Evolution of Kerogen and Bitumen during Thermal Maturation via Semi-Open Pyrolysis Investigated by Infrared Spectroscopy

Abstract

A series of artificial maturation (anhydrous, semi-open pyrolysis) experiments on Green River oil shale have been performed to simulate the thermal maturation of type I kerogen. The goals of this program were to develop a kinetic model of petroleum generation from oil shale and to characterize the yield and composition of petroleum as a function of artificial thermal maturity. The thermal maturity level (EASY%Ro = 0.48–1.28%) is based upon the kinetic model of kerogen degradation and is equivalent to vitrinite reflectance maturity. Here, we compare the structural characteristics of kerogen and bitumen during artificial maturation of oil shale using quantitative Fourier transform infrared (IR) spectroscopy. Quantitative comparison was enabled by a novel method for the preparation of bitumen for IR spectroscopy. Bitumen can be a reaction intermediate during maturation of kerogen, and the IR data indicate that bitumen has a structure intermediate between that of kerogen and generated petroleum. Moreover, the IR data reveal that the composition of bitumen changes with maturity, with trends that are similar in some aspects to those observed previously in kerogen, but different in others. Kerogen is characterized by the early depletion of oxygenated functional groups prior to petroleum generation (EASY%Ro < 0.9%) and then a late enrichment of oxygen at higher artificial maturity (EASY%Ro > 1.2%). In contrast, bitumen shows initial enrichment of oxygenated functional groups at low artificial maturity (EASY%Ro < 0.8%) and subsequent depletion at higher maturity. Kerogen evolution follows the previously observed trend with aliphatic carbon chains that became shorter and/or more branched as kerogen is consumed during all stages of artificial maturation. Bitumen, in contrast, appears to have aliphatic carbon chains that lengthen within the same artificial maturity range as bitumen is predominantly generated from kerogen. The aliphatic carbon content of bitumen is greater than that of kerogen at all levels of artificial maturity. Both kerogen and bitumen become more aromatic in character with increasing thermal maturity, especially above artificial levels EASY%Ro > 0.9%. This similarity likely results from loss of aliphatic chains from both organic fractions during petroleum generation, suggesting that both kerogen and bitumen can be direct sources for petroleum. The loss of aliphatic carbons from aromatic centers in both kerogen and bitumen leads to protonation of the residual aromatic rings. The IR spectra of kerogen and bitumen indicate very similar degrees of protonation of those aromatic rings.