only show correlation of the formations and establish the location of the oil 

 sand, they also indicate the presence of a fault, spot the depth at which it was 

 encountered, and provide a means of determining vertical displacement. Thermal 

 curves for the weathered-surface-soil mantle, an assortment of clay and organic 

 complexes, have been omitted. 



PROCUREMENT OF The first and one of the most important steps 



SAMPLES in preparing a DTA log is the proper selec- 



tion of cuttings (or cores) . It has been found 

 that with good drilling-mud control, there is little danger of bottom cuttings be- 

 ing contaminated with materials from depths higher in the hole. The main prob- 

 lems are (1) selection of significant intervals for sampling, (2) correct identi- 

 fication of depths, which can be done if periodic lag-time measurements are made 



(3) sufficient but not excessive washing to remove adhering drilling mud, and 



(4) drying at a temperature low enough (maximum of 180F or 82C) so that 

 clay water-of-hydration or mineral water-of-crystallization is not driven out. 



Of these problems, only 1 and 3 need be considered in more detail. If cut- 

 tings are used, experience has shown that the DTA run on samples at 20-foot in- 

 tervals normally provides sufficient delineation for the main body of the log. 

 However, where lithologic breaks occur, it is advantageous to have intermediate 

 samples whose thermal curves may be inserted into the log to define more sharply 

 the points of change. Therefore, 10-foot increments are desirable, or even 5-foot 

 when rate of penetration allows. 



Samples taken from cores generally require hand compositing. It is pre- 

 ferable to sample a core at 1-foot intervals, to composite 5 consecutive samples, 

 and then to treat the aggregate similarly to a 5-foot ditch-cutting sample. 



PREPARATION OF Cores require little preparation other than re- 



SAMPLES moval of the outermost mud layer, provided 



they are not saturated with oil. Cuttings, on 

 the other hand, must be washed of drilling fluid as soon as possible. If clay-base 

 mud is involved, washing should be over a 150-mesh screen and continued until 

 all the mud fluid has been excluded. Extended washing must be avoided if the 

 cuttings are clay. An experienced sampler usually can distinguish between clay 

 cuttings and balled-up fragments of partially dried drilling mud. 



Cuttings taken in oil-base mud should be washed in kerosene or some other 

 hydrocarbon solvent, then washed in water containing considerable detergent. 

 The sample is then rinsed in fresh water. 



If the cuttings or cores contain an appreciable amount of formational crude 

 oil, it is necessary to remove this oil by accepted extraction- or low-temperature 



128 



