乍得Bongor盆地Ronier油田油气成因特征与生烃演化史分析

Characteristics of petroleum genesis and evolution analysis on hydrocarbon generation in the Ronier Oilfield,Bongor Basin,Chad

Bongor盆地是在中非剪切带影响下发育起来的中—新生代陆内被动裂谷盆地,其生储盖组合以下白垩统为主,盆地整体勘探程度低,对含油气资源和勘探前景认识有限。以Ronier油田为例,通过系统有机地球化学分析,结合该区的区域地质资料,从烃源岩及生烃潜力、生物标志化合物和生烃演化等方面进行综合研究,认为盆地下白垩统烃源岩TOC含量均值可达3.5%以上,为下白垩统暗色泥岩,饱和烃气相色谱、甾烷、萜烷特征显示,烃源岩发育于半咸水-淡水、弱氧化-弱还原的湖相沉积环境,有机质发育和分布具有典型陆相湖盆特征;有机质类型Ⅰ、Ⅱ1、Ⅱ2型,烃源岩累计厚度大,生烃潜力巨大,为Ronier油田提供主要油气来源,以下白垩统湖相烃源岩生烃过程为主。西部凹陷与中部凹陷烃源岩演化存在差异,Tmax演化显示前者整体处于未成熟-成熟演化作用早期,油气发现以低成熟油气为主;中部凹陷烃源岩成熟演化程度高,对周围斜坡供烃效应明显。反转构造带与生烃演化史和有利岩相带结合控制了油气的有利富集。

The Bongor Basin is a Meso-Cenozoic intracontinental passive rift basin developed under the influence of the Central African shear zone. It is dominated by the Lower Cretaceous source-reservoir-cap assemblage. The basin is under-explored overall, and knowledges of its oil/gas resources and exploration prospects are limited. Taking the Ronier Oilfield as an example, through a systematic organic geochemical analysis and combining with regional geological data, we concluded that the Lower Cretaceous source rocks belong to the Lower Cretaceous dark mudstone with TOC content over 3.5%. The characteristics of saturated hydrocarbon gas chromatography, sterane and terpane showed that the source rocks developed in a brackish water-freshwater and weakly oxidized-weakly reduced lacustrine sedimentary environment, and the organic matter had typical continental lacustrine characteristics in its development and distribution. The organic matter is type I , II 1 ＇ 1I 2＂ It has a thick accumulation of hydrocarbon source rock with huge hydrocarbon-generating potential, and is the main source of oil and gas for the Ronier Oilfield. History of the source rock evolution suggests that the central sag differed from the western sag and reached much higher evolutionary maturity in both source rock and oil products. The main oil and gas resources in the Ronier Oilfield on the northern slope came from the central sag, as evidenced in both geological and geochemical aspects. The inverted tectonic zone, along with the trajectory of hydrocarbon evolution and favorable lithofacies zone, controlled the process of oil and gas enrichment.