Shale oil resource evaluation in complex structural belt of superimposed basin:A case study of middle Permian Lucaogou Formation in Bogda area,southeast margin of Junggar Basin
The shale oil is the liquid hydrocarbon trapped in various pores of source rock strata,and the studies on the influence of the preservation conditions of different zones on the shale oil loss and its quantitative characterization are inadequate during the shale oil resource evaluation at present. The evaluation of shale oil resource in complex structural areas is of great practical significance to guide shale oil exploration in superimposed basin that experienced multiple tectonic movements in western China. Therefore,taking the shale oil of the Middle Permian Lucaogou Formation in Bogda area,the southeast margin of Junggar Basin as an example,based on the detailed anatomy of geological structure and the division of the preservation evaluation unit,and according to the outcrop section and total organic carbon(TOC)content of the drilling rock sample,pyrolysis hydrocarbon S1 and the measured data of extraction of chloroform bitumen“A”,the relative weight assign? ment quantitation models of preservation coefficients of shale oil in different preservation evaluation units and different lithofacies are established to quantitatively characterize shale oil loss degree in different evaluation unit. Based on the calculation of original shale oil resources,the amount of residual shale oil resources is determined. The results show that:①There are four types of preservation evaluation units in Bogda area,including sag area,structural stability area,thrust transformation area,and surface outcropping area. The oil content of rocks in different units decreases with the increase of tectonic deformation strength and the deterioration of preservation conditions;②The reservoir space type and the pore structure of shale oil reservoir control the difference of TOC and oil content between mudstone and sandstone lithofacies. There is a power function relationship between mudstone and sandstone,and a linear relationship between mudstone and sand? stone shale oil;③Based on the quantitation models of TOC and oil content of shale oil of mudstone and sandstone shales in different preservation and evaluation units,the quantization of the relative weight assignment of preservation coefficient by units and lithofacies is achieved,which lays a foundation for the scientific evaluation of the amount of residual shale oil resources in the tectonic reconstruction area;④Bogda area has abundant shale oil resources. Qitaizhuang,the middle and north of Chaiwopu Sag are favorable shale oil exploration targets,and sandstone shale oil in Lu3 Member is the main exploration interval and target.