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Carbonates
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Tremendous Variety of Microstructures
Granular Inclusions
Diversity of pore shapes
Microstructural diversity leads to: Non-unique velocity-porosity relations Non-unique Vp/Vs relations Uncertainty in fluid substitution practices Non-unique porosity-permeability relations Important variations in mineral moduli
Round Grains Round Pores
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Again, carbonate data fall along modified
upper Hashin-Shtrikman line, now in theVp-Porosity plane. Shalier data fall below
it, similar to clastics.
Comparison of ellipsoidal crack models with
carbonate data, classified by pore shape.The rocks with stiffer pore shapes fit best
the spherical pore models, while the rocks
with thinner, more crack-like pores fit best
the lower aspect ratio models.
Velocity-Porosity: Interpretation Ambiguity
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Differential Effective Medium (DEM) model is superimposed for aspect ratios
[.01,.03,.1,.3,1]. Chalk data (low vclay) generally follow the DEM trend. Shales (high
vclay) follow two shale trends consisting of clay with small amounts of calcite cement.
DEM model
Shale trend
Carbonate
Velocity-Porosity
Distinct trends for carbonates and shales
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C. Scotellaro
dolomitic
micrite
Velocity-Porosity: Textural, Mineralogic Variations
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Mineral Variations
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We have observed that model-based interpretation depends quite a bit
on mineralogy. Here, we examine log data to infer mineral properties.
The assumption is that minerals represent upper bounds for data
clouds, in the limit of zero porosity. At least some of the data are too
dense and too stiff to be calcite.
Inferring Mineralogy
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The density-porosity trend should be a simple linear combination of
fluid and mineral. These plots show that low-gamma rocks are
consistent with calcite. Errors in porosity estimation (especially with
shale) will lead to incorrect intercept, and misinterpreted mineral
density.
Inferring Mineralogy
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The density-porosity trend should be a simple linear combination of
fluid and mineral.
Bootstrap Analysis for Mineralogy
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Water-saturated
Greenberg-Castagna lines
GR
Comparison of carbonate log data with Greenberg-Castagna lines.
Carbonate Vp-Vs Relations
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Water-saturated
Greenberg-Castagna lines
Looking more closely at the data in the previous slide, we can find
intervals that are more calcite-rich.
Carbonate Vp-Vs Relations
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Water-saturated
Greenberg-Castagna lines
In this interval, the data appear to be more dolomite-rich.
Carbonate Vp-Vs Relations
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The Hashin-Shtrikman bounds can help detect the presence of
dolomite in the Vp-Vs plane.
Interpreting Pore Shape from Vp-Vs Data
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For these chalks, there is an ambiguity between mineral and pore
stiffness.
Well A Well B
Carbonate Inclusion Models
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Example of substituting brine for oil
There is no fundamental reason why Gassmann theory should not applyto carbonates. Yet, there are assumptions in the model:
Homogeneous mineralogy
Isotropic
Well-connected pore spaceThe dominant consideration, as with Gassmann applied to any rock,
is the stiffness of the pore space.
Fluid Substitution
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