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

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

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

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

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

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

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

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

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 2.67 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

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.

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

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

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.

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

Results – improved gravity match Calculated

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

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.

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.

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.

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