The Occult Thermotectonic History of Cratons and Epicratonic Basins: Implications for Petroleum Generation in Williston Basin

Kirk G. Osadetz¹, Barry P. Kohn², and Shimon Feinstein^3
1Geological Survey of Canada - Calgary, 3303 – 33rd St. N.W., Calgary Alberta T2L 2A7, Canada
²School of Earth Sciences, University of Melbourne, Victoria 3010, Australia
³Department of Geological and Environmental Sciences, Ben Gurion University of the Negev, P.O. Box 653, Beer Sheva 84 120, Israel

The Williston Basin is a sub-circular Phanerozoic epicratonic basin ~800 km in diameter. It lies within a more extensive area of Phanerozoic sedimentation. Phanerozoic successions of this region overlie parts of the North American craton and contain significant petroleum provinces and mineral and coal resources, which continue to increase as additional exploration, including unconventional reservoirs and technology enhance and facilitate petroleum recovery. Epicartonic basins, of which the Williston Basin is a member, remain poorly understood tectonic features generally. While linked to large depositonal realms and successions, internally they are characterized by thin succession, far from active or passive margins, which exhibit long-lived episodic anomalous subsidence of great circular symmetry about a constant centre, regardless of the Phanerozoic “continental drift” of the craton.

The Williston Basin served as the site of William’s and Dow’s benchmark organic geochemical and petroleum geology studies which contributed greatly to our current petroleum system concept. Their conclusions were employed by Thompson and Thode, who respectively produced key studies and interpretations of gasoline range hydrocarbons and sulphur isotopes in petroleum, informed by early work on Williston Basin petroleum systems. Williston Basin petroleum occurrence exhibits striking contrasts between the American and Canadian regions. In Canada the main petroleum plays are stratigraphically entrapped in upper Paleozoic and Mesozoic reservoirs, commonly at, near, or beyond the subcrop edge of Carboniferous rocks. In contrast the American portion of the Basin contains major structures that coincide with large oil fields in lower Paleozoic rocks. Petroleum systems do not exhibit the geographic restrictions characteristic of the magnitude and stratigraphic host of the identified resources. Together the tectonics of the Williston Basin and the distribution of identified petroleum resources represent key problems that should be, respectively, considered by, and amenable to petroleum system analysis.

To elucidate the role of tectonics and thermal history we have applied apatite fission track (AFT) thermochronology to both a composite depth profile of Precambrian basement rocks underlying the Phanerozoic Canadian Williston Basin and to the outcropping Precambrian Canadian Shield, east of the Williston Basin, where the Phanerozoic depositional and erosional history can be confidently reconstructed.

Within Williston Basin AFT derived thermal histories indicate cycles of heating and cooling that mimic burial history pattern, but which also indicate profound temporal and geographic variations in the timing and degree of maximum Phanerozoic temperatures in the sedimentary succession. These thermal history variations are occult if only organic maturity indicatiors or subsidence models are considered. AFT thermochronology suggests a late Paleozoic heat flow anomaly with linear geographic characteristics following Middle Devonian-Carboniferous Kaskaskia subsidence patterns. This thermal anomaly has both economic and geodynamic significance. The revised thermal history indicates that Upper Cambrian-Lower Ordovician petroleum source rocks became fully mature during the late Paleozoic. This distinguishes the lower Paleozoic petroleum systems from those with Upper Devonian and superior petroleum source rocks, which entered the main hydrocarbon generation stage in latest Cretaceous and Paleogene time. The AFT data also implies a geodynamic coupling among the late Paleozoic heat flow anomaly, Kaskaskia subsidence, and previously inferred late Paleozoic lithospheric weakening. The time delay between the beginning of Kaskaskia subsidence and the appearance of the late Paleozoic thermal anomaly at the base of the Phanerozoic succession is consistent with lithospheric thermal parameters.

While data from the basement beneath Williston Basin suggested major time dependent heat flow changes, the nature of the composed sample profile and the lack of independent constraints on the upper Paleozoic maximum thickness precluded the outright rejection of a constant heat flow model. Howerver, the AFT analysis of a 1.15 km deep AFT profile at the Underground Research Laboratory (URL), in the Canadian Shield indicated not one, but two Phanerozoic heating and cooling episodes that also were associated with significant time-dependent heat flow variations. The URL heating and cooling episodes are temporally associated with Phanerozoic burial and erosion of the Precambrian crystalline shield and its overlying Phanerozoic successions, which are today completely eroded.

At the URL maximum Phanerozoic temperatures occurred in the late Paleozoic when the geothermal gradient is inferred to have been ~47 ± 4.6^o C/km (compared to a present day gradient of ~14±2^o C/km) and the sedimentary cover was ~800-1100 m thick, (regional stratigraphic relationships suggest that the Paleozoic succession was completely eroded prior to beginning of Mesozoic sedimentation). The second, Late Cretaceous-Paleogene heating phase occurred when the geothermal gradient is inferred to have been ~22 ± 2.1^o C/km and the Mesozoic and Cenozoic succession was ~1200 to 1400 m thick.

The URL Phanerozoic thermal history resembles that inferred for the epicratonic Williston Basin, several 100 km's to the west. This implies a common regional thermal history for cratonic rocks underlying both the epicratonic basin and the currently exposed Shield. It suggests that the morphotectonic differences between the Williston Basin and the exposed shield are due to a dissimilar thermomechanical response to similar complicated Phanerozoic geodynamic history. The two Phanerozoic episodes of increased geothermal gradient (heat flow anomaly) and the epeirogenic movements related to the deposition and erosion of sediments can be tentatively attributed to far-field effects of orogenic processes at the active plate margin (i.e. the Antler and the Cordilleran orogenies) and associated sub-lithospheric loads. While much remains to be understood regarding these process and the geographically varying lithospheric response it is clear that even in the “simplest” of tectonic settings it is necessary to adequately characterize the thermal and tectonic history if petroleum systems are to be adequately modelled.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.