Methane released from groundwater: the source of natural gas accumulations in northern West Siberia
Methane released from groundwater: the source of natural gas accumulations in northern West Siberia
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DOI：
10.1016/s0264-8172(98)00085-3
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发表时间：
1999-05-01
影响因子：
4.2
通讯作者：
Littke, R
中科院分区：
文献类型：
--
作者：
Cramer, B;Poelchau, HS;Littke, R
西西伯利亚盆地是地球上最大的大陆沉积盆地，约占世界常规天然气储量的三分之一。根据气体成分和同位素数据，这种气体可以被描述为来自下白垩纪岩石的热成因气体（三角洲（13）C（1）约为千分之-36，C2+约为10体积%）和几乎纯的甲烷（98体积%）的混合物，其相对较轻的稳定碳同位素特征平均为千分之-51（三角洲（13）C（1））。同位素轻甲烷的比例从下白垩统到白垩系烃系统的顶部（Pokur组顶部）增加，主要储层位于那里。天然气的组成和巨大的体积很难用沉积物中天然气生成的既定模型来解释。盆地的水文地质由具有几乎全盆地横向连续性的厚含水层系统决定。最大的水动力系统是白垩纪含水层，其中98%的天然气被困。白垩纪含水层内的地下水饱和有相同的同位素轻甲烷的Pokur组的水库中发现的。根据岩石物理数据，建立了最大的天然气聚集地乌伦戈伊气田下白垩纪含水层的详细水文地质模型，并将地下水释放的天然气与储层中的天然气体积进行了比较。地下水甲烷释放的可能性是由于：（1）第四系石灰岩中沉积物的冷却，（2）气田下水压的横向梯度，（3）新生代隆起引起的压力和温度降低。第四纪冷却的作用是微不足道的，根据我们的计算，地下水流的影响的贡献很小。由于新近纪隆起，从地下水中释放的同位素轻甲烷的体积非常大，可以解释整个天然气的存在。这种气体的来源必须位于气田省的南部。(C)1999 Elsevier Science Ltd.保留所有权利。
The West Siberian Basin is the largest continental sedimentary basin on earth and contains about one third of the world's conventional reserves of natural gas. From gas compositional and isotope data this gas can be described as a mixture of thermogenic gas from lower Cretaceous rocks (delta(13)C(1) approximate to -36 parts per thousand, C2+ approximate to 10 vol%) and almost pure methane (98 vol%) with the relatively light stable carbon isotope signature of -51 parts per thousand (delta(13)C(1)) on average. The proportion of the isotopically light methane increases from the lower Cretaceous to the top of the Cretaceous hydrocarbon system (top of the Pokur Formation), where the main reservoirs are situated. The composition and the huge volume of gas is difficult to explain with the established models of gas generation in sediments.The hydrogeology of the basin is determined by thick aquifer systems with an almost basin wide lateral continuity. The largest hydrodynamic system is the Cretaceous aquifer, in which 98% of the natural gas is trapped. The groundwater within the Cretaceous aquifer is saturated with the same isotopically light methane as that found in the reservoirs of the Pokur Formation. Based on petrophysical data a detailed hydrogeological model of the Cretaceous aquifer beneath the largest gas accumulation, the Urengoy gas field, has been developed and the contribution of gas release from groundwater was compared with the volume of gas in the reservoirs. The potential methane release from groundwater has been evaluated as a result of: (1) cooling of sediments in Quaternary limes, (2) the lateral gradient of the hydraulic pressure under the gas field and (3) the pressure and temperature decrease due to Cenozoic uplift. The role of Quaternary cooling is insignificant and, according to our calculations, the contribution of the effect by groundwater flow is small. The volume of isotopically light methane released from groundwater due to a Neogene uplift is very large and can explain the entire gas in place. The source of this gas must be located south of the gas field province. (C) 1999 Elsevier Science Ltd. All rights reserved.