Assessment of Vertical Salt Motion From Analysis of Footwall Bed Geometries

ABSTRACT Salt flow and gravitational gliding interact in the development of growth faults. The accumulation of salt below growth faults influences the geometry of the fault plane and therefore the footwall bed geometry. The deformation of footwall beds is measured using displacement maps which are likely to show an overall semi-elliptical pattern of the lateral variation in vertical displacement. Attributing the measured deformation to a combination of vertical salt motion, differential compaction and differential subsidence, the vertical salt growth can be calculated. The presented procedure is applied to analyze the Tertiary growth history of the salt underlying the D-1 fault, Danish North Sea. The method can be tailored to the delineation of the genesis, history and structure of fault-related hydrocarbon plays and to an evaluation of the potential production on associated prospective traps. INTRODUCTION In the last decade many papers have dealt with the analysis of the geometry of normal faults. A series of papers published by the Fault Analysis Group at the University of Liverpool have shown that the displacements along fault surfaces tend to vary in a systematic and predictable manner. By posting measured displacements along a fault surface on a vertical plane parallel to the mean fault strike, contour displacement diagrams can be constructed 1,2,3. These diagrams enable constraints to be put on the interpretation of fault geometries. The study of the deformation of sediments in the vicinity of normal faults has been the subject of many experimental and analytic/numerical studies. In general the deformation analysis has been restricted to the investigation of the deformation of the sedimentary beds in hangingwalls 4,5.6,7. Despite the often observed deformation of the footwall, and therefore also of the fault plane, the footwall has generally been assumed to be rigid in the published fault models. Salt/shale-related normal faults are common observed structures in many deltaic depositional environments such as the Niger Delta and the Gulf of Mexico. Structural traps caused by tectonic, compactional, diapiric and gravitational processes constitute the major reservoirs for already discovered oil/gas reserves. In the Gulf Coast Basin major hydrocarbon accumulations are related to growth faults and salt structures 8. Many growth faults have later been deformed by salt diapirism as discussed by 8. In the northern North Sea about 70 % of the hydrocarbon finds are in fault block traps 9. The structures of the traps are varied. The traps occur in both hangingwalls and footwalls with the most complicated traps located in the Central Graben area where the structures are strongly affected by motion of the underlying Zechstein salt. The footwall uplift is often followed by erosion of the footwall and a cut down of possible reservoir rocks. Consequently, the up-dip seal is often due to stratigraphic truncation 8. An understanding is essential of the nature of the seal and the evolution of the carrier and reservoir beds as an effect of the above processes. The purpose of this paper is to describe the displacement geometries of normal faults with the use of diagrams of the lateral variation in vertical displacement (LVD).