Micron-to nano-pore characteristics in the shale of Longmaxi Formation, southeast Sichuan Basin

Abstract

For shale of Lower Silurian Longmaxi Formation in Chongqing, southeast Sichuan Basin, characteristics of micro-nano pores in marine shale reservoirs were well studies by means of Field-Emission Scanning Electron Microscope and Low-temperature Low-pressure Adsorption Experiment of CO2 and N2. Results showed that six types of pore were developed in the shale of Longmaxi Formation, i.e., organic pores, intergranular pores, intragranular pores, intercrystalline pores, dissolution pores and microfractures, among which the organic pores and intragranular pores in interlayers of clay minerals were most developed, and a plenty of dissolution pores were also well developed because of high thermal evolution degree. BET specific surface area of the shale in Longmaxi Formation ranged from 3.5 to 18.1 m2/g, BJH total pore volume was from 0.00234 to 0.01338 cm3/g, DA specific surface area of micropores vaired from 1.3 to 7.3 m2/g, and DA pore volume ranged from 0.00052 to 0.00273 cm3/g. The specific surface area of micropores in the shale accounted for 23.1%–80.2% of total specific surface area with an average of 50.3%, and the pore volume of micropores accounted for 12.1%–48.5% of total pore volume with an average of 32.3%. Micropore was the main storage space in shale reservoir for methane adsorption, that because capacities of specific surface area provided by micropores were considerably greater than those provided by mesopores and macropores. Pore size distribution of the shale was complex, and multiple different peaks occurred in the pore size curves, showing two or three peaks in the range from 0 to 100 nm and four peaks occasionally. TOC had a good linear relationship with pore structure parameters of micropores, mesopores + macropores and total pores in the shale, indicating that TOC was the most important control factor for micron-to nano- pore structure in the shale. After normalization of pore structure parameters to TOC, the pore structure parameters of total pores and mesopores + macropores, had positive linear relationships with content of clay minerals but negative linear relationships with content of brittle minerals, indicating that clay minerals and brittle minerals mainly controlled development of mesopores and macropores in the shale.