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 Kansas Geological Survey Open-file Report 2003-32 |
The Role of Moldic Porosity in Paleozoic Kansas Reservoirs and the Association of Original Depositional Facies and Early Diagenesis With Reservoir Properties
Alan P. Byrnes, Evan K. Franseen, W. Lynn Watney, and
Martin K. Dubois
Abstract
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. As illustrated by three representative cores, the nature of the molds varies through time reflecting the change in primary carbonate grain constituents including Upper Cambrian- Lower Ordovician Arbuckle peloid and ooid molds, Mississippian carbonate/siliceous sponge spicule and echinoderm/ brachipod molds, and Pennsylvanian ooid and bioclast molds.
Reservoir properties for each system, including porosity and permeability, are strongly correlated with original depositional facies, with reservoir quality increasing from mudstone through grainstone. Most of these rocks have undergone extensive diagenetic overprinting. However, despite significant transformation, and even reversal of solid and pore space, the nature of the post-diagenetic fabric still correlates with, and reflects original textures. The final moldic rocks exhibit petrophysical-lithofacies trends that parallel those of original primary porosity carbonates as presented by Lucia (1995).
The cores from these three reservoir systems, which have produced over 4 billion barrels, played a critical role in defining reservoir performance. Integration of core petrophysical and geologic analysis provides understanding of the role of original facies and moldic porosity and definition of reservoir properties distribution. Data from core also reveal significant vertical changes in properties, such as permeability and Archie cementation exponent, which control reservoir performance or interpretation of fluid saturations, but are not fully evident in wireline logs.
Purpose
The multiple goals of this work include:
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To better understand the nature and distribution of moldic porosity in the shallow shelf carbonate systems.
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To understand the role of original depositional facies and early diagenesis on subsequent rock geologic and petrophysical properties.
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To enhance modeling relating moldic porosity, and more broadly pore architecture, to petrophysical properties.
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To compare and contrast the effects of different kinds of moldic porosity.
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To improve prediction of reservoir quality in these systems (or better define the limits of prediction).
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To improve reservoir properties models for reservoir evaluation, characterization, and simulation.
Results and Implications
- Core examples from Upper Cambrian- Lower Ordovician Arbuckle, Mississippian & Pennsylvanian shallow-shelf carbonates in Kansas indicate that moldic porosity is abundant in each system, & reservoir properties, including porosity/permeability, are strongly correlated with original depositional facies.
- Despite the nature of the molds varying through time, reflecting changes in primary carbonate grains, reservoir quality is similar in each system & increases from mudstone through grainstone.
- Despite significant diagenetic overprinting in many of the rocks, & even some complete reversal of original solid & pore space, final moldic textures exhibit petrophysical-lithofacies trends that parallel those of original primary porosity carbonates.
- 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,in some rocks, very high moldic porosity, permeability is primarily controlled by matrix properties.
- 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.
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Last updated May 2003
http://www.kgs.ku.edu/PRS/publication/2003/ofr2003-32/P1-02.html