Influence of organic matter type on porosity development in organic-rich shales: combining microscopy, neutron scattering and physisorption approaches

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Taylor & Francis
Shales are economically significant fine-grained sedimentary rocks, acting as sources and seals for energy re-sources, seals for underground carbon sequestration, and can contain metal enrichments up to low-grade ore levels. Critical to the accurate evaluation of shale prospectivity as reservoirs for energy resources and as seals for long-term underground retention of fluids and radioactive waste is the evaluation of their porosity and pore size distribution to help elucidate and appropriately model the mechanisms of gas/liquid transport and trapping over time and space. Dominant pore sizes in gas shales are generally in the sub-micron to nanometer range and include pores occurring between and within the minerals constituting the solid framework as well as pores within the organic matter dispersed in the rock. Small-scale spatial heterogeneity in organic matter-hosted porosity has previously been noted indicating that the organic matter type is likely an important control on pore formation and distribution that can offer new insights on the understanding of the overall porosity network in shales .Here we investigate the occurrence of porosity at the micro- and nano-scale in samples from the Wufeng-Longmaxi organic rich shale formation. Using a com-bination of high-resolution scanning and transmission electron microscopy (SEM and TEM), small angle neutron scattering (SANS) and Argon physisorption measurements, we first characterize the pore network of samples representative of the different lithofacies and then highlight the control of different types of organic material (primary versus secondary) on the development of the rocks pore network. SANS measurements were collected on the 40 m long Quokka instrument at the Australian Centre for Neutron Scattering, part of the Australian Nuclear Science and Technology Organization. We used the same petrographic thin sections previously used for microstructural investigations and collected data in the Q range 7 × 10−4 to 5 × 10−1 Å−1 using three detector distances, 1.3 m, 12 m, and 20 m. SANS data was used to estimate the porosity and pore size distribution of the shale samples and compared with estimates obtained from Argon sorption measurements on powdered samples, while SEM and TEM petrographic imaging provided a spatially resolved evaluation of the pores distribution and their relationships to different organic matter types. Mesopores dominate the porosity of the investigated shale samples representing between 61 and 69% of the total pore volume. The contribution of microporosity is highest for the sample richest in organic material (ca. 20% of total pore volume). The total pore volume of the organic-rich shale is 38% higher than for the organic-lean shale. Inversion of SANS data shows that for all samples, porosity detected by SANS largely lies in the mesopore size range (between 2 and 50 nm); clay-rich and intermediate composition samples display a mostly unimodal distribution with dominant peaks centered at about 3 nm. Notably, the organic rich sample exhibits additional peaks for pore sizes ranging between 6 and 40 nm, rep-resenting approximately 30% of the measured pore volume. These peaks are absent in the clay-rich, organic poor sample. High-resolution electron microscopy images indicate a noticeable difference in pore sizes be-tween the pores hosted in the organic matter interpreted as primary graptolite particles (ca. 3 nm) and those hosted in the bitumen (ca. 20–30 nm) in the organic-rich sample (Figure 1).By using organic petrology, high resolution electron microscopy petrography, neutron scattering, and Ar physisorption measurements of sample characteristics of the different lithofacies of the Wufeng-Longmaxi organic rich shale formation, we have identified two main organic matter types: primary faunal remains (graptolites) and pore-filling migrated bitumen. We observed that total porosity is directly proportional to the amount of organic material in the samples and that, porosity hosted in the organic matter contributes up to 40% of the total pore volume. High resolution imaging focusing on the organic particles indicates that the different organic matter types contribute differently to the size and distribution of the pores; characteristic pore sizes for the bitumen are in the mesopore range, while the graptolite particles mainly contribute porosity in the micro-pore range. More information on this work can be found in [1]:The team is now working on dynamic fluid (methane and hydrogen) saturation experiments under stress, measuring the resultant changes in pores shape factors and accessibility in shales and rock salt, and welcomes questions and collaborations on the current work. Reference 1. C. Delle Piane, H. Ansari, Z. Li, J. Mata, W. Rickard, R. Pini, D. N. Dewhurst, and N. Sherwood. Influence of organic matter type on porosity development in the Wufeng-Longmaxi Shale: A combined microscopy, neutron scattering, and physisorption approach. Int. J. Coal Geol., 249, 103880 (2002). © 2023 Informa UK Limited
Microscopy, Porosity, Neutron diffraction, Organic matter, Shales, Sedimentary rocks
Delle Piane, C., Ansari, H., Li, Z., Mata, J., Rickard, W., Pini, R., Dewhurst, D. N., & Sherwood, N. (2023). Influence of organic matter type on porosity development in organic-rich shales: combining microscopy, neutron scattering and physisorption approaches. Neutron News, 34(2), 8-9. doi:10.1080/10448632.2023.2191571