--> Influence of shale composition on nm-sized pore structures of longmaxi shale of lower silurian in jiaoshiba area, southeast sichuan basin, china

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Influence of shale composition on nm-sized pore structures of longmaxi shale of lower silurian in jiaoshiba area, southeast sichuan basin, china

Abstract

The shale gas production of JY-1 well, located in Jiaoshiba area, southeast Sichuan Basin, reaches ten thousand cubic meters per day, suggesting huge gas potential. The depositional environment changed from deep-water continental shelf to shallow-water continental shelf in early Silurian and deposited Longmaxi formation (S1l), which is mainly composed of argillaceous siltstone, carbonaceous shale and siliceous shale with an average thickness of 250m. To figure out the influence of shale composition on pore structure, twenty core samples from JY-1 are analyzed by a combination of focused ion beam milling, 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 is mainly distributed in 1.2∼8.0% and 0.001∼355.2mD, averaging at 4.6% and 21.93mD, respectively;(2) Total organic matter contents (TOC) range from 1.42wt% to 5.28wt% and the vitrinite reflectance varies from 2.5% to 3.0%; (3) The mineralogy 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) Micropores (<2nm) and mesopores (2∼50nm) are dominant pores and their average size is 0.19∼1.31nm and 4.67∼6.65nm, respectively. The D-R micropore volumes vary from 0.5 to 1.1cc/100g and BJH mesopore volume of 0.89∼2.64cc/100g. The BET surface areas are in the range of 12.3∼25.5m2/g. (5) Correlation analysis shows pore structures have interrelationship with TOC and inorganic mineral. The volume increases with increasing TOC indicating TOC is a primary controlling factor upon pore development. The shale's specific surface and volume also has a positive correlation with quarts content, whereas a negative correlation with clay content. It can be inferred that quartz-rich shale have higher total porosities and more nano-scale pores.