PORE STRUCTURE AND FRACTAL CHARACTERISTICS OF SHALE IN THE SECOND MEMBER OF CANGDONG SAG

Abstract

In order to study pore structure and fractal characteristics of the organic-rich continental shale，8 shale samples from Member 2 of Cangdong Sag in the Huanghua Depression, Bohai Bay Basin were investigated by X-ray diffraction (XRD) analysis, organic geochemical analysis (total organic carbon content analysis and thermal maturity) ,field emission scanning electron microscopy (FE-SEM), low-temperature nitrogen adsorption experiments. The results of XRD showed that the mineral composition of shale samples is complex, the average contents of felsic minerals, carbonate minerals, analcime, clay minerals and pyrite are 28.47%,41.58%,16.61%,12.13%, and 1.06%, respectively. Fractal dimensions D1 and D2 (at relative pressure of 0-0.5 and 0.5-1, respectively) were obtained from the nitrogen adsorption data using the fractal Frenkel-Halsey-Hill (FHH) method, of which D1 ranges from 2.44 to 2.58, with an average of 2.51. D2 ranges from 2.57 to 2.86, with an average of 2.72. The results showed that the pore diameter of shale is distributed from 1.7nm to 200nm, with an average value of 13.4nm. The specific surface area is in the range of 0.99m2/g -4.43m2/g, the total pore volume is between 0.00458cm3/g-0.01665cm3/g. The research results show that there is no significant correlation between TOC content and fractal dimensions, on account of scarcity of organic pores within shale samples. Dolomite content has no obvious correlation between D2, but negatively correlated with D1. There is no significant correlation between other minerals and fractal dimensions. D1 is negatively correlated with specific surface area, positively correlated with average pore diameter, and negatively correlated with total pore volume; D2 is positively correlated with specific surface area, negatively correlated with average pore diameter, and negatively correlated with total pore volume, indicating that the macropores and mesopores within shale samples have more complex spatial structures than micropores, and the types of micropores are relatively single, while the types of macropores are diverse, including intergranular pores, intra-granular pores, intergranular pores, dissolution pores, and micro-fractures.