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Origin of the Different Chlorite Occurrences and Their Effects on Clastic Reservoir Porosity

Abstract

Research shows that authigenic chlorite may inhibit quartz overgrowths, and thus, plays an important role in the protection of original intergranular pores in deep-buried reservoirs. However, some other scholars have an opinion that the chlorite can reduce porosity by decreasing pore throat diameters. Our study shows that the influence of authigenic chlorite on the quality of deep-buried clastic reservoir porosity depends on its occurrence. Analysis of thin sections and scanning electron microscopy images of the rocks of Xujiahe Formation in Sichuan Basin and Lucaogou Formation in Junggar Basin, respectively, reveals two types of authigenic chlorite based on occurrence: grain-coating and pore-filling. The correlations between porosity and chlorite volume vary with different occurrences. In the case of grain-coating chlorite, the correlation is positive, whereas an inverse correlation with pore-filling chlorite. Thus, grain-coating chlorite has a positive effect on reservoir quality, and pore-filling chlorite plays a negative role. Grain-coating chlorite and its growth during burial diagenesis enhance the mechanical strength of rocks and prevent contact between detrital quartz surfaces and pore water, thus preserving pores in sandstone by preventing compaction and nucleation of authigenic quartz. Pore-filling chlorite only reduce porosity by decreasing pore throat diameters. Analysis of grain size indicates that grain-coating and pore-filling chlorites always occur in high-energy sedimentary and low-energy depositional environments, respectively. The water current carries an abundance of medium-grained saltating contents in strong hydrodynamic tractive current deposition that leads to a relative velocity between the saltating population and suspended clay in the water. As indicated by Bernoulli's theory, the saltating grains are coated by the clay on the surface because of the local flow pressure resulting from relative velocity. However, in the case of a weak hydrodynamic tractive current deposition, sand is entrained with clay, i.e. both sand grains and clays are in suspension and they have no relative velocity; thus, the clay cannot completely coat the grain surface. The occurrence of smectite-dominant clays (the precursor of authigenic chlorite) determines the occurrence of authigenic chlorite during diagenesis, and therefore, determines whether the chlorite has a positive effect on the quality of clastic reservoirs.