To understand production from low resistivity-high porosity Mississippian c
hat reservoirs in south-central Kansas it is necessary to understand the na
ture of deposition and diagenesis, how tectonics is a factor, the lithofaci
es controls on petrophysical properties, and log response to these properti
es. The initial mudstones to sponge-spicule wacke-packstones were deposited
in transgressive-regressive (T-R) cycles on a shelf to shelf margin settin
g, resulting in a series of shallowing-upward cycles. Sponge-spicule conten
t appears to increase upward with increasing cycle thickness.
After early silicification, inter- and post-Mississippian subaerial exposur
e resulted in further diagenesis, including sponge-spicule dissolution, vug
gy porosity development in moldic-rich rocks, and autobrecciation. Meteoric
water infiltration is limited in depth below the exposure surface and in d
istance downdip into unaltered, cherty Cowley Formation facies. Areas of th
icker preserved chat and increased diagenesis can be correlated with struct
ural lineaments and, in some areas, with recurrent basement block movement.
Combination of folding or block fault movement prior to or during developm
ent of the basal Pennsylvanian unconformity, sponge-spicule concentration,
and possibly thickness of overlying bioclastic wacke-grainstones resulted i
n variable reservoir properties and the creation of pods of production sepa
rated by nonproductive cherty dolomite mudstones. These events also resulte
d in alteration of the depositional cycles to produce a series of lithofaci
es that exhibit unique petrophysical properties.
From bottom to top in a complete cycle seven lithofacies are present: (1) a
rgillaceous dolomite mudstone, (2) argillaceous dolomite mudstone that has
chert nodules, (3) clean dolomite mudstone that has nodular chert, (4) nodu
lar to bedded chert, (5) autoclastic chert, (6) autoclastic chert that has
clay infill, and (7) bioclastic wacke-grainstone. The uppermost cycle was t
erminated 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 cherty dolomite mudstones,
argillaceous dolomite mudstones, and bioclastic wacke-grainstones exhibit
nonreservoir properties.
Reservoir production, numerical simulation, and whole core data indicate fr
acturing can be present in chat reservoirs and can enhance permeability by
as much as an order of magnitude. Capillary pressure data indicate the pres
ence of microporosity and can explain high water saturations and low resist
ivity observed in wire-line logs. Relative permeabilities to oil decrease r
apidly for saturations greater than 60% and may be influenced by dual pore
systems. Archie cementation exponents increase from 1.8 for mudstones to mo
re than 2.5 in the cherts that have increasing sponge-spicule mold and vug
content. Detailed modified Pickett plot analysis of logs reveals critical a
spects of chat character and can provide reliable indices of reservoir prop
erties and pay delineation. Models developed provide additional insight int
o the chat of south-central Kansas and understanding of the nature of contr
ols on shallow-shelf chert reservoir properties.