Reservoir Characteristics And Accumulation Model Of Mud Shale Of Wulalik Formation In Northwestern Margin Of Ordos Basin

The northwestern margin of the Ordos Basin is between the Alxa plate and the North China plate,complex regional structural evolution,the Ordovician sedimentary period was controlled by the Qinqi epicontinental sea,the sedimentary facies were very diverse,sedimentary lithology is complex.As the exploration and development work continues to advance,In 2017,the industrial gas flow was encountered in the mud shale section of the Wulalik Formation in Well Zhong 4,realize the first breakthrough of marine shale gas in this area.However,the level of related research on this layer is relatively low,therefore,it is necessary to study its basic reservoir-forming geological conditions,provide a theoretical basis for the next step of shale gas exploration and development.Under the premise of fully understanding the previous research results,the hydrocarbon source rocks,reservoirs,main factors controlling hydrocarbon accumulation and characteristics of source-reservoir-cap assemblage of Wulalik Formation,in the northwestern margin of the Ordos Basin are studied,and the Wulalik Formation accumulation model was established.The main research content and results are as follows:In the study area,the Wulalik Formation developed deep-water slope and deep-water basin facies,which is a favorable facies belt for mud shale deposition.The type of organic matter is mainly type?followed by type?₁,organic matter maturity is in the stage of high-over-maturity,organic matter abundance is low,mainly composed of medium source rocks.The storage space types mainly include organic pores,microcracks,intergranular pores,and inner hole pores poor connectivity between pores,micro cracks can play a certain role;The aperture in the reservoir is mainly distributed around 4nm,mainly meso-porous;The micro-nano pores provided by clay minerals in the reservoir are important spaces for gas occurrence.In short,shale reservoirs are typical low porosity and low permeability reservoirs.From the perspective of reservoir forming conditions,the first line of Well YT1-Z4 is 5-110 meters high in shale thickness,3800-3900m in buried depth,high maturity of organic matter,into the dry gas stage,providing a material basis for shale gas accumulation;the low-permeability characteristics of mud shale allow shale gas to be effectively preserved,and can be used as an effective local seal with compacted limestone,the overlying Yanghugou Formation underwent compaction as a better regional cap.The combination of source-reservoir-cap assemblage is in the form of interlayer stacked on top of each other,belongs to the within oil source self-generation and self-accumulation combination,shale gas enrichment is affected by lithology and structural preservation,and is distributed in broad and gentle anticlines.After comprehensive evaluation,it is believed that the Wulalik Formation in the study area has certain exploration prospect.