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Alan P. Byrnes1, Evan K. Franseen1
(1) Kansas Geological Survey, Lawrence, KS

Abstract: Lithofacies and early diagenetic controls on reservoir properties in a complexly overprinted carbonate ramp system; Mississippian Schaben Field, Ness Co., Kansas

Many Mississippian-age shallow shelf carbonate reservoirs exhibit high water-cuts and low recovery efficiencies thereby requiring a thorough understanding of controls on reservoir properties for reservoir simulation or enhanced recovery. Detailed studies in the Schaben field of Kansas reveal the nature and distribution of reservoir properties (porosity-phi; permeability-k; pore size; capillary pressure properties, including initial water saturation-Swi), are controlled by lithofacies and early diagenesis despite overprinting by sub-Pennsylvanian subaerial exposure and burial processes.

Deposition was characterized by normal to restricted ramp environments grading up to restricted, evaporative ramp or lagoonal environments. Early stage dissolution and dolomitization enhanced porosity and permeability, especially in sponge-spicule wacke/packstones but early subaerial exposure and silicification decreased porosity, especially in many echinoderm-rich pack/grainstones.

Each lithofacies exhibits a generally unique range of porosity and permeability which together define a continuous trend: log(KlinkenbergPermeability(md))=0.19*Phi(%)–2.88. Each facies also exhibits a generally unique range of Swi with a combined trend:Swi(%)=0.31*logki(md)+80. Facies, in order of decreasing phi and k and increasing Swi, comprising the reservoir system are: (1) echinoderm pack/grainstone (rare; most are extensively silicified) (k > 100md, phi > 22%; Swi < 10%); (2) Spicule pack/wackestone (k=10-100md, phi=18-25%, Swi=15-40%); (3) Spicule wacke/mudstone (k=2-10md, phi=17-22%, Swi=30-60%); (4) Mudstone/Spicule wackestone (k=0.2-2md, phi=12-18%, Swi=40-90%); (5) Mudstone (k=0.01-2md, phi=6-12%, Swi=75-90%); (6) Neomorphic calcite (k < 0.01md, phi < 6%, Swi > 90%). Cherty facies exhibit enhanced k, above the k-phi trend, due to brecciation, but high Swi in the matrix (k=0.1-100md, phi=12-17%, Swi=50-80%).

Our results indicate depositional facies and early diagenesis can exert strong control on reservoir properties in carbonate systems despite complex diagenetic overprinting.

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