Petrogenetic Classification of Carbonate Rocks

Abstract

ABSTRACT The Petrogenetic Classification of carbonate rocks is a complex but flexible nomenclatorial system developed in an effort to organize some of the descriptive properties of this extremely complicated group of rocks into a systematic and genetically significant relationship. The properties recognized in this classification are grouped under three headings: A) Compositional elements, B) Texture, and C) Chemical composition. Winnowing, and the roundness,sorting, and size of all constituents of the rock that have been physically transported in the solid state are measured and the overall maturity ascertained. Together these properties, briefly stated, make up the first element, texture, of a tripartite Carbonate Rock Name. The second term, Main Rock Name, is based on the relativ proportions of carbonate compositional elements present in the rock under consideration. Carbonate constituents recognized in this classification are located at the poles of a ternary diagram in the following manner. All microcrystalline and sedentary fossil constituents are counted under Pole I of the triangular diagram. Under Pole II are located all allochemical elements (pellets, oolites, intraclasts, and bioclasts), and under Pole III--all pore filling cement, recrystallized and replaced minerals. The third and last part of the Complete Carbonate Rock Name is the chemical composition, commonly expressed as a calcium-to-magnesium (or Mg/Ca) ratio. The chemical make-up may also be presented as percent oxides, or mineral percentages if normative calculations are preferred. Although this classification is basically petrographic and designed for laboratory utilization, emphasis on the textural term at the expense of less easily determined compositional elements can make it a useful field tool. Application of this system to problems requiring detailed consideration of variability, rather than to studies of a reconnaissance nature, will constitute the most efficient use of the classification. Carbonate rock terminology utilized in this paper is defined and discussed in some detail, and expansion for special-purpose use is explained. Inclusion of porosity data in the textural term, for example, and correlation of pore size distribution with petrographic type may aid in oil reservoir valuation and stimulation studies, as well as gas storage problems. Similarl , addition of a term based on chemical analysis data to the petrographic name may prove valuable in the exploitation of limestone as an economic mineral.