Simple Approximations for Estimating Quickly the Motion and Timing of Salt Diapir Rise, Overhang Development, and Associated Thermal Anomalies Using Present-Day Observations: Case History from the Gulf of Mexico and Danish North Sea

Abstract

ABSTRACT The upward motion of salt through deposited and depositing sedimentary cover due solely to buoyancy forces can be split into several parts: 1. The critical thickness of sedimentary cover necessary to cause an underlying salt to become buoyant; 2. The critical thickness of sedimentary cover necessary for a salt diapir to reach the sediment mudline in the absence of an impeding pressure of competent sediments opposing salt rise, and in the absence of significant overpressure. Both differential impedance and differential overpressure will slow the rise of the salt to the mudline; 3. The effective speed of motion of the salt through the non-impeding sediments during the salt's buoyant ascent phase; 4. Current observed salt top depth below mudline versus non-impeded predicted salt top depth leading to (a) minimum estate of mechanical strength of competent resistive layers; and (b) an approximate estimate of buoyancy pressure of salt attempting to penetrate the resistive cover layer; 5. Uplift estimate of the overlying competent sediments, because of the buoyancy pressure, in relation to observed uplift, leading to an estimate of salt diapir rise speed since reaching the impeding formation; 6. Timing estimates of "mushroom cap" development of salt since emplacement of the resistive overlying layer, and an estimate of the lateral competence of sedimentary beds ahead of the mushroom salt sheet cap as a consequence of the observed mushroom extent. An estimate of mushrooming speed can also then be made. 7. An estimate of evolving thermal anomalies around the dynamical salt/sediment system as a consequence of high salt thermal conductivity. Such simple rough estimation methods are of importance in attempts to assess the local and regional factors influencing the dynamical, thermal and hydrocarbon retention factors in basinal sediments influenced by salt. Examples from the Gulf of Mexico and the Danish North Sea will be given to illustrate how to use both seismic and/or downhole data to perform the simple estimates.