Dravis, Jeffrey J.1
(1) Dravis Geological Services, Houston, TX
ABSTRACT: Deep-Burial Dissolution in Dolostone Reservoirs: Similarities Between the Devonian Keg River of Western Canada and the Ordovician Ellenburger of West Texas
Deep-burial dissolution promoted reservoir development in many Devonian Keg River
dolostone pools in western Canada. This process, and many of the resultant products, are
similar to those observed in Ordovician Ellenburger dolostone reservoirs on the Eastern
Shelf of Texas. Early, unconformity-related diagenesis was not responsible for reservoir
quality in either of these dolostone sequences. Rather, enhanced petrography demonstrates
that dolomitized fabrics were dissolved at depth.
Deep-burial dissolution in the Keg River is confirmed by several observations. First, dolomites replaced or cemented grains previously sutured by pressure solution. Their subsequent dissolution occurred during burial, after incipient pressure solution. Second, late-forming saddle dolomites were variably dissolved. Third, secondary porosity developed along stylolites, or along fractures that cut stylolites. These fractures often intersected secondary pores, implying a causal relationship. Fourth, breccia clasts contained stylolites rotated at different angles to each other and the horizon, confirming their deep-burial origin. Deep-burial dissolution and pool entrapment were related to reactivated basement faults developed off of a master wrench fault, the Hay River Fault. Hydrothermal fluids, calcium-rich and/or acidic, promoted dolomite dissolution.
Ellenburger dolostone reservoirs at Suggs and Withers Fields (Nolan and Coke Counties) underwent deep-burial replacive dolomitization and subsequent dissolution. Similar petrographic relationships noted for the Keg River dolostones are documented in Ellenburger cores and thin sections. On the Eastern Shelf, faults and fractures related to a master wrench fault, the Ft. Chadbourne Fault, apparently controlled movement of hot, deep-burial diagenetic fluids that created reservoir quality in the Ellenburger.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.