--> Abstract: Preservation of High Primary Porosity in Paleozoic Crinoidal and Bioclastic Grainstones: Mississippian and Permian Subsurface Examples from Western Canada and Texas, by D. E. Eby and K. C. Kirkby; #91004 (1991)

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Preservation of High Primary Porosity in Paleozoic Crinoidal and Bioclastic Grainstones: Mississippian and Permian Subsurface Examples from Western Canada and Texas

EBY, DAVID E., Marathon Oil Company, Littleton, CO, and KENT C. KIRKBY, University of Wisconsin, Madison, WI

Limestone reservoirs with anomalously high porosity and permeability exist in several Paleozoic basinal settings in western North America. They are commonly either crinoidal or bioclastic grainstones that escaped the varied diagenetic environments that most carbonate shelves and slopes experience during sea-level fluctuations. Thus, these diagenetically "conservative" settings can preserve most of the primary depositional porosity in calcarenites because of sedimentation and burial within normal marine basinal waters. These excellent reservoirs are often encased in deep marine, highly organic shales which may serve as both source rock and seal.

Several subsurface examples of remarkable primary porosity occur within Mississippian Waulsortian buildups of western Canada and north-central Texas. Buildups within the Pekisko Formation of Alberta, for instance, contain abundant flanking crinoidal/bryozoan grainstones with up to 25% primary porosity and 10 d permeability. Muddy core facies with productive flank and capping calcarenites are up to 300 ft (90 m) thick and 600-1200 ft (182-364 m) in diameter. The Bowar "reef complex" in the Chappel Formation of Stephens County, Texas (2.6 MMBO to date) is similar in many respects to the Canadian buildups. These limestone reservoirs resemble Waulsortian mounds in Swimming Woman Canyon, Big Snowy Range, central Montana. Diagenetic "conservatism" has also promoted excellent reservoir quali y within the outer slope to basinal lower Permian bioclastic grain-flow deposit ("Wolfcamp detrital") plays of the eastern Midland basin where significant diagenesis was arrested after basinal deposition.

Interpretations of these examples based upon core and outcrop control provide analogs for future exploration and exploitation in highly porous basinal carbonate reservoirs.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)