Eni has successfully appraised the Heidelberg discovery in the deepwater US Gulf of Mexico after the drilling of two appraisal wells (GC 903-3 and GC 903-3ST1).

The two wells have been drilled in order to appraise the extension of the Heidelberg discovery made in 2009, and are located in the Green Canyon block 903, around 210 kilometres (130 miles) off the Louisiana coast, and approximately 2 kilometres (1.3 miles) from the Heidelberg discovery well (GC859-1).

The Heidelberg appraisal well GC 903-3 was drilled in a water depth of 1,603 meters (5,260 ft.) and reached a total depth of 9,460 metres (31,030 ft.) of measured depth. The well was sidetracked down-dip (GC 903-3ST1 well) to a total depth of 9,278 metres (30,440 ft.) of measured depth, indicating an oil water contact at approximately 213 metres (700 ft.) deeper than the previous oil discovery well and significantly increasing the field's areal extent. Estimated recoverable resources could reach 200 million barrels of oil.
The data for the well is being evaluated in order to accelerate the overall sanctioning process for this project.

Eni owns a 12.5% working interest in Heidelberg. Its partners are Anadarko Petroleum Corporation (Operator with 44.25%), Apache Corporation (12.5%), Statoil (12%), Cobalt International Energy, L.P. (9.375%) and ExxonMobil (9.375%).

In the USA, Eni owns lease interests in 305 blocks in the Gulf of Mexico and is among the leading producers with a daily net production capacity of around 90,000 barrels of oil equivalent (60% of which are operated).

Eni is also present in Alaska, where it owns interests in 117 leases between offshore and onshore areas in the North Slope, where it is the operator of the Nikaitchuq oil field.

In 2009, Eni signed an agreement with Quicksilver Resources, an independent US producer of natural gas, and has a 27.5% participating interest in the development of the "Alliance" area, which produces non-conventional gas from the Barnett Shales within the Fort worth basin in Texas.