690 Subsurface Geologic Methods 



and the porosity of these rocks generally is greater than 26 percent and 

 less than 47.6 percent unless plugging of pores also has taken place 

 during geologic time. 



The porosity of massive limestones and dolomites depends chiefly 

 upon two geologic processes: namely, a fracturing, jointing, or cracking 

 of the rock matrix resulting from relief of stresses caused by movement of 

 portions of the earth's crust and a development of such porosity as may 

 exist in the matrix by weathering, erosion, solution, and leaching by the 

 circulation of waters. Reduction of porosity by crystallization and deposi- 

 tion of minerals from solution in the pores of limestones and dolomites 

 also is a common geologic phenomenon. Inasmuch as these processes are 

 sporadic in nature, in contradistinction to the orderly process of deposi- 

 tion by sedimentation in open water, the resulting porosity of limestones 

 and dolomites likewise may be greatly variable with respect to size, 

 uniformity, and continuity; and the interpretation of analytical data 

 obtained by systematic sampling of cores is difficult and subject to un- 

 certainty. In general, pores developed by separation along planes of 

 crystallization and by joint fracturing are of a distinctly diff"erent order 

 of dimensions than the pores developed by erosion, Assuring, leaching, 

 and solution. Consequently, the average porosity of a limestone or dolo- 

 mite may depend chiefly upon one or the other type — ^or both may be 

 significant. Therefore, the absolute porosity of a piece of limestone or 

 dolomite may be great or little, and the magnitude of the porosity is little 

 indication of whether the pores are intergranular or chiefly solution chan- 

 nels. The types of porosity predominating, or those which are insignifi- 

 cant, should always be determined whenever possible and stated along 

 with quantitative data for the porosity of limestones and dolomites. 



Porosity is of economic importance because it is a measure of the 



ability of rocks to contain fluids. A porosity of one percent is equivalent 



to a total pore volume of 77.58 barrels of void space per acre-foot of 



bulk volume. This useful conversion factor is of course obtained from 



the fact that the bulk volume of a rock one acre in area and one foot 



thick is 43,560 cubic feet. Only one percent of this volume is pore space, 



and a barrel is equivalent to 5.615 cubic feet, so that total pore volume 



, (43,560) (0.01), ^ , ^ ^ . 



01 the rock = barrels per acre-toot per percent porosity. 



5.615 



This conversion factor makes possible the writing of the general formula 

 Total pore volume in barrels = 77.58 at (100/) in which (3) 



A is area, acres 



t is thickness of formation in feet, and 



/ is the fractional porosity 

 The relative extent to which the pores of a rock are filled with spe- 

 cific fluids, i.e., oil, water, and gas, is expressed best as percentage satura- 

 tion of the pores. If the pores are filled with only one fluid, for example 

 oil, the saturation with respect to that fluid, oil, is 100 percent; if the 



