Permeability of reservoir rocks

The permeability of a reservoir rock is a measure of the resistance offered to movement of fluids through its pore spaces. Although the size of the grains composing a sand or sandstone if uniform, does not influence the storage capacity, the size of the pore spaces between grains will determine what fluids may enter and will have a marked effect upon the resistance offered to movement of fluids through the rock. Oil is less readily absorbed by rock pores than water and flows with greater resistance. Natural gas, although entering the rock pores more readily than water, may nevertheless be displaced by the latter in a fine-grained sandstone as a result of capillary action.

Productive Zones. – Frequently a formational interval of considerable thickness perhaps hundreds of feet of oil bearing strata, will be included within the productive formation. Seldom, however, will all the beds so included yield oil to the wells. A productive zone may be made up to lenticular bodies of sand and shale, some component members being productive, others not. Thus, a productive zone may comprise several or many individual reservoirs. Also, there may be several separate zones of production in a given structure, one below another and separated by continuous, impermeable strata, and the several zones do not necessarily have the same lateral expanse.

Impervious Cap Rocks. Cap rocks overlaying productive reservoir rocks are almost invariably argillaceous in nature-clay, shales or marls. Such rocks, in addition to being impervious to oil, do not fracture readily when bent into anticlinal folds. The thickness of the cap rock is often only a few feet, though it must be sufficient, together with the pressure of the overlying rjck masses, to withstand the fluid pressure in the underlying reservoir rock if it is to be effective in retaining oil and gas.