Page 108 - Carbonate Facies in Geologic History

Porosity and Permeability 95

uncommon. The reader is referred to several studies on this subject (Murray,
1960; Thomas, 1962; Harbaugh in Chilingar et aI., 1967; Choquette and Pray,
1970, Fig.I11-5 of this volume). Several types of porosity can be recognized and
classified genetically:
1. Secondary solution along fractures due presumably to meteoric water.
2. Secondary solution in the form of vugs or cavities presumably due to meteoric
water.
3. Primary vugs and cavities in reefframework.
4. Primary interparticle porosity in sands and gravels preserved from cementa-
tion. Primary interparticle porosity in pelagic microfossil silts (chalky).
5. Chalky (very fine porosity) caused by lack of early cementation and by replace-
ment of original lime mud fabric by blocky calcite crystals. Preservation of up
to 30% porosity, very little permeability.
6. Sucrose dolomite porosity, which forms during early dolomitization of nonli-
thified calcareous mud. This process preserves original porosity, using up car-
bonate ion as Mg replaces pure Ca in the original mud and growing dolomite
crystals to form a noncompactible framework.
Fig. III-5 is recommended as a key for describing porosity in the following
order: genetic modifier plus size plus type plus percentage of bulk volume of rock.
E.g., secondary eogenetic intercrystalline and moldic dolomitization porosity-
10% (Choquette and Pray, 1970).

<III Fig. III -5. Geologic classification of pores and pore systems in carbonate rocks. From Cho-
quette and Pray (1970, Fig. 2), with permission of American Association of Petroleum Geolo-
gists