The Effect of Water Chemistry on Thermochemical Sulfate Reduction: A Case Study from the Ordovician in the Tazhong Area, Northwest China

Hongxia Li,Chunfang Cai,Lianqi Jia,Chenlu Xu,Ke Zhang

doi:10.1155/2017/6351382

Abstract

Formation water chemistry, sulfate sulfur isotopes, and associated H2S contents and sulfur isotopes were measured from the Ordovician in Tazhong area, Tarim Basin. The aim is to elucidate the effects of geochemical composition of formation water on thermochemical sulfate reduction (TSR) and potential usage of SO4/Cl ratios as a new proxy for TSR extents in areas, where H2S and thiaadamantanes (TAs) data are not available. The formation water has SO4/Cl ratios from 0.0002 to 0.016, significantly lower than 0.04 to 0.05 from 3 to 7 times evapoconcentrated seawater. Thus, the low values are explained to result from TSR. Furthermore, the SO4/Cl ratios show negative correlation relationships to TAs and H2S concentrations, indicating that TSR occurred in a relatively closed system and SO4/Cl ratio can be used to indicate TSR extents in this area. Extensive TSR in the Cambrian in the Tazhong area, represented by low SO4/Cl ratios and high H2S and TAs concentrations, is accompanied by formation water with high TDS and Mg concentrations, indicating the effects of water chemistry on TSR under a realistic geological background. In contrast, the low TSR extent in the Ordovician may have resulted from limited TSR reaction duration and total contribution of aqueousSO42-.

Highlights

Thermochemical sulfate reduction (TSR), a process whereby aqueous sulfate and petroleum compounds react at temperatures higher than 120∘C (CnH2n+2 + SO42− → CO2 + H2S + altered petroleum), is considered to result in elevated H2S concentrations in many carbonate reservoirs [1,2,3,4,5,6,7,8]
Formation water is the solvent for sulfates, and water chemistry has a great influence on TSR which explain the low TSR extent in the Tazhong area
High Mg/SO4 ratios of the O1-2y formation water compensated the low temperature which would limit the initiation of TSR in the Tazhong area

Summary

Introduction

Thermochemical sulfate reduction (TSR), a process whereby aqueous sulfate and petroleum compounds react at temperatures higher than 120∘C (CnH2n+2 + SO42− → CO2 + H2S + altered petroleum), is considered to result in elevated H2S concentrations in many carbonate reservoirs [1,2,3,4,5,6,7,8]. A great number of organic sulfides such as thiols and thiolanes [7, 9, 10] and 1- to 3-cage thiadiamondoids with 1 to 4 sulfur atoms were detected from TSR areas [11,12,13,14]. The presence of these organic sulfur compounds, especially labile sulfur compounds such as 1-pentanethiol or diethyl disulfide, has been experimentally showed to significantly increase the rate of TSR [15]. Experiments indicated that the concentrations of MgSO4 CIP are related to temperatures and SO4/Mg ratios in the solutions [18, 19]

Results

Discussion

Conclusion