Isostacy Again Explanation of Salt Movements
Authors: Garnett M. , Kodagaii V.N. , Sankar S. Jai 
Published: October 2008

The notion of isostacy is applicable to explain vertically-rising salt movements in those situations with large lateral extrusion of salt tongues. A salt tongue maybe regarded as occurring at that depth within the earth above which the average density, regardless of lithology, is equal to that of salt, i.e., the salt tongue ‘balances’ the material above it. The supra-salt tongue section can be salt diapirs or sedimentary sequences. Accumulating sediments compact and increase in their density with depth until equaling the density of the plastic ‘balancing’ salt tongue, Under the heading of isostacy, with the balancing horizon being the salt tongue, the salt tongue is at the depth range where salt becomes buoyant relative to the overlying sediments. The isostatic depth/buoyancy level could then direct the advancing position depth of the salt tongue in the basin.

Initial sediment deposition has low density (1.5 -1.9 g/cm 3), thickness, and mechanical strength (Young's modulus). Underlying salt, nearly plastic under compression as measured over geologic time, will rise prior to isostacy buoyancy being reached, as seen from pene-contemporaneous deformation in western Louisiana offshore, Red Se% southeastern Brazilian margin, and Caspian Sea.

Computer modeling of excess pressure under moving salt tongues indicates pressure build-ups of some 170 atm (+/- 30 atm uncertainty). The excess pressure may buildup geologically ‘instantaneously’ as the laterally migrating salt over-rides another column of sediment. Presumably the excess pressure ‘evaporates’ as a discrete salt tongue leaves a supporting underlying sediment column. The hydraulic jack is an apt analog: instantaneously increasing and decreasing pressures as a load is applied and removed, respectively. The rise in pressure is an elegant testimony to the function of salt tongues as discrete units containing and distributing overlying stresses, the same function as that of an isostatic layer. A puzzling question concern show noses of salt tongues approach and even intersect the seafloor.

Determining geologic reasons for positioning of salt tongues within terrigenous sediment complexes along passive margins is important due to major changes that salt insertion causes: impermeable barrier to rising hydrocarbons, stress fractures around advancing salt noses, possible regional faulting due to stress couple developed between dynamic salt tongue and stationary ‘basement’. Predicting potential stresses and deformation above, in flont of, and below, a salt tongue is essential to successful wildcat drilling.

INTRODUCTION

The purpose of this manuscript is to explore geologic reasons for the emplacement of the Louisiana salt wedge and its various salt features along the Louisiana offshore, There are keen scientific and exploratory demands for such an exercise. As the sub-salt petroleum exploratory play enters a more mature phase, there is urgency to understand why the increasingly well-defined salt (as 3-D seismic improves resolution and becomes more ubiquitous) is found where it is.