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

Correlation of Textural Properties with Permeability

Previous investigation showed the relationship between permeability and rock textural parameters including:

Connectivity Index - An index ranging from 1 to 4 representing the degree of connection between oomolds as observed at 10X-20X:
Packing Index - An index from 1 to 4 representing the packing density of oomolds:
Size - An estimate of the average oomold diameter in phi units
Archie Matrix Porosity Index - base on Archie’s (1952) second parameter for describing matrix .

Archie Cementation Exponent

Oomoldic limestones from Kansas and globally exhibit extremely high Archie cementation exponents. This is consistent with the interpretation that the oomoldic pores are similar to micro-vugs. Modified Archie parameters for the Carter-Colliver Lease rocks are: m=1.36, a=9.59. Conversely, if m is considered to change with porosity then m can be predicted for the higher porosity rocks using: m = 0.05*Porosity(%) + 1.9. Cementation exponents are near 2.0 in the bioclastic wackestone overlying the ‘C’ zone. Cementation exponents increase into the top of the ‘C’ and then decrease with increasing depth to the base. This is associated with the higher porosity at the top of the ‘C’ zone but is also influenced by pore structure changes associated with the unconformity surface.

“Irreducible” Water Saturation

With finer pores in the matrix surrounding large oomolds it is important to understand capillary pressure relationships since high porosity may not be directly associated with effective oil porosity. Correlations of “Irreducible” water saturations indicate that Swi increases with decreasing permeability as exhibited by many rocks. Saturation increases with decreasing permeability following the relation:
log S_w50(%) = 0.22 log k(md)) - 0.43

Residual Oil Saturation to Waterflood

Residual oil saturation to waterflood (Sorw) is a critical variable. Sorw increases then decrease with increasing permeability (k). At lower k, higher Swi must be associated with lower Sorw. At high k good connectivity promotes low Sorw. Intermediate permeability rock have large pore bodies and smaller pore throats which supports oil-globule snap-off and trapping.

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Last updated May 2003
http://www.kgs.ku.edu/PRS/publication/2003/ofr2003-32/P1-09.html