COMPARISON OF PORE FRACTAL CHARACTERISTICS BETWEEN MARINE AND CONTINENTAL SHALES

摘要

Fractal characterization offers a quantitative evaluation on the heterogeneity of pore structure which greatly affects gas adsorption and transportation in shales. To compare the fractal characteristics between marine and continental shales, nine samples from the Lower Silurian Longmaxi formation in the Sichuan basin and nine from the Middle Jurassic Dameigou formation in the Qaidam basin were collected. Reservoir properties and fractal dimensions were characterized for all the collected samples. In this study, fractal dimensions were originated from the Frenkel-Halsey-Hill (FHH) model with N-2 adsorption data. Compared to continental shale, marine shale has greater values of quartz content, porosity, specific surface area and total pore volume but lower level of clay minerals content, permeability, average pore diameter and methane adsorption capacity. The quartz in marine shale is mostly associated with biogenic origin, while that in continental shale is mainly due to terrigenous debris. The N-2 adsorption-desorption isotherms exhibit that marine shale has fewer inkbottle-shaped pores but more plate-like and slit-shaped pores than continental shale. Two fractal dimensions (D-1 and D-2) were obtained at P/P-o of 0-0.5 and 0.5-1. The dimension D-2 is commonly greater than D1, suggesting that larger pores (diameter similar to 4 nm) have more complex structures than small pores (diameter similar to 4 nm). The fractal dimensions (both D-1 and D-2) positively correlate to clay minerals content, specific surface area and methane adsorption capacity, but have negative relationships with porosity, permeability and average pore diameter. The fractal dimensions increase proportionally with the increasing quartz content in marine shale but have no obvious correlation with that in continental shale. The dimension D-1 is correlative to the TOC content and permeability of marine shale at a similar degree with dimension D-2, while the dimension D-1 is more sensitive to those of continental shale than dimension D-2. Compared with dimension D-2, for two shales, dimension D-1 is better associated with the content of clay minerals but has worse correlations with the specific surface area and average pore diameter.