Ordovician petroleum source rocks and aspects of hydrocarbon generation in Canadian portion of Williston basin
Accumulation of rich petroleum source rocks - starved bituminous mudrocks in both the Winnipeg Formation (Middle Ordovician) and Bighorn Group (Upper Ordovician) - is controlled by cyclical deepening events with a frequency of approximately 2 m.y. Tectonics control both this frequency and the location of starved subbasins of source rock accumulation. Deepening cycles initiated starvation of offshore portions of the inner detrital and medial carbonate facies belts. Persistence of starved offshore settings was aided by marginal onlap and strandline migration in the inner detrital facies belt, and by low carbonate productivity in the medial carbonate facies belt. Low carbonate productivity was accompanied by high rates of planktonic productivity. Periodic anoxia, as a consequence of high rates of planktonic organic productivity accompanying wind-driven equatorial upwellings, is the preferred mechanism for suppressing carbonate productivity within the epeiric sea. The planktonic, although problematic, form Gloecapsamorpha prisca Zalesskey 1917 is the main contributing organism to source rock alginites. A long-ranging alga (Cambrian to Silurian), it forms kukersites in Middle and Upper Ordovician rocks of the Williston basin as a consequence of environmental controls - starvation and periodic anoxia. Source rocks composed of this organic matter type generate oils of distinctive composition at relatively high levels of thermal maturity (transformation ratio = 10% at 0.78% R/sub o/). In the Canadian portion of the Williston basin, such levels of thermal maturity occur at present depths greater than 2950 m within a region of geothermal gradient anomalies associated with the Nesson anticline. Approximately 193 million bbl (30.7 x 10/sup 6/ m/sup 3/) of oil has been expelled into secondary migration pathways from thermally mature source rocks in the Canadian portion of the basin.
- Research Organization:
- Canada Geological Survey, Calgary, Alberta
- OSTI ID:
- 6126708
- Report Number(s):
- CONF-8808223-
- Journal Information:
- AAPG (Am. Assoc. Pet. Geol.) Bull.; (United States), Vol. 72:7; Conference: AAPG Rocky Mountain Section meeting, Bismarck, ND, USA, 21 Aug 1988
- Country of Publication:
- United States
- Language:
- English
Similar Records
Ancestral Nesson anticline and associated geothermal anomalies: Enhanced hydrocarbon generation controlled by crustal structure
Petroleum potential of lower and middle Paleozoic rocks in Nebraska portion of Mid-Continent
Related Subjects
PETROLEUM DEPOSITS
SOURCE ROCKS
WILLISTON BASIN
CAMBRIAN PERIOD
CARBONATE ROCKS
GEOLOGIC FORMATIONS
ORDOVICIAN PERIOD
SILURIAN PERIOD
TECTONICS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
MINERAL RESOURCES
PALEOZOIC ERA
RESOURCES
ROCKS
SEDIMENTARY ROCKS
020200* - Petroleum- Reserves
Geology
& Exploration