乍得Bongor盆地天然气地球化学特征及成因

Geochemical characteristics and genesis of natural gas in the Bongor Basin

Bongor盆地位于乍得境内的中非剪切带西北部,是中新生代重要的含油气盆地之一。笔者从Bongor盆地天然气组分组成及组分碳、氢同位素分析入手,综合运用常规有机地球化学分析方法,探讨了Bongor盆地不同构造单元天然气的成因及成熟度差异。分析认为,Bongor盆地天然气化学组成主要由烃类气体、非烃类气体和微量稀有气体组成,其中烃类气体属于生物-热催化过渡带气、原油伴生气和凝析油伴生气等有机成因气,非烃气体主要包括无机成因的氮气、二氧化碳和氢气等。在天然气成因研究的基础上,笔者还开展了Bongor盆地烃类气体的组成及来源分析,包括气-气对比和气-源对比,研究认为,不同构造单元的烃类气体虽然组成相同,但相对含量略有差异,表明不同构造单元的天然气具有相似的母质来源:M组和P组的暗色泥页岩;但也不排除K组暗色泥页岩的生烃贡献。不同构造单元天然气烃类气体的成熟度差异较大,这是由于天然气源自不同层位烃源岩或同一层位烃源岩在不同演化阶段的生烃产物。

The Bongor Basin, located in the northwest part of the Central African Shearing Zone in Chad, is one of the most proliferous basins of the Mesozoic- Cenozoic era. In this study, chemical composition and carbon/hydrogen isotopic analysis were performed on natural gas samples by routine geochemical methods to investigate the origin and maturity difference of natural gas in different structural units of the Bongor Basin. The results showed that the chemical compositions of the natural gas analyzed are mainly hydrocarbon and non-hydrocarbon gases with trace amount of rare gases. Hydrocarbon gases belong to organic gas, such as biogenetic and thermal cracking gases, while inorgatlic gases included N2, CO2 and Hz, etc. Gas-gas and gas-source correlations were obtained to determine the organic source of the natural gas. It showed that the minor composition dissimilarity in these gases are attributable to facies variations in different structural units, namely the M, P and possibly K group dark clay shale facies; whereas gases generated by organic source rocks in different structural units exhibit large difference in maturity, simply because they were generated in different thermal evolution stages.