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AAPG Bulletin, V.
Calcite vein system and its importance in tracing paleowater flow and hydrocarbon migration in the Ordovician carbonates of the Tazhong area, Tarim Basin, China
Lianqi Jia,1 Chunfang Cai,2 Zhenliang Wang,3 Hongxia Li,4 Lijing Liu,5 Qingyong Luo,6 and Lei Jiang7
1State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an, China; Youth Innovation Team of Shaanxi Universities (Basin Evolution and Energy Minerals), Xi’an, China; [email protected]
2Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics and College of Earth and Planetary Sciences, Chinese Academy of Sciences, Beijing, China; Key Laboratory of Oil and Gas Resources and Exploration Technology, Yangtze University, Wuhan, China; [email protected]
3State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an, China; [email protected]
4Institute of Unconventional Oil and Gas, Northeast Petroleum University, Daqing, China; [email protected]
5State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an, China; [email protected]
6State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing, China; [email protected]
7Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics and College of Earth and Planetary Sciences, Chinese Academy of Sciences, Beijing, China; [email protected], [email protected]
It is difficult to determine flow pathways of paleofluids in sedimentary basins. Petrography, fluid inclusion, and isotopic compositions from different phases of calcite veins in the Ordovician in the Tazhong area were studied. Our results reveal a complex diagenesis process with two importantly crossformational fluid movements, coupled with three phases of calcite veins (C1, C2, and C3) precipitation and thermochemical sulfate reduction by hydrocarbons. Fluids that precipitated C2 and C3 calcites were saline and enriched in 18O and thus may have originated from underlying evaporates, whereas fluid that precipitated C3 had an additional contribution from 87Sr-rich, Ba-rich, and Zn-rich Precambrian basinal clastic units. Fluid inclusion homogenization temperatures, salinities, and δ18Owater values of C2 veins show decreasing trends with increasing distances from strike-slip faults. This may indicate that C2 fluid might migrate into Ordovician carbonate reservoirs through strike-slip faults within 6 km during late Hercynian orogeny (late Permian). The similarly decreasing trends are observed in C3 veins, indicating that the eastern part of No. 1 fault seemed to be the main fluid-flow pathway for C3 calcite precipitation during Himalayan orogeny. Hydrocarbon may have migrated along the similar pathways as supported by (1) abundant primary hydrocarbon-bearing fluid inclusions in the C2 and C3 calcites and (2) well-documented hydrocarbon migration and mixing. This is an ideal case showing how spatial variation in geochemical data from fracture fillings can be used to trace paleowater and likely hydrocarbon flows in sedimentary basins; it thus may be useful in predicting fluid flow and distribution of hydrocarbons in the Tarim Basin and other basins.
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