Pore structure and fractal characteristics are of great significance for gas adsorption and transport in shales. In order to study pore structure and fractal characteristics of shale, samples from the Upper Paleozoic in Huanjiang sag, Guizhong depression were investigated by organic geochemical analysis, X-ray diffraction analysis, methane adsorption experiments and low-pressure nitrogen adsorption experiments. The results show that the average content of total organic carbon（TOC）is 2.40%, vitrinite reflectance（Ro）is 2.65% on average, indicating the shale are at over-mature evolution stage. Mineral compositions of the samples are dominated by quartz and clay minerals. The specific surface area of shale averages 5.86 m2/g, the total pore volume averages 0.014 9 mL/g, the average pore diameter is 11.2 nm. There are a large number of mesopores in shales. The pore system is dominated by cylindrical pores and plate-like pores. The contents of TOC and quartz minerals increase with the increasing micro-mesopores in shale, the larger the specific surface area and the total pore volume, the smaller the average pore diameter. Surface fractal dimension D1（average of 2.428 4）and pore structure fractal dimension D2（average of 2.622 2）are calculated by Frenkel-Halsey-Hill（FHH）model and nitrogen adsorption-desorption curves（at relative pressure of 0～0.45 and 0.45～0.99, respectively）. With increasing the specific surface area and the total pore volume and decreasing the average pore diameter, fractal dimension D1 and D2 display an increasing trend. Meanwhile, fractal dimensions D1 and D2 have a positive linear correlation with TOC, quartz minerals content and methane adsorption, but have an infirm negative correlation with clay minerals content. Additionally, it is found that fractal dimension D1 decrease with the increasing Langmuir pressure but fractal dimension D2 increase with the increasing Langmuir pressure. In general, the higher fractal dimension of the shale in Guizhong depression, the more complicated pore structure, which made a great contribute to the gas adsorption and accumulation capacity in shales.
Mao Peixiao Jin Aimin Lou Zhanghua Zhu Rong Zhu Zhenhong. Investigation of pore structure and fractal characteristics of marine shale reservoirs of the Upper Paleozoic in Huanjiang sag, Guizhong depression[J]. Chinese Journal of Geology, 2019, 54(1): 130-144.