ABSTRACT: Microfracturing in an Overpressured Carbonate Buildup: Upper Devonian Strachan Reservoir, Deep Alberta Basin

MARQUEZ, X., E. W. MOUNTJOY, and J. E. AMTHOR, McGill University, Montreal, Quebec, Canada

The isolated Strachan buildup (Leduc Formation) occurs in the deep part of the Alberta basin (>4000 m) and contains major gas reserves. Previously undescribed abundant microfractures occur in both partially and completely dolomitized facies in the upper 100 m of the buildup. The hairline (<1 mm) microfractures are filled with bitumen and form three patterns: (1) semi-radial extending from vuggy and moldic pores, (2) random extending from irregular pores, and (3) subhorizontal extending from subvertical fractures. They crosscut all sedimentary and diagenetic products, including some late-stage anhydrite cements. Similar microfractures occur in some other buildups (e.g., Harmattan).

Overpressuring caused by thermal cracking of oil to gas during increasing burial explains most of the characteristics of the microfractures; their association with all pore types, bitumen fillings, timing, and their restriction to isolated buildups in the deep (>4000 m) Alberta basin.

Because the adjacent gas-bearing, updip buildups of the Rimbey-Meadowbrook reef trend acted as a conduit system, pressures never built up sufficiently to form microfractures. Thus thermal cracking of oil to gas appears to have been responsible for the formation of microfractures in isolated and effectively sealed buildups.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)