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Pore Characterization of Eagle Ford Shales by N2 Adsorption and Desorption Isotherms and Pore Evolution in Organic Matter During Thermal Maturation


The dominant pore system in many gas shales resides within the contained organic matter and these pores extend into sizes below the available imaging resolution (around 7 nm by field-emission SEM imaging on Ar-ion milled surfaces). It has been repeatedly observed that, in many shales, most pores are too small to be observed. Physisorption measurements can be used to address the shortcomings of imaging techniques, as they have the capability to detect pores in the range from >2 nm up to 300 nm. Adsorption and desorption isotherms together allow determination of pores from 0.38 to 300 nm, yielding both a total surface area (BET) and a pore-size distribution (PSD). The hysteresis loop (the gap between the adsorption and desorption lines) provides crucial information about pore shapes. Core samples from four wells within the Eagle Ford Formation were used for this study. The Eagle Ford samples cover burial depths from outcrop to 9200 ft. Geochemical characterization including TOC and Rock-Eval analysis was conducted, and the results suggest that our samples represent four thermal maturation stages: low mature, early-oil generation, peak-oil generation, and early-oil cracking. For oil-bearing samples, the pore volume significantly increases after removal of residual oil by solvent extraction, showing that oil is mainly stored in pores with widths larger than 20 nm. With an increase of thermal maturity, pores with widths > 10 nm increase and pores with widths greater than 40 nm are diminished. This observation suggests that the development of smaller pores is associated with high thermal maturity, and the reduction larger pores may result from burial-related compaction and cementation. Our results clearly demonstrate that thermal maturity greatly affects pore size distribution, in particular for pores with widths less than 10 nm. Integrated studies that include SEM pore imaging are required to adequately define the genetic relationships between pore sizes, pore types, mineralogy, and rock texture.