W. Lynn Watney¹, Willard J. Guy², Alan P. Byrnes²

(1) Kansas Geological Survey, Lawrence, KS
(2) Kansas Geological Survey

ABSTRACT: Characterization of the Mississippian Osage Chat in South-Central Kansas

The Mississippian "chat" reservoirs in south-central Kansas consist of a succession of mudstones to sponge spicule wacke-packstones comprising transgressive-regressive cycles on a shelf to shelf margin setting. Sponge spicule content appears to increase upwards with increasing cycle thickness.

Early silicification, followed by inter- and post-Mississippian subaerial exposure resulted in sponge spicule and carbonate dissolution, vuggy porosity development in moldic rich rocks, and autobrecciation. Meteoric water infiltration appears to have been limited in depth below the exposure surface and in distance downdip into unaltered cherty Cowley facies. Diagenetic alteration of the depositional cycles affected reservoir quality producing lithofacies that exhibit unique petrophysical properties. This alteration produced pod-shaped producing areas containing cleaner porous chert separated by non-productive cherty dolomite mudstones. These variations in reservoir quality correlate with basement lineaments and recurrent fault block movement, sponge spicule concentration, and possibly thickness of interbedded bioclastic wacke-grainstones that apparently inhibited downward percolation of meteoric waters.

The chert reservoir facies exhibit porosities ranging from 25-50% and permeabilities >5 md. The cherty dolomite mudstones, argillaceous dolomite mudstones, and bioclastic wacke-grainstones exhibit non-reservoir properties. Observed and inferred fractures based on production data can enhance permeability by up to an order of magnitude. Capillary pressure data indicate the presence of abundant microporosity and can explain high water saturations and low resistivity. Relative permeabilities to oil decrease rapidly for water saturations > 60%. Archie cementation exponents increase from 1.8 for mudstones to over 2.5 in the cherts with increasing sponge spicule mold and vug content. Detailed modified Pickett plot analysis of wireline logs reveals the chat character and can provide reliable indications of reservoir properties.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado