UNCERTAINTY ANALYSIS OF SUBSALT OVERPRESSURE DEVELOPMENT IN OFFSHORE LOUISIANA, GULF-OF-MEXICO

The confining pressure exerted by the prograding Tertiary clastic wedg e has caused extensive salt deformation in the Gulf of Mexico. The hig h rates of lateral salt motion from the Louisiana shelf break to the S igsbee Escarpment are expected to have significant impact on the dynam ic evolution of the surrounding sedimentary formations. 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 sh eers and structures offshore Louisiana. Based upon the estimated rates of salt has movement, the timing can be determined for insertion/depl etion of salt in the sediments as the salt has moved through, and by, sediments on its way basinward. An execution of a series of 1-D fluid/ flow compaction models has enabled estimation of the spatial variation in overpressure build-up with time; indicating that overpressure unce rtainty is about 10-20% about a mean of about 40 atmospheres, with unc ertainties on salt thickness and salt speed roughly comparable in impo rtance in contributing to the uncertainty. The quantitative behaviors suggest that the speed of lateral salt insertion and the thickness of the salt are the main factors causing anomalous overpressure buildup. A quantification of the dynamic behaviors is significant when modeling the timing of potential trapping of hydrocarbons beneath salt sheets and of the subsalt overpressure to be expected in subsalt drilling.