20 



SHORTER CONTRIBtTTIONS TO GENERAL GEOLOGY, 1921. 



sand at the base of unit G is the main producing 

 sand. As the Ranger sand, according to 

 Reeves, is in phices 40 feet thick it may be that 

 locally the entii-e sandy portion of the section 

 from the base of unit H to the basal bed of 

 unit G, inclusive, constitutes the productive 

 bed. 



NATURE OF THE OIL-PRODUCING BEDS. 



The lithologic characte'- of the oil-producing 

 beds seems to l)e one of the interesting subjects 

 of research in the north-central Texas fields. 

 Though I have no precise evidence as to the 

 stratigi'aphic position of the oil-bearing beds, 

 I have examined seven samples of reported t)il 

 "sands" in the collection of the United States 

 Geological Survey. In a general way these 

 samples consisted of sandy black limestone or 

 shales, in some samples coarsely sandy, in some 

 associated with coaree sandstones, l)ut the lat- 

 ter are usually rather tightly cemented with 

 calcite or silica. In thin section none of tliese 

 materials appeared porous enough to be good 

 reservoirs for oil. A. F. Melcher, physicist, of 

 the United States Geological Survey, who has 

 made determinations of the porosity of samples 

 of similar material from north-ciaitral Texas, 

 has reached the same conclusion,-- and it con- 

 sequently aj)pears probable to l)oth of us that 

 this material is not the source of the large f|uan- 

 tities of oil jiroduced in the region. In view of 

 the fact that in association with one of these 

 samples and with several basal betls in the Sea- 

 man and Rudd wells loose grains of coarse sand 

 were found, it seems more proba})le that there 

 are beds of sand or sandstone so loosely ce- 

 mented that fragments of them are not recov- 

 ered in drilling and that these beds yield the 

 oil. This is, however, a c[uestion on which 

 those who have studied it more closely and in 

 pro(hu'ing wc^lls are more competent to express 

 an o[union. 



TIME REQUIRED FOR TfflS METHOD OF WORK. 



I found that I could examine the samples 

 at the rate of about two an hour. I had 

 about two thin sections for each sample from 

 the Rudd well and studied these at the rate of 

 about five an hour, but as the materials of the 

 Seaman well were new to me, I studied the 

 thin sections at the rate of about three an 



22 Oral communication. 



hour. Probably the study of the thin sections 

 from a sample would take nearly as much 

 time as the study of the sample. This is an 

 approximate rate for establishing a type sec- 

 tion in a new field. In a small area where the 

 section is established the rate of study might 

 for merely practical pm-poses be much faster. 

 Thin sections can be prepared at the rate of 

 about one and a half an hour. In addition to 

 the petrographer probably two assistants to 

 grind thin sections and to wash, file, and per- 

 haps make certain tests on the samples would 

 be recpiired. I would urge care in washing 

 the samples, as cpiartz sand, glauconite, and 

 other important ingredients are frequently 

 among the finer parts and are likely to be 

 washed out. 



SUMMARY AND CONCLUSIONS. 



In this paper I have attempted to show 

 that the relative proportions of sand, clay, 

 and linu' as reprt>sented in a graphic log called 

 the percentage log serve to differentiate 

 distinct lithologic elements in a stratigraphic 

 section and to help in their correlation between 

 widely separated wells within a single (k'posi- 

 tional basin. The boundaries of these units 

 are defined in tiie percentage log either by 

 sharp changes in the proportions of the con- 

 stituents or by points marking the maximum 

 of an oscillation of base-level. Where the 

 break between two lithologic elements or units 

 is sharp there usually occurs in the sections 

 here described at or near the base of the upper 

 unit a tliin isolated bed containing coarse 

 glauconite, associated with abundant calcareous 

 shells or coarse shell fragments, phosphate, 

 very abundant sulphide, and coarse sand. 

 Any of these constituents may be absent or 

 may predominate. Glauconite is, however, 

 the one most likely to predominate, and after 

 that phospliate. 



Wliere the effect of maximum rise of base- 

 level is marked by the greatest proportion of 

 coarse det.rital material in a practically unin- 

 terrupted depositional sequence the conditions 

 favorable to the formation of the glauconite 

 bed may not occur just at the same time. In 

 that case the horizon of the surface of the litho- 

 sphore at that place at the time of greatest 

 elevation of base-level (the akinetic surface), 

 if it can be definitely recognized, should be 

 taken as the boundary. 



