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Evaluating Textural Changes Within Clastic Environments Using Electrical Borehole Images

B. M. Newberry1, S. M. Hansen1, and T. Perrett2
1 Schlumberger Oilfield Services, Houston, Texas
2 BHP Billiton Petroleum (Americas) Inc., Houston, Texas

Electrical borehole images have been utilized to evaluate clastic environments for over 15 years. During this period an interpreter has classified textural changes only as visual features. Now a semi-automatic product that captures high-resolution textural information from electrical borehole images, can supplant verbal descriptions of clastic textures. These attributes may then be produced in a clear, usable form. This method analyzes the total image spectrum in a 1-in. interval around the well bore. An electrical heterogeneity index is calculated from the percentile resistivity distribution of the image spectrum. This resistivity spectrum can be divided into a well sorted portion and the fractions that are either more resistive or conductive. The resistivities of these three fractions can then all be calculated. These outputs, heterogeneity-index, fractional resistivities, variability, along with layering measurements and associated open hole log data, can be combined to compute a facies description that captures the textural content of these clastic environments. This high-resolution analysis can be used to verify the variations seen in nuclear magnetic permeability and relative pore sizes in a more precise geological context. By extracting pertinent information from electrical images, this technique makes image logs more accessible for use in petrophysical as well as geological analyses.