--> Sulfate Source and Model for Thermochemical Sulfate Reduction in the Triassic Feixianguan Carbonate Gas Reservoirs, Sichuan Basin

AAPG ACE 2018

Datapages, Inc.Print this page

Sulfate Source and Model for Thermochemical Sulfate Reduction in the Triassic Feixianguan Carbonate Gas Reservoirs, Sichuan Basin

Abstract

Thermochemical sulfate reduction (TSR) has occurred in many deep carbonate reservoirs worldwide, yielding various amounts of H2S in hydrocarbons that impacts production and imposes health and safety issues. Difference in sulfate sources for TSR may create distinct predictions in the souring level of reservoirs. The Lower Triassic Feixianguan Formation (T1f) in the northeast Sichuan Basin is such a reservoir, containing different concentrations of hydrogen sulfide (H2S) in different gas fields from minor to more than 20%. Comparative analysis of the amounts of H2S accumulated and their relationship to T1f reservoir lithology in these gas fields provides new insights into the extent of TSR by linking sulfate sources with dolomitization. In this study a new model for TSR is established based on the petrographic and geochemical data and numerical simulation.

The results show that high H2S concentrations occur mainly in dolostone reservoirs, implying a potential relationship between TSR and dolomitization. The close correlation of δ13C, 87Sr/86Sr and δ34S values of matrix dolostones to the T1f seawater curves indicate reflux dolomitization of evaporated, contemporaneous seawaters in the Puguang gas field. Remnants of anhydrite occur mainly in intraparticle and intercrystal pores, suggesting their diagenetic origin and formation due to calcium release during dolomitization. These anhydrites as internal sources were thus reduced by hydrocarbons at temperatures between 150 and 200oC, leading to 13.3‰ fractionation in δ34S between H2S and sulfates. Combined with numerical simulation, the model for TSR was established. When the dolomitization scale is small, there is no anhydrite precipitation in rocks and sulfates for TSR are primarily from dolomitizing fluids. With increasing scales of dolomitization, anhydrites begin to precipitate and become a main source. In geological records, this spectrum of processes is indicated by various H2S concentrations in different scales of dolostone geobodies in the Sichuan and others basins.