Radim A. Kolarsky, Shell Deepwater Development Inc., New Orleans, LA.

       This paper presents a detailed study of the geologic history of a salt diapir from the Late Oligocene to Present. The research is based on an integrated interpretation of 3D seismic and well data with biostratigraphic support.

       The Côte Blanche Island salt diapir exhibits a notable lack of faulting. The only faults seen on 3D and supported by well data are a large down-to-northeast counter-regional fault running NW-SE through the diapir, and a few associated small synthetic and antithetic faults (Figure 1).

       This observation is in striking contrast with pre-3D interpretations of the field based solely on well data. The pre-3D interpretations proposed a large number of radial faults extending outward from the diapir. Such an interpretation was in concert with the geologic thinking of the time that all diapirs actively breached their overburden as they rose to the surface and caused intense faulting.

       In contrast, this study shows how a diapir can grow and reach the surface without causing extensive faulting at all. By using modern salt-tectonic models of reactive, active and passive diapirism (from the Applied Geodynamics Laboratory), this study suggests that those faults that are observed around the diapir are related to deep, lateral withdrawal of salt feeding the growing diapir. The faults, albeit partially coeval with diapirism, are not directly caused by salt emplacement, but are, in a broad sense, related to the diapir's growth.

       This study greatly simplifies the structural picture of the field (Figure 1). However, the overall architecture of the field is far from simple. Its reservoir complexity is caused by an intricate framework of onlapping and offlapping sequences flanking the salt. This study shows that missing section found in wells high on structure does not represent radial normal faults, but rather unconformities caused by salt movement. Individual reservoirs are not constrained laterally by faulting. Rather, their complexities stem from stratigraphic variations on the sequence and depositional-systems scales..

       Such a radical re-interpretation of the field had inevitable consequences for reservoir management and new development drilling. The changed reservoir picture contributed to an intensive workover and drilling program. The program completely revitalized this mature field and increased its total production output nearly twenty times.

Figure 1: Structural style of the Côte Blanche Island diapir.