This paper presents the results of studies of the natural fracture distribution encountered in 10 test wells drilled in three areas of the United States. Seven of the wells were drilled to depths of 200-250 m, while three were drilled to depths of about 1 kin. Using an ultrasonic borehole televiewer, fracture depths, strikes, and dips were determined. Steeply dipping fractures were found throughout each of the wells, and in general, few horizontal fractures were observed. Statistically significant fracture pole concentrations were found for each well which were basically invariant with depth, although some variation of fracture orientation with depth was found in two wells. The significant fracture orientations were not found to be the same in wells only several kilometers apart in a given region. In none of the wells did the number of observable fractures decrease markedly with increasing depth. No simple relationship of fracture orientation or fracture density with major structural features such as the San Andreas fault were observed, and no simple relation between the significant fracture orientations and either past or present regional stress fields could be determined.

A simple network model of pore space in rocks has been developed with which permeability and bulk modulus as a function of confining pressure can be calculated. Pores are modeled as straight conduits with circular, elliptic, or tapered cross sections. The interconnection of these conduits is modeled by emplacing them in regular two‐dimensional hexagonal, square, or triangular networks. Flow through each conduit is modeled using Poiseuille's law. Flow through the network is calculated based upon the analogy of fluid flow in Darcy's law to current flow in Ohm's law. An estimate of the effective bulk modulus is obtained by summing the contributions of the individual pores. The effect of confining pressure on permeability and bulk modulus is determined by the solid properties and the shape of the pores. For appropriate aspect ratio distributions, the permeability and bulk modulus characteristics of the network model are similar to those found for laboratory rock samples. For rocks of moderate porosity, like sandstone, the model predicts that the bulk modulus is most affected by small, low aspect ratio pores. In contrast, for rocks like granite and tight sandstones (permeability less than 0.5 mdarcy) in which there are relatively few round pores, the bulk modulus and the permeability are both controlled by easily deformed pores. These types of responses are observed in experimental data. The success of the model in predicting these general bulk modulus and permeability responses indicates that while the models presented are too simple to represent a rock completely, the network theory approach is a promising method for modeling porous media.

We describe a wavelet-transform-based method for automated segmentation of resistivity image logs that takes into account the apparent dip in the data and addresses the problem of discriminating lithofacies boundaries from noise and intrafacies variations. Our method can be applied to borehole measurements in general, but might have an advantage when applied to resistivity image logs as it addresses explicitly the large variability in facies segments recorded with a high-resolution multiple-sensor tool. We have developed an algorithm based on this method that might outperform other existing segmentation methods in the cases of low to moderate dip. We made a detailed comparison of the segmentation from our method with the one done by a geologist to delineate different lithofacies blocks in a well drilled in a deepwater depositional environment. Our results show considerable success rates in reproducing the geologically defined lithofacies boundaries, and the generality of our procedure suggests it could also be applied to other depositional environments.

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