Advances in Microscopic Pore Structure Characterization of Fine-Grained Mudrocks

Abstract

The microscopic structure of the pore system in fine-grained mudrocks provides spaces for hydrocarbon percolation, migration, and occurrence. It exhibits strong heterogeneity due to the intimate mingling of organic matter (OM) and inorganic minerals and the intricate architecture of the internal network and dynamic changes along the diagenetic pathway and OM thermal maturation. The present review critically summarizes the advances in structure characterization, geological controlling factors, and heterogeneity of the pore system in mudrocks. The pore system in mudrocks can be divided into micro-, meso-, and macropores by diameters, OM-hosted, interparticles, and intraparticle pores by origin and distribution and connected and nonconnected pores according to whether they are connected. The commonly used pore structure characterization techniques can be classified into three categories: i.e., imaging, fluid invasion, and ray-related techniques. Their principles, experimental conditions, applicability, required sample sizes, detectable pore types and diameter ranges, advantages, and disadvantages have been comparatively summarized. To obtain more precise and comprehensive structural properties, multiple techniques are highly recommended to validate and complement each other. The depositional environment, diagenetic events, and OM thermal maturation jointly control the formation and evolution of the pore system. Differences in provenance and depositional background affect primary grain assemblages and thus changes in physical properties during burial depth. Diagenetic transformations mainly change the morphology and structure of mineral-related pores, while OM thermal maturation mainly controls that of OM-hosted pores. Furthermore, the pore structure of mudrocks generally shows strong heterogeneity, which can be effectively assessed by a point-counting porosity quantification and multifractal theory. Finally, we propose the challenges and prospects of microscopic pore structure characterization of mudrocks. More attention needs to be paid to quantifying OM-hosted and mineral-related pore volume fractions, effectively distinguishing connected and nonconnected pores, and comprehensively quantifying the coupled effect of diagenesis and hydrocarbon generation on pore network evolution. In conclusion, the most important aspect of this review is to summarize modern characterization methods and geological controls on pore structure development and propose prospects for unconventional reservoir characterization, with the hope of providing the ultimate knowledge to researchers engaged in unconventional shale oil and gas resources.

Scale-Dependent Fractal Properties and Geological Factors for the Pore Structure in Shale: Insights from Field Emission Scanning Electron Microscopy and Fluid Intrusion

“…Various methods have been continuously explored and employed to characterize heterogeneous pores in shale reservoirs, including direct imaging, fluid intrusion, and X-ray scattering techniques. Direct imaging methods, such as field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), enable the direct acquisition of high-resolution pore images. − Fluid intrusion methods, such as the helium pycnometer, high-pressure mercury intrusion porosimetry (HP-MIP), low-pressure gas (nitrogen or carbon dioxide) adsorption (LP-N 2 /CO 2 GA), and nuclear magnetic resonance (NMR), offer a wide range of quantitative measurements for pore structures. ,, In recent years, numerous advanced methods have been utilized to assess the pore characteristics found in unconventional reservoirs, such as small-angle X-ray/neutron scattering (SAXS/SANS) , and micro/nano-computed tomography (micro-CT/nano-CT). , However, these advanced techniques are difficult to apply because of their expensive costs and arduous data processing procedures.…”

Effects of Pore Water on the Heterogeneity in Nanopore Structures of Deep Marine Shales: A Multifractal Study on As-Received Shale Samples from the Southern Sichuan Basin, China

“…Then, an LPNA experiment was conducted to analyze the pore structure distribution. The BET and BJH models were used to calculate the pore surface area and volume, respectively. , …”

“…The BET and BJH models were used to calculate the pore surface area and volume, respectively. 41,42 3.5. Multifractal Analysis.…”

Pore Characteristics of Oil Shales in Jilin Province, Northeast China: Investigations Using Gas Adsorption, Mercury Intrusion, and NMR Cryoporometry

“…Laboratory techniques that are commonly used for characterizing shale pore structures can be divided into imaging, fluid invasion, and radiation techniques. − Some of the latter two techniques can be applied to obtain quantitative information about pores. Imaging techniques primarily include field emission scanning electron microscopy (FE-SEM), focused ion beam SEM, transmission electron microscopy, atomic force microscopy, and computed tomography (CT, e.g., micro-CT and nano-CT). ,,,,− Fluid invasion techniques mainly include gas adsorption [e.g., low-temperature CO 2 adsorption/desorption (LTCA) and low-temperature N 2 adsorption/desorption (LTNA)] and mercury intrusion porosimetry (MIP). , Radiation techniques generally include small-angle X-ray scattering, ultra-small-angle X-ray scattering, small-angle neutron scattering, ultra-small-angle neutron scattering, and nuclear magnetic resonance (NMR). ,,, …”