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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
Abstract
(Begin page 85)
Characterization of the Mississippian chat in south-central Kansas
W. Lynn Watney,1 Willard J. Guy,2 Alan P. Byrnes3
1Kansas Geological Survey, 1930 Constant Avenue-Campus West,
Lawrence, Kansas, 66047; email: [email protected]
2Kansas Geological Survey, 1930 Constant Avenue-Campus West, Lawrence, Kansas,
66047; email: [email protected]
3Kansas Geological Survey, 1930 Constant Avenue-Campus West, Lawrence, Kansas,
66047; email: [email protected]
AUTHORS
Lynn Watney received his B.S. (1970) and M.S. (1972) degrees in geology from Iowa State University and Ph.D. (1985) from the University of Kansas. He worked as a petroleum geologist with Chevron U.S.A. (1972-1976) and joined the Kansas Geological Survey in 1976. He is a senior scientist, courtesy professor at University of Kansas and Kansas State University, and executive director of the University of Kansas Energy Research Center. His current research interests include reservoir characterization and stratigraphic modeling.
Willard "Bill" Guy is a research geologist with the Kansas Geological Survey in Lawrence, Kansas. He received a B.A. degree in geology in 1955 from the University of Colorado. He spent 25 years with Pure-Union Oil Company of California, 6 years with the independent oil company, and 5 years as a consultant-retired. He joined the Kansas Geological Survey in 1991. His areas of interest are in the evaluation of petroleum reservoirs using petrophysical tools such as wire-line logs, special core analyses, and drillstem tests.
Alan Byrnes received his B.S. degree in geology from the University of Illinois at Chicago and his M.S. degree in geophysical sciences from the University of Chicago. He has been a research geologist at the Institute of Gas Technology, Marathon Oil Company Research Center and owned and operated GeoCore, a special core analysis laboratory, prior to joining the Kansas Geological Survey in 1997, where he performs applied research involving the study of lithologic controls on petrophysical properties.
ACKNOWLEDGMENTS
We thank Ocean Energy, Inc. for providing access to information on Spivey-Grabs field.
ABSTRACT
To understand production from low resistivity-high porosity Mississippian chat reservoirs in south-central Kansas it is necessary to understand the nature of deposition and diagenesis, how tectonics is a factor, the lithofacies controls on petrophysical properties, and log response to these properties. The initial mudstones to sponge-spicule wacke-packstones were deposited in transgressive-regressive (T-R) cycles on a shelf to shelf margin setting, resulting in a series of shallowing-upward cycles. Sponge-spicule content appears to increase upward with increasing cycle thickness.
After early silicification, inter- and post-Mississippian subaerial
exposure resulted in further diagenesis, including sponge-spicule dissolution, vuggy
porosity development in moldic-rich rocks, and autobrecciation. Meteoric water
infiltration is limited in depth below the exposure surface and in distance downdip into
unaltered, cherty Cowley Formation facies. Areas of thicker preserved chat and increased
diagenesis can be correlated with structural
lineaments and, in some areas, with recurrent
basement block movement. Combination of folding or block fault movement prior to or during
development of the basal Pennsylvanian unconformity, sponge-spicule concentration, and
possibly thickness of overlying bioclastic wacke-grainstones resulted in variable
reservoir properties and the creation of pods of production separated by nonproductive
cherty dolomite mudstones. These events also resulted in alteration of the depositional
cycles to produce a series of lithofacies that exhibit unique petrophysical properties.
From bottom to top in a complete cycle seven lithofacies are present: (1) argillaceous dolomite mudstone, (2) argillaceous dolomite mudstone that has chert nodules, (3) clean dolomite mudstone that has nodular chert, (4) nodular to bedded chert, (5) autoclastic chert, (6) autoclastic chert that has clay infill, and (7) bioclastic wacke-grainstone. The uppermost cycle was terminated by another lithofacies, a chert conglomerate of Mississippian and/or Pennsylvanian age. The chert facies exhibit porosities ranging from 25 to 50% and permeabilities greater than 5 md. The (Begin page 86) cherty dolomite mudstones, argillaceous dolomite mudstones, and bioclastic wacke-grainstones exhibit nonreservoir properties.
Reservoir production, numerical simulation, and whole core data indicate fracturing can be present in chat reservoirs and can enhance permeability by as much as an order of magnitude. Capillary pressure data indicate the presence of microporosity and can explain high water saturations and low resistivity observed in wire-line logs. Relative permeabilities to oil decrease rapidly for saturations greater than 60% and may be influenced by dual pore systems. Archie cementation exponents increase from 1.8 for mudstones to more than 2.5 in the cherts that have increasing sponge-spicule mold and vug content. Detailed modified Pickett plot analysis of logs reveals critical aspects of chat character and can provide reliable indices of reservoir properties and pay delineation. Models developed provide additional insight into the chat of south-central Kansas and understanding of the nature of controls on shallow-shelf chert reservoir properties.
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