Causes of variation in the concentration and distribution of H2S in carbonate reservoirs of the Feixianguan Formation, Lower Triassic in the eastern Sichuan Basin

Abstract

Hydrogen sulfide (H2S) has been encountered in many oil and gas fields worldwide, causing potentially various health and safety issues; thus, it is crucial to assess its concentration before drilling. Natural gas from the Feixianguan carbonates in the Jiannan gas field is predominantly composed of methane with various H2S concentrations varying from very minor to 3.71%. However, the H2S concentrations display obvious difference, ranging from 0.18% to 0.81% in the southern zone and 0.21–3.71% in the northern zone, with high concentrations (>1%) occurring selectively in dolomitized reservoirs in the northern zone. Based on petrographic and geochemical data and reactive transport numerical modeling, the H2S origin and sulfate sources were analyzed for understanding factors controlling the H2S concentration and distribution.In the northern zone containing high H2S concentrations, there is a common mineral assemblage of pyrobitumens, burial calcite cements, pyrite, and hemispherical elemental sulfur in dolomite reservoirs. Burial calcite cements, often coated by pyrobitumens, have higher precipitation temperatures (>106 °C), and lighter δ13C ratios than host dolostones, suggesting the incorporation of organic carbon into the precipitation of these high-temperature burial calcites supplied by thermochemical sulfate reduction (TSR). Numerical modeling indicates the SO42− concentrations of pore waters would increase significantly in dolomitized domains during seawater-involved dolomitization, consisting with the fact of the higher SO42− concentrations of pore waters in the northern zone. The dolomitizing fluids delivered sulfates into reservoirs which were inevitably reduced by hydrocarbons, once the reaction temperature was reached. Comparatively, in the southern zone where carbonates were not subjected to intensive dolomitization and the SO42− concentrations of pore waters were low, the H2S originated only from the thermal decomposition of oil and organic matter (<1%). This study implies that hydrocarbon exploration in dolomitized reservoirs should be given greater caution because of the more favorable conditions for H2S generation.