Miscellaneous Subsurface Methods 691 



pores are only half filled with oil, the oil saturation is only 50 percent. 

 Consequently, the total barrels of oil in place at reservoir pressure and 

 temperature may be computed from the relation 



Total oil content = 77.58 at (100/) (O.OlSo) (4) 



in which So is the average-percentage-oil saturation of the rock. Obviously, 

 if the total oil content is desired in barrels of stock-tank oil, the oil- 

 content at reservoir conditions obtained from equation (4) should be 

 multiplied by an appropriate shrinkage factor based upon the particular 

 change in pressure and temperature appropriate to the situation. 



Equation (4) may be used to obtain the total water content at reser- 

 voir conditions for a particular bulk volume of rock by substitution for 

 So of the numerical value of S^, the percentage-water saturation of the 

 rock appropriate to the particular situation. 



The free-gas content in cubic feet at reservoir pressure and tem- 

 perature may be estimated from the relation 



Total free-gas content = 43,560 atf [1.00-0.01 (So + 5^)] (5) 



The total free-gas content at reservoir condition from equation (5) can 

 be converted to cubic feet of gas at standard conditions by use of the 

 ideal gas laws corrected for deviation therefrom. An apropriate multi- 

 plier accomplishing this objective is 



PrTi, 

 PbTrZ 



in which 



p,- is the reservoir pressure, psia 

 Pt, is the base pressure, psia 

 Tr is the reservoir temperature, °F absolute 

 Tt, is the base temperature, °F absolute, and 

 Z is the compressibility factor at Tr and pr, no unit. 

 A commercial oil-bearing rock must contain sufficient petroleum to 

 warrant development and exploitation. Consequently, the minimum eco- 

 nomic porosity of sands and sandstones is 8 to 10 percent (621-779 bar- 

 rels per acre-foot), and the maximum porosity seldom exceeds an 

 average of 35 percent (2715 barrels per acre-foot) . The porosity of com- 

 mercial oil-bearing massive limestones and dolomites extends over a 

 much larger range of values than that of sandstones. Some limestones 

 and dolomites are so cavernous and solution chasms and vugs therein so 

 large that the porosity becomes so great locally that statistical implica- 

 tions of porosity become meaningless. If, however, the porosity of oil 

 and gas-bearing limestones and dolomites results chiefly from inter- 

 granular interstices, the porosity can be as little as four to six percent 

 and still have commercial value under favorable conditions. Obviously, 

 the pores of rocks containing oil and gas must be interconnected suffic- 

 iently well to permit fluids to pass from one pore to another for an oil- 

 and gas-bearing rock to have commercial value. Some pores or in- 



