Data-driven elastic modeling of organic-rich marl during maturation

Abstract

The elastic modeling of source-rock reservoirs during maturation must incorporate microstructural and geochemical alterations. The common challenge is calibrating the volumetric proportion between each form of organic porosity along with the changes in the bulk and shear moduli of kerogen as a function of maturity. Two forms of organic porosity have generally been observed: (1) spongy and bubble pores inside kerogen and (2) low-aspect-ratio pores or gaps at the interface between shrinking kerogen bodies and the matrix. We have constructed a rock-physics model of organic-rich marl during maturation and calibrated it using rock-physics data sets from controlled pyrolysis experiments of organic-rich marl under stress. We chose these pyrolysis data because the samples provide subsequent changes in porosity and P- and S-wave velocities as a function of maturity, while evidencing minimal grain sliding and mechanical compaction due to their stiff matrix. Our calibration results indicate that spongy and bubble pores should be used as the predominant form in the model regardless of maturity. Our results also indicate a competing effect between increasing kerogen porosity and the increasing moduli of solid kerogen. Kerogen porosity mainly develops throughout the oil windows. Whereas the moduli of solid kerogen increase by a factor of two in the early-peak oil window and remain relatively constant afterward. Consequently, the effective moduli of kerogen experience minimal changes in the early-peak oil window and rapidly decrease to half of the immature values in the late oil window. These calibration results are consistent with several petrophysical and nanoindentation studies on kerogen. Finally, we used the calibrated model to build a rock-physics template of organic-rich marl during maturation. The template was tested with pyrolyzed and naturally matured samples, which showed that our model can be used to characterize reservoir properties across different maturity windows.