ABSTRACT

An integrated analysis of the petrographic characteristics and types and distribution of diagenetic alteration in the upper Paleozoic Benxi–Taiyuan, Shanxi, and Xiashihezi Formations provides insights into the controlling factors on variations in porosity and permeability in tight sandstones (85% of the sandstone samples display porosity values <10% and 90% of the samples exhibit permeability <1 md).

Diagenetic alteration includes mesogenetic compaction, cementation by dolomite, ankerite, and quartz, dissolution of feldspar, and illitization of smectite. Eodiagenesis includes compaction, development of smectite, cementation by pore-filling quartz and disordered kaolinite, and precipitation of calcite and Fe-calcite. Chlorite and quartz preserve primary pores against damage, whereas kaolinite, illite–smectite (I/S) mixed layer, and illite significantly diminish reservoir quality via permeability reduction. Chlorite and I/S content decrease abruptly as depth increases, whereas the kaolinite content remains elevated at depth because of the complete destruction of K-feldspar. The transformation from smectite to illite provides silica ions for the widely distributed quartz overgrowths. As the depositional environment transformed from fluvial (Xiashihezi) to deltaic (Shanxi) and to epicontinental (Taiyuan and Benxi), the dissolution effect increased monotonically. Feldspar dissolution is dominant in the Shanxi Formation, whereas the Benxi and Taiyuan Formations commonly contain quartz dissolution pores. The Taiyuan Formation has markedly higher porosities than in the overlying and underlying formations, caused by strong dissolution and high silica content. The decrease in porosity in the Benxi Formation results from the extensive formation of clay minerals caused by high frequency transgressions in a transitional environment.

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