Abstract: Importance of Diagenesis in Predicting Porosity in Reef Complexes--Upper Devonian Examples from Alberta
E. W. Mountjoy, R. A. Walls, G. Burrowes
Evidence from outcrops and subsurface buildups indicates a complex diagenetic history which began when sediments were deposited and extended into deep burial. Original reef porosity has been modified significantly by several solution and cementation events resulting in a porous back-reef reservoir and a nonporous reef margin. The key diagenetic events occurred in 3 stages: (1) early, near surface; (2) early burial; and (3) late burial.
Early diagenesis includes submarine, shallow-burial marine, and subaerial stages. In the reef and fore reef, submarine and shallow-burial marine diagenesis consists mainly of radiaxial and radial fibrous calcite cements that effectively fill most original void systems with geopetal internal sediments. Cements and internal sediments have positive carbon and low negative oxygen isotope values. In the back reef, submarine cementation is restricted to minor cloudy, gravitational, and isopachous-rim cements.
Subaerial diagenesis occurred around local, short-lived, small islands in back-reef shoals and lagoons. Minor laminated calcareous crusts, breccias, solution, and shaly dolomitic internal sediments are associated with gravitational and micritic cements. Calcareous crusts contain negative isotope values. Subaerial processes led to an irregular pattern of enlargement and reduction of original pore systems.
Early burial diagenesis includes pressure solution and pore filling by clear ferroan and nonferroan sparry calcites that have strongly negative oxygen isotopic values. These cements are probably pre-Late Mississippian. Late burial diagenesis is characterized by minor solution and dolomitization which slightly enhance porosity. This stage is post-Mississippian but precedes oil accumulation, which possibly occurred in the Cretaceous.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC