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Diagenesis and very low grade metamorphism in a 7,012?m-deep well Hongcan 1, eastern Tibetan plateau |
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| Authors: | Hejing Wang Yongsheng Ma Jian Zhou Tingjing Xu |
| Institution: | 1. Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Beijing, China 2. School of Earth and Space Sciences, Peking University, 100871, Beijing, China 3. Oilfield Exploration and Production Department, China Petrochemical Corporation (SINOPEC), Beijing, China 4. Chinese Academy of Geological Sciences, 100037, Beijing, China |
| Abstract: | This study uses clay mineral assemblages, illite ??crystallinity?? (IC), chlorite ??crystallinity?? (CC), illite polytypes, the b cell-dimension of K-white mica, mineral geothermo-geobarometers and homogenization temperatures of fluid inclusions to investigate the transition from diagenesis to metamorphism in a 7?km thick Triassic flysch sequence in the well Hongcan 1, eastern Tibetan plateau. The 7,012.8?m deep borehole penetrated flysch of Upper to the Middle Triassic age and represents a unique chance to characterize low temperature metamorphic processes in an unusually thick sedimentary sequence developed on thickened continental crust. Mineral assemblage analysis reveals a burial metamorphic pattern with kaolinite and chlorite/smectite mix-layer phases present in the upper 1,500?m, and illite/smectite mixed-layer phases extending to a depth of 3,000?m. The metamorphic index mineral, graphite, was detected in sedimentary rock below 5,000?m using Raman spectroscopy. There exists a good correlation between IC and CC within the prograde burial sequence; with CC anchizonal boundaries of 0.242 and 0.314°2?? (upper and lower boundaries, respectively) corresponding to Kübler??s IC limits at 0.25 and 0.42°2??. Illite polytypism also shows an increase in the 2M 1 polytype with increasing depth, with ca. 60?% 2M 1 abundance compared to the 1M type at the surface, to 100?% 2M 1 at the bottom of the borehole. Fluid inclusion analysis show HHC-rich bearing fluids correspond to the diagenetic zone, CH4-rich bearing fluids appear at transitional zone from diagenetic to low anchizone and H2O-rich bearing fluids mark the high anchizone to epizone. Based on chlorite chemical geothermometer, calcite?Cdolomite geothermo-barometers as well as homogenization temperature of fluid inclusions, a paleotemperature range of 118?C348?°C is estimated for the well with a pressure facies of low to intermediate type. |
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