The Double Edged Sword : The Impact of the Interaction between Salt and Sediment on Sub-Salt Exploration Risk in Deep Waters, from Mahogany to Jack

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

Lithology and maximum principal stress essentially control the subsurface geopressure profile development. Salt's low density is responsible for retarding the overburden gradient below the salt and, conversely, enhancing it above the salt. 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 hydrocarbon's retention capacity.

Newly developed supra- and sub-salt geopressure models were established based on several fields and dry holes data in the GoM. In each case history, these models substantiate the premise that determine the impact of interaction between salt and sediment is an essential part of risk assessment and post-drilling appraisal.

The benefits and low-risk attributes of sub-salt plays are due to the following:

• Salt enhances retention capacity in the sub-salt

• Large reserves trapped by less faulted closures

• Good flow rates of younger sediments (Plio-Pleistocene),

• Drilling does not require high mud density

On the other hand, high risk and challenges are also present:

• Drilling problems (kicks and shallow water flow) can occur above the salt due to high pressure gradient. Several casing points may be needed to drill this zone.

• Loss of circulation is common at the base of the salt

• The moderate to weak pressure gradient below the salt can be a substantial cause of seal failure and weak water drive

• Sub-salt seismic imaging quality

• Low permeability reservoirs as encountered in the frontier Wilcox-equivalent formations

This article addresses salt-related risk assessment from a geopressure standpoint.