kept in proper balance with the requirements of the exploration program. The 

 emphasis on one or the other changes from time to time because of the shifting 

 scenes of exploratory activities and the evolution of ideas on petroleum geology. 

 The subsurface geologist must exhibit a high degree of flexibility in the applica- 

 tion of his geologic knowledge if he is to keep abreast of the advances that are 

 being made in the science. In order to meet the demands of the day, the 

 microscopist must do more and better work than has been required in the past. 

 The methods employed should be consistently accurate within the limitations 

 of the instruments, yet rapid enough to turn out the necessary volume of work. 

 The purpose of this chapter is to make available to the beginning micro- 

 scopist the methods that have been successful in the course of many years of 

 application in oil geology. The attempt is to present only one phase of sub- 

 surface work — lithologic determination of well samples by means of the low- 

 power binocular microscope, the present work horse of the oil companies. 



Rotary Well Cuttings 



In the process of drilling a well with rotary tools, drilling fluid (mud) is 

 pumped down the inside of the drill pipe, thence through vents in the bit into 

 the space (annulus) between the drill pipe and the wall of the hole and up to 

 the surface again. The chips of rock cut by the bit on the bottom of the hole 

 are picked up by this constantly flowing mud stream and are carried to the 

 surface, where they are collected, washed, and put in envelopes for later 

 examination. 



If the viscosity and other desirable properties of the mud are inadequate, 

 considerable mixing of the cuttings occurs during transportation up the hole. 

 Another function of the drilling fluid is that of preventing caving and sloughing 

 from the wall of the hole during drilling operations. Caving is prevented by the 

 formation of a thin filter cake, composed of the solids in the drilling fluid, 

 which accumulates on the exposed walls at porous and permeable horizons, 

 together with the outward pressure exerted by the column of fluid in the hole. 

 When drilling mud fails to form an effective filter cake, soft formations absorb 

 large quantities of water, which causes them to swell and loosen and slough 

 into the upward-flowing mud stream. These cavings mix with the returning 

 fresh cuttings from the bottom of the hole and are collected and preserved with 

 the sample caught at the surface. At the surface the mud is often directed 

 through a series of settling pits where the chips fall to the bottom, and the 

 mud is circulated back into the hole. Occasionally the mud pits are jetted 

 violently in order to keep the solids thoroughly mixed, but in this process the 

 cuttings are also disturbed and some are picked up by the pumps and re- 

 circulated into the hole. These old cuttings mix with those freshly cut at the 

 bottom of the hole and reappear in the next sample. 



18 



