Reservoir Characteristics And Hydrocarbon Accumulation Model Of Bedrock Weathering Crust In Southern Part Of Central Paleo-Uplift Belt,Songliao Basin

In recent years,oil and gas exploration in fault and depression strata of early Cretaceous in Songliao Basin has entered the stage of high exploration.The exploration difficulty increases,and exploration effect decreases.To find a new replacement field for deep oil and gas exploration,the focus of exploration has gradually shifted to bedrock weathered crust-type oil and gas reservoirs in central paleo-uplift belt.At present,the understanding of bedrock weathering crust in the basin is not deep enough.The complexity of basement lithology,development model of weathering crust reservoir and accumulation increases the difficulty of exploration and restricts the exploration process.Therefore,by studying the lithology,structural characteristics and dominant reservoirs of basement weathering crust,we can improve the understanding of bedrock weathering crust reservoir and clarify accumulation.These studies have a certain guiding role for deep oil and gas exploration in new fields of Songliao basin.Based on the core,rock thin section,logging and seismic data in southern part of central paleo-uplift belt,this paper makes an in-depth and systematic study on the rock types,longitudinal structural characteristics of bedrock weathering crust,reservoir development characteristics,accumulation conditions and modes in the study area.We discussed the main controlling factors for the development of basement weathering crust reservoir structure,defined the development characteristics of bedrock weathering crust reservoirs with different rock types.Finally,through the study of the formation conditions of oil and gas reservoirs in the study area,we established two types of bedrock accumulation modes.The three major rocks of basement are developed,the lithology is complex and diverse.The main rock types include granite,granodiorite,diorite,andesite and tuff of igneous rocks,schist and phyllite of metamorphic rocks,limestone and glutenite of sedimentary rock,etc.The results of physical property test and analysis show that granite,granodiorite,diorite,andesite,schist and tuff are the main dominant lithology in the study area,which are easy to develop effective reservoirs.The reservoir space types of weathering crust reservoir mainly include fractures,dissolution pores and micropores between crystals.In this paper,according to the dominant lithology,the bedrock weathering crust reservoirs in central paleo-uplift belt are divided into five types: acid intrusive rock reservoirs,neutral intrusive rock reservoirs,andesite reservoirs,tuff reservoirs and schist reservoirs.Combined with lithology,physical properties,logging and other data,the vertical structure of bedrock weathering crust reservoir is divided into weathering stratified deposit layer,weathering leaching zone,fracture development zone and insider segment from top to bottom.The weathering stratified deposit layer is composed of rock masses,gravels or clay minerals.It is seriously affected by weathering,denudation,leaching and dissolution of surface water.The physical properties of the reservoir are poor.It usually does not have oil and gas storage capacity,and the thickness is generally less than 5 meters.The weathering leaching zone is the main reservoir development part,the type of effective reservoir space is mainly dissolution pores and fractures.Oil,gas and water layers are generally developed,with a thickness of 20-80 meters.The fracture development zone takes structural fractures and partial dissolution fractures as the main reservoir space types.The dissolution pores are less developed in this section due to the weakening of weathering and leaching.The reservoir heterogeneity is strong,the greater the depth,the worse the reservoir physical properties.The thickness of this section is generally 30-70 meters.The insider segment is dense rock,which is less affected by weathering and leaching.Generally,there is no reservoir or only dry layer.The development characteristics of bedrock weathering crust reservoirs are affected by many factors,mainly including lithology,the development thickness of bedrock rock mass,the height of bedrock paleo-geomorphic structure bulge,tectonic movement,etc.The basement lithology determines the physical conditions for the formation of weathering crust reservoirs.The mineral composition determines the main space types of weathering crust reservoirs and affects the physical property conditions of the reservoirs.The physical property conditions of acidic intrusive rocks are the best,while neutral intrusive rocks,andesite,tuff,schist,etc.are poor.After the formation of bedrock structure,the greater the height difference between high part of the structure and the surrounding low part,the longer the duration of weathering and denudation.In this case,the high part of bedrock structure is easy to form weathering crust structure.The quality of weathering crust reservoir conditions is mainly related to lithology.The lithology determines the physical conditions for the formation of weathered crust reservoir in bedrock.The length and degree of weathering determine the thickness of weathered crust and the structural characteristics of longitudinal weathered crust.Based on the study of configuration relationship between source rock and reservoir,the contact relationship between bedrock and surrounding rock and the accumulation conditions,we have summarized two accumulation modes in the study area: New formation hydrocarbon generation and ancient formation reservoir-near source hydrocarbon supply-deep and large faults,fracture network and unconformity surface transportation-non-source rock sealing-bedrock structure top reservoir integrated reservoir mode,New formation hydrocarbon generation and ancient formation reservoir-source internal hydrocarbon supply-faults,fracture network,unconformity surface transportation-source rock sealing-bedrock top reservoir integrated reservoir mode.Their main difference lies in the relationship between source and reservoir allocation,sealing conditions and buried depth.The source rocks of near source hydrocarbon supply accumulation model are located on both sides of the bedrock uplift structure.The direct caprock on the top of the bedrock is the sedimentary stratum of Quantou formation and Yingcheng formation.The lithology of weathered crust reservoir is usually schist,granite and diorite,with a buried depth of 880-2500 meters.The source rock directly covers the bedrock uplift structure.The top direct caprock is also the deposition of Huoshiling formation or Shahezi formation.The lithology of weathered crust reservoir is mainly tuff and limestone,with a buried depth of 3000-4000 meters.When the basement lithology is intrusive rock and metamorphic rock,it is easy to form near source hydrocarbon supply-non-source rock sealing type oil and gas reservoir on the top of bedrock.When the basement lithology is extrusive rock and sedimentary rock,it is easy to form source internal hydrocarbon supply-source rock sealing type oil and gas reservoir on the top of bedrock.