Integrating Plug to Well-Scale Petrophysics with Detailed Sedimentology
to Quantify Fracture, Vug, and Matrix Properties in Carbonate Reservoirs
Kansas Geological Survey
Open-file Report 99-47
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.
- All lithologies exhibit increasing permeability with increasing porosity
and can be broadly characterized as lying along the same general porosity-permeability
- Variance in permeability for 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 of lithologies enhances permeability directionally 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. Porosities range from 0% to 4% and absolute permeabilities range
from 0.0003md to 0.1md but are generally below 0.005md. 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. Fenestrae in this lithology 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.
- Packstone: Porosities range from 6%, for finer-grained
rock and where packstone is mottled with wackestone, to 18% for cleaner more
coarse-grained rock. Permeabilities in the packstone lithology range from
0.1md to 50md.
- 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.
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Last updated November 2002