ABSTRACT

Thermal conductivity and heat flow control temperatures within a basin. Knowledge of thermal conductivity and variations of thermal conductivity and heat flow with time are, therefore, critical factors in determining a basin's thermal evolution. A model for thermal evolution of the Williston Basin serves as a typical example of the effects of thermal conductivity structure and variation in thermal conductivity with time.

The thermal conductivity of the Williston Basin can be divided into a high-conductivity, Paleozoic carbonate section and a low-conductivity, Mesozoic and Cenozoic clastic section. This structure causes a geothermal gradient profile that essentially has two components. A gradient of about 50 K km"1 characterizes the clastic rocks and a gradient of about 20 K km"1 characterizes the carbonates. The marked vertical variation in geothermal gradient structure precludes use of bottom-hole temperature data for estimating the thermal history of strata in the basin. Variation in sediment thermal conductivity in time is one-directional and is significantly influenced by sedimentation rate, compaction rate, sediment composition and sediment thickness. Thermal conductivity of Mesozoic and Cenozoic clastic sediments in the basin increased by about 30 per cent due to compaction during sedimentation. This time-variation in conductivity caused a gradual reduction in subsurface temperatures by about 20 degrees Celsius in the primary oil source rock, the Bakken Formation, during the early Tertiary. Application of a two-dimensional thermal model that includes variable thermal properties predicts a significantly different thermal maturity for the basin than does a simple model based on present geothermal gradients.