ABSTRACT

Recent exploration in portions of the Mississippi Salt Basin indicates that some models used in the interpretation of surface and subsurface data may be inadequate to explain several salt features. Models are important in interpretation due to the depth of the objectives and the complexity of the salt ridges. Significant oil and gas reserves trapped by the salt and related faults in Upper Jurassic reservoirs were discovered from the late 1960's to the early 1980's. Early models show concordant overburden on the flanks of a simple, symmetrical salt anticline. However, in the course of further drilling, Upper Jurassic formations and the underlying salt were not encountered where predicted by the anticlinal model suggesting that many of the salt ridges in the basin are more complex. Well log data indicate that the flanks of such salt features may dip at rates greater than that proposed by the symmetrical anticline model.

A study utilizing an integrated approach to test the viability of a more complex salt ridge model was conducted to aid in the interpretation of surface and subsurface data in the Mississippi Salt Basin. Gravity data in conjunction with seismic reflection data and well log information form the data base from which salt models were created. Seismic reflection data and well control provide a reasonable estimate of the depth of the sources of the gravity anomalies. The gravity data, on the other hand, have been used to help constrain the interpretation of the seismic data by providing a better geometric framework for the salt bodies observed on the seismic data.

Eucutta Field and Diamond Field were selected as test areas for the salt models based on the availability of data. The two fields are located along different types of parallel salt ridges. Eucutta is located deeper in the basin in an area of large salt features while Diamond is located closer to the rim of the basin where smaller salt features are more common.

Results of the study indicate that an asymmetrical pattern exists in which the flanks of the ridges may dip at different rates. Both the Eucutta and Diamond salt ridges exhibit a steeply dipping flank on the basinward side of the salt ridge and a more gently dipping landward flank. The salt ridges also appear to have a complex along strike variation in the structure of the salt. The variations of the salt features in both the strike and dip directions appear to reflect the variations in the way the overlying sediments were deposited.