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<p>The lower sub-member of the fourth member of the Eocene Shahejie Formation (LSm-ESF) in the southern Bohai Bay Basin hosts red-bed sandstones that are increasingly targeted as deep hydrocarbon reservoirs, yet the quantitative controls of diagenesis on their reservoir quality remain poorly constrained below 3000&#xa0;m. This study integrates core description from 20 wells (406.72&#xa0;m), petrographic point-counting (92 samples, ~ 27,600 points), well-log and mud-log data from over 120 wells, and 450&#xa0;km² of 3D seismic data to quantify the relative contributions of compaction, cementation, and dissolution to porosity evolution. Compaction is the dominant porosity-reducing process, accounting for an average porosity loss of 13.3% and correlating inversely with initial matrix content; cementation, dominated by calcite (87.2% of cement types), accounts for a further 6.6% average porosity loss and is strongly facies-controlled, with pore-lining chlorite/illite coatings in long-axis fluvial sandstones inhibiting quartz overgrowth; dissolution of feldspar and carbonate cement, driven by organic acids from hydrocarbon charging, generates secondary porosity preferentially in overpressured zones between 3000 and 4200&#xa0;m depth. High-quality reservoirs (porosity &gt; 10%, permeability &gt; 6 mD) are concentrated in coarse-grained, well-sorted long-axis fluvial sand bodies with chlorite grain-coating and dissolution overprint, whereas alluvial-fan and fan-delta deposits are pervasively densified by compaction and calcite cementation. Compared with prior qualitative or facies-isolated studies of the Bonan Sag, this work is the first to link a revised, basal-onlap/top-truncation sequence stratigraphic framework directly to a quantified diagenetic facies model, producing a predictive map that identifies a NW-SE-trending high-quality reservoir fairway validated against well data (e.g., 19.1% porosity in Well Luo68 versus a non-productive 0.04 t/day test in Well Yi193). The resulting diagenetic facies model offers a transferable predictive framework for deeply buried red-bed reservoirs in saline lacustrine rift basins elsewhere.</p>

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Diagenetic controls on reservoir quality and heterogeneity of eocene red-bed sandstones: a case study from the Shahejie Formation, southern Bohai Bay basin, China

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  • 发表日期：

    2026年8月2日

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  • Shahab Aman e Room,
  • Zhang Liqiang,
  • Yan Yiming,
  • Ahmed E. Radwan,
  • Hassanat Ullah

摘要

The lower sub-member of the fourth member of the Eocene Shahejie Formation (LSm-ESF) in the southern Bohai Bay Basin hosts red-bed sandstones that are increasingly targeted as deep hydrocarbon reservoirs, yet the quantitative controls of diagenesis on their reservoir quality remain poorly constrained below 3000 m. This study integrates core description from 20 wells (406.72 m), petrographic point-counting (92 samples, ~ 27,600 points), well-log and mud-log data from over 120 wells, and 450 km² of 3D seismic data to quantify the relative contributions of compaction, cementation, and dissolution to porosity evolution. Compaction is the dominant porosity-reducing process, accounting for an average porosity loss of 13.3% and correlating inversely with initial matrix content; cementation, dominated by calcite (87.2% of cement types), accounts for a further 6.6% average porosity loss and is strongly facies-controlled, with pore-lining chlorite/illite coatings in long-axis fluvial sandstones inhibiting quartz overgrowth; dissolution of feldspar and carbonate cement, driven by organic acids from hydrocarbon charging, generates secondary porosity preferentially in overpressured zones between 3000 and 4200 m depth. High-quality reservoirs (porosity > 10%, permeability > 6 mD) are concentrated in coarse-grained, well-sorted long-axis fluvial sand bodies with chlorite grain-coating and dissolution overprint, whereas alluvial-fan and fan-delta deposits are pervasively densified by compaction and calcite cementation. Compared with prior qualitative or facies-isolated studies of the Bonan Sag, this work is the first to link a revised, basal-onlap/top-truncation sequence stratigraphic framework directly to a quantified diagenetic facies model, producing a predictive map that identifies a NW-SE-trending high-quality reservoir fairway validated against well data (e.g., 19.1% porosity in Well Luo68 versus a non-productive 0.04 t/day test in Well Yi193). The resulting diagenetic facies model offers a transferable predictive framework for deeply buried red-bed reservoirs in saline lacustrine rift basins elsewhere.