Grain-size analysis of mudrocks: A new semi-automated method from SEM images

There is a growing interest in mudrocks as a result of their potential as hydrocarbon reservoirs, in the storage of carbon dioxide, and as repositories for nuclear waste. Methods for characterising mudrocks are fast evolving in order to better characterise their very small grain sizes. Grain-size analysis of mudrock is challenging and time-consuming and there is need to develop a fast, effective and objective method for accurately determining the grain size of this group of rocks. We suggest that this is best achieved by using high-resolution electron microscopy to study both the microstructure and grain size of mudrocks at the same time. The contribution presents grain-size analysis from scanning electron microscopy (SEM) through image analysis of the Feret (or calliper) diameter of grains. The method has been tested on 7 mudrock samples from two IODP Expeditions and compared with results from standard laser diffraction granulometry. Image analysis shows that all the samples fall within the clayey silt to silty clay range with average grain size from fine silt to medium silt. Closely comparable results and statistical parameters were obtained by laser diffractometry. Linear plots of grain percentage at corresponding phi values show strong positive correlation between the two techniques with R-square values typically ranging between 0.76 and 0.96. Image analysis of grain size as described herein gives comparable and generally smoother normal distribution curves than the laser diffraction technique for all the seven samples. The procedures involved in the proposed method for analysing grain size of fine-grained sediments are rapid, automated, devoid of human subjectivity and precise.