Potential of Light Oil and Condensates from Deep Source Rocks Revealed by the Pyrolysis of Type I/II Kerogens after Oil Generation and Expulsion

Light
and condensable oils derived from mature kerogen and residual
oil in deep source rocks have contributed strongly to a rapid increase
in oil production. In this study, oil expulsion from kerogen and shale
was simulated by selective solvent extraction (hexane/toluene, 9:1
v/v, instead of the commonly used dichloromethane) and by returning
the extracts (so-called “oil a”) back into a controlled
mass of mature kerogen. The maturity intervals and potential of the
light oil and condensates were investigated by analyzing the yields
of different hydrocarbons from the subsequent pyrolysis of mature
kerogen-“oil a” mixtures. The gas-to-oil ratio was used
to constrain the maturity range for the light oil, condensate, and
gas stages. The lowest equivalent vitrinite reflectance (EVRo, 1.9–2.1%) for the gas stage was compatible with commonly
accepted models. The EVRo cutoff of 1.55–1.75% between
light oil and condensate was higher than that in traditional models,
although this depended primarily on the generality of the “condensate”
definition. An EVRo ranging between 1.35 and 1.55–1.75%
was defined in this work as the “light oil/gas” substage
within the commonly accepted condensate/wet gas stage. Moreover, yields
of hydrocarbons from the cracking of “oil a” were distinctly
affected by mature kerogen. This effect showed little difference on
the yield of C6–14 hydrocarbons and C15+ hydrocarbons but notable difference for the gases between types
I and II kerogens. The release of C6–14 hydrocarbons
was promoted when the release of C15+ hydrocarbons was
notably inhibited. Approximately linear relationships were established
between maximum yields of liquid hydrocarbons and the carbon content
of “oil a” (selective solvent extraction products) in
the mixtures. This relationship was helpful in estimating both oil
and total petroleum potential of deep source rocks that have undergone
oil generation and expulsion, but it was dependent on the composition
of the solvents used in extraction.