Characterization of free and bound bitumen fractions in a thermal maturation shale sequence. Part 2: Structural evolution of kerogen and bitumen during shale oil generation, expulsion and retention

Abstract

Knowledge of the formation and enrichment mechanisms of shale oil is essential to understanding its mobility and hence a critical aspect for evaluating shale oil exploration and production. Semi-open hydrous pyrolysis experiments were performed on an organic-rich shale to simulate the petroleum generation and expulsion processes of Type II kerogen. The residual rocks matured to different levels (Easy%Ro = 0.50–1.37) were thereafter treated with stepwise extraction to obtain free and bound bitumen fractions. This study focuses on the structural characteristics of kerogen, bitumen fractions, and expelled oil during shale maturation using Fourier transform infrared (FTIR) spectroscopy and quantitative flash pyrolysis–gas chromatography (Py–GC). The results reveal that the composition of expelled oils changes with increasing maturity, whereas an opposite trend in structural composition was observed for kerogen after significant oil expulsion from the shale occurs, beginning at Easy%Ro = 0.79. A notable change is that aliphatic carbon chains of kerogen became shorter and/or more branched while those of expelled oil appeared to be more aliphatic with longer carbon chains at this stage. However, the overall compositional structures of shale bitumen fractions, both in free and bound phases, show only slight changes within the oil window stage. The bound bitumen is generally enriched in oxygenated functional groups as compared to free bitumen, indicating that the bound bitumen is related mainly to the interaction between the mineral surface and the bitumen–kerogen matrix. During maturation, the cracking (including defunctionalization) and condensation of kerogen and bitumen fractions, as well as a compositional fractionation due to oil expulsion, are likely to be the main factors influencing the opposite trend observed in structural compositions between kerogen and expelled oil. These processes may control the quality and content of expelled oil and bitumen in a petroleum system. Nevertheless, kerogen, expelled oil and bitumen show a gradual trend of decarboxylation during shale maturation in general.