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Open AccessArticle

Full-Scale Pore Structure and Fractal Dimension of the Longmaxi Shale from the Southern Sichuan Basin: Investigations Using FE-SEM, Gas Adsorption and Mercury Intrusion Porosimetry

by 1,2, 1,2,*, 3,4,5,*, 1,2, 1,2, 1,2 and 1,2
1
State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China
2
Unconventional Oil & Gas Institute, China University of Petroleum, Beijing 102249, China
3
Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China
4
School of Earth Resources, China University of Geosciences, Wuhan 430074, China
5
Energy & Geoscience Institute, University of Utah, Salt Lake City, UT 84108, USA
*
Authors to whom correspondence should be addressed.
Minerals 2019, 9(9), 543; https://doi.org/10.3390/min9090543
Received: 2 August 2019 / Revised: 3 September 2019 / Accepted: 4 September 2019 / Published: 9 September 2019
(This article belongs to the Special Issue Mineralogy of Shale Gas and Other Low Permeability Reservoirs)
Pore structure determines the gas occurrence and storage properties of gas shale and is a vital element for reservoir evaluation and shale gas resources assessment. Field emission scanning electron microscopy (FE-SEM), high-pressure mercury intrusion porosimetry (HMIP), and low-pressure N2/CO2 adsorption were used to qualitatively and quantitatively characterize full-scale pore structure of Longmaxi (LM) shale from the southern Sichuan Basin. Fractal dimension and its controlling factors were also discussed in our study. Longmaxi shale mainly developed organic matter (OM) pores, interparticle pores, intraparticle pores, and microfracture, of which the OM pores dominated the pore system. The pore diameters are mainly distributed in the ranges of 0.4–0.7 nm, 2–20 nm and 40–200 μm. Micro-, meso- and macropores contribute 24%, 57% and 19% of the total pore volume (PV), respectively, and 64.5%, 34.6%, and 0.9% of the total specific surface area (SSA). Organic matter and clay minerals have a positive contribution to pore development. While high brittle mineral content can inhibit shale pore development. The fractal dimensions D1 and D2 which represents the roughness of the shale surface and irregularity of the space structure, respectively, are calculated based on N2 desorption data. The value of D1 is in the range of 2.6480–2.7334 (average of 2.6857), D2 is in the range of 2.8924–2.9439 (average of 2.9229), which indicates that Longmaxi shales have a rather irregular pore morphology as well as complex pore structure. Both PV and SSA positively correlated with fractal dimensions D1 and D2. The fractal dimension D1 decreases with increasing average pore diameter, while D2 is on the contrary. These results suggest that the small pores have a higher roughness surface, while the larger pores have a more complex spatial structure. The fractal dimensions of shale are jointly controlled by OM, clays and brittle minerals. The TOC content is the key factor which has a positive correlation with the fractal dimension. Clay minerals have a negative influence on fractal dimension D1, and positive influence D2, while brittle minerals show an opposite effect compared with clay minerals. View Full-Text
Keywords: Longmaxi shale; Sichuan Basin; pore structure; fractal dimension; controlling factors Longmaxi shale; Sichuan Basin; pore structure; fractal dimension; controlling factors
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Wang, X.; Jiang, Z.; Jiang, S.; Chang, J.; Zhu, L.; Li, X.; Li, J. Full-Scale Pore Structure and Fractal Dimension of the Longmaxi Shale from the Southern Sichuan Basin: Investigations Using FE-SEM, Gas Adsorption and Mercury Intrusion Porosimetry. Minerals 2019, 9, 543.

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