Challenges in Structure Prediction When Developing Remaining Field Reserves Using 3D Seismic, Gulf of Mexico Shelf, Offshore Louisiana

Ingo Steinhoff and William A. Hill
BP America, Inc., 200 Westlake Park Blvd, Houston, Texas 77079

On the Gulf of Mexico Shelf, operators still encounter structural surprises when drilling for remaining reserves in well established, mature fields with 3D seismic coverage. These surprises are associated with the nature of remaining development opportunities along major fault systems where attic reserves are targeted.

Wells targeting both shallow and deep reservoirs to develop stacked attic reserves often find shallow structures as mapped. Deeper horizons, below major faults within fault shadows, are more problematic. Seismic structure maps may show conforming crestal positions in all target horizons whereas subsurface control may render a scenario where structural apexes seem less conformable between mapped horizons.

Errors in seismic depth conversion of only fractions of a percent at depths around 10,000 feet or more can make or break a given development prospect if structural gains (height) are merely in the tens of feet to an offsetting well. New well paths must be carefully designed when developing stacked pay.

This case study shows that within 3D seismic coverage, subsurface control is an integral part of subsurface mapping especially when targeting multiple fault-sealed reservoirs. This is specifically important when the target horizons are located in areas of limited or masked seismic resolution due to "fault shadowing". The case presented here is a superb example for changes not only in structural morphology but also reservoir quality over only a few hundred feet governed by local facies changes. Understanding both mitigates drilling risk significantly.

AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana