The effect of gravitational compaction on estimation of vertical salt structure growth

Abstract

The growth history of salt structures is reflected in the variation of the present thickness of the overlying and surrounding sediments in relation to the regional normal. A method for estimating the growth history of salt structures in the pillow and post-diapiric phase, including the effect of differential syn- and post-depositional compaction, is described. The assumption that stratigraphic thinning and established structural relief is a result of vertical salt movement alone is inadequate. A quantification of the true vertical movement of the salt crest necessitates a correction for compaction and differential basement subsidence. The on-structure thickness is compared to the regional normal for the time-stratigraphic interval under consideration. On the basis of the dominant porosity-reducing process, the minimum and maximum net growth can be calculated. The true net growth is assumed to be contained within these limits, probably closest to the minimum net growth. Two shallow salt diapirs in the Danish Basin exemplify the relationship between the observed stratigraphic thinning and structural relief and the differential compaction, differential basement subsidence, and vertical growth of the salt surface. The average minimum and maximum net growth of the salt surface at the Batum and Mors salt diapirs in the post-diapiric phase are calculated. The minimum and maximum net growth values for the Batum salt diapir are 731 m and 1303 m respectively, and for the Mors salt diapir 276 m and 996 m respectively, thus emphasizing the importance of including the effect of compaction in the calculations of salt structure net growth. The implications for storage of hydrocarbons, radioactive and chemical wastes, as well as for petroleum exploration, are emphasized.