Deep-burial dissolution and brecciation in dolostone reservoirs: Striking similarities between the Devonian Keg River Formation of western Canada and the Ordovician Ellenburger group of west Texas

Abstract

Deeper burial dissolution of dolomitized fabrics created most of the reservoir quality in several Devonian Keg River oil pools on the Comet Platform in northwestern Alberta, Canada. This process also created reservoir quality in two Ordovician Ellenburger dolostone oil pools on the Eastern Shelf of the Midland Basin in west Texas, and provided an alternative explanation for reservoir development unrelated to the subaerial karstification model. Several observations confirmed deeper burial dissolution of Keg River dolostones. First, dolomites replaced, or cemented, grains previously sutured by pressure solution. Their subsequent dissolution postdated this pressure solution, confirming burial dissolution. Second, late-forming saddle dolomites dissolved. Third, secondary porosity developed along, or cross cut, stylolites, often in zebra dolostone fabrics, or formed along fracture planes that cut stylolites. These burial fractures often intersected secondary pores, implying a causal relationship. Fourth, dolostone breccia clasts contained stylolites rotated at different angles to each other and the horizon, confirming deeper burial brecciation. In the Keg River Formation on Comet Platform, proximity to conjugate reactivated basement faults that developed off of a master wrench fault, the Hay River Shear Zone, controlled deeper burial dissolution and pool entrapment. Hydrothermal fluids, either calcium rich or acidic in composition, were moved along these faults and promoted extensive dissolution of completely dolomitized fabrics. Ellenburger dolostone reservoirs at Suggs and Withers Fields on the Eastern Shelf of Texas also resulted from deeper burial dissolution of completely dolomitized facies. The Keg River dissolution fabrics were replicated in Ellenburger samples, with the exception of zebra dolostones. Ellenburger reservoir development was closely aligned to faults and fractures associated with a master wrench fault, the Ft. Chadbourne Fault. These faults were inferred to have moved hot (possibly hydrothermal?), deeper burial diagenetic fluids that were calcium-rich, or periodically acidic, promoting dolomite dissolution and secondary porosity development.