鄂尔多斯盆地天环坳陷南段侏罗系原油油源分析

Oil Sources Analysis of the Jurassic Crude Oil in the Southern Tianhuan Depression,Ordos Basin

天环坳陷南段位于鄂尔多斯盆地西南部,跨西缘冲断带和天环坳陷两个构造单元,地质条件复杂。近年来针对该区侏罗系低幅度构造岩性油藏的勘探取得了重大突破,钻遇多口工业高产油流,但目前对侏罗系油气的来源问题仍存在不同的认识,制约了勘探的进一步深入。应用色谱/质谱和碳同位素等分析方法和手段,对侏罗系原油与三叠系延长组长4+5段、长6段和长7段3套烃源岩从生物标志化合物、碳同位素等方面进行对比研究,并结合侏罗系油气成藏特征,探讨了侏罗系原油的油源。结果表明:天环坳陷南段侏罗系原油和长6段、长7段烃源岩在母源性质、沉积环境、成熟度等方面具有相近似的特征,二者具有较好的亲缘关系;区域性不整合面、裂缝成为沟通侏罗系储层与长6段、长7段油源的运移通道;侏罗系原油主要来源于长6段和长7段烃源岩。

The southern Tianhuan Depression is located in the southwest of the Ordos Basin,stretching across both of the west thrust belt and the Tianhuan Depression with complex geological conditions.In recent years,the exploration of the Jurassic low-amplitude structural-lithologic reservoirs has gained a great breakthrough with several high productive oil-flow wells found in this region.However,there are some different points of view about the resources of the Jurassic oil and gas,which has constrained the further exploration.In this paper,comparative studies have been done about the Jurassic crude oil and three series of source rocks which are separately from the Chang 4＋5,Chang 6 and Chang 7 Sections of the Triassic Yanchang Formation from the aspects of biomarker and carbon isotope,etc.with the methods of GC-MS and carbon isotope analysis.Besides,the oil sources of the Jurassic crude oil have been discussed according to the Jurassic oil and gas accumulation characteristics.The study results show that the Jurassic crude oil and source rocks in the sixth and seventh members of Yanchang Formation in the southern Tianhuan Depression have similar characteristics in the aspects of resource properties,sedimentary environments and maturity;meanwhile,the regional unconformities and cracks formed the migration pathway of the Jurassic reservoir beds and the crude oil of the sixth and seventh members of the Yanchang Formation;the Jurassic crude oil is mainly derived from the source rocks of the sixth and seventh members of the Yanchang Formation.