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

Lithofacies, Permeability, Porosity

Lithofacies and early diagenesis are major controls on permeability (k) and porosity () despite complex diagenetic overprinting by sub-Pennsylvanian subaerial exposure and burial processes.

• k and decrease significantly and continuously with decreasing grain/mold size from packstone to mudstone ( a trend exhibited by many other carbonates) and from echinoderm-rich to spicule-rich facies
• An exception is the echinoderm grainstone facies which is silica cemented and exhibits very low k and

The permeability-porosity trend for all lithofacies are approximately bounded within two orders of magnitude by trendlines defined by:

log k_{in situ} = 0.25 _{in situ} - 2.5
log k_{in situ} = 0.25 _{in situ} - 4.5

• Between these bounding trends each lithofacies exhibits a generally unique range of k and which together define a continuous trend, with k decreasing with decreasing grain/mold size for any given porosity. Each individual lithofacies exhibits a unique sub-parallel trend to the general trend. Statistically the general trend is dominated by the large number of spicule-rich samples and is strongly influenced by mudstone and cemented echinoderm grainstone properties:

log k_{in situ} (md) =0.24 _{in situ}(%) - 3.78

Subtrends for clusters of facies or individual facies may also be defined and are significantly more accurate with standard error of prediction of permeability decreasing with increasing selectivity of lithofacies characteristics. Standard error for a specific lithofacies is generally less than a factor of 3. Linear regression trends for spicule- and echinoderm-rich facies are:

log k_{in situ} (md) =0.19 _{in situ} (%) - 2.88 [Spicule-rich]
log k_{in situ} (md) =0.12 _{in situ} (%) - 1.04 [Echinoderm-rich]

Other Mississippian fields’, including Bindley and Ness City field, permeability (k)-porosity () trends are similar to those of the Schaben for similar lithofacies. One significant difference is that calcite cementation of spicule-rich pack-wackestones significantly occludes porosity and reduces permeability. The bounding trends can be considered to define the range of porosity for a given lithofacies trend. The low k- slopes of individual lithofacies trends indicate that increasing porosity does not significantly increase permeability compared with the influence of grain size. This is consistent with porosity development through dissolution of pores surrounded by permeability-controlling matrix.

Trends for echinoderm-rich and spicule-rich facies are significantly different:

log k_{in situ} (md) =0.157 _{in situ} (%) - 1.87 [Echinoderm-Bindley Field]
log k_{in situ} (md) =0.147 _{in situ} (%) - 1.50 [Echinoderm-Ness City Field]
log k_{in situ} (md) =0.230 _{in situ} (%) - 4.04 [Spicule-Bindley Field]
log k_{in situ} (md) =0.170 _{in situ} (%) - 2.76 [Spicule-Ness City Field]

Standard error of prediction of k ranges from a factor of 2 to 4.8. For all fields the lowest k- slope and highest predictive accuracy is obtained for a single lithofacies. With successive addition of more lithofacies into a statistical analysis the resulting trend-line slope approaches that of the bounding trends. The intercept varies as a function of the nature of the population grain/mold size.

e-mail : webadmin@kgs.ku.edu
Last updated May 2003
http://www.kgs.ku.edu/PRS/publication/2003/ofr2003-32/P2-07.html