Valuation and Subsurface Geology 797 



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 per- 

 haps 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 con- 

 tinuity of the pore spaces, which permits passage of fluids through the 

 pores) does not exist, the fluids in the porous reservoir cannot be pro- 

 duced — or they can be produced only so slowly that they are not com- 

 mercial. 



Fracturing may contribute to the porosity of limestones. It is no- 

 toriously 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 any- 

 thing if you let them make a few innocuous "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 esti- 

 mates 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 promise of enabling one to make a good estimate of the amount 

 of permeable zone in such a section, although the percentage porosity still 

 must be obtained by coring or estimation. If one has micrologs nearby, 

 he can pretend that the section will be the same elsewhere. If one has only 

 electrical logs, he just does his best, remembering that assuming 50 per- 

 cent of the reef to be porous seems to be about the average used. 



Porosity in producing fields varies from^ 2 to 40 percent, with the 

 average being between 20 and 25 percent. It is a great variable. The 

 porosity between wells is not known, even though one may have measured 

 it in cores from the producing section of the wells. It is always assumed 

 that the well bores are representative of the reservoir. Performance of 

 the wells themselves is often some help — good, high-capacity wells are 

 not made in thin, nonporous sections. 



Connate or interstitial water (denoted by / in the formula) is non- 

 producible water contained in the reservoir rock along with the oil and/or 

 gas. It is thought that the formations, when laid down, contained seawater 

 in the available pore spaces and that the reservoir rocks retain this seawater 

 (sometimes called fossil water) until it was driven out by the inmigrating 

 gas and oil. The gas or oil displace most of the sea water, but not all 

 of it — some of it is held by capillary forces and remains as a coating of 

 water around each sand grain. Producing formations nearly always con- 



