Geologic Nomenclature and Classification of Porosity in Sedimentary Carbonates

Pore systems in sedimentary carbonates are generally complex in their geometry and genesis, and commonly differ markedly from those of sandstones. Current nomenclature and classifications appear inadequate for concise description or for interpretation of porosity in sedimentary carbonates. In this article we review current nomenclature, propose several new terms, and present a classification of porosity which stresses interrelations between porosity and other geologic features. The time and place in which porosity is created or modified are important elements of a genetically oriented classification. Three major geologic events in the history of a sedimentary carbonate form a practical basis for dating origin and modification of porosity, independent of the stage of lithification. These events are (1) creation of the sedimentary framework by clastic accumulation or accretionary precipitation (final deposition), (2) passage of a deposit below the zone of major influence by processes related to and operating from the deposition surface, and (3) passage of the sedimentary rock into the zone of influence by processes operating from an erosion surface (unconformity). The first event, final deposition, permits recognition of predepositional, depositional, and post epositional stages of porosity evolution. Cessation of final deposition is the most practical basis for distinguishing primary and secondary (postdepositional) porosity. Many of the key postdepositional changes in sedimentary carbonates and their pore systems occur near the surface, either very early in burial history or at a penultimate stage associated with uplift and erosion. Porosity created or modified at these times commonly can be differentiated. On the basis of the three major events heretofore distinguished, we propose to term the early burial stage the late stage telogenetic, and the normally very long intermediate stage mesogenetic. These new terms are also applicable to process, zones of burial, or porosity formed in these times or zones (e.g., eogenetic ceme tation, mesogenetic zone, telogenetic porosity). The proposed classification is designed to aid in geologic description and interpretation of pore systems End_Page 207------------------------------ and their carbonate host rocks. It is a descriptive and genetic system in which 15 basic porosity types are recognized: seven abundant types (interparticle, intraparticle, intercrystal, moldic, fenestral, fracture, and vug), and eight more specialized types. Modifying terms are used to characterize genesis, size and shape, and abundance of porosity. The genetic modifiers involve (1) process of modification (solution, cementation, and internal sedimentation), (2) direction or stage of modification (enlarged, reduced, or filled), and (3) time of porosity formation (primary, secondary, predepositional, depositional, eogenetic, mesogenetic, and telogenetic). Used with the basic porosity type, these genetic modifiers permit explicit designation of porosity origin and evolution. Pore shapes are classed as irregular or regular, and the latter are subdivided into equant, tubular, and platy shapes. A grade scale for size of regular-shaped pores, utilizing the average diameter of equant or tubular pores and the width of platy pores, has three main classes: micropores (< 1/16 mm), mesopores (1/16-4 mm), and megapores (4-256 mm). Megapores and mesopores are divided further into small and large subclasses. Abundance is noted by percent volume and/or by ratios of porosity types. Most porosity in sedimentary carbonates can be related specifically to sedimentary or diagenetic components that constitute the texture or fabric (fabric-selective porosity). Some porosity cannot be related to these features. Fabric selectivity commonly distinguishes pore systems of primary and early postdepositional (eogenetic) origin from those of later (telogenetic) origin that normally form after extensive diagenesis has transformed the very porous assemblage of stable and unstable carbonate minerals into a much less porous aggregate of ordered dolomite and/or calcite. Porosity in most carbonate facies, including most carbonate petroleum reservoir rocks, is largely fabric selective.