3d Gravity Modeling in Deepwater Gulf of Mexico

Harold L. Yarger¹, Jerry Hensel¹, Robert Pawlowski¹, and John Bain^2
1Earth Science Dept., Chevron Energy Technology Co., Houston, TX
²Bain Geophysical Services, Inc., Houston, TX

Gravity modeling (GM), at prospect scale, helps to reduce risk during the prospect maturation process. GM uses the large density contrast between salt and sediment to guide interpretation of canopy base salt where seismic imaging is ambiguous. Deep autochthonous Louann salt rollers are rarely imaged well by 3D seismic, but are assumed to underpin interpreted Wilcox structures. GM ensures that the entire salt model, canopy and rollers, is consistent with observed gravity.

Workflows and software used for GM have evolved over recent years to yield more consistent and reliable results. With input from Chevron and other industry experts, gravity modeling vendor software continues to improve. Our in-house GM tools allow for quick evaluation of density and velocity data at an appropriate scale. Sediment density control has evolved from single density-depth functions to the current use of density voxets tied to velocity via Gardner fit coefficients. An uplift from this is better gravity match over mini-basins that may have anomolous velocities & densities. The first step in GM is to achieve a long wavelength best fit between modeled and observed gravity. Analysis of signal power spectra, from various parts of the full earth model, minimizes the potential that deep gravity signals from Moho and/or basement horizons influence salt horizon inversion results.

Before a prospect is placed in drill queue it should be consistent with observed gravity. If the earth model, containing a consensus full salt model, does not fit observed gravity, then basement density inversion and/or mini-basin density inversion is used to seek reasonable alternative explanations. Examples, using these work flows and others, will show how GM is benefiting prospect maturation in deepwater GOM.

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