Rock physics of sand-shale mixtures: Classifications, theoretical formulations and study on real dataset

Abstract Shale and clay minerals as common constitute in the sandstones can be distributed in various scenarios which in turn can result in different reservoir and elastic properties. A new classification with distinct subclasses for clay distribution has been proposed based on the plausible configuration of shale/clay particles within the sandstone reservoirs. These subclasses give more quantitative insights into laminated, structural and dispersed clay scenarios by taking into account more details of the shale and clay microstructures. Some rock physics-based workflows were proposed and implemented on a real dataset to validate the given strategies to model different clay distribution types. These workflows can be used to generate a template in the elastic domain in order to differentiate various clay distributions. Accordingly, our dataset which was taken from a shaly sandstone in the Persian Gulf, was investigated in more details using the generated template for detecting different clay distribution types as well as observing different elastic and reservoir properties when clay distribution type is changed. We observed that there are four different classes (class I, II, III and IV) of properties with different trends in the crossplot of P-wave velocity versus porosity. Class I includes properties which their P-wave velocity increased when porosity is reduced. In contrast, class II includes properties which show a decreasing trend for P-wave velocity when porosity is reduced. Class III belongs to the properties with two different trends towards structural and dispersed clays. These properties do not follow the same trends for P-wave velocity changes versus porosity similar to classes I and II. However, when the given property decreases or increase by moving toward dispersed clay or structural clay, their P-wave may increase or decrease, respectively. Finally, class IV includes properties which their values do not show any particular trend within the given clay distribution types template.