LERCHE, I., University of South Carolina, Columbia, SC, and R. O. THOMSEN, Saga Petroleum, Sandvika, Norway

ABSTRACT: 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

Estimates of the upward motion of salt, due solely to buoyancy forces, through deposited and depositing sedimentary cover 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 nonimpeding sediments during the salt's buoyant-ascent phase;

(4) Current observed salt-top depth below mudline versus nonimpeded predicted salt-top depth leading to (a) minimum estimate 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 (one can then estimate mushrooming speed); and

(7) An estimate of evolving thermal anomalies around the dynamic salt/sediment system as a consequence of high-salt thermal conductivity.

Such simple rough estimation methods are important in assessing the local and regional factors influencing the dynamic, thermal, and hydrocarbon retention factors in basinal sediments influenced by salt. Examples from the Gulf of Mexico and the Danish North Sea illustrate how to use both seismic and/or downhole data to perform the simple estimates.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.