ObjectiveThe Benxi Formation in the East Yanchuan Block of the southeastern Ordos Basin has emerged as a significant exploration target for tight gas reservoirs. Despite its strategic importance, the sandstone reservoirs in this interval are typically characterized by low porosity, poor permeability, strong heterogeneity, and complex diagenetic histories. These characteristics limit their development potential and make accurate reservoir prediction challenging. This study aims to thoroughly investigate the reservoir characteristics and diagenetic evolution of the Benxi Formation sandstones, with the goal of clarifying how multi-stage diagenetic processes control pore evolution and reservoir quality differentiation.
MethodsAn integrated analytical approach was applied, including casting thin section petrography, scanning electron microscopy (SEM), and high-pressure mercury intrusion (HPMI). These techniques were used to examine pore types, mineral assemblages, diagenetic alterations, and pore-throat distributions, allowing for a detailed reconstruction of the diagenetic evolution and its influence on petrophysical properties.
ResultsThe study reveals: (1) The Benxi Formation sandstones have undergone a multi-stage diagenetic evolution involving three principal phases: mechanical compaction, cementation enhancement, and dissolution modification. Each stage had a distinct influence on the pore structure and reservoir performance. (2) Quartz-rich sandstones showed relatively high resistance to mechanical compaction but experienced substantial porosity loss during the middle diagenetic stage due to the pervasive precipitation of quartz and carbonate cements. This stage led to the formation of a rigid framework, where the intergranular pores were severely occluded, significantly reducing effective porosity and permeability. (3) Lithic quartz sandstones exhibited a much more favorable diagenetic trajectory. These rocks experienced strong dissolution of unstable components, such as feldspar and lithic fragments, resulting in the formation of secondary intragranular and intergranular pores. This process markedly improved pore connectivity and storage capacity, making this lithofacies the most favorable for tight gas accumulation. (4) Lithic sandstones suffered from intense early-stage compaction and cementation, which drastically reduced both primary and secondary porosity. As a result, these rocks exhibit extremely poor reservoir quality and are considered the least favorable reservoir type. (5) Based on the observed differences in diagenetic intensity and pore evolution characteristics, four distinct diagenetic facies types were identified.
Conclusion(1) The reservoir quality of the Benxi Formation sandstones is primarily controlled by the intensity and sequence of compaction, cementation, and dissolution. (2) Different mineral compositions lead to divergent diagenetic pathways and contrasting pore evolution patterns. (3) Lithic quartz sandstones hold the highest potential due to extensive dissolution, while quartz sandstones are more susceptible to cementation. (4) Lithic sandstones, dominated by early-stage densification, represent poor-quality reservoirs. (5) The proposed four-type diagenetic facies framework effectively reflects the reservoir heterogeneity and offers practical criteria for facies-based reservoir prediction. Significance This study advances the understanding of diagenetic control on pore structure in tight sandstone reservoirs and provides a robust geological foundation for classifying and predicting reservoirs and targeting sweet spots in future exploration efforts.
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