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Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin
Chemical Geology ( IF 3.9 ) Pub Date : 2020-02-01 , DOI: 10.1016/j.chemgeo.2019.119453
Marta Sośnicka , Volker Lüders

Abstract Upper Permian Zechstein carbonate Ca2 gas reservoirs in the southern part of the Pompeckj Block in the North German Basin locally contain up to 36 vol% hydrogen sulfide (H2S) produced by thermochemical sulfate reduction (TSR). TSR was triggered by migration of dry to extremely dry coal gas from Upper Carboniferous into the Zechstein carbonate reservoirs. Methane reacted with dissolved sulfate at temperatures of The results of this study decipher three major stages of gas migration in the Pompeckj Block. Stage I commenced in the Late Triassic during burial when Zechstein Ca2 reservoirs were charged with dry CH4-CO2 ± N2 gas sourced from mature Upper Carboniferous coals. Burial continued through the Jurassic and caused alteration of Ca2 reservoir gas by sulfate reduction reactions due to increasing temperatures. Entrapment of CH4-H2S-CO2-N2 gases in fluid inclusions, hosted in cements and fracture-filling minerals, occurred at temperatures between 100 and 152 °C and was related to Stage II uplift in the Early Lower Cretaceous. In the Late Cretaceous (Stage III) deep burial of the Pompeckj Block led to charge of the Zechstein Ca2 carbonate reservoirs with Upper Carboniferous-derived CH4-CO2 ± N2 ± C2+ coal gas and/or dilution of existing reservoir gas at temperatures of 144–167 °C. Highly variable δ13CCH4 values from −18.7 to −8.7‰ and very negative δ13CCO2 values (−22.4 to −18.9‰) of H2S-rich fluid inclusion gases as well as negative δ13C values (−10.4 to −4.6‰) of host calcites reveal compelling evidence for participation of methane in TSR. Fluid inclusions imply that CH4-dominated TSR proceeded at Tmin of 135 °C in the presence of catalyzers such as H2S and dissolved Mg2+. This study demonstrates that fluid inclusions serve as an excellent and accurate tool for tracing H2S concentrations in hydrocarbon gases through time and space, which is not possible using the present-day compositions of natural reservoir gases. It also contributes to the understanding of carbonate reservoir-hosted hydrocarbon-bearing fluid systems and processes that significantly control the quality of reservoir gases.

中文翻译:

北德盆地上二叠统碳酸盐岩储层 (Zechstein, Ca2) 干煤气低温热化学硫酸盐还原 (TSR) 的流体包裹体证据

摘要 德国北部盆地 Pompeckj 区块南部的上二叠统 Zechstein 碳酸盐 Ca2 气藏局部含有高达 36 vol% 的硫化氢 (H2S),由热化学硫酸盐还原 (TSR) 产生。TSR 是由干燥至极干燥的煤气从上石炭统运移到 Zechstein 碳酸盐岩储层引发的。甲烷与溶解的硫酸盐在 20 ℃ 的温度下反应 本研究的结果解释了 Pompeckj 区块中气体运移的三个主要阶段。第一阶段开始于晚三叠世埋藏期间,当时 Zechstein Ca2 储层充注了来自成熟上石炭纪煤层的干燥 CH4-CO2±N2 气体。埋藏一直持续到侏罗纪,并且由于温度升高,硫酸盐还原反应导致 Ca2 储层气蚀变。CH4-H2S-CO2-N2 气体夹杂在流体包裹体中,存在于水泥和裂缝填充矿物中,发生在 100 至 152 °C 之间,与早下白垩世的第二阶段隆起有关。在晚白垩世(第三阶段),Pompeckj 区块的深埋导致 Zechstein Ca2 碳酸盐岩储层充注上石炭统衍生的 CH4-CO2 ± N2 ± C2+ 煤气和/或在 144– 温度下对现有储层气体的稀释167℃。富含 H2S 的流体包裹体的 δ13CCH4 值从 -18.7 到 -8.7‰ 的高度变化和非常负的 δ13CCO2 值(-22.4 到 -18.9‰)以及宿主方解石的负 δ13C 值(-10.4 到 -4.6‰)显示引人注目甲烷参与 TSR 的证据。流体包裹体意味着 CH4 主导的 TSR 在 135 °C 的 Tmin 下在催化剂(如 H2S 和溶解的 Mg2+)存在下进行。这项研究表明,流体包裹体是一种极好的、准确的工具,可以通过时间和空间追踪碳氢化合物气体中的 H2S 浓度,而使用当今的天然气储层成分是不可能做到的。它还有助于理解碳酸盐岩储层承载的含烃流体系统和过程,这些系统和过程显着控制储层气体的质量。
更新日期:2020-02-01
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