Microfractures due to overpressures caused by thermal cracking in well-sealed upper Devonian reservoirs, deep Alberta basin

Journal Article · Mon Apr 01 04:00:00 EST 1996 · AAPG Bulletin

OSTI ID:249787

Marquez, X M; Mountjoy, E W ^[1]

---

1. McGill Univ., Montreal, Quebec (Canada)

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 microfractures 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 scaled reservoirs.

• OSTI ID: 249787
• Journal Information: AAPG Bulletin, Journal Name: AAPG Bulletin Journal Issue: 4 Vol. 80; ISSN 0149-1423; ISSN AABUD2
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
ALBERTA
DEVONIAN PERIOD
ENHANCED RECOVERY
OIL FIELDS
PETROLEUM
PRODUCTION
RESERVOIR ENGINEERING
RESERVOIR ROCK
SEDIMENTARY BASINS
THERMAL CRACKING