Some Theoretical Considerations of Tilted Water Tables

An analysis has been made of the factors responsible for tilted fluidcontacts in petroleum reservoirs. The factors are static and dynamic with theformer being controlled by those variables responsible for the capillary riseof fluids, and the latter include rate of formation tilt and hydrauliccirculation. The explanation that rate of formation tilt may be responsible fortilted contacts appears untenable. The role of some of these factors in themigration, fractionation, accumulation, and possibly the prospecting for oil isindicated. Introduction While it is generally considered that the segregation of gas, oil, and waterin a petroleum reservoir results in approximately horizontal planes separatingthese phases, there are factors which modify this simplified picture to a verygreat extent. Some of these factors are static, and the departure of the fluidinterfaces from horizontal may represent a static equilibrium. Some of thesefactors may be dynamic and the interface positions may be non-equilibrium andrepresent a steady state condition. Of course combinations of these twosituations may also exist. The reasons most commonly given for this behaviorare hydraulic circulation of fluids, change in character of the formation, anda rate of tilt of the formations more rapid than the rate at which the fluidsmay re-establish an equilibrium. Russell's Principles of Petroleum Geologycontains an extensive discussion of tilted fluid interfaces and theirimportance in the migration and accumulation of petroleum. Russell citesexamples of tilted water tables at Lance Creek, Wyo.; Rock River, Wyo.; CutBank, Mont.; Cushing, Okla.; and Graham, Okla. Also given was the Wasson Fieldof West Texas which has saucer shaped oil-water and gas-oil interfaces with thesaucer being deeper in the case of the oil-water contact. The Bradford Field inPennsylvania has a tilted water table. San Ardo, Coalinga Nose and some otherCalifornia fields have tilted contacts. A recent paper by King Hubbertdiscusses the effect of hydraulic circulation of the water underlying the oilin a given stratum and its implications in the accumulation of and drilling forthe oil. The foregoing would indicate that tilted fluid interfaces are not uncommonand that it would be of importance to understand the forces which areresponsible. Further, it would be of interest to examine certain idealizedmodels of situations which may cause tilting with the thought in mind ofderiving relationships between the important variables. These relationships mayonly hold exactly for the models assumed but the results may applysemi-quantitatively to actual formations and at least indicate trends. Thediscussion will be divided into the effect of static factors and the effect ofdynamic factors. T.P. 3564