Abstract
The effects of brittle minerals in shale diagenesis on shale pores remain controversial and it is difficult to quantify directly. However, the relationship between brittle minerals and shale pores could provide indirect guidance regarding diagenesis processes in post-mature marine shales. In this study, the pore size distribution was determined, and the relationship between pore volume and shale composition was examined in shale samples with different total organic carbon contents from the Wufeng and Longmaxi Formations, with the objective of distinguishing pore size ranges in organic matter and inorganic minerals, respectively, and studying shale diagenesis. The samples of the Wufeng and Longmaxi shales are composed of clay minerals, calcite, dolomite, quartz, feldspar, and some minor components. The pore size distributions, which were determined using nitrogen adsorption isotherm analysis of shale and kerogen, show similar trends for pore sizes less than approx. 6.5 nm but different trends for larger pore sizes. Mercury injection saturation shows that macropores account for 14.4–22% of the total pore volume. Based on a series of crossplots describing the relationships between shale composition and pore volume or porosity associated with different pore sizes as well as on scanning electron microscopy observations, organic matter pores were found to comprise most of the micro-mesopores (pore diameters < 6.5 nm). Organic matter pores and intraparticle pores associated with carbonate constitute the majority of mesopores (pore diameters 6.5–50 nm). Finally, interparticle pores associated with quartz comprise the majority of the macropores. The mesopores associated with carbonate were formed by dissolution during diagenesis, whereas the macropores associated with quartz are the remainders of the original interparticle pores. Mesopore volumes increase with increasing carbonate content while macropore volumes decrease due to the ‘pore size controlled solubility’ effect, which causes dissolved calcium carbonate to precipitate in larger macropores.
Highlights
The great success of shale gas development in North America has attracted considerable attention from other countries, including China
The black shales of the Wufeng and Longmaxi Formations are mainly composed of quartz, carbonates, clay, feldspar, and pyrite
pore size distributions (PSDs) (Figures 4 and 5) show kerogen has multimodal distributions with three pore size peaks, the high total organic carbon (TOC) shale samples (TOC >5%) display bimodal distributions, and the low TOC shale samples (TOC
Summary
The great success of shale gas development in North America has attracted considerable attention from other countries, including China. The pore size classification of the International Union of Pure and Applied Chemistry (IUPAC) divides pores into micropores (less than 2 nm in diameter), mesopores (pores with diameters of 2–50 nm), and macropores (larger than 50 nm in diameter) This classification system has been recommended for characterizing shale pores by Chalmers et al (2012), and it has been widely used in recent years (Klaver et al, 2015; Mastalerz et al, 2013). Both qualitative and quantitative approaches such as field emissionscanning electron microscopy (FE-SEM); transmission electron microscopy, atomic force microscopy, and helium ion microscopy; low-pressure nitrogen and CO2 gas adsorption; and high-pressure mercury intrusion provide geologists with convenient methods for determining pore properties such as pore size, specific surface area, pore volume, and pore connectivity
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