Porosity is of great importance in calculating the volume of oil in the sand, as by means of it a valuable approximation may be made as to the value of a field or property. Even in cases where decline curves are used in such valuation, the porosity should also be considered and the two methods used together.
Oil and gas might have been formed in the sand where they are found or again might have originated in the nearby strata and have migrated into the sand body.• The common conception is that the oil has migrated into the sand body, however, a few are of the opinion that it has originated in the sand. That move ments of oil and gas are possible are due to several factors. The weight of the overlying sediments will cause compacting and force the contents of a stratum out, through the road of least resistance, into the sandstones as they offer better chances for accumulations. The constantly increasing pressure of the over lying rocks due to further sedimentation may be one of the factors of motion (Fig. 14) as well as many others, such as the increasing temperature at greater depths, the formation and pressure of newly formed gases (dynamo chemical gases),* the effect of capillary attraction in the pores, hydrostatic pressure, the specific gravity of oil and water, and earth movements (diastrophism). One or a combination of these factors may cause migration. As to compacting, sandstones and limestones offer greater resistance than shales, therefore they are not so easily and firmly compacted, allowing for greater pore space.
The pressure of gases may be said to depend on various factors, and some of those that will cause migration may also produce or increase pressure. Thus, the formation of new gases will exert a certain amount of additional pressure, as well as the closing up of the pore-space due to compacting, or the addition of more cement, which will reduce the size of the spaces, compelling the same volume of gas to occupy less space. The
weight of the overlying strata exert pressure and hydrostatic pressure no doubt plays an important part in this connection.
Many, if not all these factors are required to explain pressure, and as most of these are not obtainable, to calculate possible pressure, only such methods are used that have been obtained from the study of actual conditions in the field, based mainly on the figures obtained from pressure readings, and those figures form the basis of the formula: that are in use. Only an approxi mate calculation is possible, and as it has been found that pressure and depth seem to have some relation to each other, this has been used for calculating pressure. The possible pressure may be approximated by multiplying the calculated depth of the sand from the average altitude by the weight of a column of water one square inch at the base, one foot in height, plus the addition of a constant. This constant can be determined only after a field has been partially developed, or the constant of a near-by pool may be used. The weight of a column of pure water as above mentioned is .43 pound, and that of salt water more, in some fields a weight of .57 has been found. A formula of Dr. Edward Orton, worked out for the Trenton fields of Ohic, uses .48 for the weight of this column of water, and his constant is .41 thus his formula may be expressed as follows: Pressure =0.48X depth from average altitude +41.
Methods like the above will give results that will be suffi cient for most purposes, but they only apply to the original pressure in the reservoir; after the pool is drawn upon the pressure becomes less.