Organic maturity in the western Canada sedimentary basin

Abstract

Organic maturity of strata in the Western Canada Sedimentary Basin has been investigated in numerous studies using petrological, chemical and physical methods. Compilation of these data has facilitated a regional analysis of the maturity patterns and maturation history. Variation in maturity patterns can be divided into three levels; basin wide (1st order), regional (2nd order) and local (3rd order). First order variations are manifested by an overall increase in maturity of strata of the same age from east to west from the Plains to the Rocky Mountain Foothills and Front Ranges in response to progressively deeper burial and higher paleo-geothermal gradients. Superimposed on this first order (basinal scale) variation in maturity are second and third order variations which reflect local differences in depth of burial, conductive and advective heat transport or effects of thrust faulting. Maturity gradients in the axis of the basin are low, averaging 0.10 log% R omax/km, whereas in the Front Ranges and Foothills the gradients are substantially greater (average 0.25 log% R omax/km. Variations in maturity gradients reflect lower paleo-geothermal gradients resulting from rapid sediment loading and subsequent unloading in the Plains, and higher conductive heat transport in the deformed belt. The main component of maturity of most Phanerozoic strata occurred during deep burial under Upper Cretaceous and/or Paleogene molasse in foredeeps developed in response to crustal loading during the easterly migration of the foreland fold and thrust belt. As a result of the west to east propagation of deformation during the Laramide Orogeny, deep burial, maturation, hydrocarbon generation and uplift occurred earlier in the foreland belt (Late Cretaceous) than in the Plains to the east where the main component of maturity and hydrocarbon generation occurred as late as Eocene. In the Plains, because of the low maturity gradients, a thick succession of strata currently is within the oil window. In the deformed belt however, higher maturity gradients have resulted in a thinner succession of strata within the oil window. In response to higher paleo-geothermal gradients, strata in the deformed belt matured more quickly leading to more rapid hydrocarbon generation and migration than areas to the east. In the southern part of the Cordillera a significant component of maturation post-dates structural deformation of the strata whereas in northeastern British Columbia and adjacent parts of Alberta, maturity primarily pre-dates structural deformation. Data from some deep bore holes and surface samples in the southern part of the Cordillera provide evidence for maturity post-dating or accompanying emplacement of major overthrust sheets (tectonic burial). In most areas however, there is no clear evidence for the timing of maturity relative to faulting or evidence of frictional heating. The maturity gradients and levels of maturity of surface and near surface strata in the western Plains and Rocky Mountain Foothills indicate that a succession (several kilometres) of Paleogene strata were deposited in these regions (foredeep) and subsequently eroded in response to post-Laramide uplift.