Study On Characteristics And Effectiveness Of Lacustrine Shale Reservoir

Aiming at problems of weakness of the basic research,uncertainty of effective reservoir space types and its genesis and incomplete evaluation of reservoir effectiveness of lacustrine shale,this paper takes shale of E₂S₄^S Submember-E₂S₃^X Submember in Dongying sag and Zhanhua sag in Jiyang depression and E₁f₂ Member in Subei basin as the examples for study.The thesis has comprehensively used core,geophysical and other test data,and applied the theoretical knowledge of sequence stratigraphy,sedimentology,mineral petrology and rock mechanics.On the basis of multi-scale observation of core-thin section-SEM,the study has carried out the research on sequence stratigraphy,sedimentary environment,diagenesis and reservoir space of shale by means of thermal simulation experiment and digital core technology.Finally,from perspective of geological genesis,a method for evaluating the effectiveness has been put forward after summarizing the characteristics of shale reservoir.Based on the assumption that tectonic extensional subsidence leads to falling of the relative lake-level in stagnant lacustrine basin,the paper has determined forming mechanism of the sequence boundary for E₁f₂ Member and E₂S₄^S Submember and then divided systems tract by using the four-division model of sequence stratigraphy which is suitable for lacustrine shale.Through analyzing seismic reflection characteristics,range of stratigraphic distribution,lithological association and logging curve response characteristics,parasequence sets and parasequence types of shale in different structural belts of basin have been identified in single well combined with water salinity change and ideal sedimentary sequence that is summarized on the basis of core observation.Based on this,the study has finished sequence stratigraphic correlation in connecting wells by using sedimentary sequence and logging curve response.Finally,it has be determined that the E₁f₂ Member and E₂S₄^S Submember consist of a third-order sequence and four-component systems structure respectively,which include lacustrine lowstand systems tract,lacustrine transgression systems tract,lacustrine highstand systems tract and lacustrine regression systems tract.Organic-rich shale mainly develops in lacustrine transgression systems tract and lacustrine highstand systems tract.Taking E₁f₂ Member as the examples,the thesis has analyzed sedimentary conditions,established the integrated sedimentary model of shale and also determined sedimentary environment of organic-rich shale.Shale in E₁f₂ Member is totally shore continental lacustrine deposits forming under dry hot climate of central to south subtropical climate with climate regularly changing wet later.Lake basin extended continuously with brackish water,strong oxidation and intermediate water delimitation.With short transgression,shale in E₁f₂ Member is characterized by sedimentary provinces,which is divided into muddy water desalination deposit and clear water salinization deposit and also divided into muddy water desalination deposit in flooded area,clear water salinization deposit in transition area and still water salinization deposit in clear water.Under paleogeomorphic settings of local uplift of E₁f₂Member,combining characteristics of vertical stratification and plane division of the lake,an integrated sedimentary model of shale in flat platform has been established under climate control.The organic-rich shale forms in still,brackish,strong reducing and stratified water under humid climate or during climate transition periods between dry hot and humid climate.Organic-rich shale is distributed mainly in clear water and end of muddy water which is far from coarse-grained clastics.The lithofacies classification scheme of shale has been established by combining core macrostructures and rock types.The thesis has proposed the principle of“four-component and three-terminal element”to determine rock types with the introduction of organic matter content considering importance and genetic significance of it.Based on whether organic matter content is over 15%,the shale is firstly divided into two major classes,organic-rich shale and organic-poor shale.Then organic-poor shale is divided into subclasses following the principle of“three-level naming”.On this basis,considering the practicality and reservation of the traditional names,classification names have been further simplified.Finally,lithofacies classification of shale has been finished in E₂S₄^S Submember-E₂S₃^X Submember and E₁f₂Member,which includes oil mudstone?shale?,sheet-like lime?dolomitic?claystone and so on,altogether seventeen types.Organic-rich shale usually has laminated and sheet-like structure,high content of organic matter and is also lime?dolomitic?.Moreover,shale forms under mixed sedimentation and its rock composition and macrostructure have a significant indication to sedimentary environment and intensity of sedimentationTaking E₁f₂ Member as the examples,the study has identified diagenetic types,diagenetic stages and diagenetic evolutionary sequences.The main types of reservoir space have been determined.On this basis,the study has carried out thermal simulation experiment of diagenesis,thus summarizing evolution model of diagenesis and determining genesis of organic pores in shale.The study shows that the main diagenetic types of E₁f₂ Member include compaction,dissolution,cementation,metasomatism,dolomitization,synaeresis and transformation of clay minerals and thermal evolution of organic matter.The main diagenetic stage is A period of the middle diagenesis,partly belonging to B period of the early diagenesis and B period of the middle diagenesis.The diagenetic evolutionary sequences are as follows:in the early stage,the collapse and shrink of smectite occurred;in the middle stage,illite mixed layers rapidly formed and transformed,and organic matter generated a great deal of hydrocarbons with dissolution of unstable components.in the late stage,fractures were filled by authigenic minerals.Dissolution pores and fractures are the most important types of shale reservoir space,followed by organic pores.On the basis,organic-rich shale samples have been selected for thermal simulation experiment of diagenesis.The results show that as the experimental temperature increases,clay minerals transform with dissolution of unstable components becoming stronger.Organic pores resulting from hydrocarbon generation during kerogen degradation increase with different characteristics.Kerogen type?generates hydrocarbon in the way of“dissociation”and shrinks as a whole,thus producing pores at the boundary between organic matter and grains.While kerogen type?generates hydrocarbon in the way of“paralleled removal of functional group”,producing pores both inside and at the edge of organic matter with residual inert carbon skeleton.Kerogen type?is between type?and type?,and it can produce pores inside and at the edge of organic matter and at the boundary between organic matter and grains.This thesis has systemically studied types,charateristics,influence factors and evolution law of organic pores by taking E₁f₂ Member as the examples.Organic pores have both primary and secondary genesis.Secondary organic pores display various shapes and distribute heterogeneously.Especially when maturity of organic matter reaches a certain degree,the internal structure of kerogen type?is a honeycomb connecting body,which is verified by3D reconstructed model of organic pores by digital core based on FIB-SEM.Temperature,overburden pressure,inorganic minerals and fluid are the external conditions influencing the development of organic pores.Organic matter types,maturity and content are the internal factors controlling the development of organic pores.Organic pores have the characteristics of episodic evolution and differential evolution,which is closely related to fabric evolution and hydrocarbon-generating mechanism during the process of hydrocarbon generation from different types of kerogen and is also verified by thermal simulation experiment.The thesis has systemically summarized types,charateristics,origins of natural fluid pressure fractures by taking shale of E₂S₄^S Submember-E₂S₃^X Submember as the examples.Early sluiced fractures have the typical characteristics of snaking morphology,and bedding-parallel vein fractures filling with fibrous calcite vein are closely related to calcite crystallization and fracture development caused by fluid overpressure.Hydrocarbon generation and expulsion fractures generated by hydrocarbon-generating pressurization of kerogen are generally filled by bitumen.Aiming at charateristics and origin of hydrocarbon generation and expulsion fractures,samples of organic-rich shale have been selected for thermal simulation experiment under high temperature and pressure.The results shows that hydrocarbon generation and expulsion fractures initiate from kerogen,extend in laminae-parallel direction,expand along grain boundary and become curved at the end,thus finally connecting with longitudinal fractures.The pore fluid pressure and rock fracturing strength are the main influencing factors for formation of hydrocarbon generation and expulsion fractures.Pressurization by the rapid generation of hydrocarbons provides the impetus for fracture initiation.The directional distribution of composition in shale results in lower fracturing strength in laminae-parallel direction.The flat kerogen helps hydrocarbon generation and expulsion fractures to initiate from tips and edge of kerogen.The fractures extend along the laminae when tensile strength is lower in the vertical direction than in the horizontal direction.Mechanical property is weak at grain contact boundary,thus allowing fractures to expand around grains.The direction change of stress intensity factor at fracture tips controls fractural diverting.When pore fluid pressure fields at different fracture tips overlap,fractures will interconnect into a network.Oil mudstone?shale?is high-quality source rock and can self-generate hydrocarbon.Inorganic mineral pores,organic pores and fractures are developed with self-storage capacity.Clay-organic rich soft rock facies are interbedded with carbonate-rich hard rock facies to form sandwich-type source-reservoir-cap assemblages with self-sealing property.Asphalt is mainly in the form of free state,followed by adsorption state and shows flow characteristics.Asphalt migration resistance is the displacement pressure,including capillary force and adsorption resistance,and the main driving force is pore fluid pressure.The asphalt migration modes include both“pore”and“piston”types,which are controlled by pore fluid pressure,displacement pressure and rock fracture strength.Because of multi-scale of shale reservoir space,vein-type model of multi-scale progressive transport network develops obeying Murray's law with dominant migration path in shale reservoir.Shale reservoir is a special lithologic reservoir controlled by sedimentary environment and has characteristics of self-generation,self-storage and self-sealing with development of pore fluid overpressure and multi-scale transport network.Based on the characteristics of shale reservoir,by taking lithofacies as the basic evaluation unit,the study has chosen source quality,reservoir property,alteration and fluidity as evalution contents.Then influencing factors have been analyzed,thus optimizing total organic carbon content,potential hydrocarbon generation amount,fracture types and its developing intensity,pore types and plane porosity,diagenesis,relative brittleness coefficient,brittleness index and critical pore diameter of flow as evaluation parameters.Finally,comprehensive evaluation of effectiveness of shale reservoir has been finished with fuzzy-grey mathematic method.Based on evaluation results,shale reservoir can be divided into type of oil generation,type of oil generation and accumulation,type of oil accumulation and invalid type.E₁f₂ Member mainly belongs to type of oil generation and accumulation and type of oil accumulation.E₂S₄^S Submember-E₂S₃^X Submember mainly belongs to type of oil generation and type of oil generation and accumulation.