Rock Fabric and Texture from Digital Core Analysis

Descriptions of rock fabric and texture are of great value to geologists and petrophysicists as they can be used in facies analysis and in the interpretation of the environment of deposition. Grain shape and size information is used to correlate to petrophysical properties. They are also of importance to the production technologist for completion design and sand strength/failure prediction. Textural descriptions are traditionally obtained via petrographic and petrological analysis including thin section analysis, particle sieving techniques and laser diffraction studies. All methods have limitations in quantitatively describing the full 3D rock fabric and assumptions and interpretations in the processing of data can skew or distort predictions of textural data. We have previously demonstrated the ability to image, visualize and characterise sedimentary rock in three dimensions (3D) at the pore/grain scale via X-ray computed microtomography (Arns et al., 2004; Arns et al., 2005). We now demonstrate the ability to directly measure rock fabric and texture from 3D digital images of core fragments. The mathematical procedure to extract individual particles from a full core image is described and its accuracy demonstrated. A single core fragment image can yield more than 8,000 individual grains. We describe methods for mathematically characterizing the individual grains including grain size (max/min and mode, skewness, sorting, kurtosis) and shape (sphericity, roundness). These are measured in parallel with textural information (sorting, grain contacts, matrix/grain support). A comparison of grain size analysis from digital image data to laser diffraction studies on sister core material is shown; good agreement between estimates of grain size is obtained. Comprehensive grain shape data obtained over thousands of grains shows significant variability within samples and systematic shape changes with grain size. Measures of grain contacts and grain overlap area show that many grains are loose within the pack and larger grains can have coordination numbers greater than 20. Anisotropy in grain orientation is also directly measured. The rock fabric and texture derived from digital 3D images is more comprehensive, systematic and quantitative than current analysis techniques. This analysis, coupled with studies of 3D pore structure and the ability to directly measure petrophysical properties from 3D images, will enable one to embark on a systematic study of the effect of grain size, shape and cementation on transport and elastic properties of core material.