Understanding the origin of hydrogen sulphide in South-Western Gissar, Southern Uzbekistan

Hydrogen sulphide (H2S) concentrations across three adjacent gas condensate fields in Southwestern Gissar produced from fractured Middle-Upper Jurassic reef-carbonates with anhydrite-bearing beds have been analysed. Wellsite testing measurements followed by iodometric titration showed concentrations of H2S below 0.2 mole%. Statistical analyses and cross plots of H2S vs. depth, reservoir temperature, porosity, anhydrite content, well tests and water saturation did not reveal linear correlations. Uniform gas condensate compositions and constant souring levels through time rule out secondary bacterial or/and technogenic processes, while temperatures bellow 90–110°С are against thermochemical sources. Low H2S values could result from mineral-fluid interactions within the host rock. However, low abundance of H2S and carbon dioxide (CO2) (<0.2 and <1 mole%) is also typical for thermogenic gases. Sour gas could be sourced from the deeply subsided up to 8–10 km axes of the Afghan-Tajik petroleum system. H2S concentrations here are generally around 1–2%. Diminished H2S concentrations in the fields are explainable by long distance migration losses from the source area 50 km apart. TSR is unlikely to be important contributor from the downdip part due to replacement of lagoonal anhydrites by distal carbonates. Interpreted logs, calibrated against XRD in the fields reveal anhydrite drop basinward from 30–100% to 0%.