Capturing Key Attributes of Fine-grained Sedimentary Rocks in Outcrops, Cores, and Thin Sections: Nomenclature and Description Guidelines

An integrated nomenclature scheme is proposed to capture the inherent heterogeneity of fine-grained sedimentary rocks at the 102 to 1023 mm scale and to assist the evaluation of these rocks as sinks of organic carbon, barriers to fluid flows, and reservoirs of oil and gas. This scheme incorporates previous knowledge and the latest field, petrographic, and laboratory observations. We propose to name fine-grained sedimentary rocks using a root term based on their texture (grain size), which is modified by description of bedding, composition, and grain origin. Regarding texture, we suggest the use of ‘‘mudstone’’ as a class name for the entire spectrum of fine-grained sedimentary rocks. We define mudstone as a rock in which more than fifty percent of its grains are mud (clay and silt) size (< 62.5 mm). Similar to the approach used for the description of sandstone texture, mudstone texture can be refined by a ‘‘coarse,’’ ‘‘medium,’’ or ‘‘fine’’ size-range term. Regarding bedding, we follow Campbell’s (1967) genetic approach to define laminae, laminasets, and beds, and describe lamina geometry, continuity, and shape. Regarding composition, we propose terms such as ‘‘siliceous,’’ ‘‘calcareous,’’ ‘‘argillaceous,’’ and ‘carbonaceous’’ to capture differences in rock composition. The name of a mudstone can be further modified by additional attributes that detail the form and origin of the rock components. Application of this approach to the Cretaceous Eagle Ford Shale illustrates the variability typically present in mudstone successions and demonstrates how our detailed characterization can be used to decipher and predict rock properties of economic interest.