--> Influence of Shale Composition on Nano-Scale Pore Structures of Longmaxi Formation of Lower Silurian in JiaoShiba Area, Southeast Sichuan Basin, China

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Influence of Shale Composition on Nano-Scale Pore Structures of Longmaxi Formation of Lower Silurian in JiaoShiba Area, Southeast Sichuan Basin, China

Abstract

The shale gas production of Jiaoye 1 well, located in Jiaoshiba area, southeast Sichuan Basin, reaches ten thousand cubic meters per day, suggesting huge gas potential in China. The depositional environment in research area changed from deep-water continental shelf to shallow-water continental shelf in early Silurian and deposited Longmaxi formation (S1l), which is rich in graptolite fossils. The upper part of S1l is mainly composed of argillaceous siltstone and gray mudstone with an average thickness of 160m. The lower part is is mainly composed of carbonaceous shale, siliceous shale with an average thickness of 90m. To figure out the influence of shale composition on pore structure in S1l, twenty core samples from Jiaoye 1 well are analyzed by a combination of focused ion beam milling(FIB), SEM technology, X-ray diffraction, low pressure CO2 and N2 sorption, high pressure Hg porosimetry. The results are summarized to five points: (1) The porosity and permeability of S1l shale is mainly distributed in 1.2∼8.0% and 0.001∼355.2mD, averaging at 4.6% and 21.93mD, respectively;(2) The total organic matter contents (TOC) range from 1.42wt% to 5.28wt% and the vitrinite reflectance (Ro) varies from 2.5% to 3.0%; (3) The mineralogy of S1l is dominated by quartz and clay mineral. Quartz ranges between 18% and 71%, varying inversely with clay contents, which range from 17% to 63%. Andreattite is the dominant clay mineral, but illite may also be present in significant quantities (up to 39%); (4)The pores in the shale are mainly micropores (<2nm) and mesopores (2∼50nm), and their average size is 0.19∼1.31nm and 4.67∼6.65nm, respectively. The shales have Dubinin–Radushkevich(D-R) micropore volumes varying from 0.5 to 1.1 cc/100g and Barrett–Joyner–Halenda(BJH)mesopore volume of 0.89∼2.64cc/100g. The N2 and CO2 Brunauer-Emmett-Teller (BET) surface areas are in the range of 12.3∼25.5m2/g and 11.3∼34.1 m2/g, respectively. (5)Correlation analysis shows pore structures have interrelationship with TOC and inorganic mineral. The surface area and volume increase with increasing TOC indicating that the organic matter is a primary controlling factor upon pore development. The shale's specific surface and volume also has a positive correlation with inorganic minerals and quarts content, whereas a negative correlation with clay content. It can be inferred that quartz-rich shales have higher total porosities and more nano-scale pores