A Methodology for Defining the Base and Geometry of Salt Bodies in the Deep Water Gulf of Mexico

By

COBURN, GARY W.

Bell Geospace, Inc., Houston, TX

Exploration and development of new reserves in the deepwater GOM is often hampered by extensive salt canopies, sheets and other salt bodies which absorb or redirect seismic energy resulting in poor seismic imaging. Interpretation of base of salt and sub-salt structures can be extremely difficult. Factors affecting sub-salt imaging are steep dips of the top of salt, rugose top of salt, salt structures with embodied sediments, multiples and velocity insensitivity. Sediment floods, salt floods and prestack depth migrations (PSDMs) are used to overcome the salt ambiguity. The use of Full Tensor Gradient (FTG) data in conjunction with seismic interpretation and processing has proven extremely effective.

Full Tensor Gradiometry data represents the first derivative of the gravity (vector) field, and describes the spatial rate of change (of the vector field components) in all three dimensions. Gradiometer data differs in many aspects from conventional high-resolution gravity data because of bandwidth, retention of high frequency short wavelength signal (generated by shallow to intermediate geologic features), and much greater resolution. Thus, enabling its incorporation in the seismic interpretation at a prospect level.

An initial Top of Salt horizon, Base of Salt horizon, Bathymetry and density grids are derived from seismic data. A geologic model is constructed comprised of depth layers and associated density grids. A 3-D forward gravity gradient model is then calculated and its response subtracted from the measured FTG data. The resulting difference maps show residual anomalies; areas where mass needs to be increased (positive anomaly) or decreased (negative anomaly) within the geologic model. In most areas a base of salt can be determined somewhere in the seismic model area providing a calibration point for fine-tuning of the density grids. The remaining anomalies relate to salt or sub-salt features. The modified horizon is then exported to the seismic interpretation platform and compared to the seismic. The horizon is further modified based on the fit with the seismic where possible and the model recalculated in an iterative process until the difference between the calculated and measured data is no more than 5 Eötvös (1 Eötvös = 0.1 milligals per kilometer). The final density and geologic models are then used in the seismic interpretation and processing (i.e. PSDMs).

In two recent Green Canyon projects, a complete base of salt could not be determined by seismic data alone. In the first example, FTG data was able to determine an accurate base of salt, denote suture zones and salt keels as well as define a sub-salt sand fairway, which was subsequently verified via seismic and well data. The FTG interpretation helped drive the PSDM resulting in the need for fewer iterations. In the second example, FTG-enhanced seismic analysis indicated a salt thickness three times the original seismic interpretation. The well was drilled and confirmed the FTG-enhanced interpretation to within 400 ft.