Three-Dimensional Seismic Data Reveals the Finer a Piercement Salt Dome
Authors: Blake Bruce A. , Jennings James B. , Curtis Michael P. , Philipson Rhona M. 
Published: October 2008

Eugene Island Block 77 field is a shallow pier cement salt dome with a low relief overhang that is productive from Upper Miocene sands at depths between 13,300 and 15,800 feet (3.260 - 3.690 seconds). These hydrocarbon accumulations are trapped in steeply dipping beds between the salt mass and the rim syncline. Varied gas-water contacts in field wells suggest that radial faults also control reservoir limits.

Maps produced from 2D seismic data provided limited structural detail due to:The wide spacing between linesThe presence of migration artifacts and sideswipe problems that mask minor faults and make salt interface maps ambiguous. To resolve these problems, a three-dimensional seismic survey was performed using a bay-cable technique. The increased density of subsurface sampling, together with the advantages of 3D migration and the ability to view and interpret the data from the horizontal plane, enabled a highly detailed interpretation to be performed.

Maps produced from the 3D volume far surpass the detail possible from the previous 2D data. The salt-mess boundary, together with a complex system of regional growth and radial faults associated with the dome, has been mapped.

Introduction

Eugene Island Block 77 Dome is 18 miles off the Louisiana coast approximately 90 miles southwest of New Orleans in the federal waters of the Gulf of Mexico. Figure 1 details the location of the area. The dome and surrounding field are in blocks 62, 63, 76, and 77 of the Eugene Island area, all of which are leased to Hunt Oil Company.

The dome has apparently emerged from the deepest point of a localized syncline, as can be seen from the generalized regional structure map of Figure 2. Regional faulting is closely associated with the many surrounding salt features, which are both shallow pier cement and deep seated. The Block 77 dome is a shallow pier cement dome that comes to within 750 feet of the surface.

Figure 3 is a well traverse through the southeast flank of the dome between depths of -8000 feet and -15,000 feet. The line of section illustrates the salt, wells 77 #1 and 77 #2, and the rim syncline beyond. The #1 well was drilled without encountering any salt; it penetrates the shallow (brown) horizon at approximately -12,700 feet and the deep (green) horizon at -14,600 feet. The #2 well is only 1400 feet to the northwest, yet it penetrates 6000 feet of salt before encountering climatic sediments beneath the salt overhang. This well also penetrated the shallow horizon but TD? end before reaching the deep horizon. Diameter ridings suggest dip close to the salt to be around 40.

A complete interpretation of a salt dome prospect such an this requires not only the structural interpretation of horizons of interest but also a detailed interpretation of the salt interface. In this study, this knowledge of the salt face was required to aid in planning well deviations around the salt stock so that future wells may be positioned close to the up dip limit of any reserves to maximize potential recoveries.