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Abstract: Microfractures Due to Overpressures Caused by Thermal Cracking in Well-Sealed Devonian Reservoirs, Deep Alberta Basin, Canada

Xiomara M. Marquez, Eric W. Mountjoy

Microfractures (< 1 mm in width) filled with reservoir bitumen crosscut all diagonetic 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 perpendicular to subvertical fractures, (2) random in the matrix, and (3) radial around vugs and fossil molds. Subhorizontal microfracturing is most common and radial is the least common.

Overpressuring caused 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. Thermal cracking of crude oil to gas during burial is also indicated by finely and coarsely deformed lamellar textures of the reservoir bitumen that fill the microfractures in the Strachan buildup. Also, the stress field was modified by tectonic compression during the Late Cretaceous and Early Tertiary Laramide orogeny. Pressures generated during thermal cracking of oil together with tectonic compression probably created the microfractures in he isolated and effectively sealed reservoirs. The increased horizontal stresses resulted in subhorizontal microfractures, whereas rare radial and random microfractures formed under conditions of more uniform stress. The lack of microfractures in adjacent gas-bearing and updip buildups along the Rimbey-Meadowbrook reef trend is likely due to the connection of these buildups to a regional conduit system in the underlying Cooking Lake platform, preventing them from developing sufficient overpressures.

AAPG Search and Discovery Article #90951©1996 AAPG International Conference and Exhibition, Caracas, Venezuela