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
Summary of Petrophysical Trends
Petrophysics of lithofacies at the core-plug scale, and for
many lithologies at the whole-core scale, are dominantly controlled by grain
size. Each lithology exhibits a generally unique range of petrophysical properties
modified by the presence of fractures, vuggy porosity, and grain size variation
within the lithologic class. Facies comprising multiple lithologies of differing
grain size exhibit bulk properties that are scale-dependent and are a function
of the architecture of the constituent facies.
Variance in permeability at any given porosity is approximately
two orders of magnitude and may be primarily attributed to the influence of
such lithologic variables as the ratio and distribution of matrix and fenestral/vuggy
porosity, grain size variations, and subtle mixing or interlamination of lithologies.
Fracturing enhances permeability but does not add significantly to porosity.
Vuggy porosity is largely isolated in mudstones, even up to vuggy porosities
as high as 8%, but is better connected in wackestones. Vuggy pores can be
well connected where vuggy porosity is extensive near the unconformity surface.
- Peloidal Packstone-Grainstone: Cementation of matrix
has resulted in nearly total occlusion of porosity and destruction of reservoir
properties. In the absence of cross-cutting vugs or fractures these beds
may act as seals. Fenestrae within this facies may range up to several centimeters
in length and may enhance porosity by several percent but are not interconnected
but are isolated by low permeability matrix.
- Mudstones: Without fractures or fenestrae these exhibit
porosities ranging from zero to 10% and absolute permeabilities ranging
from <0.0001md to 0.1md. Where fenestrae are present, porosity may be
enhanced up to values as high as 17%, however, the fenestrae are primarily
isolated and permeabilities are not increased significantly. Threshold capillary
entry pressures for this lithology are sufficiently high that these can
act as seals in the absence of fractures.
- Wackestones: Without vugs these exhibit porosities ranging
from 2% to 11% and permeabilities ranging from 0.01md to 1md. Where vugs
are present, porosities can range from 9% to 17% and permeabilities can
range from 1md to 1,000 md.
- Ooid Packstone-Grainstone: Generally contain little to
no vuggy porosity but exhibit intercrystalline and moldic porosities ranging
from 11% to 30%, associated permeabilities range from 10md to 1,500md. The
highest porosity and permeability values are exhibited by clean, homogeneous,
medium-grained moldic packstones.
- Algal boundstones: Exhibit both reservoir and non-reservoir
properties. Laminated muddy algal boundstones exhibit porosities generally
less than 6% and permeabilities below 0.1md. Where fenestral or vuggy porosity
is developed these may exhibit high permeability at the core-scale but it
is unlikely that these permeabilities represent higher scales. Laminated
grainy algal boundstones represent some of the best reservoir rock ranging
in porosity up to 32% and permeability up to 1,500md. This lithology may
extend laterally to the inter-well scale.
e-mail : email@example.com
Last updated June 2003