Investigation of pore structure characteristics of marine organic-rich shales using low-pressure N-2 adsorption experiments and fractal theory

摘要

The pore structure and fractal characteristics of the Lower Cambrian marine organic-rich shale in southern China were comprehensively studied using low-pressure N-2 adsorption and organic geochemical experiments, X-ray diffraction, petrophysical property tests, and scanning electron microscope observations. The results indicate that the total organic carbon (TOC) content of the study shale varies between 0.45% and 8.50%, with an average value of 3.9796. The adsorption isotherm of the shale samples belongs to type IV, and slit-type pores are the predominant pore type in these shales. The shale has a Brunner-Emmet-Teller specific surface area ranging from 1.83 to 28.14 m(2)/g, a pore volume ranging from 0.00398 to 0.01848 cm(3)/g, and an average pore diameter ranging from 3.61 to 15.19 nm. Organic matter pores (OMPs) are the main contributors to the specific surface area and the pore volume. The organic matter is closely symbiotic with the epigenetic quartz. We have obtained two fractal dimensions (D-1 and D-2) of the shale using the Frenkel-Halsey-Hill method. It was found that D-2 is suitable for the quantitative characterizing of the pore structure of nanopores inside the shale due to its good correlation with the TOC content and pore structure parameters. When the TOC content of the shale exceeds 4%, the main pore type inside the shale is OMP and the D-2 value mainly reflects the fractal characteristics of OMP. Moreover, we analyzed the seepage characteristics of different types of pores. It was found that the parallel plate-like pores and the slit-type pores are more favorable for fluid seepage than the ink bottle-like pores. The shale with H-3 and H-4 type pore structures should be the key exploration targets for the target shale in the study area.