2024 M. KING HUBBERT

CONCLUSION

At the outset of the present paper we proposed to investigate the forces which cause petroleum to migrate and the characteristics of the positions in which it will become entrapped under the general conditions when the environmental ground water is in some state of motion. This we have done and our formulations reduce, as they should, in the special case for which the water is at rest, to the results with which we are already familiar: vertical and parallel impelling forces, and traps for both oil and gas in the spaces between downwardly concave impermeable barriers and horizontal surfaces.

If the water is in motion, however, in a non-vertical direction, which often is the case, our formulation leads to consequences which are by no means familiar. Oil and gas equipotentials are no longer horizontal but are inclined, with the angle of inclination of the equipotentials for oil greater than that of the equipo-tentials for gas. The paths of migration for oil and gas in the same space are no longer vertical, nor are they parallel, the paths for oil being deflected away from the vertical by an angle greater than that of the paths for gas. Likewise the traps for oil and gas no longer coincide and may in fact be separated entirely. In the latter event a trap for oil will not hold gas nor a trap for gas, oil; the fluids will migrate to their respective traps instead.

The oil- and gas-water interfaces will not be horizontal but inclined at an angle given by

dz dhtan 0 = =___________

dx Po dx

where po is the density of the oil or gas, respectively.Under such circumstances oil or gas entrapments will not occur in the con-

ventional positions. They may occur in anticlines in asymmetrical positions with the water high on one side and low on the other, or in completely unclosed structures such as noses or terraces, with the water flowing down the dip. Again, if the closing dip of an anticline in the downstream direction is less than the angle of tilt, this structure will not hold the specified fluid under the conditions prevailing.

These theoretical deductions have been confirmed experimentally, and the predicted phenomena have also been sought in the field. Not only have they been found, but the frequency of their occurrence has exceeded expectations, and in every major oil-producing area so far examined, hydrodynamic conditions in at least some reservoir formations, with oil-field tilts ranging from tens to hundreds of feet per mile, have been observed. We are thus led to the suspicion that many off-structure accumulations of oil and gas, which, on the basis of hydrostatic premises, have been classified as fault or stratigraphic traps, may in fact be hydrodynamic traps instead.

From such considerations it becomes evident that in the prospecting for petroleum in any area, as complete a knowledge as possible, in three-dimensional

ENTRAPMENT IT 'DER HYDRODYNAMIC CONDITIONS 2 0 2 5

space, of the ground-water hydrology is of importance comparable with a knowl-edge of the stratigraphy and the structure. If conditions can be demonstrated to be very nearly hydrostatic, then our customary procedures are appropriate; if hydrodynamic conditions prevail, it is important that these be determined in detail, stratum by stratum, over the given basin in order that the positions of the traps may better be determined.

For this purpose regional geology and topography constitute the initial and most readily available information. Next comes the information obtainable from wells of which the most informative are widely spaced wildcats. Water samples for analysis and density determination should be taken in such wells in every regional sand or permeable formation. In addition, in the same formations, accurate shut-in pressures, together with the precise elevation of the point of measurement, should also be taken. This information is essential for the computation of the po-tential or the head h„, by means of the equations

(D v, = gz —P.

h,„= z P —P w g

where p is the undisturbed pressure in the formation and z the elevation of the point of measurement.

The systematic assembling of data of this kind is appropriately a cooperative enterprise for the whole petroleum industry, and such data should be taken and exchanged between various groups in the same manner that well-log information is now exchanged. It will be found that our present procedures in taking pressure measurements in wildcat wells are inadequate, both as to frequency and accuracy. Since pressure measurements are most often made incidental to drill-stem tests, there is need for an improvement in the pressure measurements and procedures in making such tests. This includes both an improvement in the precision of pressure measurements, and also a change of the routine so that shut-in pressures may be taken prior to the drastic disturbance produced by the withdrawal of fluids, rather than afterward.

In the light of the evidence before us it appears essential that in addition to our customary procedures in petroleum geology, involving principally stratigraphy and structure, we must now add regional ground-water hydrology if many otherwise obscure accumulations of petroleum are not to be overlooked.

REFERENCES CITEDADAMS, JOHN EMERY, 1936, "Oil Pool of Open Reservoir Type," Bull. Amer. Assoc. Petrol. Geol.,

VOL 20, pp. 78o-96.BEAL, CARL H., 1917, "Geologic Structure in the Cushing Oil and Gas Field, Oklahoma, and Its Relation

to the Oil, Gas, and Water," U. S. Geol. Survey Bull. 658. 64 pp.CHAMBERS, L. S., 1943, "Coalinga East Extension Area of the Coalinga Oil Fields," Geologic Forma-