The Research Of Electrical Characteristics And Prospecting Significance Of Organic-rich Shale In Southern Sichuan And Eastern Chongqing Area

The middle-upper Yangtze area has better shale gas exploration and development prospect in our country, it develops many sets of marine shale, contains organic matter, it is one of the main shale gas reservoirs in our country. But it develops limestone near-surface, and its terrain conditions are very bad, there are many influences in seismic exploration, it has an effect on seismic prediction of shale gas sweet spots. In recent years, the study of the electrical property parameters in reservoir description has made great progress. The research result of the electrical characteristics and origin of organic-rich shale make electrical prospecting be a potential method of shale gas interpretation. The research emphasis of this paper is study the electrical characteristics of organic-rich shale in southern Sichuan and eastern Chongqing area, provide physical basis of shale gas distribution and evaluation based on rock electrical characteristics.We did regional geological survey and field profile measuring, collected outcrop shale and underground shale core samples of Longmaxi formation, and selected representative samples for testing. Using X-ray diffraction(XRD) and organic carbon detection to research the organic carbon content, mineral composition and brittle characteristics of the shale samples. Using scanning electron microscopy(SEM) analysis and casting thin sections to analysis the micro pore types of the shale samples. Using high pressure mercury injection method, nitrogen and carbon dioxide adsorption experiments to test the microscopic pore structure and adsorption ability of the shale samples. Using 1260 A type impedance phase analyzer to measure electrical characteristics of the shale samples. The results of these experiments indicate that:(1), the mineral composition of Longmaxi formation shale in the study area is complex, and organic carbon content is higher, the average brittleness mineral content is more than 60%, brittleness is very good.(2), The Longmaxi formation shale in the study area has many nanoscale pores, pore types can be divided into matrix pore and fracture pore. Matrix pore is divided into inorganic and organic pore, the main pores are organic pore, intergranular pore, intragranular pore.(3), The microscopic pore structure of organic-rich shale in the study area is complex, reservoir pore morphology is usually open type, such as cylinder pore and parallel plate pore, open at both ends, connectivity is good. The Longmaxi formation shale in the study area has relatively large pore volume and specific surface, the average pore size is small, micropore is well-developed, it is good for gas adsorption.(4), The characteristic of organic-rich shale in study area is low resistance and high polarization.According to these analysis results, we researched the relationship between electric property and mineral composition, TOC content, pyrite content, pore type of the organic-rich shale in study area. There are negative correlation between resistivity and TOC content, pyrite content, positive correlation between polarizability and pyrite content. These shale samples which mainly consist of organic pore have low resistivity, which mainly consist of intergranular pore and intragranular pore have middle resistivity, which mainly consist of intercrystalline pore have high resistivity. These shale samples which connectivity is good have low resistivity, which connectivity is bad have high resistivity. We also used statistical comparison analysis to research the electrical characteristics and origin of organic-rich shale in southern Sichuan and eastern Chongqing area. According to the electrical characteristics, we made a preliminary division of organic-rich shale in three different areas.