阿姆河盆地别什肯特坳陷及邻区中下侏罗统烃源岩地球化学特征与勘探意义

Geochemical characteristics and exploration significance of Middle–Lower Jurassic source rocks in the Beshkent Depression and adjacent areas of the Amu Darya Basin

阿姆河盆地别什肯特坳陷及邻区中下侏罗统发育一套潜在烃源岩,此烃源岩也是一套潜在的含油气系统,但是目前研究仍然薄弱。为此,通过钻井岩心和岩屑有机地球化学分析及盆地模拟方法,研究了别什肯特坳陷及邻区中下侏罗统烃源岩的有机质丰度、类型与成熟度等地球化学特征及烃源岩的形成环境,并明确了油气勘探的方向。研究结果表明:①中下侏罗统海陆过渡相泥岩段烃源岩有机质丰度介于0.50%~3.53%(平均值为1.27%),有机质类型为偏腐殖混合型,有机质热演化程度为介于0.7%~1.1%,为有效烃源岩;②泥岩段上部的浅海相泥质石灰岩段有机质丰度低,为非烃源岩;③泥岩烃源岩可溶有机物以低碳数正构烷烃为主,呈典型单峰,C27甾烷含量高于C29甾烷,三环萜烷和四环萜烷含量高,但是饱/芳比低,孕甾烷含量高,芳烃硫芴含量高,因此,有机质可能来自浅水的宏观底栖藻类,形成于潟湖环境。结论认为,别什肯特坳陷中下侏罗统烃源岩在早白垩世晚期已成熟,以生气为主,生油为辅,现今仍处在生烃高峰阶段,与构造圈闭形成时期配置好,具备较大的成藏潜力;致密砂岩气是该套含油气系统的重要勘探方向。

The Middle–Lower Jurassic in the Beshkent Depression and its adjacent areas of the Amu Darya Basin develops a set of potential source rocks,which is also a set of potential petroleum system,but the current research is weaker.Therefore,through organic geochemical analysis of drilling cores and cuttings and basin modeling methods,the geochemical characteristics of organic matter(such as abundance,types and maturity)of the Middle–Lower Jurassic source rocks in the Beshkent Depression and adjacent areas are studied,as well as their formation environment,and the exploration direction is clarified.The results show that the organic matter abundance of source rocks in the marine-continent transitional mudstone section of the Middle–Lower Jurassic is 0.50%–3.53%(averaging 1.27%),the organic matter type is II2–III,and the thermal evolution degree of the organic matter is 0.7%–1.1%,thus they are effective source rocks.The shallow marine argillaceous limestone in the upper part of the mudstone section is a non-effective source rock with low organic matter abundance.The soluble organic matter in mudstone source rocks is dominated by low-carbon n-alkanes with a typical single peak.The content of C27 sterane is higher than that of C29 sterane,and the content of tricyclic terpane and tetracyclic terpane is high,but the saturation/aromatic ratio is low,the content of pregnane is high,and the content of aromatic sulfur fluorene is high.Therefore,organic matter may come from shallow-water macrobenthic algae,and was formed in the lagoon environment.It is concluded that the Middle–Lower Jurassic source rocks in the Beshkent Depression had matured in the late stage of the Early Cretaceous,mainly generating gas and supplemented by oil.They are still in the peak stage of hydrocarbon generation and are well configured with the formation period of structural traps,with large potential to form a reservoir.Tight sandstone gas is an important exploration direction of the Middle–Lower Jurassic petroleum system.