Classification and Description Guidelines for the Spectrum of Fine-Grained Sedimentary Rocks: Simplicity and Order out of Chaos

Lazar, Remus O.; Bohacs, Kevin; Macquaker, James; Schieber, Juergen; Demko, Timothy

More than two thirds of the sedimentary record is composed of rocks whose grain size is largely smaller than 62.5 μm. Because new technologies have unlocked significant hydrocarbon reserves in these rocks, interest in their sedimentology, diagenesis, and stratigraphy has expanded at a rapid pace in the last years. These rocks are commonly described as ‘shales' and routinely applied attributes include ‘homogenous', ‘structureless', ‘massive' or ‘laminated'. Our examination of more than 10 km of Paleozoic to Cenozoic cores and outcrops over the last three decades in pursuit of source, reservoir, and seal studies revealed that these rocks are typically heterogeneous. Detailed study of this heterogeneity suggests that these rocks formed under more variable sedimentologic and diagenetic conditions than typically envisioned. In our experience, ‘shales' have physical, biological, and chemical properties that vary in systematic ways and can be deciphered and predicted within an integrated, genetic sequence-stratigraphic framework.

In this contribution, we provide definitions and description guidelines, and suggest a simple classification in which the name of a fine-grained sedimentary rock has a root based on texture (grain size) and is modified by bedding and composition. In exact analogy to the approach used for sandstone texture, we classify these rocks into three main categories, using the root name ‘mudstone' modified by a size-range term: ‘coarse', ‘medium', or ‘fine'. The full name can be further modified by attributes such as color, degree of bioturbation, fossils, sedimentary structures, and diagenetic components. A fine-grained rock may be described, for example, as a "dark-brown, bioturbated, discontinuous wavy non-parallel bedded, calcareous medium mudstone with abundant brachiopod fragments and pyrite nodules". The same rock can be also referred to using a shortened ‘nickname' as a "bioturbated calcareous medium mudstone".

The proposed nomenclature and classification captures the vertical and lateral variability in rock properties at millimeter to meter scales in a consistent and concise matter. This, in turn, provides the basis for sound interpretations of the primary controls on the formation of these rocks and has the potential to enhance predictions of their quality and distribution as sources, reservoirs, and seals of hydrocarbons.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013