Buildup Of Overpressure Beneath Offshore Salt Sheets:Case Histories And Models

Abstract

ABSTRACT The presence of a salt sheet in the subsurface provides a permeability barrier to the flow of fluids, which can lead to the development of an overpressure condition and slower rate of compaction for formations underlying the salt sheet. These effects are demonstrated through (a) numerical simulations and (b) observations from four wells in the Gulf of Mexico which penetrated salt sheets and tested the underlying formations, The numerical models simulate the time-dependent fluid flow in a sedimentary basin containing a salt sheet; these models predict a significant increase in excess pore pressure in sub salt formations relative to the regional trend and also relative to the formations overlying the salt The well data confirm the model results. Based on analysis of sonic log, density log, and mud weightdata the sub salt formations in three of the four wells are determined to be extremely over pressured while the fourth well is also highly over pressured, but not to the same extent. This work shows that pore fluid pressure constitutes a significant risk component in assessing a sub salt prospect and needs to be analyzed carefully before drilling. INTRODUCTION Owing to the impermeable nature of salt, saltbodies have long been recognized as providing an excellent sealing mechanism for trapping hydrocarbons in the subsurface. While numerous oil and gas fields have been discovered on the flanks of salt structures, other salt configurations also offer the potential of trapping significant hydrocarbon accumulations. In recent years the formations underlying buried salt sheets have received some attention and present an exciting exploration play. While a salt sheet may provide an efficient trapping mechanism, it may also affect thephysical properties of the surrounding formations, thereby impacting the economic risk associated with a sub salt prospect. In particular, salt sheets provide a permeability barrier to the flow of fluids in the subsurface, which condition may lead to overpressure in the underlying formations. The degree of overpressure may affect the economics of a sub salt prospect as well as influencing the drilling program, and needs tobe addressed in evaluating the prospect. In this paper the question of the pressure development in the sedimentary section underlying a buried salt sheet is considered. The approach consists, firstly, of modeling the evolution of a sedimentary basin which contains a salt sheet. The numerical models provide an understanding of the physical processes which are at work and provide gauges for the ensitivity to various controlling factors. Secondly, data are analyzed from wells whichhave been drilled through salt sheets in the Gulf of Mexico and which tested the underlyingformations. These wells provide a "ground truth" comparison to validate the numerical models. Combining the results of numerical modeling and well data analysis yields a picture of the fluid pressure distribution underlying aalt sheet and of the factors which control this distribution.