Microfractures (<1 mm in width) filled with reservoir bitumen occur and crosscut all sedimentary and diagenetic phases in the upper 200 m of the partially to completely dolomitized Upper Devonian (Leduc Formation) Strachan buildup and other buildups in the deep Alberta basin. They display three patterns: (1) subhorizontal, extending from intraskeletal pores and subvertical fractures, (2) radial around vugs and molds, and (3) random in the matrix. Subhorizontal microfracturing is the most common, and radial is the least common.

Overpressuring by thermal cracking of crude oil to gas during burial can produce most of the characteristics exhibited by these microfractures: their association with all pore types, bitumen fillings, and relatively late diagenetic timing. Microfractures are restricted to isolated buildups below depths of about 3800 m in the Alberta basin. The lack of microfractures in adjacent gas-bearing and updip buildups along the Rimbey-Meadowbrook reef trend is likely because of the connection of these buildups to a regional conduit system in the underlying Cooking Lake platform, preventing them from developing sufficient pressures. Thermal cracking of crude oil to gas during burial is also indicated by finely and coarsely deformed lamellar textures of the reservoir bitumen that fills the microf actures in the Strachan buildup. This thermal cracking took place during the Late Cretaceous when the buildup was buried deeper than about 3500 m; however, tectonic compression occurred immediately west of these areas during the Late Cretaceous and early Tertiary Laramide orogeny, modifying the stress field. Suprahydrostatic (abnormal) pressures generated during thermal cracking of oil in conjunction with Laramide tectonic compression probably created the microfractures in isolated and effectively sealed reservoirs. The subhorizontal orientation of the microfractures is a result of increased horizontal stresses. Horizontal fractures formed around cylinder-like pores where ^sgr₁/^sgr₃ > 3. This condition is favored by high pore pressures, but failure occurs before pore-fluid pressures reach the lithostatic pressure. Radial and random microfractures are rare and of uncertain origin.