Much of the growth in offshore exploration and production is expected from Nigeria and Angola, a report from the U.S. federal government said.

Oil field services company Baker Hughes reported 31 rigs actively exploring for or producing natural gas in U.S. waters for the week ending Sept. 18. That's down by half from the same week last year.

A daily brief from the U.S. Energy Information Administration said the Gulf of Mexico's share in the global offshore rig mix has declined from about half the world total in 2000 to less than 20 percent in the years since 2008.

"Most of the more recent growth in active offshore rigs outside the United States has occurred in Africa. Angola and Nigeria account for much of the growth in the region after 2010," the EIA's report said.

The Nigerian National Petroleum Corp. said this week it secured more than $1 billion in financing for the development for 23 onshore and 13 offshore wells in Nigeria. Though production is down from the peak 2.1 million barrels per day in 2011, the Organization of Petroleum Exporting Countries said member state Nigeria was among those contributing most to the group's overall gains in recent output.

At least three deepwater projects are on tap in Nigeria within the next five years.

For Angola, OPEC said production has declined slightly, but was still on par with the historic average of around 1.7 million bpd. OPEC estimates Angola has proven crude oil reserves of 9 billion barrels and the EIA's report said more than 10 offshore projects are slated for development within the next five years.

In general, most energy companies are spending less on oil and gas exploration because of lower crude oil prices. Only five companies came forward with bids on the 33 tracts available in the U.S. waters of the Gulf of Mexico put on the auction block in August.