Formation mechanism of authigenic chlorite and relation to preservation of porosity in nonmarine Triassic reservoir sandstones, Ordos Basin and Sichuan Basin, China
HUANG Si-jing, XIE Lian-wen, ZHANG Meng, WU Wen-hui, SHEN Li-cheng, LIU Jie on, Chengdu University of Technology, China)
The occurrence and formation mechanism of authigenic chlorite are studied in this paper. The cases are from Upper Triassic nonmarine reservoir sandstones of Ordos Basin and Sichuan Basin, China. Pore lining and grain coating are the most important occurrences of chlorite, which are similar to the (marine) sandstones reported by published literature. Chlorite precipitation occurred at the early stage of (diagenesis), but continued chlorite growth would have been localized on existing chlorite surfaces. This (occurrence) of chlorite plays an important role in protection of porosity in deeply buried reservoir sandstones by the following mechanism: (1) the pore lining and grain-coating chlorite and its overgrowth during buried diagenesis have enhanced the mechanical strength of rocks and ability of resistant to compaction; (2) the chlorite isolates detrital quartz surfaces from contact with pore water, thus preventing nucleation of authigenie quartz and consequently preserving pores in sandstone. It has been found that more authigenic chlorites are distributed in the propulsive delta environment, especially the delta front in the setting of lakes. This shows that Mesozoic-Cenozoic lakes in China are not really freshwater lakes. They have more or less salinity and some successive character for ocean-origin fluid.