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The AAPG/Datapages Combined Publications Database

Williston Basin Symposium

Abstract

SKGS-AAPG

Sixth International Williston Basin Symposium, October 7, 1991 (SP11)

Pages 293 - 311

RELATIONSHIPS AMONGST OIL QUALITY, THERMAL MATURITY AND POST-ACCUMULATION ALTERATION IN CANADIAN WILLISTON BASIN (SOUTHEASTERN SASKATCHEWAN AND SOUTHWESTERN MANITOBA)

KIRK G. OSADETZ, Institute of Sedimentary and Petroleum Geology, 3303 - 33rd Street N.W., Calgary, Alberta T2L 2A7
LLOYD R. SNOWDON, Institute of Sedimentary and Petroleum Geology, 3303 - 33rd Street N.W., Calgary, Alberta T2L 2A7
PAUL W. BROOKS, Institute of Sedimentary and Petroleum Geology, 3303 - 33rd Street N.W., Calgary, Alberta T2L 2A7

ABSTRACT

Oil quality in the northeastern Williston Basin is controlled by source rock characteristics, maturity, and to a lesser extent post accumulation processes, especially biodegradation. Previous studies of Mission Canyon oil quality and compositional variations attributed changes in API gravity, saturate/aromatic hydrocarbon ratio and n-alkane depletion to progressive biodegradation accompanied by meteoric water washing. Source composition and maturity can produce similar effects, but it was previously difficult to characterize oil pool thermal maturity. The trisnorhopane thermal maturity indicator (Ts/Tm) can be used to characterize oil pool maturity and is resistant to biodegradation. Revised oil-source associations quality variations are inferred to be dominated by source rock compositional variations and thermal maturity. Previously proposed gasoline range thermal maturity indicators are of limited value. The few Bakken-sourced oils discovered to date are of uniformly high quality (> 35° API) because of a delayed expulsion threshold (0.9 percent Ro) controlled by source rock composition. These oils are commonly connate water washed, but are not biodegraded. Lodgepole-sourced oils are expelled at the beginning of mainstage hydrocarbon generation (0.7 percent Ro). Their API gravities show a slight sensitivity to thermal maturity that follow progressive changes in saturate/aromatic ratio. Biodegradation, a subsequent and lesser process, overprints a northwestwardly decrease in Madison Group oil pool thermal maturity, but the effects of thermal maturity are inferred to dominate. Some Madison pools are connate water washed without being biodegraded and this probably reflects secondary migration pathway length. Winnipegosis-sourced oils are very sensitive to maturity variations, and oil quality and saturate/aromatic ratio both increase rapidly with maturity. There is no evidence that these oils are biodegraded, although longer migrated oils tend to be more connate water washed. The few discovered Ordovician-sourced pools yield only a limited sample of expected thermal maturities. Ordovician-sourced oil quality is also strongly dependent on thermal maturity. High quality oil pools are expected to be discovered in stratigraphic and diagenetic traps along the eastern flank of the basin, because of the timing and kinetics of Ordovician hydrocarbon generation.

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