Diagenesis of the Fengcheng Formation in the Mahu Sag and its impact on physical properties

doi:10.3969/j.issn.1006-6535.2025.05.009

Abstract

Abstract: The lithofacies of the Permian Fengcheng Formation in the Mahu Sag,Junggar Basin are complex,and the types of diagenesis are diverse.Its reservoir capacity is significantly affected by diagenesis.However,the differences in diagenesis among different lithofacies and their evolutionary sequences are unclear,and the controlling effects on reservoir capacity are not well understood,hindering an in-depth understanding of the reservoir formation mechanism of the Fengcheng Formation.In response to this issue,through image processing technology to analyze rock optical thin sections and scanning electron microscope (SEM) images,minerals and pore-fracture characteristics were quantitatively identified,the types of diagenesis and their evolutionary sequences were clarified,and the impact mechanisms of diagenesis on the reservoir capacity of shale oil reservoirs were systematically analyzed in combination with core descriptions,high-pressure mercury injection experiments,etc.The research results show that the lithofacies of the Fengcheng Formation shale oil reservoir can be divided into three types:Type Ⅰ siltstone facies has the highest porosity and permeability and the strongest reservoir capacity,Type Ⅱ carbonate rock facies comes second and Type Ⅲ mud shale facies has the lowest.The three lithofacies share similar diagenetic evolutionary sequences,but the dominant diagenetic processes differ. The Fengcheng Formation reservoir is in the middle diagenetic stage B. Its diagenetic evolutionary sequence includes early mechanical compaction,calcite and dolomite cementation,pyritization,early hydrocarbon charging,dissolution,fracturing,late hydrocarbon charging and dissolution.Compaction and carbonate cementation are the main factors for densification,while dissolution and fracturing are key to the diagenetic modification of tight reservoirs,contributing significantly to the formation and improvement of reservoir space.The research results have important guiding significance for deepening the understanding of shale oil reservoir formation mechanisms,improving the shale oil reservoir evaluation system,and "sweet spot" optimization.

Key words: diagenesis, reservoir physical properties, diagenetic evolution, Mahu Sag, “sweet spot”, Fengcheng Formation

CLC Number:

TE122.2

ZHANG Lulu, YANG Zhi, GAO Zhiye, ZHANG Hong, MA Guoming, JIA Lidan, LIU Zhaochen, XIN Haotian. Diagenesis of the Fengcheng Formation in the Mahu Sag and its impact on physical properties[J]. Special Oil & Gas Reservoirs, 2024, 32(5): 76-84.

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