The petrographic description of carbonate facies: are we all speaking the same language?

Both academia and industry require consistent and repeatable carbonate lithofacies classifications as a primary input to the construction of depositional, diagenetic and reservoir models, and the Dunham System has long been held to satisfy this requirement. However, ambiguities in the petrographic description of carbonates are widespread. This study investigates the classification of carbonate lithofacies across academia and industry at a wide range of experience levels in order to quantitatively assess reproducibility. Some 241 volunteers, with over 4200 years of combined experience, examined a range of synthetic rock textures and natural lithologies and assigned textures as they saw appropriate. The results of the study identify the situations where classification ambiguities and inconsistencies are most common. The Dunham classification system was proved to be the scheme of choice with 89% of the classifications using some form of the Dunham system. However, all of the 24 samples yielded a wide variability in assigned texture with between 22 and 131 different names being assigned to a single specimen. The most common causes of inconsistency are; errors in assessing the mode of support, mistakes in estimating the size and volume of grains within the lithology, and confusion as to how to classify lithologies in which more than one texture is present. The textures of the modified Dunham Classification System are redefined in order to clarify any classification criteria that have been identified as points of confusion. Detailed classification guidelines are offered in order to minimize the possibility of misidentification or confusion. It is suggested that the term bafflestone is redundant and should be removed from the classification system. The adoption of these guidelines will increase confidence, reliability and value in the petrographic classification of carbonate lithologies, thereby enhancing communication and facilitating the development of more realistically constrained depositional, diagenetic and reservoir models.