Characterization of Mississippian Osage Chat in South-Central Kansas

Kansas Geological Survey Open-file Report 2002-50

Characterization of Mississippian Osage Chat
in South-Central Kansas

Introduction

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 (Cowley Formation).. 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.

1. Mississippian “Chat” is a low resistivity, porous and generally permeable, highly weathered chert developed near the subcrop of the Mississippian beneath the basal Pennsylvanian
   unconformity, usually in proximity to prominent structures which exhibit early and late deformation.
2. Osage “Cowley Facies”, also referred to as the Cowley Formation, is a relatively thick, low shelf,
   spiculitic cherty dolomite exhibiting intermediate resistivity that is highly porous with low permeability.
   The updip limits of the “Cowley Facies” are believed to serve as the siliceous precursor to “Chat”
   in south-central Kansas. The “Cowley Facies” is interbedded with open shelf crinoidal wackestones
   and grainstones creating internal layering of the reservoir. These later facies are marginal or
   non-reservoir rock. The “Cowley Facies” is more massive on the lower shelf and becomes
   increasingly interbedded with open shelf deposits higher on the shelf.
3. Mississippian “Chat” reservoirs serve as hosts to prolific, combination structural-stratigraphic
   oil and gas fields in south-central Kansas. Fields are positioned along the subcrop of weathering
   of host cherty facies including the “Cowley Facies”.
4. Problems: While primary production has been prolific, operators have experienced limited
   success in improved recovery applications. Issues include: A) characterization of effective porosity;
   B) adverse relative permeability behavior of reservoirs experienced during enhanced recovery
   operations, and C) identifying additional fairways of potential “Chat” reservoirs.