Comments on Sedimentary Rocks 77 



erate original textures and structures and develop new ones. Carbonates 

 accumulate under various environmental conditions: stable shelf, mildly 

 unstable shelf, intracratonic basin, and geosyncline. 



The color of carbonate rocks varies from white to black; crystallinity 

 ranges from fine to coarse. Limestones and dolostones may be thick- 

 bedded to finely laminated, vuggulated, oolitic, or pisolitic. Foreign con- 

 stituents as shown by insoluble-residue work include chert, clay, quartz, 

 pyrite, glauconite, arenaceous Foraminifera, and silicified fragments. 



During the past several years, oil companies have shown intense in- 

 terest in reef limestones and dolostones as possessing great oil and gas 

 potentials. Systematic surveys of all possible reef areas are being made 

 and criteria are being collected to evaluate these deposits more accurately. 



The following techniques have been followed in the study of car- 

 bonate rocks: thin-section, polished surface, insoluble residue, chemical- 

 stain test, spectrochemical, porosity and permeability, relative solubility, 

 petrofabrics, and chemical analysis. In addition to these methods, elec- 

 trical, radioactive, and micrologging have added considerably to our 

 knowledge of subsurface carbonate characteristics. Detailed carbonate in- 

 vestigations contribute to improvement of more efficient production and 

 well-completion methods. 



Evaporites 



The sediments composing the evaporites are primary precipitates 

 which have resulted for the most part from the evaporation of saline solu- 

 tions. The best-developed evaporite sections in North America are found 

 in the Permian Basin of west Texas and eastern New Mexico, and in the 

 Silurian salt basins of Michigan and New York. The evaporites are rep- 

 resented by the sulphates (anhydrite, gypsum), chlorides ( mainly halite), 

 and minor carbonates. The sodium and potassium sulphates and the 

 nitrates are also included in the evaporites. Rock salt, anhydrite, and 

 gypsum are the most commonly encountered. 



Rock-salt beds, ranging from inches to 80 feet in thickness, are gen- 

 erally associated with anhydrite and gypsum. They are frequent in red- 

 bed sections. Their purity, color, and texture are variable. Crystallinity 

 of the salt varies from fine to coarse. Great masses of this material have 

 flowed and produced salt domes in the Gulf Coast area of Texas and 

 Louisiana and in Germany, Iran, and Russia. 



Anhydrite and gypsum occur as bedded deposits mainly in nonmarine 

 stratal sequences and are commonly associated with dolostone and shale 

 phases. Frequently these rocks exhibit normal lamination or corrugated 

 lamination. Anhydrite generally ranges in color from white to dark gray 

 and assumes a fine to coarse crystallinity. The structural and textural 

 features are sometimes appreciably modified by conversion to gypsum 

 upon hydration. Gypsum frequently occurs in argillaceous strata as trans- 

 parent crystals of selenite. 



