--> Abstract: Mississippian Debolt Formation, Northwest Alberta: New Perspectives Add Significant Reserves to an Established Shallow Gas Reservoir, by C. S. Fic; #90987 (1993).

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FIC, CHRISTOPHER S., Mobil Oil Canada, Calgary, Alberta

ABSTRACT: Mississippian Debolt Formation, Northwest Alberta: New Perspectives Add Significant Reserves to an Established Shallow Gas Reservoir

The Mississippian Debolt Formation in northern Alberta and northeast British Columbia consists of a series of stacked shoaling upward carbonate ramps. Pre-Cretaceous cordilleran tectonism gently tilted the strata to the southwest and concomitant erosion truncated the beds on their updip reaches. This created a series of backstepping southeast striking outcrop edges that were subsequently incised by Lower Cretaceous fluvial channels. A regional transgression during the Lower Cretaceous deposited fine mudstones over the Mississippian strata effecting an updip seal. Karsting which formed during the erosive phase of development greatly enhanced porosity and permeability along the Mississippian outcrop belt.

A detailed geologic study of the Mississippian subcrop edge established the existence of unique reservoir qualities in the Upper Porous member of the Debolt Formation. The Upper Porous has a complex lithology variably consisting of a mixture of dolomite, limestone and tripolitic chert. Intercrystalline and moldic porosities average 15%, with up to several darcies of permeability. Conventional rotary mud drilling of this reservoir induces severe formation damage and produces inconclusive and misleading drill stem test results. Tripolitic chert further complicates evaluation of this reservoir by muting the resistivity logging tool response and masking the presence of low resistivity gas pay zones.

This study emphasizes how a unique perspective can transform an exploration play with limited upside potential to a prime exploration target.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.