Abstract: A Comparative Examination of Organic Geochemistry and Petrology of Upper Devonian to Early Mississippian Epicontinental Black Shales from Canada

OBERMAJER, MARK, LAVERN D. STASIUK, MARTIN G. FOWLER, and KIRK G. OSADETZ

Epicontinental, black shales of Upper Devonian to Early Mississippian age are widespread on the North American continent and are important sources of oil and gas. Three organic-rich black shale units of this age from the Alberta Basin (Exshaw Formation), the Michigan Basin in Ontario (Kettle Point Formation) and the Williston Basin in Saskatchewan (Bakken Formation) are compared in terms of their organic facies as defined by organic geochemical (biomarkers) and organic petrological (macerals) characteristics and parameters. A range of kerogen types is present within the black shales ranging from Type I to Type III, although Type II kerogen typically dominates. Organic facies, based on the distribution and type of alginite, acritarch and sporinite macerals, shows a good correlation with paleodepositional setting, relative water depth and agitation rates (i.e. outer shelf to open basin) for the Kettle Point and Bakken Formations whereas the Exshaw shale only partially conforms to this model.

The biomarker characteristics show similarities for the Kettle Point and Bakken shales. Both units were deposited in a marine clastic algal-dominated environment. Their terpane fingerprints typically show a smooth homohopane profile, a predominance of Tm over Ts and low gammacerane. The concentration and ratio of acyclic isoprenoids (i.e. pristane and phytane) indicate dysoxic conditions in the water column and sediments (Pr/Ph >1). In contrast, the geochemical signatures of the Exshaw shales are more typical for carbonate sequences, with prominent gammacerane and C34 hopane. The greater variability of biomarker parameters in the Exshaw shales probably results from the much broader depositional extent of this unit, reflecting more open (to the north) and more restricted (to the south) conditions.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky