Low-resistivity Origin And Evaluation Of Gas-bearing Properties Of Marine Shale In Southern Sichuan Basin

Resistivity logs usually reflects the gas content of shale reservoirs,and low resistivity response is often considered as one of the characteristics of dry gas layers.With the deepening of shale gas exploration,it has been found that aboundant shale gas occurred in low-resistivity shale,and high-production gas flows have been obtained in local areas.However,the causes of low resistivity in shale reservoir has not been revealed yet,and the relationship between low-resistivity characteristics and gas-bearing properties is also unclear,which significantly restricts the process of low-resistivity shale gas exploration and development.Therefore,it is of great theoretical and practical significance to reveal the mechanism of how low-resistivity origin controls the gas-bearing property of the reservoir.Hence,this article collected samples from the Longmaxi Formation shale in the southern Sichuan Basin and utilized a series of experimental methods,such as petrological analysis,geophysical logging,field emission scanning electron microscope,high pressure mercury injection,gas adsorption,transmission electron microscope and X-ray photoelectron spectroscopy analysis.The results show that low-resistivity shale has a good hydrocarbon generation potential,and the total organic carbon content is generally higher than 2%.The organic matter has high thermal maturity,with Easy-R_oin the range of 2.18%to 3.51%.The low-resistivity shale reservoir is dominated by siliceous minerals and clay minerals,and the clay minerals are generally higher than other shales.Shale pores mainly consist of micropores and mesopores,characterized by complex pore structure.The specific surface area is mainly provided by micropores,and the pore surface is relatively flat.Based on resistivity well-logging data intergrated with statistical results,when Easy-R_o>2.9%,a large amount of graphite structure occurs in the organic matter.There is a negative correlation between logging resistivity and graphite proportion,and graphitization of organic matter is one of the main influencing factors leading to the decrease in resistivity of shale.When Easy-R_o<2.9%,the graphite structure in shale organic matter is extremely rare,no obvious correlation between logging resistivity and graphite ratio has been found.But under the joint control of high clay content and high formation water saturation,logging resistivity decreases as a power function with increasing water saturation.Based on the gas content testing results and statistical analysis results,TOC,porosity,clay mineral content,siliceous mineral content,and water saturation were selected as the evaluation parameters of the gas-bearing property of low-resistance shale reservoir.Due to the different causes of shale resistivity reduction,gas-bearing comprehensive evaluation models and classification evaluation standards were established for over-mature(Easy-R_o>2.9%)and high-mature(Easy-R_o<2.9%)low-resistivity shales.Evaluation results show that high-mature(Easy-R_o<2.9%)shale in the southern Sichuan Basin is the main target for low-resistivity shale gas exploration and development,and low-resistivity shale reservoirs(1<Z_g<2)are the most favorable.Gas-bearing properties of low-resistivity shale in over-mature zone(Easy-R_o>2.9%)are complex and there are certain exploration risks.Among them,layers 1,2,and 3 have exploration potential,while 4layers are high-risk layers for low-resistivity shale gas exploration.