--> --> Diagenesis and its Impact on the Reservoir Quality of Low Permeability Reservoirs: A Case Study of Eocene Liushagang Formation of Wushi Depression, Beibuwan Basin, the South China Sea

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Diagenesis and its Impact on the Reservoir Quality of Low Permeability Reservoirs: A Case Study of Eocene Liushagang Formation of Wushi Depression, Beibuwan Basin, the South China Sea

Abstract

Wushi depression is a typical example of small rift basin. Based on core observation, thin section examination, scanning electron microscopy, X-ray diffraction analysis, porosity-permeability test and other analytical methods, combined with the histories of burial evolution and organic matter thermal evolution, the petrologic features, pore structure characteristics and the effect of diagenesis on reservoir quality of low-permeability sandstone are studied of Liushagang Formation in Wushi Depression. The results show as follows: (1) the reservoir sandstones are classified as litharenite and feldspathic litharenite. The sandstone of the eastern and central part of the depression, which sourced from Qixi uplift in the northern sag, is mainly feldspathic litharenite with high contents of feldspar. Whereas the sandstone of the southern which sourced from Liusha low uplift is dominated by litharenite. (2) Mechanical compaction occurs by grain rearrangement, ductile grain compaction and brittle grain fracturing throughout the whole burial process. Clastic particles are cemented by kaolinite, illite, carbonate cements and a spot of quartz overgrowth. Kaolinite is probably the result of feldspar dissolution resulting from Organic acids that produced by the maturation of organic matter. (3) The evolution of interval transit time along with the depth shows that the values significantly increased in the Liushagang formation, indicating the development of secondary porosity in the longitudinal direction. (4) The diagenetic environment evolution of the reservoir in the study area is early alkaline, interim acid and late alkaline, forming the diagenetic sequence of early calcite and siderite cementation, feldspar dissolution accompanied by quartz overgrowth and authigenic kaolinite precipitation, late period ferrocalcite and ankerite cementation. (5) Mechanical compaction is the dominating factor for the loss of intergranular pores, which leads to a reduction in porosity of 10.8% ~31.5%, with an average of 23.4%. During cementation, the loss rate of porosity is 1.0%~26.5%, with an average of 7.16%. In addition, the proportion of secondary porosity, caused by the dissolution of feldspar and feldspar-bearing volcanic debris, takes up to 2.8%~8.4% with an average of 5.3%.