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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 the Vp-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 apply to carbonates. Yet, there are assumptions in the model:

•  Homogeneous mineralogy •  Isotropic •  Well-connected pore space The dominant consideration, as with Gassmann applied to any rock,

is the stiffness of the pore space.

Fluid Substitution