--> Impact of Diagenesis on Deeply Buried Sandstone Reservoir of Nanpu Sag, Bohai Bay Basin, East China

2018 AAPG International Conference and Exhibition

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Impact of Diagenesis on Deeply Buried Sandstone Reservoir of Nanpu Sag, Bohai Bay Basin, East China

Abstract

Digenesis has a significant impact on porosity enhancement, destruction, and preservation in deeply buried reservoir sandstone. The methodology includes core description, thin section petrography, Fluorescence, Scanning Electron Microscope (SEM), Cathodoluminescence (CL), fluid inclusion and isotope/electron probing analysis to evaluate the reservoir characteristics by a different diagenetic process. The studied sandstone consists of medium to coarse-grained, slight to moderately sort lithic arkose and feldspathic litharenite. Quartz, calcite, and clay are more dominant pore occluding cement and occur as euhedral to subhedral crystals. Porosity and permeability values range from 0.5% - 30% and 0.006mD -7000mD respectively. Main diagenetic events include compaction, dissolution of cement and unstable mineral in grains and fracturing. Compaction resulted in densification and significantly reduced the primary intergranular pores. Calcite cement in intergranular pores and precipitation of quartz cement along detrital sandstone grain is a significant porosity destroying process in deeply buried sandstone at high temperature. Dissolution of volcanic lithic fragments and pressure solution of feldspar grains were the key sources of quartz cement whereas carbonate cement is derived from an external source. Kaolinite clay minerals formed from the alteration of feldspar and volcanic lithic fragments. Clay coating (Chlorite), micro-quartz, early hydrocarbon emplacement and reservoir overpressure are a major factor that prevents the formation of quartz overgrowth and preserve the reservoir quality. Porosity and permeability decrease with cementation either related to quartz overgrowth or precipitation of calcite, whereas increase with leaching of metastable grains, dissolution of cement and pore-lining by chlorite. Thicker sandstone bodies, medium to coarse-grained, well-sorted with low primary ductile grains and containing a minor amount of calcite cement are acting as a best reservoir. The deeply buried reservoir quality is significantly affected by these diagenetic processes, and the overall quality of reservoir is sufficient to produce Hydrocarbons of cost-effective value.