Hydrothermally Fluoritized Ordovician Carbonates as Reservoir Rocks in the Tazhong Area, Centraltarim Basin, Nw China

Abstract

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 including pyrite, quartz, sphalerite and chlorite accompany the fluorite.Two generations of fluid inclusions are present in the fluorite. Homogenization temperatures (Th) for primary inclusions are mostly between 260°C and 310°C and represent the temperature of the hydrothermal fluid responsible for fluorite precipitation. Th for secondary inclusions range from 100°C to 130°C, and 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 the 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 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. Plate 1 (left). Photographs of hydrothermal minerals and hydrocarbons in wellTazhong 45 cores: A. Fluorite vein in limestone at 6,081.2m, fluorite replaces limestone; B. Chert and fluorite at 6,101.2m; C. Fluorite at 6,106.8m (partially cross polarized light); D. Fluorite and calcite at 6,106.8m (plane polarized light); fluorite replaces calcite and there are more intercrystalline cavities in the fluorite than in the calcite. E. Remnant calcite and quartz in fluorite at 6,106.8m (cross polarized light); F. Hydrocarbons in microfractures in fluorite at 6,106.8m, UV epifluorescence; G. Chert at 6,101.2m (plane polarized light); H. Quartz and fluorite at 6,100.5m, quartz is deposited among the fluorite (plane polarized light); I. Hydrothermal fractures at 5,298.0m filled with pyrite contemporaraneously precipitated from the hydrothermal fluid (reflected light); J. Sphalerite and chlorite in well Tazhong 12, 5,217.0m, core photograph; K. Sphalerite in well Tazhong 12, 5217.0m, EPMA secondary electron image. Plate 2 (above). Primary and secondary fluid inclusions in fluorite in well Tazhong 45. A, B. Primary fluid inclusions (transmitted light) at 6,100.9m and 6,104.2m, ...

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