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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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