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 Kansas Geological Survey Open-file Report 2003-32 |
Discussion & Conclusions - All Systems
- Multi-scale carbonate-dominated sequences were deposited in subtidal to supratidal environments on the broad shallow Kansas shelf throughout the Paleozoic.
- A repeating association of original depositional facies and early diagenesis
for these rocks produced lithofacies ranging from mudstones to grainstones
with abundant moldic porosity. The nature of the molds varied through time
reflecting the change in primary carbonate grain constituents:
- U. Cambrian-L. Ordovician Arbuckle peloid and ooid molds
- Mississippian carbonate/siliceous sponge spicule and echinoderm/ brachipod molds
- Pennsylvanian ooid and bioclast molds
Primary Factors Controlling Reservoir Properties
- Depositional Facies - grainstones/boundstones exhibit best properties, properties improve from mudstone to grainstone/boundstones.
- For the systems investigated, depositional facies are dominant control even with: 1) extensive and various, early and late diagenesis; 2) biotic constituent differences; 3) warm-cool water environments; 3) karst overprinting; 4) burial overprinting.
- Reservoir properties for each system, including porosity and permeability, are strongly correlated with original depositional facies despite significant fabric transformation, and in some cases even complete reversal of solid and pore space, with reservoir quality increasing from mudstone through grainstone.
- The final moldic rocks exhibit petrophysical-lithofacies trends that parallel those of original primary porosity carbonates.
- Understanding facies locations is important for both stratigraphic and structural plays.
Secondary Factors Favoring Reservoir Properties
- Stratigraphy - e.g., shallowing upward high frequency cycles
- Diagenesis - e.g., dissolution of carbonate grains to form molds, extensive dissolution to establish direct mold-mold connections
- Paleotopography - e.g., local relief that accentuates diagenetic processes
- Structure/Burial - e.g., fracturing, crushing to establish direct mold connectivity
Synthesizing Lithofacies-Petrophysical Properties for All
Systems
A Pore Understanding
- Correlation of permeability and pore throat size in moldic-porosity rocks is similar to that of intergranular-porosity rocks. This can be interpreted to indicate that, despite some rocks having very high moldic porosity, permeability is primarily controlled by matrix properties. However, the strong association of increasing permeability with increasing grain size and packing (i.e. mudstone to grainstone) indicates that matrix pores must also be increasing in size.
- Enhanced mold connectivity resulting from extensive dissolution, crushing, or fracturing creates high permeability parallel flow systems.
- Dominant control of matrix properties in rocks with high moldic porosity is consistent with a pore-scale series-flow model of low permeability matrix and high permeability moldic pore bodies. The strong correlation of permeability with connectivity may result from the establishment of increasing pore-scale parallel flow, effectively “short-circuiting” the series-flow dominated system.
- Lithofacies progression from mudstone through grainstone results in a
greater change in permeability than increasing porosity. For a given facies,
increasing mold content and porosity results in a k increase that is subparallel
and at a lower slope to the general k-
trend. This is consistent with matrix properties being the dominant control
on flow. - Although permeability in moldic porosity-dominated rocks is strongly controlled by matrix properties, and is correlated with porosity, permeability is also controlled by other rock textural parameters including:
- High sponge-spicule mold content in Chat and oomold content in the L-KC is associated with Archie cementation exponents ranging up to 3 for the Chat and 3.6 for the L-KC. These values are consistent with high micro-vug content with electrical current flow dominated by matrix pores. Knowing these values is critical to quantitative wireline log water saturation interpretation.
- Moldic rocks play an enormous role in Kansas oil and gas production. The diverse nature of the matrix, molds and mold content with continued study is providing better understanding of the role of moldic porosity and of pore architecture to fluid flow in porous media.
Connectivity Index - A 1-4 index for the degree of connection between molds
Packing Index - An index from 1 to 4 for the packing density of molds
Size - An estimate of the average mold diameter in phi units
Archie Matrix Porosity Index - Archie's 2nd parameter for matrix porosity
Portions of this research were derived from programs funded in part by the U.S. Department of Energy - National Petroleum Technology Office. We are grateful for their support
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Last updated June 2003
http://www.kgs.ku.edu/PRS/publication/2003/ofr2003-32/P3-07.html