Abstract:

The marine organic-rich shales of Niutitang and Longmaxi formation in south China became the key horizons for shale gas
exploration. But the thermal evolution degree of the shales is very high and its pore structure is complicated. In order to deepen the understanding of this complexity, we analyzed the whole-aperture pore structure characteristics of two sets shales using a combination of field-emission environmental scanning electron microscope (FEI-SEM), high-pressure mercury injection and low-temperature gas (N2 and CO2) adsorption experiments. Results show that the pore volume of the Niutitang and Longmaxi formation were in the range of 0.0202~0.0402 mL/g and 0.0255~0.0310 mL/g, respectively. Mesopores had the largest proportion in two sets of shale pore volume. The average specific surface area of two sets shales were in the range of 13.74~41.26 m2/g and 21.42~27.82 m2/g, respectively. Micropores and mesopores amounted to almost 95% of the specific surface area in shale. The pore structure characteristics of two sets shales were very different. The dissolution pores and interparticle pores were developed in the Niutitang formations while organic-matter pores were unevenly distributed. And organic-matter pores, interparticle and intraparticle pores in the mineral matrix were well-developed in the Longmaxi formations. Micropores in the Niutitang formations were well-developed while mesopores and macropores in the Longmaxi formations were better developed. According to the result of the experiment data and past literature, the level of maturity and buried depth played a decisive role in the difference of pore structure characteristics.

Key words: shale gas, pore structure, whole-aperture, Niutitang Formation, Longmaxi Formation, difference contrast