Carbonate Classification: Porosity
Although most of this book focuses on the identification of grains and cements, what is often of prime interest to hydrocarbon explorationists is understanding the absence of those materials — in other words, the origin and history of open primary or secondary pore space. This chapter, therefore, will deal with recognition of different types of porosity; the chapters on diagenesis will cover the mechanisms and relative timing of porosity creation, retention, reduction, or destruction.
A number of classifications of porosity in carbonate rocks have been proposed (see citations at end of section), but only the Choquette and Pray (1970) scheme has met with widespread acceptance. Thus, it will be the only one described and applied in this book. This classification combines terms that encompass four separate categories of observations. The main term (called the “basic porosity type”) codifies the location and type of pore space. That term is prefaced with a genetic modifier or modifiers that relate to the process, direction or stage (enlarged, reduced or filled) of porosity evolution, and the time of pore formation; an additional term describing pore sizes can also be added. Finally, an abundance term can be appended at the end of the name to describe the percentage of pore space. In practice, most geologists simply specify the basic porosity type along with the one or two modifiers that are best suited to their needs.
The basic porosity types are illustrated in two diagrams (below and at the top of the next page). The basic porosity types are organized according to whether they are fabric selective, not fabric selective, or either fabric selective or not. The modifying terms are shown in the middle diagram (next page). Examples of the major porosity types (and some more minor ones) are given in subsequent illustrations.
A final note: the proper classification of porosity requires accurate observation of the amount and nature of pore spaces. Some porosity is either too large or too small to be recognizable in thin section (see upper photograph on the title page of this chapter), but most is visible at thin-section scales. To recognize and measure porosity properly under the microscope, one MUST use thin sections prepared from rock chips that were pressureimpregnated with color-dyed epoxy. Grains or crystals commonly are plucked out of sections during cutting and grinding; only with colored impregnation media can one distinguish pre-sectioning “real” pores from ones created during section preparation. Intensely dyed epoxy also lends emphasis to porosity and helps to reveal micropores that could otherwise be overlooked. To do quantitative or semiquantitative measurements of porosity using microscopy, one must mathematically correct the observations made in two-dimensional space (see, for example, Halley, 1978); modern digital image analysis methodologies can also be applied to this process (e.g., Anselmetti et al., 1998).
Choquette & Pray (1970) basic fabric-selective porosity types
A diagrammatic representation of the basic fabric-selective porosity types used in the Choquette and Pray (1970) carbonate porosity classification. What is meant by fabric selectivity is that the porosity is controlled by the grains, crystals, or other physical structures in the rock and the pores themselves do not cross those primary boundaries.
Choquette & Pray (1970) basic non-fabric-selective or variable porosity types
A diagrammatic representation of the basic non-fabric-selective or variably fabric-selective porosity types used in the Choquette and Pray (1970) carbonate porosity classification. These are all porosity patterns that actually or potentially can cross-cut primary grains and depositional fabrics. They also include porosity types that potentially can be much larger than any single primary framework element.
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks