ABSTRACT

The confining pressure exerted by the prograding Tertiary clastic wedge has caused extensive salt deformation in the Gulf of Mexico. The high rates of lateral salt motion from the Louisiana shelf break to the Sigsbee Escarpment are expected to have significant impact on the dynamic evolution of the surrounding sedimentary formations including fluid pressure. The evolution of fluid pressure in supra- and subsalt sediments has been modeled in a series of pseudo-wells along a profile intersecting numerous salt sheets and structures offshore Louisiana. Based upon the estimated rates of salt movement, the timing can be determined for insertion/depletion of salt in the sediments as the salt has moved through, and by, sediments on its way basinward. A series of 1-D fluid-flow models incorporating compaction enables estimation of the spatial variation in overpressure build-up with time. The models indicate that overpressure uncertainty is about 10-20% around a mean of about 200 atmospheres (1 atm 14.7 psi 0.1 MPa), with uncertainties on salt thickness and salt speed roughly comparable in importance in contributing to the uncertainty. The quantitative behaviors suggest that the speed of lateral salt insertion and the thickness of the inserted salt are the main factors causing anomalous overpressure build-up. Quantification of the dynamic behavior is significant when modeling both the timing of potential trapping of hydrocarbons beneath salt sheets and the sub-salt overpressure to be expected in sub-salt drilling.