1. Gravity modeling as guidance for salt interpretation: a case study from the Western Gulf of Mexico
Irina Filina* (formerly at Hess Corporation, currently at the University of Nebraska at Lincoln),
Nicholas Delebo, Gopal Mohapatra, Clayton Coble, Gary Harris, John Layman, Mike Strickler and JP Blangy, Hess Corporation

2. Outline 1 Study area – Western Gulf of Mexico 2
Gravity model built from seismic salt horizons
Velocity to density conversion - modified Gardner equation based on well data and public domain datasets 3 4
Results - how gravity can help with seismic salt/velocity model

3. Study area – Western Gulf of Mexico
East Breaks
Gulf of Mexico
Bathymetry map of WGoM
Western Gulf of Mexico
Seismically challenged due to presence of salt
Seismic reprocessing effort
380 blocks, shown by red outline
3D seismic data from multiple vendors merged for proprietary reprocessing
Gravity modeling iterating with seismic to obtain more confident salt model
East Breaks
Seismic
reprocessing
15,000
Alaminos Canyon
contour interval 250 ft

4. Example of density cross-section
Salt in Gulf of Mexico
Example of density cross-section
Water
Salt resides in two layers:
Allochthonous (shallow)
Autochthonous (deeper)
Seismic image of salt:
Top of allochthonous (shallow) salt is the most confident in seismic;
Base of shallow salt may be ambiguous, although is mapped relatively confidently in some areas;
Top and base of deep salt is somewhat ambiguous.
Allochthonous
salt
20,000
Depth below sea level in feet
Autochthonous
salt
40,000
Basement

5. Gravity data Bouguer gravity map
contour interval 2 mGal
Bouguer correction density is 1.9 g/cc
shown with permission from CGG GravMag Solutions
Seismic
reprocessing
Bouguer gravity map
Gravity data was acquired with seismic 2D survey (phase 45, available from CGG GravMag Solutions)
Regional increase from NW to SE due to crustal thinning
Oceanic crust confirmed by refraction data in SE corner
Local negative anomalies due to presence of salt; the known “salt wall” province trending SW-NE

6. Salt in Gulf of Mexico First vertical derivative of Bouguer gravity
Allochthonous salt thickness from seismic
Seismic
reprocessing
Contour interval 2.5 Eotvos
Contour interval 1000 ft

7. First vertical derivative of Bouguer gravity
How gravity can help
First vertical derivative of Bouguer gravity
Rock salt has low density
2.15 g/cc assumed for this study based on core data
For most of section salt is less dense than the surrounding sediments => source of negative gravity anomaly
Gravity data may be used as an independent tool to
(1) test the existing salt model
(2) guide seismic Base of allo-salt (shallow) interpretation
(3) test different geological hypotheses (e.g., presence of salt roots, rafted sections, subsalt minibasins).
Seismic
reprocessing
Contour interval 2.5 Eotvos

8. Building a gravity model From Horizons to Continuous Layers
Seismic input - horizons
For this project three pairs Tops and Bases were used in order to describe complex geometries of allochthonous (shallow) salt
Gravity model – continuous layers
Salt
Water
The patched horizons need to be organized into continuous layers for gravity modeling

9. Building a gravity model - layers
Water
What goes in the model:
Bathymetry (from seismic)
Allochthonous salt – three tops and three base horizons from seismic data
Autochthonous salt – top from seismic (where interpretable)
Base of auto- salt is assumed to be the top of acoustic basement (inferred as local lows of Top_auto, with some seismic control)
Upper crust with densities varying from g/cc to 2.9 g/cc;
Lower crust of density 2.9 g/cc;
Mantle, 3.3 g/cc
Allochthonous
salt
20,000
Autochthonous
salt
Basement
Depth below sea level in feet
60,000
Continental crust
Moho
Mantle
100,000

10. shown with permission from CGG GravMag Solutions
Deep sources of gravity anomaly
Bouguer gravity map
Basement
Partially from seismic as local lows of autochthonous slat
Moho
Very limited refraction data - [Nakamura, Y., et al., 1988] shown as white lines
Oceanic crust (~ 7 km thick) at the SE corner of the model from refraction data
From gravity modeling to fit the regional trend
Crustal thickness
thinned continental crust is assumed in order to satisfy regional gravity trend of ~ 100 mGal
Seismic
reprocessing
Seismic refraction data (Nakamura et al, 1988)
contour interval 2 mGal
shown with permission from CGG GravMag Solutions
Nakamura, Y., et al., 1988, Gulf Coast Association of Geological Societies Transactions, 38, 207.

11. shown with permission from CGG GravMag Solutions
Deep layers - Moho
Bouguer gravity map
Depth to Moho
Seismic
reprocessing
90 kft
Moho is at ~ 67 kft depth, crust is ~7 km thick from refraction
70 kft
contour interval 2 mGal
shown with permission from CGG GravMag Solutions
contour interval 1000 ft

12. shown with permission from CGG GravMag Solutions
Deep layers - basement
Bouguer gravity map
Depth to Basement
Seismic
reprocessing
45 kft
contour interval 2 mGal
shown with permission from CGG GravMag Solutions
contour interval 250 ft

13. Gardner Coefficient versus depth
Classic Gardner equation [Gardner et al, 1974]
The well data in the study area suggest that the constant value of 0.23 is oversimplified
Depth dependent Gardner coefficient was derived based on wells and public domain data [Hilterman et al, 1998, Fleming et al, 2005]
Gardner, G., et al., 1974, Geophysics, 39, no. N 6: 770.
Fleming, P. et al., 2005, Integrated Ocean Drilling Program: Expedition 308.
Hilterman, F., et al., 1998, proceedings of the 14th Annual SEG Gulf Coast Technical Meeting, Geophysical Society of Houston, May 1998.

14. Results – base of salt sensitivity
Free-Air gravity
Free-Air gravity
2 mGal confidence interval
2 mGal confidence interval
Calculated
Calculated
8 mGal
Observed
Observed
Water, 1.03 g/cc
Water, 1.03 g/cc
Salt 2.15g/cc
Modified
Gardner equation
Salt 2.15g/cc
20,000
20,000
Removed
salt
40,000
40,000
Autochthonous salt
Autochthonous salt

15. Results – improved gravity match
Calculated

16. Results – improved subsalt image
These seismic data are owned by and proprietary to MKI (MultiKlient Invest AS). Shown with permission of MKI/PGS.

17. Gravity mismatch (observed – calculated)
Initial model
After several iterations
Contour interval 1 mGal
Yellow color – mismatch within confidence interval;
Green/blue areas – need more low density (such as salt);
Orange/red – need high density, or remove salt.

18. Summary The 3D model integrates gravity, seismic, and well data
Iterative process – used to guide seismic salt interpretation
Independent tool to test different geological scenarios, such as presence of salt wings, overhangs, pedestals, sub-salt minibasins, etc.
Resulted in more confident salt model and lead to significant improvements in subsalt imaging.

19. Acknowledgements
The authors are grateful to Hess Corporation for allowing them to publish this study.
We thank CGG GravMag Solutions for the use of the multi-client marine gravity data.
Hess would like to thank MKI/PGS for permission to publish the cross- section through a multi-client seismic survey.
Tim Grow, Ken Kemp and Keith Katahara from Hess Corporation for valuable input and constructive discussions.

20. Thank you! 20 20