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Unconformity-associated replacement limestones after anhydrite in Mississippian of Williston Basin

Conference · · Am. Assoc. Pet. Geol., Bull.; (United States)
OSTI ID:6182636
Locally in southeastern Saskatchewan, Mississippian nodular anhydrites (after subaqueous gypsum) beneath an unconformity have been altered to limestone-limestones that are commonly porous and oil-bearing. In the region, carbonates beneath the unconformity are normally overlain by red beds and have been completely dolomitized and plugged with anhydrite to form an impermeable caprock. Mississippian anhydrites subcrop at the unconformity surface and reveal little evidence of alteration-even to gypsum. Textures in replaced anhydrites indicate that calcitization involved both creation of porosity and in-situ (small-scale) replacement leading to retention of anhydrite (and later gypsum) fabrics. Celestite formed as strontium was released from anhydrite during replacement by gypsum and calcite. Sulfur in associated pyrite is isotopically lighter than the anhydrite, suggesting anhydrite-alteration involved the activities of sulfate-reducing bacteria. Evidently, H/sub 2/S liberated during the reaction migrated across the unconformity to reduce overlying red beds. Limestones of this type do not appear to have been reported previously. Stratigraphic and petrographic evidence indicates replacement, although spacially related to the unconformity, was not a weathering phenomenon. It occurred after the unconformity was buried. Unexpectedly heavy delta/sup 13/C and delta/sup 18/O values (+ 1.22 to 1.54, and -1.0 to -3.7) obtained from the replacement limestones seem to preclude the utilization of organic carbon in the reaction. The source of carbonate and of the energy required for sulfate-reducing bacterial activity is therefore problematic.
Research Organization:
Amoco Canada, Calgary
OSTI ID:
6182636
Report Number(s):
CONF-8304200-
Conference Information:
Journal Name: Am. Assoc. Pet. Geol., Bull.; (United States) Journal Volume: 67:3
Country of Publication:
United States
Language:
English

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Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
& Exploration
03 NATURAL GAS
030200 -- Natural Gas-- Reserves
Geology
& Exploration
ALKALINE EARTH METAL COMPOUNDS
ANHYDRITE
CALCIUM COMPOUNDS
CALCIUM SULFATES
CANADA
CARBONATE ROCKS
DISSOLUTION
ENERGY SOURCES
FOSSIL FUELS
FUELS
GEOLOGIC AGES
GEOLOGIC STRUCTURES
LIMESTONE
MINERALS
MISSISSIPPIAN PERIOD
NORTH AMERICA
OXYGEN COMPOUNDS
PALEOZOIC ERA
PETROLEUM
POROSITY
PRECIPITATION
RESERVOIR ROCK
ROCKS
SASKATCHEWAN
SEDIMENTARY BASINS
SEDIMENTARY ROCKS
SEPARATION PROCESSES
SULFATE MINERALS
SULFATES
SULFUR COMPOUNDS