Late Devonian and Early Mississippian Bakken and Exshaw Black Shale Source Rocks, Western Canada Sedimentary Basin: A Sequence Stratigraphic Interpretation

Abstract Petrographic analyses of dispersed organic matter (including macerals and palynomorphs), siliceous and calcareous microfossil assemblages and microtextures (e.g. stromatolitic) have been used to define and interpret five organic facies and regionally map their distribution for the following informal groupings of potential hydrocarbon source rocks in the Western Canada Sedimentary Basin: Upper Devonian Woodbend group, Upper Devonian Winterburn group and Upper Devonian to Lower Mississippian black shales of the Exshaw and Bakken formations. Five petrographic organic facies (A–E) are defined for the potential source rocks based on assemblages of alginites, acritarchs, sporinites, siliceous microfossils and algal mat microtextures. Organic facies A, B (prasinophyte alginites and acritarchs) and C (coccoidal alginite), represent accumulation in relatively deep (basin), intermediate (shelf-platform), and shallow water depths (bank-reef margin to lagoonal). Organic facies D is defined by siliceous microfossils (e.g. Radiolaria) and accumulated in deep basinal to outer shelf settings immediately east of an ancient Pacific Ocean, or south of an ancient Arctic Ocean. This facies may reflect regions of upwelling which extended into intracratonic and epicontinental settings. Organic facies E, characterized by stromatolitic microtextures with or without coccoidal alginite, only occur within Upper Devonian Winterburn Group shallow water, restricted shelf to lagoonal dolostones associated with evaporites. As a whole, the regional distribution of organic facies is related to paleogeography, paleobathymetry or paleostructure in the source rocks. Surprisingly, petrographic organic facies do not show strong positive correlation with kerogen type as defined by Hydrogen-Oxygen indices or TOC-S2 plots.

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Western Canada Sedimentary Basin petroleum systems: A working and evolving paradigm

Interpretation ◽

10.1190/int-2017-0165.1 ◽

2018 ◽

Vol 6 (2) ◽

pp. SE63-SE98 ◽

Cited By ~ 2

Author(s):

Kirk G. Osadetz ◽

Andrew Mort ◽

Lloyd R. Snowdon ◽

Donald C. Lawton ◽

Zhuoheng Chen ◽

...

Keyword(s):

Sedimentary Basin ◽

Anaerobic Biodegradation ◽

Source Rocks ◽

Western Canada ◽

Biogenic Gas ◽

Western Canada Sedimentary Basin ◽

Petroleum Systems ◽

Migration Pathways ◽

Canada Sedimentary Basin ◽

Secondary Biogenic Gas

Western Canada Sedimentary Basin (WCSB) crude oil source rocks accumulated typically in “starved” depositional settings of Sloss outer detrital facies belts and lesser stratigraphic cycles. These produced petroleum from marine type II organic matter in response to burial by commonly westward-thickening overlying successions. Oil occurs commonly within the “Sloss” sequence containing its source rock, often up dip from the “petroleum kitchen.” Migration pathways cross stratal contacts, unconformities and structures, and much oil migrated into adjacent sequences, especially into Lower Cretaceous Mannville Group reservoirs. Anaerobic biodegradation affects oil quality and generates secondary biogenic gas. The WCSB oil system paradigm predates the recognition of anaerobic biodegradation. Biodegradation in post-Mannville reservoirs remains underappreciated. Natural gases originate by thermogenic and biogenic mechanisms from kerogens, coals, and crude oils. Gases are variably altered: physically, microbially, and inorganically. Few oil studies addressed solution and associated primary thermogenic or secondary biogenic gas. Gas studies are independent of oil studies and none recognize secondary biogenic gas even in association with biodegraded oils. We hypothesize that secondary biogenic gas occurs commonly, often mixed with other gas, to produce hydrocarbon isotope ratios and variations distinctive from primary biogenic and thermogenic gases. Where Mannville oil pools have sources in underlying marine rocks, Mannville gases are attributed largely to nonmarine sources. Currently, cross-stratal migration is inferred less commonly for gas than for oil. The inference of gas stratigraphic immobility is problematic for biodegradation studies that infer large secondary biogenic gas fluxes into soil and atmospheric sinks, the migration pathways of which pass through Cretaceous strata. In some unconventional plays, gas isotopic “rollover” and “reversal” due to thermal cracking has implications for reservoir performance. Efforts to understand Cordilleran petroleum systems merit investigation to extend unconventional resource plays westward from Interior Platform.

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