The Influence Of Different Depositional Environments On The Resistivity And Polarizability Characteristics Of Shale

The Silurian Longmaxi formation shale in Sichuan Basin is an important series with high exploration degree and commercial development in China.The use of geophysical methods is a powerful means for shale gas exploration.Facing the complex terrain of southern Sichuan,electromagnetic method is widely used in shale gas exploration because of its advantages of economy and large detection depth.However,the electrical conduction mechanism and polarization mechanism of shale are extremely complicated,which makes the research on the electrical properties of shale rock relatively weak,which greatly restricts the relevant theoretical breakthroughs in rock electromagnetism and the application of electromagnetic exploration.In this paper,Longmaxi formation shale is selected as the research object.Firstly,the reservoir characteristics are characterized quantitatively and qualitatively,and the dispersion characteristics and influencing factors of shale are analyzed through complex resistivity experiment;Then,the geological events are identified and the sedimentary paleoenvironment is reconstructed,and the effects of sedimentary environment on mineral types and components,pore types and connectivity,organic matter abundance and so on are clarified;Finally,the differences of electrical properties of shale under different sedimentary backgrounds are comprehensively discussed,and the rock electrical mechanism model is established,so as to provide rock electrical basis for shale electromagnetic exploration and achieve the scale of core scale.Through the research,the following understandings are obtained:(1)The minerals,pores,and organic geochemical characteristics of Longmaxi Formation shale reservoirs are clarified,and four lithofacies types are classified.The main mineral composition of Longmaxi Formation shale in southern Sichuan and the periphery is quartz and clay minerals,and contains a small amount of dolomite,calcite,feldspar,pyrite,etc.The bottom shale has higher brittle minerals,the middle and upper shale has higher clay minerals.The Longmaxi Formation shale in the study area has good hydrocarbon generation potential,with an average TOC content of 1.97%and an average maturity of 2.96%.Through the argon ion polishing scanning electron microscope analysis technology,three main matrix pores(intergranular pores,intragranular pores,organic pores)and fractures were identified in the Longmaxi Formation shale.According to the mineral composition,sedimentary structure and pore type,the Longmaxi Formation shale is divided into four lithofacies: argillaceous shale,argillaceous/siliceous mixed shale,argillaceous-rich mixed siliceous shale and mixed siliceous shale.The mixed siliceous shale is characterized by high quartz content,high TOC content and organic matter pore development,which is the dominant lithofacies type in the shale of Longmaxi formation.(2)By measuring the complex resistivity of the rock,the factors affecting the frequency dispersion of shale are analyzed,and the resistivity and polarizability characteristics of different lithofacies are clarified through model fitting.Through the core complex resistivity test,it is found that the real part,imaginary part and phase of shale samples change with the change of frequency,that is,the characteristics of frequency dispersion occur.It is found that both mineral composition and organic carbon content have a certain impact on the dispersion characteristics of shale,among which pyrite content and organic matter content have a great impact on the dispersion characteristics of shale.Through model optimization and calculation,it is concluded that the damping least square method combined with double Cole Cole model has the best fitting effect.Through the resistivity and polarizability parameters,it is found that different types of shale show different electrical properties.Among them,the resistivity of mixed siliceous shale is the lowest(34.53 Ω· m)and the polarizability is the highest(0.159).(3)Four types of geological events were identified by means of sedimentology,and the shale paleosedimentary environment was reconstructed.By means of element geochemical analysis and testing,combined with core and outcrop observation,four main geological events in the sedimentary period of Late Ordovician and Early Silurian are identified: volcanic movement,hernantian ice age,upwelling event and turbidity current.Mud shale sedimentary paleoenvironment is affected by geological events,topography and other factors.By selecting appropriate geochemical indexes,the Paleoenvironment of shale sedimentary period of Longmaxi formation is reconstructed.The shale of Longmaxi formation is generally in warm and humid climate conditions,and only semi dry and cold climate conditions can be seen at the bottom.Paleoproductivity is generally high,especially the bottom mixed siliceous shale.Volcanic movement and upwelling provide rich nutrients for its biological growth.The redox conditions change greatly longitudinally,and gradually transition from reduction and secondary oxidation to oxidation conditions from bottom to top.And the terrigenous input of bottom mixed siliceous shale is low,while the terrigenous input of middle and upper part is high.(4)The influence mechanism of sedimentary environment on rock electrical properties under macro micro joint control is established.Firstly,the sedimentary environment affects the composition and content of minerals: the bottom shale has strong paleoproductivity,low terrigenous input and a large number of organisms,so that the mixed siliceous shale quartz is mainly biogenic and the quartz content is high;At the same time,the reduction environment is conducive to the formation of strawberry pyrite,which promotes the high content of pyrite and the small particle size of strawberry pyrite.Secondly,the sedimentary environment affects the degree of organic matter enrichment: reducing sulfide conditions,high paleoproductivity and low terrigenous input are the key factors for organic matter enrichment of mixed siliceous shale.Then,in the diagenetic process,the mixed siliceous shale with high TOC content will lead to a large number of organic matter pores under the effect of kerogen pyrolysis and hydrocarbon generation.Thanks to the strong anti compaction ability of biological siliceous,the pore connectivity of the bottom shale is good.Finally,the conductive mechanism and polarization mechanism of shale are clarified: the resistivity change of Longmaxi formation shale is mainly related to the pore network formed by inorganic and organic pores,high pyrite content and organic surface functional groups with high specific surface area.Under the condition of power on,the path length of conductive circuit is related,That is,its conductive mechanism is pore fluid ionic conductivity,which mainly controls the conductivity of rock.The polarizability of shale in Longmaxi formation is mainly affected by the interfacial polarization of electric double layers of clay minerals and pyrite.Its polarization mechanism is that electronic conductors and ionic conductors jointly control the polarization characteristics of rocks.Finally,the influence mechanism of sedimentary environment on rock electrical properties under macro micro joint control is established.