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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 54 (1970)

Issue: 2. (February)

First Page: 207

Last Page: 250

Title: Previous HitGeologicNext Hit Previous HitNomenclatureNext Hit and Previous HitClassificationNext Hit of Previous HitPorosityNext Hit in Previous HitSedimentaryNext Hit Previous HitCarbonatesNext Hit

Author(s): Philip W. Choquette (2), Lloyd C. Pray (3)


Pore systems in Previous HitsedimentaryNext Hit Previous HitcarbonatesNext Hit are generally complex in their geometry and genesis, and commonly differ markedly from those of sandstones. Current Previous HitnomenclatureNext Hit and classifications appear inadequate for concise description or for interpretation of Previous HitporosityNext Hit in Previous HitsedimentaryNext Hit Previous HitcarbonatesNext Hit. In this article we review current Previous HitnomenclatureNext Hit, propose several new terms, and present a Previous HitclassificationNext Hit of Previous HitporosityNext Hit which stresses interrelations between Previous HitporosityNext Hit and other Previous HitgeologicNext Hit features.

The time and place in which Previous HitporosityNext Hit is created or modified are important elements of a genetically oriented Previous HitclassificationNext Hit. Three major Previous HitgeologicNext Hit events in the history of a Previous HitsedimentaryNext Hit carbonate form a practical basis for dating origin and modification of Previous HitporosityNext Hit, independent of the stage of lithification. These events are (1) creation of the Previous HitsedimentaryNext Hit 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 Previous HitsedimentaryNext Hit 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 Previous HitporosityNext Hit evolution. Cessation of final deposition is the most practical basis for distinguishing primary and secondary (postdepositional) Previous HitporosityNext Hit. Many of the key postdepositional changes in Previous HitsedimentaryNext Hit Previous HitcarbonatesNext Hit and their pore systems occur near the surface, either very early in burial history or at a penultimate stage associated with uplift and erosion. Previous HitPorosityNext Hit 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 "eogenetic," the late stage "telogenetic," and the normally very long intermediate stage "mesogenetic." These new terms are also applicable to process, zones of burial, or Previous HitporosityNext Hit formed in these times or zones (e.g., eogenetic ceme tation, mesogenetic zone, telogenetic Previous HitporosityNext Hit).

The proposed Previous HitclassificationNext Hit is designed to aid in Previous HitgeologicNext Hit 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 Previous HitporosityNext Hit 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 Previous HitporosityNext Hit. 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 Previous HitporosityNext Hit formation (primary, secondary, predepositional, depositional, eogenetic, mesogenetic, and telogenetic). Used with the basic Previous HitporosityNext Hit type, these genetic modifiers permit explicit designation of Previous HitporosityNext Hit 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 Previous HitporosityNext Hit types.

Most Previous HitporosityNext Hit in Previous HitsedimentaryNext Hit Previous HitcarbonatesNext Hit can be related specifically to Previous HitsedimentaryNext Hit or diagenetic components that constitute the texture or fabric (fabric-selective Previous HitporosityNext Hit). Some Previous HitporosityNext Hit 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. Previous HitPorosityTop in most carbonate facies, including most carbonate petroleum reservoir rocks, is largely fabric selective.

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