possible to identify their positions in the zone, although the zone itself could be 

 identified. If the samples overlapped into zone 2, then the boundary could be 

 recognized, and it could be stated that 300 feet or more of zone 1 was absent. 



Many cherts are alike in color and texture, and similar cherts in two differ- 

 ent zones would prevent identification, unless an associated residue was diagnostic 

 or a zone boundary was passed. The similarity of the brown, smooth chert in 

 the Lower Devonian and the Upper Ordovician in West Texas has been men- 

 tioned previously. If a chert in zone 1 was similar to a chert in zone 4 in the 

 section postulated in the last paragraph, the two zones could easily be confused. 

 If the set of 10 samples was identified as belonging to zone 1, but actually zones 

 1, 2, 3, and part of 4 had been eroded, an error of at least 650 feet in correlation 

 would result. The correct identification of zone 4 would show a structural upfold. 

 If the samples were identified as zone 4 and the producing bed was zone 2, the 

 absence of zone 2 would be concluded and deeper drilling prevented. 



The foregoing discussion shows the necessity of having some associated 

 diagnostic residue or a zone boundary included in the sample interval for positive 

 identification. Knowledge of the similarity of two zones would call for careful 

 drilling and a postponement of identification until the underlying zone was en- 

 countered. With lithologic examination no zones could be identified. 



Pyrite, regenerated sand grains, a sandy chert or sandy zone, a shale break, 

 or other detritus often present clues to a formational change, which is some cases 

 can be confirmed by other evidence. 



The use of residues is not restricted to the laboratory. A microscope, a jug 

 of acid, and a half-dozen beakers may be carried to the field. Water from a drill- 

 ing well may be used for washing, and heat from an automobile-engine head 

 or a drilling rig will dry the samples for examination on location. Obviously, a 

 geologist attempting such work must be familiar with residue zones and se- 

 quences, as the necessary samples for comparison may not be available. 



New workers with residues should be well aware that successful correlation 

 by residues comes only after a thorough knowledge of residue types, principles 

 of secondary replacement, and facies changes and the examination of many 

 samples. Experience with residue material is prerequisite to the successful cor- 

 relation and identification of beds. Of course, the foregoing statement is true for 

 lithologic examination, but an inexperienced geologist can soon learn the sur- 

 ficial characteristics of rock fragments and correctly correlate, but he would find 

 it difficult to correlate with residues without experience or the supervision of one 

 experienced in residue work. 



The use of residues for correlation has been successful in the thick calcareous 

 sections of most Paleozoic rocks but has had little success in the thick Permian 

 section of West Texas and New Mexico. Residue work has been especially use- 

 ful in subsurface work and petroleum geology in Texas, Oklahoma, Kansas, 

 Missouri, and Illinois and has contributed much to geologic science in the states 



88 



