Stress, Strain, and Fracturing Around Self-Consistent Models of Salt and Sediments
Authors: Petersen K. , Lerche I. 
Published: October 2008

As salt diapers and salt sheets move through sediments, both the sedimentary horizons being deposited as well as Pre-existing sedimentary beds are distorted by the moving Salt and, in turn, influence the shape of salt bodies. Self-consistent Models have been constructed to examine this Interactive situation. The model behaviors include Sedimentary strain and associated stress of the distorted Beds, so that rock failure under Mohr's criterion can be evaluated for timing and spatial location of the sedimentary Beds in relation to the evolving salt. In this way likely locations of fractured formations and their relation to the Timing of hydrocarbon generation and migration can be Evaluated in order to increase the chance of finding Commercial oil and gas in sub-salt prospects. Two Examples are presented to illustrate the diversity of Fracturing patterns that can arise dependent on sediment deposition rates, mechanicall strength of sediments, and on Salt motion and shape changes with time.

The main results indicate that around and under the Moving "nose" of a salt overhang is the dominant ovations for shear failure of contiguous sedimentary beds. Secondary locations for failure are associated with Withdrawal of the salt from rim syncline areas and infill of Sediments into such regions.

The ability to set the timing and location of rock failure in Relation to evolving salt masses and sedimentary distortion Provides information on likely sub-salt fracture permeability And on likely leakage of hydrocarbons into and out of such Fractured formations. This relative timing pattern then Improves the chances for risking sub-salt hydrocarbon Prospects.

INTRODUCTION

An inverse quantitative procedure (Larches and Petersen,1995) allows for estimation of the interactive evolution of Salt and sediments through time. The larger the number of Sedimentary beds, the larger the number of time steps Required and the closer the changing shape of the salt Surface can be modeled in time. Accordingly, the deformation of the surrounding sediments can be followed with the same accuracy. This capability provides the Option of inferring and following the strain in the formations with time, unlike the majority of mathematical models that Use the estimated stress (based on assumptions of the Behavior of the dynamical system) to infer the resulting deformation in the sediments. The deformation of rocks, Whether the deformation is permanent or not, and the Mode of deformation (for instance folding and/or faulting) Depend on the interaction behaves a number of physical and chemical factors such as fluid pressure, rates of Deforming processes, rock composition, cementation, Temperature, etc. The stresses that generated the Deformations in the formations can themselves then are Inferred through application of the theory of elasticity or Any other proposed connection between strain and stress; the state of stress during time of salt rise can thus be Estimated. This approach was presented by O'Brien and Lerche (1987) where draping of sediments around a simplified salt shape was examined.

The progressive deformation of sediments in the vicinity of a rising salt structure can be any combination of an overall translation of a sediment unit.