Genetic Mechanism Of Low-resistivity Marine Shale And Its Influence On Gas-bearing Properties

The marine shale in southern China has experienced multiple cycles of tectonic and thermal evolution events.The abnormally low resistivity encountered in the Longmaxi Formation shale in southern Sichuan has become one of the biggest problems in the field.The unclear shale low-resistivity genesis and large differences in low-resistivity shale production have greatly restricted the exploration and development of shale gas reservoirs in the Longmaxi Formation.Research on the genesis mechanism of shale low-resistivity and its control on gas-bearing properties should be carried out immediately.Therefore,the marine low-resistivity shale of the Longmaxi Formation in the Sichuan Basin was selected as the research object.The research results can be divided into the following points: the boundary of low-resistivity was defined;the controlling mechanism and main controlling factors of shale low-resistivity formation were clarified;the storage capacity development of different low-resistivity shales were clarified;the control mechanism of reservoir characteristics and gas-bearing property by low-resistivity genesis was determined;and also new evaluation methods in well logging and geological field were established.The dissertation established a shale resistance evaluation scheme with the limit of 5,15 Ω·m to distinguish the ultra-low resistance,low resistance,and conventional shale.It also respectively clarified the mechanism of different rock components to reduce the resistivity of shale and completed the identification of the main controlling factors of low resistivity.And 4 kinds of ultra-low resistivity and low resistivity shale genetic models are established in this chapter.Specifically,aiming at the mechanism of reducing resistivity of organic matter,the dissertation established a qualitative-quantitative experimental identification method for graphitization of organic matter,and clarified the resistivity reduction mechanism of graphitization of over-mature organic matter and changing the arrangement of carbon atoms.The graphitization degree of shale formation with low and ultra-low resistance is determined to be 15% and 25%,respectively.The research content of this chapter reveals the conduction mechanism of two kinds of minerals,namely,pyrite through internal iron-containing ions and clay minerals through cation exchange ability and occurrence bound water conduction.Meanwhile,it also illustrates the understanding that pyrite cannot cause the whole shale low resistivity and high clay mineral layer can lead to low resistivity in a thick stratum.A set of process is formed to identify the cause of reservoir low resistivity based on three factors: clay mineral content,water saturation and resistivity.According to the main low resistivity causes and resistivity classes,low resistivity shale can be divided into four types.The tectonic water formed by deep and large faults and the graphitization of over-mature organic matter is the main low-resistivity reason for Type A shale ultra-low resistivity shale(resistivity <5Ω·m).Type B shale ultra-low resistance(resistivity <5Ω·m)is only caused by graphitization of over-mature organic matter.Type A shale low resistance(resistivity of 5～15Ω·m)is caused by graphitization of high mature organic matter.Type B low resistance(resistivity of 5～15Ω·m)is caused by bound water of clay mineral.The dissertation elucidates the pore structure development characteristics of different low-resistivity shales.And the gas adsorption experiment before and after organic matter separation was used to determine the variation interval of organic pores in low-resistivity shale and the controlling factors of low-resistivity shale is expounded.Type A ultra-low resistance shale has the worst porosity and pore structure.Compared with conventional shales,the main contribution intervals of pore volume and specific surface area below 100 nm in low-resistivity shale reservoirs affected by graphitization have a forward trend,and small-scale pores contribute more pore volume and specific surface area.After graphitization,the organic matter becomes more plastic,pores are prone to deformation under compaction,and the size of macropore contributors is reduced,which is more significant under the influence of tectonic.The pore development of low resistivity shale reservoir is mainly controlled by the total organic matter content and the degree of graphitization(maturity).After graphitization exceeds 18%,the porosity,pore volume and specific surface area increase first and then decrease,which is closely related to the hydrocarbon generation of asphalt and the pore deformation after the completion of the secondary hydrocarbon generation.With the decrease of resistivity,the connectivity of oil-wetting pore decreases,and the wettability changes first to oil wetting and then to neutral wetting.Taking pore volume and specific surface area as evaluation parameters,the evaluation method of low-resistivity shale reservoir capacity is established,and it is stated that Type A ultra-low resistivity shale has the worst reservoir capacity and high exploration risk,and low-resistivity shale and Type B ultra-low resistance shale in tectonic stable areas still have good reservoir capacity.The dissertation also determined the main controlling factors of gas-bearing properties in different low-resistivity shale and established low-resistivity shale logging and geological evaluation methods.The total organic matter content,graphitization degree,water saturation,porosity and distance to major faults are the most important factors for gas bearing properties of low-resistivity shale.In view of the inapplicability of Archie’s formula for low resistivity shales,non-electrical saturation logging with the best response effect was used to determine the gas-bearing logging evaluation method.All the non-electric saturation models for different low resistivity shale have good application effects.Compared with the conventional methods,the dissertation introduces the graphitization degree and distance to major faults as important parameters for the gasbearing property geological evaluation for shale with low resistivity.The low resistance shales in different areas were evaluated by normalized treatment.The low resistivity shales of Changning Shuanglong syncline and Luochang syncline have poor gas content and great exploration risk,while the low resistivity shales of Tiangongtang anticline,northeast Luochang syncline and east Changning still have good gas content,which is the sweet spot for low resistivity shale gas exploration in the future.