Organic microfacies and basinal tectonic control on source rock accumulation; a microscopic approach with examples from an intracratonic and extensional basin

• Published in: International journal of coal geology 1991-12, Vol.19 (1), p.457-481
• Main Authors: Stasiuk, L.D., Osadetz, K.G., Goodarzi, F., Gentzis, T.
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Hydrocarbons; Sedimentary rocks; Tectonics. Structural geology. Plate tectonics
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/0166-5162(91)90030-M

Relationships among organic petrographic maceral assemblages and hydrocarbon source rock depositional environments distinguish broad marine, terrestrial and mixed kerogen accumulation settings. Organic microfacies from Paleozoic and Mesozoic source rock intervals in the intracratonic Williston Basin and the extensional Sverdrup Basin have been studied using incident white and fluorescent light microscopy. Distinct anoxic organic microfacies are defined in Upper Ordovician Bighorn Group subtidal, platformal kukersites and Middle Devonian upper Elk Point Group carbonate lithologies that preserve a progression of depositional environments from platformal to starved basinal. Paleozoic carbonate platform microfacies are characterized by well preserved cyanophyceae alginite stromatolitic mats displaying upper surface textural features such as pustules, pinnacles, endolithic algal borings and interlaminated vertical and horizontal alginite resulting from phototactic growth. Potential source intervals in Middle and Upper Triassic shelf shales from the Sverdrup Basin and Middle Devonian carbonate platform marginal mudstones in the Williston Basin are both characterized by high concentrations of sapropelic, bituminite kerogen hosting unicellular marine alginite. Similar bituminite-rich organic microfacies occur in Middle Devonian basinal settings associated with larger, more abundant alginite and significant amounts of dasycladacean calcareous algae. The main controls on source rock accumulation are water depth and chemistry as controlled by depositional setting and basin restriction.