The high rewards of finding hydrocarbons in sub-salt plays in the deepwater mini-basins and frontier salt toe belts make it very attractive for exploration endeavors. However, complex subsurface geopressure can cause hydrocarbon breaches and recurrent drilling challenges that drastically increase the operation costs of reaching the objective targets.

After the discovery of Mahogany Field, fifteen years ago, special attention was directed to the sub-salt plays and gradually shifted from the shelf to the deepwater. The striking success of the Jack prospect along the Sigsbee Escarpment (salt toe) gave hope for a substantial replenishment of national petroleum reserves. Deepwater (>1000-ft water depth) yearly production increased from 21 mmbo and 33 bcfg in 1985 to 339 mmbo and 1.1 tcfg in 2006 (MMS). The presence of intricate geopressure compartmentalization in a salt environment, deep mud line, increased target depth and shallow sediment hazards in this more challenging environment has resulted in dramatically increased costs to acquire and test prospects.

In the Gulf of Mexico (GoM) Tertiary- Quaternary system, geopressured sediments are caused mainly by compaction disequilibrium phenomena. Lithology and maximum principal stress essentially control this process. Salt’s unique petrophysical properties contribute to substantial changes in the pore-pressure gradients. Its low density is responsible for retarding the overburden gradient below the salt and, conversely, enhancing it above the salt. The negligible permeability of salt creates a perfect seal. The salt’s ductile nature substantially dictates the path and magnitude of the subsurface stress cage. Consequently, it impacts the integrity of the sealing caps and retention capacities to trap oil and gas in a specific structural closure.

Lower Tertiary Fold Belts New Discoveries

Newly developed supra- and subsalt geopressure models were established based on several fields and dry hole data in the GoM. In each case history, these models

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substantiate the premise that the impact of interaction between salt and sediment is an essential part of risk assessment and postdrilling appraisal.

The benefits and low-risk attributes of sub-salt plays are due to the following:
• Salt enhances the retention capacity in the sub-salt and the sealing capacity in supra-salt.
• Large reserves are trapped by less faulted sub-salt structural closures as a result of salt swells in the mini-basins area.
• High sealing capacity of the thrusted fault system characterizes the salt toe belt fairways.
• Good flow rates occur due to the high permeability of the younger sediments (Plio-Pleistocene), particularly above the salt.
• Drilling does not require high mud density to reach objective targets.

On the other hand, high risk and challenges are also present:
• Drilling problems (pressure kicks and shallow water flow) can occur above the salt due the high principle stress and pressure gradient. Several casing points may be needed to drill through this zone.
• Drilling problems are common at the salt-sediment interface at the base of the salt, especially in the gouge zones.
• The moderate to weak pressure gradient and sealing capacity below the salt can be a substantial cause of seal failure and weak water drive in the production phase.
• Sub-salt seismic imaging quality and accuracy are compromised.
• Low-permeability reservoir as encountered in the Wilcoxequivalent formations is the new frontier sub-salt play. While a great deal is known about salt body delineation from geological and geophysical data, this presentation addresses salt-related risk assessment from a geopressure standpoint.

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