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Alan P. Byrnes1, W. Lynn Watney1, Willard J. Guy2, Paul Gerlach2
(1) Kansas Geological Survey, Lawrence, KS
(2) Kansas Geological Survey

Abstract: Oomoldic Reservoirs of Central Kansas: Controls on Porosity, Permeability, Capillary Pressure and Architecture

Cyclic carbonate strata of the Pennsylvanian Lansing-Kansas City groups represent important enhanced oil recovery targets on the Central Kansas Uplift and thus require understanding of controls on reservoir properties and architecture. Interaction of changing sea level and episodic local processes, such as tidal currents along a broad topographically high shelf area, led to accumulation, and local reworking and redeposition of multicycle elongate stacked, shingled, and cross-cutting oolite sand bars (0.5-10 m thick). Subaerial exposure and meteoric water percolation led to microporous cementation around the aragonite ooids and often dissolution of the ooids and variable development of vuggy porosity. Resulting oomoldic grainstones, the principal reservoir lithofacies, underwent variable degrees of early or later fracturing and crushing, providing connection between otherwise isolated oomolds. Grain size variation, location on oolite buildups and local topography, and interbedded carbonate mud (aquitards) influenced the nature and extent of diagenetic overprinting and resulting permeability-porosity, and capillary pressure properties.

The relation between permeability (0.01-400md) and porosity (5-35%) is significantly influenced by the connectivity of the oomoldic pores complicating the use of porosity as an effective predictor of permeability without information about lithology and pore geometry. For “average” connectivity: logPermeability(md)=0.16*Porosity(%)-2.32. Vug and fracture enhanced connectivity and oomold isolation can respectively increase and decrease permeability by a factor of up to 25X. Irreducible water saturation (Siw) and residual oil saturation after waterflooding (Sor,w) are also strongly controlled by connectivity and correlate highly with permeability: Siw(%)=35.7*exp(-0.46*logPermeability(md)), Sor,w(%)=-20.9*logPermeability(md)+59. These oomoldic reservoirs provide insight into the interactions of rock fabric-architecture-diagenesis on reservoir properties.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana