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首页> 外文期刊>Geochimica et Cosmochimica Acta: Journal of the Geochemical Society and the Meteoritical Society >TSR versus non-TSR processes and their impact on gas geochemistry and carbon stable isotopes in Carboniferous, Permian and Lower Triassic marine carbonate gas reservoirs in the Eastern Sichuan Basin, China
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TSR versus non-TSR processes and their impact on gas geochemistry and carbon stable isotopes in Carboniferous, Permian and Lower Triassic marine carbonate gas reservoirs in the Eastern Sichuan Basin, China

机译：川东地区石炭纪，二叠纪和下三叠统海相碳酸盐岩气藏的TSR与非TSR过程及其对天然气地球化学和碳稳定同位素的影响

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摘要

The Palaeozoic and lowermost Mesozoic marine carbonate reservoirs of the Sichuan Basin in China contain variably sour and very dry gas. The source of the gas in the Carboniferous, Permian and Lower Triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking- and TSR-related processes for these gases. Sixty-three gas samples were collected and analysed for their composition and carbon stable isotope values. The gases are all typically very dry (alkane gases being >97.5% methane), with low (<1%) nitrogen and highly variable H2S and CO_2. Carboniferous gas is negligibly sour while the Lower Triassic gas tends to be most sour. The elevated H_2S (up to 62%) is due to thermochemical sulphate reduction with the most sour Triassic and Permian reservoirs being deeper than 4800m. The non-TSR affected Carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen-derived oil and primary gas and is highly mature. Carboniferous (and non-sour Triassic and Permian) gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane (a reversal of typical low- to moderate-maturity patterns). The gas in the non-sour Triassic and Permian reservoirs has the same geochemical and isotopic characteristics (and therefore the same source) as the Carboniferous gas. TSR in the deepest Triassic reservoirs altered the gas composition reaching 100% dryness in the deepest, most sour reservoirs showing that ethane and propane react faster than methane during TSR. Ethane evolves to heavier carbon isotope values than methane during TSR leading to removal of the reversed alkane gas isotope trend found in the Carboniferous and non-sour Triassic and Permian reservoirs. However, methane was directly involved in TSR as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded. CO_2 increased in concentration as gas souring proceeded, but typical CO_2 carbon isotope ratios in sour gases remained about -4‰ V-PDB showing that it was not solely derived from the oxidation of alkanes. Instead CO_2 may partly result from reaction of sour gas with carbonate reservoir minerals, such as Fe-rich dolomite or calcite, resulting in pyrite growth as well as CO_2-generation.
机译：中国四川盆地的古生界和中生界最低层的海相碳酸盐岩储层含各种酸性和非常干燥的天然气。石炭纪，二叠纪和下三叠统储层中的气体来源尚不确定，事实证明很难区分和区分与这些气体的热裂解和TSR相关过程的影响。收集了63个气体样品，并对其成分和碳稳定同位素值进行了分析。这些气体通常都是非常干燥的（烷烃气体> 97.5％甲烷），氮气含量低（<1％），H2S和CO_2高度可变。石炭系气体的酸度可以忽略不计，而下三叠纪天然气的酸度往往最高。 H_2S升高（高达62％）是由于热化学硫酸盐的减少，最酸的三叠纪和二叠纪储层的深度超过4800m。不受TSR影响的石炭系气体是一种二次气体，其衍生自腐泥质干酪根衍生的油和一次气体的裂解，并且非常成熟。石炭系（和非酸性的三叠纪和二叠纪）气体具有不同寻常的碳同位素，甲烷和丙烷的同位素比乙烷重（典型的中低熟模式的逆转）。非酸性三叠纪和二叠纪储层中的天然气具有与石炭系天然气相同的地球化学和同位素特征（因此具有相同的来源）。在最深，最酸的储层中，最深的三叠纪油藏的TSR改变了气体成分，达到了100％的干燥度，表明在TSR期间，乙烷和丙烷的反应速度比甲烷快。在TSR期间，乙烷演变为比甲烷重的碳同位素值，从而消除了石炭纪和非酸性三叠纪和二叠纪油藏中发现的逆烷烃同位素趋势。但是，随着气体酸化的进行，甲烷的碳同位素比逐渐增加，表明甲烷直接参与了TSR。随着气体酸化的进行，CO_2的浓度增加，但酸性气体中典型的CO_2碳同位素比仍约为-4‰V-PDB，这表明它并非仅源自烷烃的氧化。相反，CO_2可能部分是由于酸性气体与碳酸盐储层矿物（例如富铁白云石或方解石）反应而导致的，从而导致黄铁矿的生长以及CO_2的产生。

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