Characterization of Green River Kerogen upon Induced Maturation

Abstract:

The aim of this research is to investigate the effects of organic maturity on the elastic properties of kerogen, and then model its effect on the rock elastic responses. To fulfill this objective, we present a workflow that combines nano-scale Secondary Ion Mass Spectrometry (nanoSIMS), nanoindentation, SEM, ex situ maturation experiments, RockEval analysis and Self-Consistent modeling (SC).

First, we used SEM and nanoSIMS to identify the organic rich-kerogen bodies. NanoSIMS provided maps of the secondary ion intensity distribution of H^-, C^- and O^- which show a uniform distribution of these ions within the immature kerogen body. The measured H^- /C^- and O^-/C^- ionic intensity ratios range between 1.40 ±0.86 -1.69 ±0.61 and 0.77 ±0.72 - 1.04 ±0.44 respectively.

Next, we used the nanoindentation technique to measure the elastic properties of an immature Green River kerogen, which had an average bulk modulus (K) of 3.11 ± 0.23 GPa. Then, we induced maturation using a High Temperature-High Pressure vessel that mimics reservoir conditions. Ex situ maturation resulted in a strong hydrocarbon smell, oil staining, and the expulsion of an oil-like viscous fluid. Geochemical analysis confirmed that the sample had successfully matured to the oil window. SEM time-lapse images show porosity (ϕ) development within the kerogen and surrounding matrix as a result of maturation.

Once maturation was complete, we re-measured the elastic properties of the kerogen in the sample using the same nanoindentation technique. The average value of K of the mature kerogen (oil window) was 3.65 ±0.67 GPa. Therefore, we conclude that changes in the elastic properties of solid kerogen in the oil window are negligible. However, ϕ development within the kerogen, the shape of kerogen and its pores, and the presence of fluid can affect the composite rock stiffness.

Therefore, we used SC modeling to investigate the effect of ϕ development within the kerogen associated with ex situ maturation, on the elastic properties of the porous kerogen and the rock composite. Our results show that the increase in kerogen ϕ and the presence of fluid during the oil and gas generation scenarios cause a -6% and -44% change in the kerogen effective K. The model indicates that the increase in kerogen ϕ during oil and gas generation causes the rock
K to decrease by 2% and 5% respectively.