MICROFRACTURES DUE TO OVERPRESSURES CAUSED BY THERMAL-CRACKING IN WELL-SEALED UPPER DEVONIAN RESERVOIRS, DEEP ALBERTA BASIN

Microfractures (<1 mm in width) filled with reservoir bitumen occur an d crosscut all sedimentary and diagenetic phases in the upper 200 m of the partially to completely dolomitized Upper Devonian (Leduc Formati on) Strachan buildup and other buildups in the deep Alberta basin. The y display three patterns: (1) subhorizontal, extending from intraskele tal pores and subvertical fractures, (2) radial around vugs and molds, and (3) random in the matrix, Subhorizontal microfracturing is the mo st common, and radial is the least common. Overpressuring by thermal c racking of crude oil to gas during burial can produce most of the char acteristics exhibited by these microfractures: their association with all pore types, bitumen fillings, and relatively late diagenetic timin g. Microfractures are restricted to isolated buildups below depths of about 3800 m in the Alberta basin, The lack of microfractures in adjac ent gas-bearing and updip buildups along the Rimbey-Meadowbrook reef t rend is likely because of the connection of these buildups to a region al conduit system in the underlying Cooking Lake platform, preventing them from developing sufficient pressures, Thermal cracking of crude o il to gas during burial is also indicated by finely and coarsely defor med lamellar textures of the reservoir bitumen that fills the microfra ctures in the Strachan buildup. This thermal cracking took place durin g the Late Cretaceous when the buildup was buried deeper than about 35 00 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 gen erated during thermal cracking of oil in conjunction with Laramide tec tonic compression probably created the. microfractures in isolated and effectively sealed reservoirs. The subhorizontal orientation of the m icrofractures is a result of increased horizontal stresses. Horizontal fractures formed around cylinder-like pores where sigma(1)/sigma(3) > 3. This condition is favored by high pore pressures, but failure occu rs before pore-fluid pressures reach the lithostatic pressure. Radial and random microfractures are rare and of uncertain origin.