Miscellaneous Subsurface Methods 687 



The solution of this problem is a goal toward which petroleum geologists 

 and engineers are striving to arrive analytically and experimentally. 



Degree of Consolidation 



The sedimentary materials found in petroleum reservoirs range in 

 degree of consolidation from loose, unconsolidated materials to hard, 

 competent, and even very dense sandstones, quartzites, dolomites, and 

 limestones (oolitic and massive), the extreme conditions occuring less 

 commonly — dolomite and limestone excepted. Generally, some 90 percent 

 or more of the granular material in a true sand or sandstone is quartz; 

 and, consequently, the physical properties of quartz contribute greatly to 

 the resultant. Usually something less than five percent of the remaining 

 material is composed of feldspar, heavy minerals in considerable variety, 

 and micaceous minerals. Clay minerals, shale, silt, and calcareous ma- 

 terial usually make up the remaining five or more percent which usually 

 also serve as a cement which binds together the component particles of 

 a consolidated sedimentary rock. For example, the common bonding 

 agent in the Gulf Coast reservoir rocks is clay and, more specifically, 

 usually is the clay mineral, illite.^ '^ Those sandstones, containing five per- 

 cent or somewhat less of clay minerals or shale and little or no calcareous 

 matter, also may be consolidated and can be clean in appearance. The 

 conclusion may be drawn that something less than five percent of clay 

 or shale is the minimum amount required for consolidation. Generally 

 more than five percent of these materials occur in sandstones, the amount 

 being large in so-called "dirty" sandstones. Loose sands and relatively 

 unconsolidated sandstones frequently contain more than five percent of 

 these bonding agents, the amount being larger in "dirty" sandstone. 



Unconsolidation must be attributed to some diff"erence in geo-history 

 as well as to some difference in physical properties of the materials. The 

 cementing material in a rock is one of the more important factors which 

 affect the behavior of fluids therein. 



Structural Configuration 



The shape and size of sand grains range between wide limits. The 

 component particles usually possess the approximate form of parallelepi- 

 peds having various axial dimensions with rounded sides and corners. The 

 particles range in size from small pebbles to that characteristic of fine- 

 grained silt. A preponderance of either extreme in a sand or rock is un- 

 usual. Screen-analysis data for various sands may be found in the litera- 

 ture.^^ ^^ ^^ The diversity and magnitude of sizes reported in the literature 



" Fancher, G. H., and Oliphant, S. C, AIME, Trans., vol. 151, pp. 221-232, 1943. 



*" Fancher, G. H., Lewis, J. A., and Barnes, K. B., The Pennsylvania State College, Mineral Indus- 

 tries Experiment Station Bull. 12, pp. 65-171, 1933. 



" Halbouty, M. T., Oil Weekly, vol. 83, no. 11, pp. 21-24; no. 12, pp. 22-26; no. 13, pp. 36-48, 1936; 

 vol. 84, no. 1, pp. 34-50; no. 2, pp. 36-46; no. 3, pp. 36-42, 1936. 



" Muskat, Morris, The Flow of Fluids through Porous Media, McGraw-Hill Book Company, 1933. 



" Muskat, Morris, Physical Principles of Oil Production, McGraw-Hill Book Company, 1949. 



