Ch. 31] HEAVY MINERALS 571 



study of heavy minerals, relative amount of insoluble residues, and 

 for detailed correlation of individual strata of a producing forma- 

 tion within a reservoir. 



Textuke 



Some textural properties of sandstones and shales are of considerable 

 interest in studies of basin configuration, direction of source, etc. 

 Such properties include grain size, shape and roundness, surface texture, 

 orientation of elongate grains, and degree of cementation. To date, 

 the results of studies of these properties have been of little direct 

 value in the business of finding oil, but the potentialities are great. 

 Numerous techniques have been developed for the preparation of 

 sample materials and their study for such textural parameters. For a 

 short historical development of this subject the reader is referred to 

 Krumbein (1932, pp. 89-124). 



MlCROPALEONTOLOGY 



Microfossils, principally foraminifera, have been used widely in many 

 regions as an aid in stratigraphic correlation. Their minute exoskele- 

 tons frequently occur in limestones, muds, and silts. Many forms are 

 widely scattered, both vertically and horizontally in the rock, whereas 

 others are quite restricted in their vertical range, abundant and wide- 

 spread in their horizontal distribution. The latter forms are true index 

 fossils and are of great value in establishing correlations from well to 

 well. Microfossils may be separated from either cores or cuttings by 

 partially disaggregating the sample and boiling it in water containing 

 a little sodium bicarbonate. Alternating washing and decanting of 

 the sample material under a stream of water will remove most of the 

 non-fossiliferous rock, leaving behind a concentration of foraminifera. 

 Such materials are then ready for binocular examination by a micro- 

 paleontologist, who identifies the index forms and notes their strati- 

 graphic position and relative abundance. 



Heavy Minerals 



All sandstones and sandy limestones contain minor and variable 

 amounts of "heavy minerals" which afford another tool in stratigraphic 

 correlation. Heavy-mineral assemblages may vary considerably from 

 formation to formation but remain quite characteristic over a con- 

 siderable area within a single formation. In making a heavy-mineral 

 study of a formation, the usual procedure consists in determining the 

 relative amounts of the different minerals present. A heavy-mineral 

 separation is effected by immersing a disaggregated rock sample in a 



