152 Subsurface Geologic Methods 



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 associ- 

 ated diagnostic residue or a zone boundary included in the sample inter- 

 val 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 encountered. With lithologic examination no 

 zones could be identified. 



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

 break, or a detritus often present clews to a formational change, which in 

 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 drilling well may be used for washing, and heat from an automo- 

 bile-engine head or a drilling well will dry the samples for examination 

 on location. Obviously, a geologist attempting such work must be familiar 

 with residue zones and sequences, as the necessary samples for compari- 

 son would not likely be available. 



New workers with residues should be well aware that successful cor- 

 relation 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 pre- 

 requisite to the successful correlation and identification of beds. Of 

 course, the foregoing statement is true for lithologic examination, but an 

 inexperienced geologist can soon learn the surficial 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 useful in subsurface work and petroleum geology in 

 Texas, Oklahoma, Kansas, Missouri, and Illinois and has contributed 

 much to geologic science in the states between the Appalachian and the 

 Rocky Mountains. The beds receiving the most attention have been Upper 

 Cambrian and Lower Ordovician, but Silurian, Devonian, and Mississip- 

 pian beds have been extensively studied. 



The space allotted here would be inadequate to give worth-while 

 descriptions of the subdivisions of the thick sections of calcareous rocks 

 in the various parts of the United States. Anyone concerned would profit 

 more to confer with workers familiar with local areas and sections. 



