HANNA, MARIAN C., Enterprise Oil; and ANDY McGRANDLE, JOHN JEAGER, PHILL HOUGHTON, ARK Geophysics, Milton Keynes, UK

Abstract: Using Gravity Modeling to Enhance Salt Interpretation and Influence Pre-stack Depth Migration

For years gravity data has been used in the Gulf of Mexico to aid interpreters in defining regional features, salt bodies and the subsequent impact on hydrocarbon accumulation. Even with the best seismic, 3D volumes can present a technical challenge in defining salt boundaries. A marriage of gravity modeling and 3D seismic can provide the interpreter with an alternate definition of salt and the surrounding sediment. This union can be extremely beneficial with aiding the interpretation, since modeling can now be conducted on the workstation and is closely integrated with the seismic data. Gravity modeling can therefore be vital for influencing accurate processing of a Pre-Stack Depth Migration volume or in some cases eliminating the need for PSDM.

Introduction

This case study involves salt structures in the geologically contorted area of Garden Banks, Gulf of Mexico. Definitions of the salt bodies were tricky at best due to poor seismic imaging of the salt boundaries even on modern 3D seismic data. High dips of both the salt and sediment have limited imaging both due to acquisition and processing of the seismic. Integrated gravity modeling along several key profiles highlighted some pitfalls in the original seismic interpretation and pointed towards a radically different interpretation, particularly of the base of the salt.A plan-view interpretation of some enhanced gravity grids showed fault trends and lineations that were poorly imaged by seismic and also indicated links between the salt body in the project area and salt bodies in adjacent blocks.

Interpretation Workflow

Non-exclusive 2D gravity data over the project area were purchased, reprocessed and enhanced with the production of residual gravity and terraced grids. These were interpreted within the ARKFIELD^TM workstation package and integrated directly with the seismic data and original seismic interpretation. The terraced gravity data provided a new insight into the overall shape of the main salt body and particularly the areas where it thickened and thinned. This qualitative model was later confirmed by the integrated seismic and gravity modeling.

Originally, the salt was interpreted from seismic to have a shallow base, slight overhang to the east and a thick neck therefore limiting upside potential for the discovered reservoirs. For this case study, the gravity modeling indicated that the main salt structure did indeed have an overhang to the east but with a thinner neck below therefore indicating potential upside for reservoir presence. Appropriate risk must be assigned for reservoir presence and quality in the updip location area that is gained by the thinner salt neck. Gravity modeling also indicated that the base of salt is much thicker than originally interpreted and the tabular salt bodies in the area are thinner than predicted from seismic interpretation. It is recognized that many iterations of the modeling could match the observed versus calculated gravity response. However it needs to be stated that modeling should be finalized based upon seismic integration and geologic relevance, thereby converging on a more constrained interpretation.

This modeling had guided pre-stack depth migration processing on the area to better.define the salt structures and their effect on the surrounding sediment. Unfortunately, the PSDM processing could not image the salt underhang and steep neck and the base of salt modeled is much deeper than the seismic recorded.

One of the goals of the continued work on this project is to back out the velocity data from the gravity model and compare it to the velocity modeling done for the PSDM. Our goal is to reduce cycle time on the velocity model building for depth conversion or seismic processing that is influenced by a more relevant geologic model.

Our industry is progressing into areas with more challenging technical plays than ever before and with increasing exploration costs the issue becomes one of cost versus value. Gravity modeling can help to make those bottom line business decisions when integrated with the total subsurface work.The gravity work carried out in this study amounted to 1/120 th of the total PSDM costs and provided a better constrained interpretation. In times where reduced exploration risk, integrated interpretation and cost effective solutions become the "key words" used by exploration managers, integrating gravity data into the seismic workflow can have a significant impact.

The authors would like to thank Enterprise Oil Gulf of Mexico Inc, Houston, for their permission to present the findings of this case study. We would also like to thank Austin Exploration Inc for supplying the potential field data used in this study.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England