Kerogen structure and porosity in Woodford Shale before and after hydrous closed-system pyrolysis

Abstract

Micro-plugs of the Upper Devonian to Lower Mississippian Woodford Shale from the Ardmore Basin in southern Oklahoma were analyzed before and after hydrous pyrolysis (300, 320, 330, and 340 °C for 24 h) by using a combination of μ-FTIR, CP-Py-GC/MS, BIB-SEM-EDS techniques to investigate kerogen transformation and evolution of pore space.Tasmanites and Leiosphaeridia in Woodford Shale are abundant macerals characterized by relatively long, unbranched alkyl chains, with an additional carboxyl component, and a minor contribution of aromatic structures based on μ-FTIR spectra results. The aliphatic νasCH2/ νasCH3 ratios show almost no correlation with pyrolysis temperatures, except for a minor increase of the ratio for both samples after pyrolysis at 330 °C, which may be attributed to cracking of C-C bonds next to a tertiary carbon atom. A decreasing aliphaticity and increasing aromaticity for alginite with increasing pyrolysis temperature and thus thermal maturity is indicated by the reduction of aliphatic CHx stretching bands and increasing γCH/ νCHx ratios. The relative abundance of olefinic bonding (νC=C) decreases in alginite compared to aromatic hydrogen (γCH) with higher pyrolysis temperature, and thus formation of monoaromatic rings is indicated by the condensation ratios (γCH/ νC=C); similar observations have been made on natural maturation series.This is in line with the CP-Py-GC/MS results on whole kerogen showing an increase of aromatic structures over aliphatic pyrolysis products with increasing thermal maturities, though with some differences between the two samples investigated. The differences are probably related to higher alginite/bituminite ratios in one of the samples, which also contain more N-compounds. In contrast, the other sample enriched in bituminite contains more S-compounds. Both FTIR spectra and EDS results document a loss of carbonyl/carboxyl C=O functional groups and/or O content with thermal maturation. The hydrocarbon generation potential parameter “A-Factor” (νCHx/ νCHx + νC=C), analyzed on alginite in this study, is rather consistent at different pyrolysis temperatures with a value of about 0.9 and with a slight tendency of higher values at higher pyrolysis temperatures.SEM observations show that there is no significant occurrence of microfractures in the initial samples and after pyrolysis at 300 °C, while microcracks and organic pores formed pore networks when pyrolysis temperature increased from 320 °C to 340 °C. Most of the newly generated cracks are parallel to the bedding. Mineral-filled Tasmanites cysts in Woodford Shale are silica-rich and may be related to the dissolution of siliceous tests (e.g., radiolarians).