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Relationships Among Oil Density, Gross Composition, and Thermal Maturity Indicators in Northeastern Williston Basin Oils and Their Significance for Expulsion Thresholds and Migration Pathways

OSADETZ, KIRK G., LLOYD R. SNOWDON, and PAUL W. BROOKS, Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, Calgary, Alberta, Canada

Oil density ( degreeAPI), gross composition, and biological marker thermal maturity variations in northeastern Williston basin have stratigraphic and geographic significance controlled by migration pathways and source rock composition as it affects hydrocarbon generation and expulsion characteristics. The trisnorhopane thermal maturity indicator, Ts/Tm, exhibits strong correlations with oil density and saturate/aromatic hydrocarbon ratio that can be used to infer the equivalent vitrinite reflectance of an oil. This shows that oils derived from source rocks in the Lodgepole and Winnipegosis formations are expelled at approximately 0.7% Rv, the conventional intense hydrocarbon generation threshold. Expulsion from Bakken Formation source rocks is delayed until much higher thermal maturit es, approximately 0.9% Rv, near the transition zone. Delayed expulsion is attributed to lithological differences among source rocks and is probably instrumental in controlling the hydraulic fracturing in Bakken shales so important to the horizontal-well play. It also accounts for the relative efficiency among source rocks as shown by the small number of Bakken-sourced conventional oil pools.

When the depth and density of oil pools is compared to relationships predicted using the correlation between source rock thermal maturity and oil density, several different migration pathways can be inferred. Winnipegosis source oils indicate four paths. Most small pinnacle reef pools are sourced locally, but larger coalesced reefs contain oils migrated long distances through the Lower Member Winnipegosis Formation. Among oils that have migrated past Prairie salts, both locally sourced oils, like those on the flank of the Hummingbird Trough, and more mature, longer migrated oils in Saskatchewan Group reservoirs can be identified. Bakken oils have the longest migration pathways, controlled primarily by a lowstand shoreline sandstone on the eastern side of the basin. Lodgepole-sourced o ls dominate Madison Group plays. Northwest of Steelman field, oil density increases primarily due to thermal maturity differences but also because of increasing biodegradation and water-washing that affect the western edge of the play trend. Along the margin of the Hummingbird Trough are a number of deep, medium-gravity pools whose oil compositions are entirely attributable to low thermal maturity and local migration pathways.


AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)