Carbon and hydrogen isotopic characteristics of natural gases from the Luliang and Baoshan basins in Yunnan Province, China
Carbon and hydrogen isotopic characteristics of natural gases from the Luliang and Baoshan basins in Yunnan Province, China
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DOI：
10.1007/s11430-006-0938-8
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发表时间：
2006-09
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通讯作者：
Yongchang Xu;Wenhui Liu;P. Shen;Wanchun Wang;Xiaofeng Wang;Tenger;Yaomin Yan;Ruobin Liu
中科院分区：
文献类型：
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作者：
Yongchang Xu;Wenhui Liu;P. Shen;Wanchun Wang;Xiaofeng Wang;Tenger;Yaomin Yan;Ruobin Liu
云南陆良盆地和保山盆地是晚第三纪在石炭系和泥盆系基底上发育的两个小型陆相含油气沉积盆地，面积分别为325 km 2和254 km 2。90年代以来，这两个盆地相继发现了小型天然气藏。天然气主要由烃类气体组成，非烃类气体占不到2%。烃类气体中甲烷占99%以上，C2以上组分占0.2%以下。在前人地质背景、天然气组成及碳同位素研究的基础上，确定这两个气藏为细菌成因。本文对这两个盆地天然气的碳、氢同位素组成进行了全面的测定，并对天然气的生成机理进行了详细的探讨。陆良盆地天然气的δ 13 C1值在−72.1‰ ~ −73.3‰之间，δ DCH 4值在−242‰ ~ −234‰之间，表明细菌气的生成以CO2还原为主。已经证明，在陆相淡水条件下，确实发生了作为细菌气体生成过程的CO2还原。保山盆地天然气的δ 13 C1值为−62.5‰ ~ −63.5‰，δ DCH 4值为−252‰ ~ −260‰。这些同位素特征属于Whiticaret等人定义的乙酸发酵和CO2还原的过渡阶段。（1986年）。吕梁盆地的一个重要发现是纯生物成因乙烷的碳同位素组成，δ 13 C2值在−61.2‰-−66.0‰之间。这些碳同位素值在国内尚属首次报道。与世界上以前遇到的两种情况的δ 13 C2值小于−55‰相比，一个显著的区别是，后两种情况有明显的热源乙烷污染迹象。因此，不能排除轻δ 13 C2值是热源乙烷同位素分馏的结果。陆梁盆地天然气的δ 13 C2值均小于−60‰，热成因乙烷几乎不可能垂向和水平向运移进入盆地内的气藏。这说明陆良盆地中δ 13 C2值在−61.2‰ ~ −66.0‰之间的乙烷是纯生物成因的，为细菌作用产生乙烷提供了科学依据。这是自然科学中的一个重要发现，也为全面研究乙烷的形成机理提供了线索。
The Luliang and Baoshan basins of Yunnan Province are two small-sized continental oil/gas-bearing sedimentary basins, which were developed at the bases of the Carboniferous and Devonian systems during the Late Tertiary, covering an area of 325 km2and 254 km2, respectively. Since the 1990s, there have been discovered small-sized natural gas pools in these two basins. The natural gases are composed mainly of hydrocarbon gases, with nonhydrocarbons accounting for less than 2%. Of the hydrocarbon gases, methane accounts for more than 99%, and the components above C2account for less than 0.2%. On the basis of previous studies of geological background, the composition of natural gases and their carbon isotopic composition, it has been defined that these two gas pools are of bacterial origin. In this work we have comprehensively measured the carbon and hydrogen isotopic composition of natural gases from these two basins and have gone into the details of the mechanism of gas generation. Theδ13C1values of natural gases from the Luliang Basin are within the range of −72.1‰–−73.3‰, and theδDCH4values, −242‰–−234‰, indicating that the bacterial gas generation is dominated by the way of CO2reduction. It has been evidenced that under continental-facies fresh water conditions there did occur the CO2reduction as a process of bacterial gas generation. Theδ13C1values of natural gases from the Baoshan Basin are within the range of −62.5‰–−63.5‰, and theδDCH4, values, −252‰–−260‰. These isotopic characteristics are fallen into transitional phase of acetate fermentation and CO2reduction as defined by Whiticaret al.(1986). An important discovery in the Luliang Basin is the carbon isotopic composition of ethane of purely biogenetic origin, i.e., itsδ13C2values are within the range of −61.2‰–−66.0‰. These carbon isotopic values have been reported for the first time in China. As compared to theδ13C2values of less than −55‰ for the two cases encountered previously in the world, a significant difference is that in the latter two cases there are obvious signs of contamination caused by ethane of thermal origin. So it cannot be ruled out that the lightδ13C2values are the result of isotope fractionation of ethane of thermal origin. Theδ13C2values of natural gases from the Luliang Basin are all less than −60‰, and there is almost no possibility that ethane of thermal origin migrated vertically and horizontally into gas pools within this basins. This demonstrates that in the Luliang Basin the ethane, whoseδ13C2values are within the range of −61.2‰–−66.0‰, is of purely biogenic, hence providing scientific evidence that ethane can be produced by the bacterial process. This is an important discovery in natural science, and also provides clues to the comprehensive study of the mechanism of formation of ethane.