Estimating modern equilibrium temperatures in the Bakken Formation of North Dakota, USA: Application of an analytical solution to depth dependent changes in thermal conductivity

Abstract

The mid-Paleozoic through late Cretaceous section in the Williston Basin records a transition from carbonate dominated marine sedimentation to that dominated by terrestrial and marine shales. Even though the intervening stratigraphy is complex the overall thermal profile is not. Equilibrium temperature profiles for 20 wells distributed across the North Dakota portion of the Williston may be closely approximated by a two or three-layer thermal model. This model consists of one or two upper layers with constant thermal conductivities that extend through the Tertiary and upper Cretaceous. A third layer from the top of the Cretaceous Greenhorn Formation to the top of the Mississippian-Devonian Bakken Formation exhibits a thermal gradient that is consistent with a gradual and constant increase in thermal conductivity with depth. This gradient is consistent with a nonlinear analytical solution that relates temperature and depth to a progressive increase in thermal conductivity with depth. These results provide an alternative approach to estimating temperature profiles from the Greenhorn Formation to the top of the Bakken Formation than those using more complex models consisting of many discrete layers. The method also provides a means of extrapolating temperatures recorded above the Madison Group to the top of the Bakken Formation. • Equilibrium temperature logs are nonlinear with depth between the Cretaceous and Miss-Dev. Bakken Fm. • Explained by an analytical expression for a temperature profile caused by gradual change in thermal conductivity with depth. • Nonlinear regression fits observed temperatures for more than a kilometer. • .Provides a means of extrapolating temperature profiles from the Madison to Bakken Fm. in the Williston Basin.