天然气碳、氢同位素部分倒转成因——以苏里格气田为例

Partial reversal cause of carbon and hydrogen isotope compositions of natural gas:a case study in Sulige gas field,Ordos Basin

通过对苏里格气田上古生界天然气碳、氢同位素及轻烃分析,认为其天然气为来自石炭系—二叠系煤系地层的煤成气。将苏里格气田上古生界天然气与其成熟度相近而沉积环境不同、以及沉积环境相近而成熟度不同的天然气进行对比发现,氢同位素值随着母质沉积环境由湖相、陆相、海陆交互相和海相的变化而变重,并随着成熟度的增加而变重,这说明应用氢同位素进行沉积环境分析时一定要考虑成熟度的因素。苏里格气田上古生界天然气有碳、氢同位素序列同时出现部分倒转的现象。通过分析认为,碳、氢同位素同时倒转的主要原因可能是"累积"聚气不同阶段煤成气混合造成的。

This study analyzes the carbon - hydrogen isotope and light hydrocarbon compositions of natural gas in the Upper Paleozoic strata of Sulige gas field, Ordos Basin. Evidence shows that natural gas in this basin is coal-derived gas from the Carboniferous-Permian coal meas- ures. Then, a comparative analysis is conducted on natural gas in the Upper Paleozoic strata in Sulige gas field and those with similar maturi- ty but different depositional environment and similar depositional environment but different maturity. Results show that the hydrogen isotope value becomes heavier with the transition of depositional environment from lacustrine, terrestrial, and paralic to marine environment, as well as with increasing maturity of natural gas. Thus, maturity should be taken as an important factor in application of hydrogen isotope analysis for identification of depositional environment. Natural gas in Sulige gas field is associated with partial reversal of both carbon and hydrogen i- sotope compositions which mainly due to the mixing of coal-derived gases from cumulative hydrocarbon accumulation at different stages.