AAPG Middle East Region Geoscience Technology Workshop:
3rd Edition Carbonate Reservoirs of the Middle East

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Understanding the Origin of H2S in Carbonate Oil and Gas Reservoirs: A Review

Abstract

Hydrogen sulfide (H2S) can be an important component of oil and gas production, reaching concentrations up to percent levels. This is a toxic and corrosive gas that impacts commercial viability of petroleum resources. Therefore, understanding the root cause of H2S is instrumental for the field development plan and an important issue in the industry. This sour gas can be generated in reservoirs through natural mechanisms or as a result of some production methods. The main reactions that lead to H2S production in oil and gas reservoirs are bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR). These reactions occur in mutually exclusive thermal regimes but generate similar products (e.g. H2S, CO2, and altered hydrocarbons). Also, some recovery methods can enhance the generation of H2S through these reactions. H2S can also be produced as a result of aquathermolysis during steam assisted heavy oil recovery, or hydraulic fracturing use in production of shale oil/gas accumulations. Source organic matter or oil thermal cracking can also be the origin of H2S in reservoirs. The information obtained from stable isotopes analysis, as well as fluid and rock composition can be utilized to differentiate between various H2S production mechanisms. High concentrations of H2S in carbonate reservoirs at elevated temperatures are generally associated with TSR, which generally consists of the reduction sulfate minerals and oxidation of hydrocarbons. In-reservoir TSR will impact both rocks and fluids. The threshold TSR temperature is a matter of debate and may vary from system to system, but generally assumed to be at least > 100 ⁰C. Various factors may affect H2S production and concertation in reservoir fluids, including rock mineralogy and texture, formation water composition and pH, and the presence of H2S scavengers. This results in a high vertical and lateral variability in H2S concentrations in sour reservoirs. This talk will review the main mechanisms of H2S generation in oil and gas accumulations and factors that can lead to reservoir souring, with emphasis in carbonate reservoirs, such as, for example, the Khuff and Arab formations in Abu Dhabi. It will present an overview of geochemical and other criteria used to discriminate between different H2S generation processes and a workflow for the investigation of the origin of this gas in reservoirs.