One of the ﬁrst, if not the ﬁrst, carbonate porosity classiﬁcation, was developed by G. E. (Gus) Archie (1952). He also pioneered the study of electrical resistivity in rocks, developed the principles that led to the Archie saturation equation, and investigated methods to integrate geological data with petrophysical laboratory data and well-log signatures. His objective was to illustrate relationships between a rock and petrophysical properties in reservoirs.
Archie's porosity classiﬁcation is based on textural descriptions of the carbonate reservoir rock as it relates to the "character" of any visible porosity (Table 1). Three textural categories are termed Type I, II, and III, and four classes for visible porosity are identiﬁed as classes A through D. Class A has no visible porosity at 10x magniﬁcation, class B has visible pores between 1 and 10 μm, and class C has visible pores larger than 10 μm but smaller than well cuttings or chips (roughly, about 2.0 mm diameter). Class D includes large visible pores, such as solution vugs larger than cutting chips.
Table 1. Carbonate classification of Archie (1952).
Archie described Type I carbonates as "crystalline, hard, dense, with sharp edges and smooth faces on breaking." Under the binocular microscope, these rocks have a matrix made of tightly interlocking crystals that do not exhibit visible intercrystalline porosity. For practical purposes, these rocks correspond to today' s mudstones and dolomudstones.
Type II rocks are described as "earthy" or "chalky" with grains not larger than about 50 μm in diameter (just below the ﬁnest silt size), and they are composed of "ﬁne granules or sea organisms." These rocks correspond to chalk, or mudstones and wackestones that have probably undergone diagenetic alteration to attain the chalky appearance.
Type III carbonates are "granular or saccharoidal." Saccharoidal is used to define a "sugary" texture typical of many dolomites with intercrystalline porosity whereby such dolostones, are composed of crystalline mosaics that reﬂect light like so many sugar crystals. His term granular carbonates encompasses most grainstones and packstones.
Archie, G. E., 1952, Classiﬁcation of carbonate reservoir rocks and petrophysical considerations: Bulletin American Association Petroleum Geologists, v. 36, p. 278-298.
At Saltworks, the aim of all our training modules and workshops is two-fold. 1) give an understanding of the relevant process, 2) train participants in the application of the skill sets tied to the concept and prioritise the skill sets needed to apply this understanding. We illustrate the skills and knowledge necessary to recognise subsurface evaporite salts using a conventional suite of well logs.
If you want to know more, please download the relevant saline geosystems or carbonate geosystems catalogue and choose a combination of the various training modules that best suite your company needs.