Pore Systems of the Different Lithofacies of the Longmaxi Formation at Depths Exceeding 3500 m in the Zigong Area, Sichuan Basin

摘要

The deep Longmaxi Formation shale in the Zigong area is widely distributed and has a huge resource potential, which has enabled it to become one of the most important replacement areas for shale gas exploration in China. This article investigates the key shale gas evaluation well, Well Z1, in the Zigong area using field emission-scanning electron microscopy, mercury intrusion capillary pressure, low-pressure N-2/CO2 adsorption, and fractal theory to determine the pore systems of the different lithofacies of the deep O(3)w-S(1)l shale. The mineral composition and total organic carbon (TOC) content (1.79%- 8.16%/3.48%) are highly heterogeneous, and five types of shale lithofacies including mixed shale, mixed siliceous, clay-rich siliceous, argillaceous/siliceous shale, and mixed carbonate shale were identified. The deep shale mainly develops interpores and microcracks associated with the organic matter (0M)-mineral (carbonate, pyrite, and clay minerals) aggregates and a small number of mud pores. The surface areas and total pore volume vary from 16.29 to 41.76 m(2)/g and from 1.19 to 3.49 cm(3)/100 g, with averages of 25.03 m(2)/g and 1.99 cm(3)/100 g, respectively. The pore distribution exhibits significant multimodal and fractal characteristics. The distribution morphology, peak values, and fractal dimensions vary with lithofacies. The mesopores provide the main pore volume (average of 53.99%), and the micropores provide the main surface area (average of 73.95%). The TOC content and biogenic quartz are beneficial to the development of micro- and mesopores. In the Zigong area, mineral aggregates composed of OM and brittle minerals (quartz, feldspar, and carbonate) are more favorable for pore development than the clay mineral-brittle mineral aggregates.