Estimation of Formation Pressures From Electrical Surveys-Offshore Louisiana

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Abstract
Discussion: Background Theory: Origin of Abnormal PressuresThe origin of abnormal pressures has been discussed by numerous researchers and several theories, well summarized by Watts in 1948, have been advanced to explain such pressures. The generally accepted theory explaining the occurrence of abnormally high pressures in the offshore area is paraphrased below. Consider a layer of sediments deposited at the bottom of the sea in a fluid environment. As more and more sediments are added above this layer, the grains of the sediments will pack closer together and accept the added weight of the overburden. Some of the water within the pore spaces will be squeezed out by the compaction. Since the water is continuous with (or in communication with) the water in the sediments and sea above, the pressure with-in the water is hydrostatic. As long as this process continues through geologic time so that water in the pore spaces is free to move out with the addition of overburden weight, the pressure of the fluid in the pores will remain hydrostatic or normal. As reported by Cannon and Craze in 1938, the normal pressure gradient in the Gulf Coast region approximates 0.465 psi/ ft, or the weight of a column of water with 80,000 ppm salt. JPT P. 165ˆ

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