Miscellaneous Subsurface Methods 



689 



non-spherical particles such as rods or plates are mixed with the spheres, 

 pores can either be partially filled or bridging can result in the formation 

 of even larger pores with the result that porosity would tend to be either 

 decreased or increased, depending upon which phenomenon predominates. 

 The resultant effect of both factors usually is such that the porosity of 

 oil-bearing clastic sedimentary rocks is much lower than the maximum 

 of 47.6 percent for wide-packed spheres of equal diameter because of 

 the effectiveness of plugging and the ability of forces of compaction to 

 minimize bridging. 



The clastic sediments are differentiated by names somewhat according 

 to particle size and include the gravels, sandstones, shaly sandstones, 



Cubic or wide packed 



Rhombohedral or close packed 



Figure 371. Unit cells and groups of uniform spheres for cubic and 

 rhombohedral packing. (After Graton and Fraser, Jour. Geology.) 



and clays. The porosity of the finest grained sediments, such as the muds 

 and oozes which eventually become clays and shales, originally was high 

 (40 to 85 percent) because of the great degree of hydration of these tiny 

 particles when deposited. Prolonged compaction under the great weight 

 of overlying sediments reduces porosity and decreases permeability of 

 these rocks, sometimes almost to the vanishing point. The porosity of 

 shale, for example, decreases exponentially with depth, according to 

 Athy.^^ The coarser grained sediments, sands, sandstones, sandy shales, 

 and oolitic limestones, were better able to resist forces of compaction; 



^''Athy, L. F., Am. Assoc. Petrol Geologists Bull., vol. 14, no. 1, pp. 25-35, 1930. 



