Depositional process and its control on the densification of coal-measure tight sandstones: insights from the Permian Shanxi Formation of the northeastern Ordos Basin, China

• Published in: International journal of earth sciences : Geologische Rundschau 2023-09, Vol.112 (6), p.1871-1890
• Main Authors: Chen, Yuhang, Zhao, Jingzhou, Guo, Mingqiang, Wang, Yingbin, Wu, Heyuan, Li, Jun, Wu, Weitao
• Format: Article
• Language: English
• Subjects: Carbon dioxide; Carbonates; Cementation; Channel pores; Channels; Coal; Compaction; Densification; Diagenesis; Dissolution; Dissolving; Earth and Environmental Science; Earth Sciences; Fluid flow; Geochemistry; Geology; Geophysics/Geodesy; Hydrodynamics; Lithology; Logging; Mineral Resources; Organic acids; Organic matter; Original Paper; Permian; Pores; Rocks; Sandstone; Sedimentary rocks; Sedimentology; Structural Geology; Swamps; Thermal degradation
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s00531-023-02329-y

The densification of coal-measure sandstones is important to the exploration of tight sand gas. Previous studies have focused on the influence of diagenesis on sandstone densification, however neglecting the role of depositional processes. Based on an integrated analysis of logging, core, and rock and mineral data, this study depicted the lithological distribution and petrological characteristics of the coal-measure sandstones of the Shanxi Formation in the northeastern Ordos Basin, with revealing the influence of depositional process on diagenesis, and thus identified the densification of the sandstones. The results show that the sandstones were deposited in fluvial-delta settings and were isolated vertically and interbedded with the organic-rich carbonaceous mudstones and coal seams deposited in swamps. Depositional process exerts significant impact on the diagenesis of coal-measure sandstones by determining the sandstone composition. The sandstones are characterized by high argillaceous matrix content that sourced from both the weathered parent rock and the erosion of swamps. The well-sorted coarse and medium sandstones deposited in the middle of channels have relatively low argillaceous matrix contents due to high hydrodynamic energy, which preserved partial intergranular pores after compaction owing to grain supporting. The residual pores facilitated the acidic flow from organic matter in coal seams and mudstones, which led to dissolution and produced dissolved pores. However, the CO 2 generated by the thermal degradation of organic acids in carbonaceous mudstones and coal seams contributed to ferroan carbonate cementation, which led to sandstone densification. The poorly sorted fine sandstones deposited in channel margins or crevasse splays present high argillaceous matrix content due to low hydrodynamic energy. The mixed muddy and carbonaceous clasts due to the erosion of swamps by channels further increased the content of argillaceous matrix. High content of argillaceous matrix occupied most intergranular pores after strong compaction and restricted acidic fluid flow, therefore, dissolution and ferroan cementation were limited.