Subsurface Logging Methods 487 



SPECTROCHEMICAL SAMPLE LOGGING 

 L. L. SLOSS AND S. R. B. COOKE 



Subsurface stratigraphers have long encountered difficulties in dif- 

 ferentiating and correlating thick stratigraphic sections involving mo- 

 notonous successions of similar lithology which lack lithologic or paleon- 

 tologic markers or identifiable zones. In some areas and in some parts of 

 the rock column, heavy-mineral analyses, insoluble residues, and electrical 

 or radioactive character, singly or in combination, have served effectively 

 in the solution of such problems. In other cases, notably where thick 

 shale or carbonate sequences are concerned, these methods have failed to 

 achieve a satisfactory basis for detailed structural and stratigraphic in- 

 terpretations. In these cases differentiation of stratigraphic units and their 

 correlation have, perforce, been based on tedious microscopic examination 

 of samples. Microscopic sample study has added greatly to our under- 

 standing of sedimentary petrography, but as a correlation technique it 

 often fails in the very cases that demand its application — that is, in rock 

 sequences not adapted to "in-the-hole" techniques or analyses of special 

 mineralogic attributes. 



These commonly encountered circumstances require an approach to 

 stratigraphic differentiation and correlation based on characteristics other 

 than gross lithology, observable mineralogy, or the electrical or radio- 

 active constants of the rock or its contained fluids. Further, the applic- 

 ability of any new technique would be increased if easily reproducible 

 data were involved and if the impact of the personal equation were held 

 at a minimum. 



While the writers were associated at the Montana School of Mines 

 they became involved in correlation problems that arose from detailed 

 studies of Mississipian limestones in the subsurface and outcrop areas of 

 Montana and southern Alberta. In this area the Mississippian rocks do 

 not contain suitable microfossils in sufficient numbers to permit the estab- 

 lishment of paleontologic zones. Lithologic aspect varies from place to 

 place in response to tectonic influences, which range from the border of a 

 geosyncline, across a stable shelf area, and into an active intracratonic 

 basin. Correlation is sufficiently difficult within any single tectonic pro- 

 vince and becomes increasingly complex as provincial boundaries are 

 traversed. 



It occurred to the writers that since the entire area was submerged 

 by a continuous seaway any changes in the chemical composition of the 

 marine waters would be transmitted throughout the area and be reflected 

 more or less simultaneously in the concurrent sedimentation. To test this 

 hypothesis qualitative spectrochemical analyses of the trace and minor- 

 element constituents of successive samples were run. The results proved 

 negative in that all samples contained about the same suite of elements, 

 and no data of significance to correlation were obtained. 



