Isomerization and Aromatization of Hydrocarbons and the Paleothermometry and Burial History of Alberta Foreland Basin

Abstract

The hydrocarbon fractions extracted from 34 Cretaceous shales from a cross section of the Alberta basin have been analyzed using gas chromatography-mass spectrometry. Particular attention has been paid to the extents to which three temperature-dependent geochemical reactions have occurred: isomerization of steranes at C-20; isomerization of hopanes at C-22; and aromatization of C-ring monoaromatic steroid hydrocarbons. Estimates of the kinetic constants of these reactions have been published previously; therefore, constraints may be put on the maximum temperature experienced by the sediments and the time for which this temperature existed. The subsidence history of the Alberta basin is simulated using a lithospheric flexural model in which the lithosphere is downwarped by the loading of overthrusts in the adjacent Cordilleran orogene. When overthrusting ceases, erosion of the overthrusted region lightens the load and the lithosphere rebounds, causing uplift and erosion of the foreland basin. Major overthrusting and therefore deep burial occurred during the Laramide orogeny of Late Cretaceous to mid-Tertiary time. The extent of chemical reactions suggests that the present maturity of the Cretaceous shales is almost entirely a reflection of burial and heating during the Laramide orogeny in a manner that is consistent with the lithospheric flexural model for the basin. The chemical reactions suggest that the paleogeothermal gradient increased systematically away from the foothills of the Rocky Mountains along the profile studied. That a similar trend exists today suggests this situation has persisted at least since the early stages of the Laramide orogeny. This variation is best explained by advective heat transfer. Water entering the basin in the faulted and topographically higher foothills is heated at depth, thereby depressing the geothermal gradient. This hot water then migrates updip and away from the foothills to enhance the geothermal gradients in the Edmonton area and beyond. The route of the migrating hot water is similar to that taken by hydrocarbons from Devonian and Cretaceous source rocks in the deep basin, updip to the oil and tar-sand accumulations of Alberta.