ABSTRACT

Apatite fission-track (AFT) thermochronology and organic maturity data indicate spatial and temporal variations in the thermal history of the Williston Basin. Variations of oil windows are associated with crustal structure. Oil windows are elevated by -750 m to -1250 m and heat flow, as determined from maturity models, is at least 18% higher in the North American Central Plains Conductivity Anomaly than in adjacent regions. The persistent difference in heat flow between regions is attributed to crustal compositional changes originating during the Precambrian Trans-Hudson Orogeny. In the same region, some deduce that current geothermal gradients are still high. Analysis of apatite fission track (AFT) data indicates complicated temporal and geographical heat flow variations in the Williston Basin and on the adjacent Canadian Shield. On the Canadian Shield and in the thin wedge of peripheral Williston Basin strata, AFT data typically record a Late Precambrian epeirogeny followed by episodic Phanerozoic heating and cooling, the pattern of which corresponds to Williston Basin sedimentation history. Elsewhere the record of Late Precambrian cooling is obliterated by a resetting of AFT clocks during the Permian. The regions affected include both a basin central zone, with a radius of more than 300 km, where the Phanerozoic succession is more than 1500 m thick; and a linear zone at shallower depths, trending northwesterly from Yorkton to Lake Athabasca in Saskatchewan. The Permian thermal anomaly is intrinsic to the mechanism of Williston Basin subsidence, in that, in addition to the foregoing, it is associated with a major erosional event and originates in the basement. With geohistory constraints, the AFT models suggest Permian heat flows that were at least 184% of Mesozoic to present values. Neither the timing nor magnitude of this thermal event is considered by current tectonic models. Everywhere the pre-Mesozoic thermal history is partially to totally obscured by heating accompanying the Rocky Mountain Foreland sedimentation that extended beyond the Williston Basin onto the Canadian Shield. In most locations and strata the main stage of hydrocarbon generation began no sooner than latest Cretaceous to Early Tertiary time, although Permian heating has a significant potential to affect organic maturity histories in select locations and in lower parts of the Phanerozoic succession. The Deadwood Formation, which may be a source of a regionally significant, but poorly described hydrocarbon system, is predicted to have reached the main hydrocarbon generation stage during the Late Paleozoic, as a direct consequence of the Permian heating.