Salt structures in the deepwater Gulf of Mexico

Gulf of Mexico map showing distribution of salt structures and the various salt provinces. [49,503 bytes] Gulf of Mexico salt in its various forms (Images courtesy of Geco-Prakla). [16,002 bytes] Increased geologic and seismic work is being done with the petroleum industry's move from the shelf into deepwater areas. This work is showing that the salt features in deepwater, generally speaking, are very different from those on the shelf. Therefore, we need to have a better understanding of

Salt's character and history key to search

James Fox
Reading & Bates Development

George Jamieson
Schlumberger Geco-Prakla

Increased geologic and seismic work is being done with the petroleum industry's move from the shelf into deepwater areas. This work is showing that the salt features in deepwater, generally speaking, are very different from those on the shelf. Therefore, we need to have a better understanding of the salt in the deepwater and its relationship to the petroleum system.

Salt not only has an impact on the geology of the hydrocarbon system; it also has a critical role on operational considerations for drilling, pipe-laying and platform location.

In the past, many geologists believed that the deepwater salt was generally just an immature version of the shelf salt system. Because a large amount of geologic work is done by analogy, geologists would use their knowledge of shelf salt systems to predict how deepwater salt would act, and how it controlled the surrounding sediments and hydrocarbon system.

New data show that the shelf salt model by itself is inadequate to explain the deepwater salt system. It also shows that geoscientists made some incorrect assumptions and therefore some poor decisions by using that model to the exclusion of others.

Salt differences

Some generalizations can be made concerning the differences between the deepwater and shelf salt. In the deepwater, the salt is closer to the surface, and more massive. Thus, it is thicker to drill through than shelf salt. On the shelf, much of the salt was evacuated to form salt welds and collapse features. If we compare the initial 10,000 ft of section from the shelf and deepwater, we can see that there is a much greater percentage of salt in the deepwater across this zone.

Salt has a dynamic relationship with the surrounding sediment. Salt grows most actively when the sedimentation rate is slightly less than the rate at which the salt mass can move. Deepwater salt has continued to grow, especially in a lateral direction, because most areas are sediment-starved compared to the shelf. In many areas it is actively moving, creeping basinward at rates of about a centimeter per year.

At the present time some salt sheets are at the mudline and there are salt glaciers exposed to the water with actively dissolving salt and brine basins. The implications are large, especially for the drilling and setting of facilities and pipelines.

On the shelf, salt's low density allows it to reach equilibrium with the surrounding sediments at about 3,000 ft below the mudline. In the deepwater, the greater water column and its attendant pressure allows the salt to reach this equilibrium much closer to the sea bottom, sometimes within 500-1,000 ft of the mudline.

Heat flow, source maturity

Salt conducts heat better than typical sediments. It transfers heat upward away from underlying sediments and cools them relative to the surrounding sediments. Therefore, where there is more salt present, as in the deepwater, the salt will have a much larger impact on the heat-flow.

We can see from deep wells drilled in the deepwater, that there is a depressed heat flow in basins floored by the massive salt. Salt is insulating these basins and wicking the heat away to the shallower surrounding canopies. This may depress local petroleum generation and migration.

Salt provinces

The map of the extent of salt feature extent and general geometry shows that all of the deepwater salt is not the same. The deepwater separates into several main provinces with each province having a different set of associated rules and risks.

  • Mississippi Canyon: Salt structures in the northern part of Mississippi Canyon are similar to salt features on the shelf, with the exception that sediments have not prograded across them and stopped their movement. There was also not as much original salt deposited as elsewhere in the deepwater, and generally the salt has not moved as much laterally.
Exploration has focused on this area because it is easier to map and understand, has well-developed minibasins between the salt structures, and has multiple direct hydrocarbon indicators. It was a major depocenter of Miocene sediments that provided excellent stacked reservoirs. The shelf and slope were narrower at the time of deposition, and sands could move directly from the delta into deepwater minibasins caused by salt movement. Overall, there is a good balance of salt and sedimentation, creating an ideal petroleum habitat.

  • Garden Banks, Green Canyon, and East Breaks: These areas are more complex. The salt is much more massive and the sedimentation history more complicated. Large basins in these areas either trap sediment or direct it basinward, depending on the balance between salt growth and sedimentation throughout geologic time.
Significant sediment pathways extend for many tens of miles between salt sheets and canopies in this region, the longest ones with a characteristic NNW-SSE or NW-SE trend. These fairways sometimes terminate against shallow salt massifs causing extensive sediment ponding.

The salt was dominant in these areas during the Miocene, and most of the sedimentary section was deposited during the Pliocene and Pleistocene. In some places large basins can appear isolated as sediment fairways are often obscured by later salt sheet/canopy emplacement. This makes the geologic play much different than the Mississippi Canyon deepwater corridor.

  • Keathley Canyon, Walker Ridge: These deeper areas are even more complicated. Huge canopies of salt cover large expanses. These occur relatively close to the seabed such that the suprasalt sediment cover is relatively thin with occasional, but significant, minibasins floored by welded salt. Some of these basins can be so thick that a salt floor cannot be confirmed seismically. These basins are often quite circular in plan-view, but can be elongated. Salt highs between these basins are occasionally deflated, forming "colliding mini-basins."
There is a large subsalt play evident on the new seismic data shot across the area that will require new technology. It is in over 6,000 ft of water, and there is on average over 2,000 ft of salt to drill through. Even with these challenges, there has been huge interest in these areas in the lease sales. There is a lot of geologic and geophysical work to be completed to understand the risk of these areas before drilling will commence.

Developing histories

Much of this work will concentrate on the reconstruction of the salt's historic movement. Because the salt controls deposition of sand-prone sediments, it is critical to understand what the salt looked like when the sand was being deposited. The majority of the salt in deepwater has had stages of lateral movement, and a reconstruction of its past location is necessary to reconstruct sediment pathways.

Geoscientists will have to determine how long the salt sheets have been present, and the salt's impact on both sedimentation and the rest of the oil system. This is one of the keys to understanding the deepwater play in the Gulf of Mexico.

There will be numerous technologies needed to reduce the exploration, drilling and operational risks in this deepwater salt province. These include:

  • Seismic - more detailed 2D data and regional 3D data.
  • Depth migration - because salt has twice the seismic velocity of the surrounding sediment section, it distorts the seismic reflections that lead to erroneous views of the subsurface geology. Only by correcting for this distortion with depth conversion and migration can the geophysicists make the time sections useful for integrated project teams.
  • Regional interpretation and reconstruction - this will lower reservoir and trap risk prior to drilling.
  • New drilling technologies - setting surface casing, riserless drilling, temperature and pressure problems all require new techniques for deepwater. Large bathymetric relief, slides and slumps will make surface positioning that much more difficult and important.
  • Pipelaying and facilities - infrastructure planning is critical not only across the salt features, but also around them. The actively growing salt creates very steep sided valleys and active faults that must be crossed or avoided. FPSOs may become the technology of choice in the Gulf of Mexico.

Conclusion

There are many benefits and detriments due to the differing geology of salt features in the deepwater Gulf of Mexico. The salt has helped to develop the very prolific petroleum system. To date, fields in the deepwater have proved to be larger on average than those found on the shelf.

This is probably because the salt in the deepwater has allowed larger traps to form, and allowed the traps to remain competent for longer periods of time. This is fortunate, because economics dictate that this play will only work if field size is large, and deliverability per well is better than on the shelf.

The big question, whether large fields will continue to be found in the more complex regions of the deepwater, will only be answered through a better understanding of the salt in the deepwater and its impact on the petroleum system.

The big question, whether large fields will continue to be found in the more complex regions of the deepwater, will only be answered through a better understanding of the salt in the deepwater and its impact on the petroleum system.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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