ABSTRACT: Microfracturing in Deeply Buried Carbonate Buildups: Upper Devonian Strachan and Ricinus Reservoirs, Alberta Basin

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

Major gas reservoirs in partially and completely dolomitized buildups (Leduc Formation) occur in the deep part of the Alberta basin (>4000 m). Cores from two buildups are illustrated: Strachan (10-31-37-9W5) and Ricinus (10-33-36-10W5). In Strachan, 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 occur in three different patterns. They crosscut all sedimentary and diagenetic products. In Ricinus, carbonates have been completely dolomitized, and microfractures, although present, are minor. The microfractures are restricted to the upper part of the reservoir and occur mostly in a semi-radial pattern around vugs and moldic pores.

In addition to the microfractures, the types and macroscopic characteristics of the dolomites including selective replacement, late cementation, and pore types in both buildups are shown. Thin-section and cathodoluminescence photomicrographs highlight the similarities between the dolomites and other diagenetic products.

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, and timing. However, they are restricted to isolated buildups in the deeper (>4000 m) part of the 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)