Interactive salt and sediment evolution: self-consistent quantitative models

Abstract

Assessment of the combined evolution of salt and surrounding sediments is one of the major problems in salt-dominated basins. A quantitative approach, guided by observations from seismic and/or well data, is presented. The modeled present-day shape of a salt structure is constrained by an inverse non-linear procedure which determines the parameters controlling the salt shape. The non-linear procedure guarantees the shape to be in close correspondence with the observed present-day salt shape. By introducing time dependence of each of the parameters controlling the salt shape, the procedure permits an assessment of an evolving salt shape with time. By letting sedimentary beds move around the developing salt structure, the influence of the uprising salt on the evolving bed geometries above and around the salt is depicted. As the predicted present-day bed geometries must also be in accord with the observed rim syncline bed geometries, additional constraints are put on the parameters controlling the dynamic evolution of the salt structure. The inverse procedure automatically determines, in a self-consistent way, resolution and sensitivity of all parameters in the quantitative model, and also of the dynamic evolution of both the salt and the surrounding sedimentary formations. The procedure handles symmetric and asymmetric two-dimensional structures as well as symmetric three-dimensional structures. Applications are given to two salt structures in the North Louisiana salt basin. The first structure is a tall diapir with clearly defined rim synclines. By varying the parameters in the non-linear inverse procedure, a consistent reproduction of the observed data is obtained on both sides of the asymmetric diapir. In addition, in the regions where absence of data is greatest, the consistent model provides a measure of the degree, extent and location of allowed salt overhang in order that the observed and predicted salt shapes are in accord. Therefore, the procedure produces a quantified measure of the salt stock stem width to overhang width and the locations of both. The second structure consists of two salt pillows. Here the quantitative model provides a consistent picture of the dynamic shaping of each mound in relation to their common rim syncline. A synthetic case exemplifies the capability of modeling the combined salt-sediment evolution with the constraint that the depositional surfaces through time are horizontal. The modeled self-consistent dynamical evolution then permits a better understanding of evolving stress-strain, thermal focusing and hydrocarbon entrapment to be realized.