604 russell-dickey. POROSITY AND PERMEABILITY [Ch. 32 



cept of packed spheres. The experimental data given above show that 

 natural reservoir rocks depart considerably from this ideal. The less 

 consolidated sands, such as the Woodbine (Upper Cretaceous), ap- 

 proach it most closely, while the hard, calcareous Weber (Pennsyl- 

 vanian) diverges widely. The divergences are caused by petrographic 

 textures in the rocks that make the pores very different from the in- 

 terstices between spheres. 



Gkoss Dimensions of the Reservoirs 



It was once generally believed that the normal hydrostatic pressures 

 encountered within the reservoir when it is first tapped by the drill 

 were maintained by meteoric water percolating down from the out- 

 crop. The behavior of a great many pools, however, shows that there 

 is seldom continuity through the aquifer, or water-bearing portion of 

 the porous reservoir, to the outcrop (Bugbee, 1943). Oil occurs very 

 commonly in sands of limited lateral extent. Even wide, sheet-like 

 sands are generally interrupted by faults, if not stratigraphic discon- 

 tinuities, between the pool and the outcrop. It appears that the en- 

 closing strata, although they are impermeable to oil and gas, have a 

 small but finite permeability to water. Thus, as the overburden is 

 slowly increased by sedimentation or removed by erosion, normal 

 hydrostatic pressure is maintained in the system by the addition or 

 withdrawal of small amounts of water. 



Bedding 



Beds and laminae of shale, or even very thin micaceous partings, 

 greatly reduce the permeability of a sand body to fluids. These lam- 

 inae impede the flow in any direction not parallel to the bedding in 

 nearly all sandstone reservoirs. In the process of water-flooding by 

 the injection of water into input wells, the behavior of the producing 

 wells strongly indicates that a sand body acts like a large number of 

 small, independent reservoirs of differing permeability. The most 

 permeable bed is flushed of its oil and starts to produce water, and 

 then the other units successively produce oil followed by water. The 

 least permeable beds may produce oil 15 or 20 years after the more 

 permeable strata are flushed. 



Appreciable permeability normal to the bedding is more common in 

 limestone than in sandstone reservoirs. Fractured and cavernous zones 

 generally occur as stratigraphic units, but the fractures and caverns 

 themselves may transect the bedding planes. In certain pools in Mex- 

 ico, west Texas, and the Middle East, the oil occupies large open cav- 

 erns or fractures underground. Certain oolitic lime pools also have 



