Roger M. Wright

Interactive seismic mapping of net producible gas sand in the Gulf of Mexico

In the Garden Banks area of offshore Louisiana several gas sands have been drilled and found productive. However, the sands are laterally variable in thickness and effectiveness. An improved understanding of the spatial distribution of net producible gas sand is highly desirable for reservoir management. The bright reflections from the top and the base of each sand were tracked automatically on an interactive interpretation system. This yielded time structure maps and hence isochron maps for each gross sand interval. The horizon Seiscrop™ sections diplaying amplitudes over the sand interfaces were then summed, adjusted for tuning effects, and smoothed to yield estimates of net gas/gross sand ratio over the area under study. By combining these with the corresponding isochron maps and an appropriate gas sand interval velocity, we obtained net gas sand isopach maps which tie acceptably with well data. Integration of these provided total reservoir volumes. ™Trademark of Geophysical Service Inc.

Modern technology in an old area: Bay Marchand revisited

Bay Marchand, a giant oil field in the Gulf of Mexico, is undergoing renewed drilling activity as the result of a recently acquired 3-D seismic survey. This classic mature field had seemingly entered its last stages of production in the mid-1980s. However, an integrated team effort by geologists, geophysicists, and engineers revitalized the area by innovative use of the new 3-D data.

The shapes of Gulf Coast salt intrusions related to seismic imaging

In the Gulf of Mexico, it is important to know the salt‐sediment interface as accurately as possible because of its effect on depth imaging. 3-D depth migrations are rapidly changing our understanding of subsalt structure, since they reveal that 3-D time‐migrated subsalt images are confusing or totally misleading (Figures 1–2). With common velocity contrast ratios of 1.5–2.5 at the salt boundary, Snell’s law implies large refraction angles. Complex 3-D shapes cause data to refract over long lateral distances at sharp angles. Examples of many carefully studied salt intrusions show little hope of invoking 2-D symmetry of the salt for acceptable depth imaging.

Application of Turning Wave Migration to a 3-D Seismic Survey In Main Pass Block 299, Offshore Louisiana, And Its Impact On Field Development

In 1988 a turning wave migration was applied to a 3D seismic survey covering the Main Pass Block 299 salt dome, offshore Louisiana. Chevron chose to apply this technology due to the known structural complexity of this salt dome. Previously acquired 2D seismic and salt proximity data had indicated that this salt dome was overhung in several ocations. In addition, the salt dome in question had numerous points of well control for salt which would aid in the evaluation of the migration. The results indicate that the turning wave migration was successful in imaging the overhung flanks of the salt dome, where salt flank dips approached 130 degrees. In addition, the migration also imaged salt geometries described as “dikes” or “wings” interpreted to be the extrusion of salt into nearby fault zones. These newly recognized geometries have significantly impacted the development of this field by providing additional locations for drilling. 5. A 3D turning wave migration algorithm was requireu to correctly handle dips from 0 to 130 degrees with appropriate amplitudes (Hale et al., 1991).