煤成甲烷、乙烷碳同位素动力学研究和应用——以鄂尔多斯盆地上古生界煤成气为例

Carbon Isotope Modeling of Coal-derived Methane and Ethane from the Upper Paleozoic of the Ordos Basin,China

同位素动力学模型的发展和应用使天然气的研究工作进入了一个动态的研究阶段。迄今为止,大多数研究工作集中在甲烷的碳同位素动力学模拟。而少有地球化学家对乙烷及其他重烃气组分的碳同位素组成的动力学模拟加以重视。本文在甲烷碳同位素动力学模型基础上,编制了限定体系下乙烷的同位素动力学模拟软件。并在热解模拟实验的基础上对鄂尔多斯盆地上古生界煤成气甲烷、乙烷的同位素演化进行了模拟,获得长期累积甲烷、乙烷的同位素值分别为δ13C1=-33.46‰、δ13C2=-23.1‰,与上古生界储层中天然气的组分特征(δ13C1=-32.95‰、δ13C2=-23.41‰)基本一致,与下古生界天然气的同位素组成存在着明显的差别,说明上古生界煤与上古生界天然气存在着源-气的对应关系;而对下古生界天然气来说,上古生界煤系不是主要的气源岩。

Kinetic modeling of stable carbon isotope ratios of methane is a useful method in simulating the accumulative history of a gas pool. The usage of modeling of stable carbon isotope ratios of other heavy components （C2-C5） is rare because of their low content in natural gases. Comparison between δ^13C2 in confined system and in open system showed that δ^13C2 is sensitive to the openness of a system. As δ^13C2-5 is much more stable than δ^13C1 in migration and bacterial contamination, δ^13C2-5 is as valuable as δC1 in assessing the history of a gas accumulation. In this paper, coal from the Upper Paleozoic of the Ordos basin was pyrolyzed in a confined system and the kinetic modeling of δ^13C2 was developed. The modeled values of δC1 （-33.46‰） and δC2（-23.10‰） from this coal were close to the data of natural gases produced in the Upper Paleozoic, but different from those in the Low Paleozoic. A conclusion can be drawn that natural gases in the Upper Paleozoic are mainly derived from long in-situ cracking of coal, while gases in the Low Paleozoic were from the overmature marine organic matters of the Low Paleozoic.