What Differences Will They Make: Depositional Vs Dynamic Salt Models?

Changrui Gong, Rich Gibson, Terry Fitzpatrick, John Stephens, and Mark Steuer
BP America Inc.

The characterization of salt kinematic evolution is an integral part of the deepwater Gulf of Mexico exploration process. Its dynamic nature in geological history impacts sediment thermal history and source rock maturation as well as structural styles, trap geometry, and sediment distributions. When building a 3D basin model to simulate burial history and thermal evolution in salt basins it is not uncommon to treat autochthonous and allochthonous salt as depositional, i.e., they were deposited at the same time as the sediments surrounding them, though piercing/invading salt can be much younger.

In this presentation we are applying a concept of “regional”, “local”, and “actual” surfaces to reconstruct autochthonous and allochthonous salt volumes through time and use these thickness variations in 3D basin models. Comparison between this dynamic salt approach and the depositional salt approach will be presented.

“Regional” surface is a stratigraphic surface of a basin before salt deformation, commonly reflecting the original depositional attitude due to thermal and flexural and sediment fill (Marshak and Woodward, 1988) (red in Figure 1). “Local” surface is constructed by extending interpreted surface into salt/weld, an approach similar to depositional salt model (cyan in Figure 1). “Actual” surface is represented by connecting an actual interpreted surface to the base of salt or the base of weld (yellow in Figure 1)

Paleo-salt thickness can then be determined from subtractions of these three conceptual surfaces. The difference between the “local” and “regional” is the salt thickness removed since this horizon was deposited, and adding this salt thickness to an existing autochthonous salt would indicate autochthonous salt thickness at the time of this horizon’s deposition. The difference between the “actual” and the “local” is allochthonous salt thickness at the time of this horizon deposition. Applying this same technique to all horizons intercepted by piercing/invading salt gives us thickness through time of both autochthonous and allochthonous salt layers.

Both depositional and dynamic salt approaches were applied in the Gulf of Mexico Basin deepwater. Given that all other model parameters are identical, the differences in thermal history results from both approaches are very evident. For example the predicted Jurassic source rock maturity is significantly less in the depositional salt model than the dynamic salt model. We are also going to present variations/uncertainties in other aspects of burial and thermal history. The implication of these differences in source rock maturity, generation timing, generation volume, and fluid quality will be discussed.

Reference:

Marshak, S., and Woodward, N., 1988, Introduction to cross-section balancing: in Basic Methods of Structural Geology, Marshak, S., and Mitra, G., eds., Prentice Hall, Englewood Cliffs, p. 303-314.

Figure 1. Illustrating the concept of regional (red), local (cyan), and actual (yellow) surfaces. Left hatched area represents autochthonous salt removed and cross hatched area allochthonous salt at the time of this horizon deposition

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.