The A is the number of acres in the reservoir. This factor requires the most 

 careful and detailed kind of subsurface geology. Some important information 

 is almost always lacking, a fact which requires the most thoughtful consideration 

 of what is known and the drawing of the most reasonable inferences. It has 

 been said by some of our most eminent valuation experts that properly dimen- 

 sioning the reservoir is the greatest single source of error in reserve estimates. 



The T is the thickness of the producing measure. This factor is ordinarily 

 arrived at from electric logs, supplemented by such coring or cuttings and core 

 analysis as it obtained from that property or any related property. This pro- 

 cedure always involves the picking of the top of the formation, frequently in- 

 volves the location of a gas-oil contact, and usually requires the selection of an 

 oil-water contact. Experience in the general area of the property is of com- 

 manding importance in the making of these various judgments. There is much 

 to know that is not written in books. An opportunity to see many wells cored 

 and logged, to produce them, and to be intimately acquainted with their entire 

 history — this type of experience, to which geologists with major oil companies 

 are daily exposed, is the best possible teacher in such matters. 



The P in the formula is the porosity. Frequently no information is avail- 

 able from the particular wells — then an estimate for normal porosity in that 

 formation at that depth is used. Often a little core analysis at one or two points 

 in the producing horizon is available — this fact is carefully placed on the log 

 and an effort made to realize whether the data represent normal porosity or 

 whether the cores were from a good streak, or perhaps a tight part of the horizon. 

 Often only porosity data from other scattered wells in the field are available — in 

 such cases an educated guess is made and porosity is assumed to be the same 

 under the entire property. 



Although a rock may be porous, it also must be permeable to be a producible 

 petroleum reservoir. If sufficient permeability (the continuity of the pore spaces, 

 which permits passage of fluids through the pores) does not exist, the fluids in 

 the porous reservoir cannot be produced — or they can be produced only so slowly 

 that they are not commercial. 



Fracturing may contribute to the porosity of limestones. It is notoriously 

 hard to recover cores from lime, and usually cores give no information as to 

 total amount of fracturing. The amount of fracturing is sometimes estimated 

 by computing what the capacity of the well should be from its total section, bore 

 hole, porosity, and permeability — then attributing any excess capacity to 

 fractures. (Engineers can compute anything if you let them make a few innocu- 

 ous assumptions while they get their slide rule out of the case.) Cores taken 

 with a diamond bit quite often result in the recovery of fractured cores intact, 

 so that estimates of fracture porosity can be made. 



Measurement of porosity in the now popular reef, conglomerate, or other 

 "heterogeneous" reservoirs introduces new problems. The microlog shows 



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