Descriptions of rock fabric and texture are of great value to 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 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. 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. From a typical core fragment image one can extract more than 100,000 individual grains. We then describe methods for mathematically characterizing the individual grains including grain size (max/min and mode, skewness, sorting and shape (sphericity, roundedness. These are measured in parallel with textural information (sorting, grain contacts, matrix/grain support.
The rock fabric and texture derived from digital 3D images is more comprehensive, systematic and quantitative than current analysis techniques. It also allows anisotropy in grain orientation to be directly measured. A comparison of grain size analysis from digital image data to particle sieving and laser diffraction studies on the same core material is shown; strong differences in the predictions due to non-sphericity of grains are noted. Correlations relating grain size distributions and skewness to depositional environment are considered. Grain contacts, used as a measure of the intensity of compaction, are directly measured in 3D and a comparison of grain and pore sizes in samples are considered. Finally, relationships between grain size and permeability are compared.