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.
Highlights
The shale revolution in the USA has changed the global energy landscape with profound impact on international economies [1, 2], which has led to the assessment and exploration of shale oil and gas in basins worldwide
Low-pressure gas (N2 and CO2) adsorption methods were applied to shale samples from the upper part of the Sha-4 Member of the Paleogene Shahejie Formation (Es4U) in the Dongying Depression, after the Soxhlet extraction of residual oil
Our samples from the Es4U shale in the Dongying have an average brittle mineral content of 68.3 wt%, higher than that reported for the Chang-7 shale
Summary
The shale revolution in the USA has changed the global energy landscape with profound impact on international economies [1, 2], which has led to the assessment and exploration of shale oil and gas in basins worldwide. Low-pressure CO2 adsorption can be used to analyze micropores Both N2 and CO2 techniques need to be used to study pore properties in shale. The geological factors controlling pore properties are usually studied using the bivariate cross-plots and univariate regression analysis [23, 25, 30, 31]. PLSR analysis needs to be used with VIP evaluation when the univariate correlation relationships are poor between pore properties of the Soxhlet-extracted samples and geological factors. The pore properties in fifteen Soxhlet-extracted samples of the Es4U shale were analyzed using N2 and CO2 adsorption techniques. These samples were analyzed for geological factors including mineral composition, Ro, and IOC. The analysis of the shale pore evolution helped us find the shale with the modest micromesopore volume, brittle mineral, and clay mineral contents which is conducive to the development of the Es4U shale oil
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