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The common occurrence of hydrodynamic conditions causing tilted oil-water contacts, especially in the Rocky Mountain area, requires the use of new exploration and prospect-evaluation methods. The physical principles of oil entrapment under hydrodynamic conditions are easily understood when explained qualitatively by means of vector-force diagrams. That is, the buoyancy of oil is a vector force acting in a vertical direction, the movement of water in the aquifer produces a vector force acting in a direction approximately parallel with the bedding plane, and the oil-water contact is perpendicular to the vector sum of these two forces. This combination of forces, resulting in a tilted oil-water contact, may (1) cause oil production to extend in the gradient direction beyond he previously assumed limiting contour, (2) force the oil pool away from the highest point of the structure and to the position of minimum potential energy on the flank or nose of the structure, or (3) tilt the oil pool completely out of the structure, leaving it barren of oil production.
When the pertinent forces and structural features are properly scaled, a three-dimensional plastic flow model can be used to study the location and distribution of oil in the structure. In addition, these hydrodynamic scale models can be used to study the effect of local geological factors (faulting, permeability changes, and sand thickness changes) on the hydrodynamic gradient and consequently on the location of the oil pool in the structure.
Electrical and mechanical potentiometric models have been constructed to make regional studies of the potentiometric surface and to evaluate the average hydrodynamic gradient in any specific area of interest. Careful analysis and evaluation of all available data are critical requirements for this study.
It is therefore concluded that in the Rocky Mountain area hydrodynamic gradient analyses must be integrated with structural mapping and stratigraphic studies to properly evaluate all prospects and to find new prospects including nose or flank possibilities on structures previously condemned by dry holes.
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