HYDROTHERMALLY FLUORITIZED ORDOVICIAN CARBONATES AS RESERVOIR ROCKS IN THE TAZHONG AREA, CENTRAL TARIM BASIN, NW CHINA

Zhijun Jin*, Dongya Zhu*+, Xuefeng Zhang**, Wenxuan Hu** and Yucai Song**

* Exploration and Production Institute, SINOPEC, Beijing 100083, P. R. China.

** Dept of Earth Sciences, Nanjing University, Nanjing 210093, P. R. China.

+ corresponding author, email: zhu_dongya@sohu.com

Reservoir rocks at the Tazhong 45 oil pool, central Tarim Basin, consist of fluoritized carbonate strata of Middle-Late Ordovician age. Petrological observations indicate that the fluorite replaces calcite. Several other hydrothermal minerals accompany the fluorite and include pyrite, quartz, sphalerite and chlorite.

Two generations of fluid inclusions are present in the fluorite. Homogenization temperatures (Th) for primary inclusions are mostly between 260C and 310C, representing the temperature of the hydrothermal fluid responsible for fluorite precipitation. Th for secondary inclusions range from 100C to 130C, and they represent the hydrocarbon charging temperature as shown by the presence of hydrocarbons trapped in some secondary inclusions. The mineral assemblage and the homogenization temperatures of primary fluid inclusions indicate that the precipitation of fluorite is related to hydrothermal activity in the Tazhong area. Strontium isotope analyses imply that the hydrothermal fluids responsible for fluorite precipitation are related to late-stage magmatic activity, and felsic magmas were generated by mixing of mafic magma and crustal materials during the Permian.

Theoretical calculations show that the molecular volume of a carbonate rock decreases by 33.5% when calcite is replaced by fluorite, and the volume shrinkage can greatly enhance reservoir porosity by the formation of abundant intercrystalline pores. Fluoritization has thus greatly enhanced the reservoir quality of Ordovician carbonates in the Tazhong 45 area, so that the fluorite and limestone host rocks in the area have become an efficient hydrocarbon reservoir. According to the modelled burial and thermal history of the Tazhong 45 well, and the homogenization temperatures of secondary fluid inclusions in the fluorite, hydrocarbon charging at the Tazhong 45 reservoir took place in the Tertiary.

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