--> Calculation of a Rate Index for the Bakken Formation (Devonian-Mississippian) in North Dakota Using Experimentally Determined Kinetics, Current Formation Temperatures, and Total Reactive Kerogen Mass

AAPG ACE 2018

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Calculation of a Rate Index for the Bakken Formation (Devonian-Mississippian) in North Dakota Using Experimentally Determined Kinetics, Current Formation Temperatures, and Total Reactive Kerogen Mass

Abstract

The most basic kinetic analysis of source rocks typically uses some variant of the Arrhenius Equation to combine source rock properties (kinetics), organic richness (reactive mass) and formation temperatures to produce a corresponding reaction rate. Unfortunately the basic relationship between kinetic properties, time and temperature are complicated by fundamental questions concerning the value of the so called frequency or pre-exponential factor (A) that is derived from experimental data. A significant part of the debate is whether or not the pre-exponential factor is consistent with one derived from Transition State Theory. In particular, a simple dependency of A on the thermal vibrational frequency ( A = kb/hTH where kb is the Boltzman constant, h is Planck’s constant and TH is the harmonic mean of peak reaction temperatures (oK) from a series of nonisothermal pyrolysis experiments).

This study presents data from over 50 samples of the Bakken Formation of North Dakota that, in spite of a significant statistical compensation effect (extremely elongated error ellipse in the Ea – ln (A) plane), produces an average frequency factor that is remarkably consistent with the thermal vibrational frequency. Interestingly the slope of the statistical compensation effect is also a simple function of the harmonic mean of reaction temperatures. Recalculating experimental results using the statistical compensation effect and thermal vibrational frequency produces corrected activation energies that are highly correlated with Tmax , whereas uncorrected values are not. When mapped, corrected activation energies from the Bakken Formation, vary smoothly across the Williston Basin and bear several commonalities with published maps of kerogen maturity, reservoir overpressures and production.

Modern reaction rate indices found using corrected kinetics, subsurface temperature profiles from thermally equilibrated wells and reactive kerogen assays show that theoretical reaction rates increase by at least an order of magnitude between a well that is immature and a another that is mature. These results suggest that the combination of kinetic properties, formation temperature and total reactive kerogen mass may provide a useful metric related to the dynamics of oil generation within the Bakken Shale and potentially other Basin Centered Petroleum Systems.