Origin of deep sour natural gas in the Ordovician carbonate reservoir of the Tazhong Uplift, Tarim Basin, northwest China: Insights from gas geochemistry and formation water

Deep sour gases are found in the Ordovician carbonate reservoir of the Tazhong Uplift in the Tarim Basin, northwest China. All sour gases in this field are dominated by gas hydrocarbons (C-1-C-4) with varying amounts of H2S. Most of the gases in the eastern Tazhong are characterized by high gas dryness coefficients ( > 0.95), C-13-enriched methane (delta C-13(1) ranging from -44.6 parts per thousand to -35.8 parts per thousand), and small difference between values of the delta C-13(2) and delta C-13(1) (delta C-13(2-1)). In contrast, the gases in the western Tazhong are characterized by relatively low gas dryness coefficients (mostly < 0.90),C-13-depleted methane (delta C-13(1) ranging from -54.9 parts per thousand to -35.7 parts per thousand), and high values of delta C-13(2-1), indicating relatively lower maturity. Gas compositions, carbon isotope ratios, and diamantane data together with thermal pyrolysis experiments using sealed gold tubes suggest that the Ordovician gases in the Tazhong Uplift belong to oil cracking gases mixed with high-maturity C-13-enriched dry gases, which originated from the deeper paleo-reservoirs cracking. The H2S (0.0008%-23.1000%) is attributed to thermochemical sulfate reduction (TSR); in contrast to typical TSR-altered gases, the natural gases in the western Tazhong are characterized by low dryness coefficients, relatively C-13-depleted methane, and high delta C-13(2-1), suggesting a low degree of TSR alteration. Observations suggest that the H2S contents are generally high in the western Tazhong, with positive correlation to Mg2+ concentrations and total dissolved solids (TDS) in the Ordovician formation water, indicating that TSR has entered early period of SO42- contact ion pairs (CIPs) oxidation. Compared with the eastern Tazhong, the Mg2+ concentrations and TDS in the western Tazhong are higher and form more CIPs that accelerate TSR and produce H2S. In addition, based on the analysis of the Cambrian formation water and the Cambrian gypsiferous salt, TSR might have also exist in the deeper Cambrian reservoirs, and the TSR-altered gases with high content of H2S migrated along faults to Ordovician reservoirs.