the Taiyuan sandstone reservoirs from 176 Ma to 80 Ma when the reservoirs were already tight (porosity <10%). Gas expansion forces provided by source rocks were the main driving force for gas charging instead of buoyancy. Since the late Cretaceous, uplift activity had little influence on Taiyuan tight gas-sands due to self-generation and self-storage. Overlapping sandstone bodies interbedded with high-thickness and good-quality source rocks jointly determined development of tight gas sands. These study results may explain the gas accumulation process and provide insights into gas exploration and development when this method is applied elsewhere. • A new method to indicate the type of gas accumulation. • Three stages of gas accumulation process for Taiyuan Formation in Kangning area. [ABSTRACT FROM AUTHOR]