Impact of fluorine-bearing hydrothermal fluid on deep burial carbonate reservoirs: A case study from the Tazhong area of Tarim Basin, northwest China

Abstract

Deep burial carbonate reservoirs (>4000 m) have great potential of reserves for oil and gas exploration, although the matrix reservoirs generally become extremely heterogeneous and tight due to various diagenesis. Hydrothermal dissolution is supposed to be important for formation of high-quality deep burial carbonate reservoirs. However, few studies have focused on influence of minor fluorine-bearing fluid on carbonate reservoir quality. This study integrates petrography, geochemistry and fluid inclusion data to understand impact of fluorine-bearing fluid on deep burial carbonate reservoirs. Mineralogy and mineral distribution suggest that fluorite and concomitant calcite are found in the Ordovician and Cambrian strata of the whole Tazhong area, especially in the vicinity of faults. Calcite associated with fluorite is probably derived from progressive mixture between formation water and fluorine-bearing hydrothermal fluid, which is supported by REY pattern of concomitant calcite, surrounding limestone and fluorite, and fluid inclusion microthermometric data of concomitant calcite and fluorite. Fluorine-bearing hydrothermal fluid was responsible for the concomitant C2 calcite of high Y/Ho ratio. Therefore, the superchondritic Y/Ho ratio of concomitant calcite can be probably used as proxy for identifying distribution of minor fluorine-bearing hydrothermal fluid. For thick fluorite bed, influx of fluorite-rich fluid resulted in surrounding limestone dissolution and altered zones between fluorite and limestone. This process simultaneously improves porosity and permeability of high-energy sedimentary facies reservoirs. In contrast, the fluorine-rich hydrothermal fluid through the low-energy sedimentary facies mainly enhanced permeability of the tight carbonate reservoirs rather than porosity. For widespread tight reservoir of high-energy and low-energy sedimentary facies, even minor fluorite-bearing fluids still enhance permeability. The fluorine-bearing hydrothermal fluid may influence carbonate reservoirs away from hydrothermal-altered zones. Significance of minor fluorine-bearing hydrothermal fluid for widespread tight carbonates has probably been underestimated for deep burial carbonate reservoir.