Reservoir Characteristics And Oil-bearing Evaluation Of Laminated Shale In Chang 7 Member Of Ordos Basin

As a typical sedimentary structure in fine-grained sediments,laminae are widely found in mud shale deposits.In recent years,a large number of exploration and production practices show that laminated shale is a favorable lithofacies for shale oil production.Therefore,in this paper,the shale of the Chang 7 Member of Yanchang Formation in Ordos Basin is taken as the research object,and the development lamina,types,reservoir properties and oil-bearing characteristics with different laminated shale are determined through core description,thin and light section identification,determination of organic carbon content,soxhlet extraction,rock pyrolysis,XRD analysis,scanning electron microscope,nitrogen adsorption,and nuclear magnetic resonance-centrifugation finally the occurrence mode of shale oil is established.The development frequency of laminae in the macro-core shale of the Chang 7Member is highly heterogeneous,and the laminae under the micro-slices have six microshapes: flat,wavy,grain-ordered,lens-shaped,spotted and weak laminae.The types of laminae is divided into organic laminae,felsic laminae and tuffaceous laminae,and the combination types of laminae is divided into block structure,"organic matter+felsic" and "organic matter+tuffaceous".According to the characteristics of the development of the laminae combination,the laminated shale is divided into homogeneous shale,felsic laminated shale and tuffaceous laminated shale.Among them,homogeneous shale and felsic laminated shale are widely distributed in plane with large thickness,while tuffaceous laminated shale is only developed in Wuqi-Luochuan area with small thickness.Laminated shale is mainly composed of quartz,pyrite and clay minerals.According to the order of felsic laminated shale-homogeneous shale-tuffaceous laminated shale,the content of clastic minerals,specific surface area and pore volume decrease in turn,while the average pore diameter,clay minerals and pyrite content increase in turn.Shale mainly develops intergranular(crystalline)pores,intragranular pores,dissolution pores,microcracks and a small amount of organic matter pores.Pore morphology includes parallel plate-shaped slit,unilateral slit and ink bottle shape.Pore development is mainly affected by organic matter abundance,clastic minerals and pyrite content.Among them,the pores of homogeneous shale are concentrated in the range of 30 nm ～ 80 nm,with complex pore structure,while those of felsic laminated shale are widely distributed in the range of3 nm ～ 100 nm,with complex pore structure,while those of tuffaceous laminated shale are concentrated in the range of 7 nm ～ 30 nm and 1 nm ～ 3 nm,with simple pore structure.All the three shale types have reached the level of good source rocks and have great hydrocarbon generation potential.Homogeneous shale has good oil content and mobility,with the least amount of mobile oil.Most shale oil resources are potential to be mined,and its occurrence mode is "micro migration of page storage+long migration of sand storage".The felsic laminated shale has poor oil content,good mobility and high movable oil content.The shale oil resources are mainly potential to be mined and potential recoverable,and its occurrence mode is "storage and migration of foliated sand".The tuffaceous laminated shale has good oil content,poor mobility,and the highest amount of movable oil.The shale oil resources are mainly concentrated and potential to be produced,and its occurrence mode is "page generation and page storage micro migration".The oil content of laminated shale is mainly controlled by organic matter abundance,type,maturity,clay mineral content and pore structure,and its mobility is mainly controlled by organic matter abundance,maturity and clay mineral content.