Pore Properties of the Lacustrine Shale in the Upper Part of the Sha-4 Member of the Paleogene Shahejie Formation in the Dongying Depression in East China

Abstract

Lacustrine shales hold a huge potential oil resource in China. Pore properties (pore volume, diameter, specific surface area, and fractal dimensions) and their relationships with geological factors (mineralogy, insoluble organic carbon, burial depth, and vitrinite reflectance) are critical for evaluating shale oil resource. However, the factors controlling pores for lacustrine shale oil remain unclear, as the relationships between pore properties of Soxhlet-extracted samples and geological factors have not been studied using multivariate analytical methods. In this paper, the samples from the lacustrine shale in the upper part of the Sha-4 Member of the Paleogene Shahejie Formation in the Dongying Depression were tested with a set of experiments including Soxhlet (solvent) extraction, X-ray diffraction mineral analysis, insoluble organic carbon, vitrinite reflectance, and low-pressure CO2 and N2 adsorption experiments. The micromesopore volume varies from 0.003 cm3/g to 0.045 cm3/g. The relationships of pore properties with geological factors were studied with partial least square regression analysis (PLSR analysis, a powerful multivariate regression analysis). The results of the PLSR analyses indicate that clay minerals and carbonates are two key factors affecting the pore properties of the lacustrine shale. Compared with marine shales, more clay minerals in the lacustrine shale make them become more important for controlling pores than organic matter. The PLSR results also illustrate that the shale with higher pore volume contains more clay minerals and fewer carbonates and thus is unfavorable for hydraulic fracturing. Therefore, the shale with high micromesopore volume may be unfavorable for shale oil production. The shale with the modest micromesopore volume (~0.036 cm3/g), relatively high content of brittle minerals (~71 wt%), and low clay mineral content (~29 wt%) is conducive to both oil storage and hydraulic fracturing for the development of the Es4U shale oil in the Dongying Depression in East China.

Efforts to untie the multicollinearity knot and identify factors controlling macropore structures in shale oil reservoirs

“…And the noted negative correlation between pyrite content and macropore volume has been linked to the macropore blockage caused by idiomorphic pyrites in the Chang-7 shale (Huang et al, 2023;Wang et al, 2023a). Similar negative correlations and interpretation have also been applied to the Shahejie shale of the Dongying Depression (Wang et al, 2021b;Khan et al, 2022). However, it is imperative to emphasize the substantial interconnections among geological factors.…”

“…For example, in an examination of the Bakken shale in North America , linear relationships were assumed across all pore and geological factors, leading to a linear PLS-regression analysis based on the original parameters. In another study focusing on the Shahejie shale in East China (Wang et al, 2021b), researchers contemplated relationships as linear or nonlinear, with the choice of functional forms in the subsequent PLS-regression analysis contingent upon the goodness-of-fit derived from univariate regression models (linear, logarithmic, exponential, and power models). Given the absence of a definitive consensus on the most appropriate functional form, this study embraces the consideration of both assumptions in the two phases of PLS-regression analysis conducted herein.…”

“…Essentially, identifying geological factors that exhibit significant correlations with pore-related properties may not necessarily indicate their role as the principal controllers of pore development. Despite the prevalence of studies based solely on correlation analysis to discern controlling influences Wang et al, 2021b;Huang et al, 2023), the conclusions drawn from such investigations often give rise to disagreements due to the oversight of multicollinearity. This phenomenon, wherein interdependencies exist among pore-related parameters and geological factors, remains unaddressed.…”