Pore Structure and Heterogeneity in the Lacustrine Shale of the Second Member of the Paleogene Funing Formation, Subei Basin, China

Abstract

Characterization of pore structure and heterogeneity is crucial for exploring and evaluating shale oil and gas resources. Existing methodologies exhibit various limitations, and intuitive, quantitative description techniques are lacking. To address these issues, shales from the second member of the Funing Formation (E1f2) in the Subei Basin were studied in this study. A comprehensive research methodology was employed that combined petrological analysis, physical structure testing, and SEM images supplemented with multifractal analysis and partial least squares regression (PLSR) data processing to achieve nuanced qualitative and quantitative characterization of the heterogeneity in lacustrine shale pore structures. This study revealed that E1f2 shale pores predominantly consist of mesopores (2–50 nm) and macropores (>50 nm). A positive correlation was observed between the development of mesopores and calcite content, while a negative correlation exists with orthoclase content. Conversely, a positive correlation with plagioclase content was found in macropore development. The growth in total pore volume is inhibited by the total organic carbon (TOC) content. Employing multifractal methods to analyze pore morphology data extracted from scanning electron microscopy (SEM) images facilitated a quantitative characterization of the heterogeneity within the pore structures. The results showed that organic-medium mixed shale (OMMS) has the strongest heterogeneity. The weakest heterogeneity is exhibited by the organic-medium calcareous shale (OMCS). PLSR analysis indicates that the structural heterogeneity of the E1f2 shales is positively correlated with TOC content and negatively correlated with orthoclase content. Additionally, the type of pore influences the degree of heterogeneity. Increasing the total and macropore volume reduces the heterogeneity, while increasing the micropore (<2 nm) volume enhances it in the E1f2 shales.