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

Abstract

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 N2/CO2 adsorption, and fractal theory to determine the pore systems of the different lithofacies of the deep O3w–S1l 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 (OM)–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 m2/g and from 1.19 to 3.49 cm3/100 g, with averages of 25.03 m2/g and 1.99 cm3/100 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.

Differences in the Nanopore Structure of Organic-Rich Shales with Distinct Sedimentary Environments and Mineral Compositions

“…It is interesting to observe that all of the samples exhibit hysteresis loops, formed by the divergence between adsorption and desorption curves (Figure ). The shape and size of hysteresis loops in marine shales indicate that pores are mainly inkbottle-shaped and the pore structure may be more complicated than other samples. ,, …”

“…The shape and size of hysteresis loops in marine shales indicate that pores are mainly inkbottle-shaped and the pore structure may be more complicated than other samples. 20,44,45 Based on the BJH model, the pore parameters of shale samples are extracted from N 2 adsorption (Table 2). The pore volumes of marine shales vary from 19.85 × 10 −3 to 34.63 × 10 −3 cm 3 /g, with an average of 29.27 × 10 −3 cm 3 /g.…”

Shale Gas Exploration and Development in China: Current Status, Geological Challenges, and Future Directions

“…Plus, how to recognize the effective reservoir types especially in the deep shale gas reservoirs is still unclear. 163 The research foundation in the regional scale is so weak in terms of diagenesis, diagenetic evolution, reservoir type, and pore system, together with organic−inorganic interactions, that the problem has become one of the important factors impeding the progress of shale gas exploration and development. With the generation and expulsion of hydrocarbon coupled with the pressure changes in the shale reservoirs, how did the organic and mineral pores interact and coevolve?…”

“…The method of assigning weight considers little about shale diagenesis, reservoir type, pore structure, and pressure. Plus, how to recognize the effective reservoir types especially in the deep shale gas reservoirs is still unclear . The research foundation in the regional scale is so weak in terms of diagenesis, diagenetic evolution, reservoir type, and pore system, together with organic–inorganic interactions, that the problem has become one of the important factors impeding the progress of shale gas exploration and development.…”

Effect of Division Methods of the Adsorption Isotherm on the Fractal Dimension of Clay Minerals Calculated Based on the Frenkel–Halsey–Hill Model

“…Based on the observation of the FHH model fitting lines on the adsorption isotherms in different samples, ,− it is found that there is no uniform method for dividing the adsorption isotherms when this model was applied. Moreover, the division method has great effect on the calculation result of the fractal dimension.…”