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Hydrogen Sulfide in the Permian Basin


Hydrogen sulfide (H2S) is a hazardous and toxic gas component, which increases the cost of oil and gas exploration, development and production. Meanwhile, H2S is a natural tracer reflecting hydrocarbon flows during geological period and during production. Investigating and understanding the distribution of H2S concentration may increase the reliabilities of H2S-risk prediction and hydrocarbon resource assessment. In this work, we focus on the interpretation of H2S in the Permian Basin (western Texas and southeastern New Mexico). H2S concentration in natural gas (associated or non-associated) produced from the Permian Basin varies from several parts per million (ppm) to over 100,000 ppm (10 %) volumetrically. Over 85% of the gas streams are H2S-rich (>100 ppm), and over 40% of the gas streams are extremely rich in H2S (>10,000 ppm, or 1 %). H2S-rich gas exists in both conventional and unconventional reservoirs, and in all sub-basins and shelf areas. Generally, H2S concentration is higher in the shallower formations (Guadalupian) than in the deeper ones (Leonardian and Wolfcampian), and it is higher in the Central Basin Platform than in the Midland and Delaware sub-basins. The high concentration levels indicate that the main source of H2S is neither bacterial sulfate-reduction (BSR) nor the reactions involving sulfate components in drilling fluids. Consequently, modifying drilling fluids or adding biocides (to kill sulfate-reducing bacteria) has no impact on H2S concentration. The general trend of decreasing H2S concentration with thermal maturity and with gas-to-oil ratio (GOR) indicates that H2S is unlikely from thermochemical sulfate-reduction (TSR). The widely spread anhydrite and gypsum layers in the Permian formations are not a significant source of H2S due to the low thermal stress experienced by these formations. Cracking of Type IIS kerogen during source rock maturation is most likely the main source of H2S in the Permian Basin. H2S vertical distribution along the boundary of the Central Basin Platform reflects fluid migration paths. The increase of H2S concentration during the course of production could be due to the heterogeneous distribution of H2S in the reservoir, and to the adsorption of H2S on fresh rock surfaces after hydraulic fracturing.