Estimates Of Salt Dynamical Motion And Hydrocarbon Trapping Around A Salt Diapir In The Gulf Of Mexico
Authors: Lerche I. , Petersen K. 
Published: October 2008

On the basis of Iithology determinations assessed from seismic, together with the geological situation inferred around a salt structure in the Viosca Knoll region of the Gulf of Mexico, quantitative analysis of both salt and sediment motion, together with thermal effects and hydrocarbon kinetic modeling, would seem to imply that:the salt stock rose syndepositionally with sedimentation throughout Jurassic time, with the salt top maintained at the sediment/water interface;at Cretaceous time a competent chalk deposition terminated salt's rise;Miocene and later deposition raised the temperature regime around the salt so that hydrocarbon generation occurred from about 10 MYBP until present day, with the gas window being entered about 3-5 MYBP;post-Miocene salt uplift suggests weakening of the Cretaceous chalk, implying CO2 production near the salt top if leaching caused the weakening;the observed salt 'mushroom' implies a fractured or incompetent Jurassic carbonate underlying the Top Cretaceous chalk;A secondary target for hydrocarbon retention is the Jurassic sediments with the salt mushroom as a tight sealing cover, and the salt stock as a lateral updip boundary impeding migration.

Downhole information to improve the likelihood of estimating economic reserves of liquid condensate or gas hydrocarbons for this diapir, and possibly others, includes:Vitrinite reflectance with depth to better assess paleotemperatures;Downhole temperatures (corrected) with depth, giving a better assessmentof the local thermal anomaly with depth;Fluid pressure measurements with depth, which provide information on fluid-flow through formations, together with inferred excess heating and maturation of sediments with time caused by lower than normal thermal conductivitiesTOC and Rock-Eva] pyrolysis measurements with depth to estimate regional and local source potential for hydrocarbon production.

Introduction

The purpose of this paper is to present some rough rules-of-thumb by which approximate estimates can be made of factors influencing the dynamical, thermal and hydrocarbon maturity aspects of a basin influenced by salt, and to illustrate their application to a Gulf of Mexico salt structure in the Viosca Knoll area. While a self-consistent treatment of the combined interactive evolution of salt and sediments is always required (Lerche and Petersen, 1995) nevertheless, rough estimates can be made of dynamical, thermal and hydrocarbon maturity which are invaluable in providing measures of the magnitude, timing, and relative evolution of salt and sediments. The self consistent modeling can then be better tailored to reconstruct the pale history of the combined salt sediment evolution with present-day data of limited quantity, imprecise in quality, and finitely sampled.

Formulae for making estimates

Dynamical Aspects. The upward motion of salt through sedimentary cover due solely to buoyancy depends on:The critical thickness of sedimentary overburden to cause buoyancy of salt;The critical thickness of sedimentary cover necessaiy for a salt diapir to reach the sediment mudline in the absence of impeding pressure of competent overlying sediments and of significant overpressure, both of which slow the rise of the salt;