--> Differences of the Chlorite Origin, Occurrence and Their Influence on Petrophysical Properties in the Paleogene Deep Reservoir, Offshore Bohai Bay Basin

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Differences of the Chlorite Origin, Occurrence and Their Influence on Petrophysical Properties in the Paleogene Deep Reservoir, Offshore Bohai Bay Basin

Abstract

There are a number of chlorite-rich reservoirs in the Bohai Sea area, Bohai Bay Basin. However, the physical properties of chlorite-rich reservoirs make significant differences. Some of them have a porosity over 20% under deep buried condition showing a better ability to maintain the petrophysical properties, but other chlorite-rich reservoirs have very low porosity and permeability, or become tight reservoirs. This study makes full use of scanning electron microscopic (SEM) and thin section data of two kinds of reservoirs to carry out a comparative study. It ha sbeen found that the occurrence, types, and content of chlorite have significant effect on the reservoir properties. The three factors above are also affected by reservoir mineral compositions and deposition conditions. Thin sections and SEM analysis show that the chlorite occurrence can be grouped into particle coating, pore lining, pore filling, and particles alternation. Among them, particle coating chlorite shows better quartz overgrowth inhibition, which is favorable for higher porosity preservation. Pore lining chlorite is favorable for the intergranular pore preservation. The pore filling chlorite type can be subdivided into prochlorite, Pennine, and floccular chlorite. Prochlorite and Pennine are derived mainly from intergranular tufaceous filling alteration. The reservoirs with the development of this kind of chlorite are relatively tight with low porosity and permeability. Floccular chlorite is mainly of diagenetic origin, generally filling residual intergranular pore, and this kind of chlorite generally has a negative effect on reservoir. Because of the limitation in the grains, the effect of grain-alteration chlorite on physical properties is generally not obvious. However, if the content of grain-alteration chlorite is higher, the compressive strength of sandstone will be significantly reduced, which generally result in strong compaction, and poor petrophysical property. The extensive development reservoir of grain-alteration chlorite, ripidolite and Pennine often appear in alluvial fan deposits, where the provenance is generally of volcanic rock. Grain coating and pore lining chlorite often appear in traction current deposits, such as the delta front.