The Role of Moldic Porosity in Paleozoic Kansas Reservoirs and the Association of Original Depositional Facies and Early Diagenesis With Reservoir Properties

Kansas Geological Survey
Open-file Report 2003-32

Depositional Facies

Facies Descriptions

Sponge Spicule-rich Wacke-Packstone (SWP):

Mudstone-Wackestone (MW):

Echinoderm Wacke-Pack-Grainstone (EWPG):

Sponge Spicule-rich Wackestone-Packstone (SWP) and
Mudstone-Wackestone (MW) Facies

SWP Facies. In this sample the sponge spicules are preserved, much of the matrix is silicified and only a minor amount of intercrystalline porosity is present. Width 5 mm. SWP Facies. Abundant sponge spicule moldic and intercrystalline porosity in dolomite matrix which tend to form the most favorable reservoir facies. Width 5 mm.
SWP Facies. Burrow mottling created variably tight and porous (blue) areas with abundant moldic and intercrystalline porosity. Width 5 mm. SWP facies. Sponge spicules have been dissolved leaving molds (dark blue areas) and the surrounding matrix has been mostly replaced by chert. The upper right corner was not replaced by silica and is dolomitic. Width 5 mm.
MW Facies. Wispy lamination imparted by clay and horsetail stylolites. This facies is typically tight. Width 5 mm. Silica-replaced bladed and radiating bladed crystal textures of original evaporite (anhydrite/gypsum) minerals in SWP facies. This sample exhibits displacive growth of crystals and formation of nodules in dolomitic sediment. Preservation of these fabrics suggests early replacement by silica prior to any significant compaction. Width 5 mm.

Echinoderm Wacke-Pack-Grainstone (EWPG) Facies

Silicified EWPG Facies. Packstone-grainstone texture has largely been preserved. Echinoderm fragments with textures preserved or molds filled by cement predominate with some identifiable sponge spicule molds filled with cement. Note chalcedony cement (brown) lines primary pores followed by later pore-filling clear megaquartz cement. Width 5 mm. Silicified EWPG Facies. Skeletal grains, inluding echinoderm, bryozoan, and other unidentifiable grains, have been preserved by silica replacement whereas surrounding matrix has been dissolved leaving abundant interparticle, vuggy, and some intercrystalline porosity (tripolitic texture). Width 5 mm.
EWPG Facies. Echinoderm fragments and other skeletal fragments, including sponge spicules, have been dissolved leaving abundant moldic porosity (blue areas) in relatively tight dolomitic matrix. Width 5 mm.


Shale and Siltstone Facies

Shale and siltstone layers are locally present, typically interbedded with SWP or MW facies. Much of this material is likely related to post-depositional fill, but some layers appear to be associated with deposition during Osagian time. This sample contains mostly angular-subround silt-very fine sand-size quartz grains with minor clay. Width 5 mm. Crossed nicols.

top of report

e-mail :
Last updated May 2003