QUANTITATIVE MODELING OF SALT AND SEDIMENT INTERACTIONS - EVOLUTION OF A NORTH LOUISIANA SALT DIAPIR

The restoration of sections involving salt structures is complicated, because it is often difficult to understand fully the dynamics of the system. The occurrence of hydrocarbons is often associated with traps in the vicinity of or directly adjacent to, salt structures; different methods to delineate the combined evolution of salt and sediments hav e therefore been developed. Because dynamical parameters are rarely kn own more accurately than within an order of magnitude, intrinsic uncer tainties are introduced in any model based on assumptions concerning t he dynamical behaviour of salt and sediments. A quantitative approach is presented here, guided by observed geometries of the salt-sediment interface and of the bed horizons from seismic and/or well data. This novel procedure considers the salt-sediment system as composed of geom etric shapes that are allowed to vary with time when calculating the s alt-sediment evolution. By letting sedimentary beds move around the de veloping salt structure, the influence of the uprising salt on the evo lving bed geometries above and around the salt is depicted. Because th e predicted present-day bed geometries must also be in accord with app lied control criteria (such as the change in salt volume with time, th e geometry of the depositional surface with time, and the compaction h istory), additional constraints are put on the parameters controlling the evolution of the salt shape. The capability of being able to follo w the changing bed geometries with time allows the strain in the sedim ents through time to be traced The stresses that generated the deforma tions in the sediments can then be inferred through application of the theory of elasticity. Furthermore, the combined evolutionary picture may be used to assess thermal ''focusing'' through time by a highly co nductive salt structure, because the salt and bed geometries are known at each instant. A case history from the North Louisiana Salt Basin s hows aspects of different combinations of control criteria on salt-sed iment evolution. Information is thus provided on the importance of eva luating the data and control criteria necessary for constraining the p ossible evolution histories. It is suggested that the novel inverse pr ocedure presented here will improve understanding of the combined evol ution of salt and sediments, and is therefore of significance in hydro carbons exploration.