Characterization of Mississippian Osage Chat in South-Central Kansas

Kansas Geological Survey Open-file Report 2002-50

Rock Petrophysics

Spivey-Grabs Field Core and Petrophysical Analysis of Miss. “Chat” General Atlantic Tjaden #A-1 WIW #1 ne sw sw Section 24, T30S, R8W Kingman County, Kansas

While the Mississippian chat reservoirs of south-central Kansas reflect a specific depositional and diagenetic history, the lithofacies present, the nature of chert formation and dissolution and fracturing, and the log and core petrophysical properties are likely to be representative of other shallow shelf chert systems or may shed light on controls in these systems. The most productive and economic chat reservoir is a tripolitic chert exhibiting variable degrees of sponge spicule mold content, chert microporosity, dissolution vuggy porosity, autoclastic brecciation, and clay infill of fractures.

A core described below from the General Atlantic Tjaden 1-A WIW contains a succession of four stacked shallowing-upward cycles in which a complete cycle comprises beds dominated by the following lithofacies from bottom to top: 1) argillaceous dolomite mudstone; 2) argillaceous dolomite mudstone with chert nodules; 3) clean dolomite mudstone with nodular chert; 4) nodular to bedded chert; 5) autoclastic chert; 6) autoclastic chert with clay infill; and 7) bioclastic crinoidal wackestone to grainstone.

• Chert conglomerate (overlying the chat) conglomerate consisting of dense and highly porous chert clasts in matrix of clay, quartz silt, and smaller chert particles; corresponds to Type 3 chat defined by Montgomery et al. (1998).
• Autoclastic chert with clay infill brecciated insitu highly porous tripolitic chert with dessication fractures in which clay has infilled; birefringent clay cutans, grumulous and clotted fabric, shelter porosity and circum-clast fractures, silicified sponge spicules or sponge spicule molds varying in concentration from abundant felted masses to wackestone, variable vuggy porosity resulting both from dissolution of spicules, bioclasts and matrix, solution enhanced porosity along fractures, scattered isolated sucrosic dolomite
• Autoclastic chert brecciated insitu chert with the above characteristics except fractures and microcracks contain no infilling clay, corresponding to Type 2 chat defined by Montgomery et al. (1998).
• Nodular to bedded chert tripolitic chert with massive mottled appearance to nodular that appear overpacked and fitted, no significant vuggy porosity content, variable amounts of sponge spicule molds, mottled areas with more micritic carbonate.
• Cherty dolomite mudstone light gray dolomite mudstone with variable large (up to 10 cm), white microporous chert nodules, no distinct bedding
• Argillaceous cherty dolomite mudstone similar to cherty dolomite mudstone, but with increased argillaceous content and thin wispy horizontal bedding.
• Sucrosic dolomite massive, medium grained sucrosic dolomite with porosity between 10-20%.
• Bioclastic crinoidal wacke-grainstone limestone with abundant crinoids and occasional bryozoans and other bioclasts, generally tightly cemented with calcite, patchy chalcedony and drusy quartz cement and replacement of calcite, occasional fractures and vugs.