Generation of Calcrete with Chalky Porosity in Upper Ordovician Red River Grainstone Reservoirs, Cedar Creek Anticline, Southwestern Williston Basin

Anna Dombrowski

Red River dolomites produce from subtidal burrowed grainstones and algal-laminated shallow subtidal to intertidal grainstones. As the result of a detailed reservoir study on the Cedar Creek anticline, chalky porosity was recognized to originate from weathering of grainstones.

Three shoaling-upward sedimentary cycles each ended with subaerial exposure. Weathering features are concentrated immediately below disconformity surfaces and decrease downward. These features include: manganese dioxide dendrites, degrading neomorphism of carbonate crystals, extensive micrite cement precipitation, chalky weathered nodules, recrystallization and leaching along vertical microfractures, organic-rich calcrete soil horizons with calcrete floatstone breccias, and secondary chalky porosity that cuts across depositional features.

Weathering features were superimposed on grainstones after incomplete vadose cementation and leaching but prior to dolomite replacement. Locally, at least one post-dolomite weathering episode also occurred. Extensive weathering of grainstones resulted in development of white chalky beds of calcrete 1-3 ft thick, which are highly porous but impermeable; these are interbedded within the saccharoidal reservoir and are recognizable on wireline logs.

Recognition of the diagenetic origin of chalky porosity permitted use of a more realistic depositional model for the Red River (carbonate ramp with grainstone-dominated shoreline similar to Shark Bay) rather than the one previously used (carbonate shelf-slope model with mud-dominated shoreline). Diagenetic chalky porosity development also explained abrupt lateral and horizontal variations in reservoir quality observed throughout fields on the Cedar Creek anticline.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.