Oil, water a tense mix

Proposed legislation would still ban partnerships. But the consensus to permit some exception in the gulf region is growing as oil companies move closer to Mexican territory. The U.S. has issued drilling rights on dozens of parcels less than 10 miles from Mexican waters. Shell, BP, Chevron and Exxon Mobil, plus independents including Houston's Bois d'Arc Energy, have secured acreage adjacent to the boundary.

"The pressure is forcing [legislators] to do something," said Mexico City attorney David Enriquez, a maritime law expert who will testify at a Senate hearing today on transborder reservoirs. "It's the one area where they are unified."

It's unclear whether big shared deposits even exist in the Gulf of Mexico. Historically, the region's deepwater finds have been isolated pockets of petroleum, not mega-fields.

Officials at the U.S. Minerals Management Service, the federal agency that regulates U.S. offshore production, said they had no knowledge that any gulf reservoirs now under development crossed the international divide.

Shell, which is developing its Perdido platform with Chevron and BP, said the deposits they were targeting were confined to U.S. territory.

Mexicans are skeptical. A recent editorial cartoon showed a greedy Uncle Sam sucking from a straw plunged deep into the gulf. But Pemex hasn't done the seismic and drilling work needed to determine if there is crude on its side.

All the more reason, Enriquez said, for Mexico to collaborate with the U.S. to find out what lies near the 470-nautical-mile gulf border and end all the speculation.

A spokesman for Minerals Management said his agency had worked with Mexico before on boundary issues and was open to discussing cross-border fields. "It's the neighborly thing to do," said Dave Cooke, deputy regional supervisor for resource evaluation for the agency in New Orleans.

Oil and gas fields straddle international borders all over the globe. Countries typically strike a "unitization agreement" to share the costs to extract the deposits and split the proceeds based on how much lies in each nation.

Britain has partnered with the Netherlands and Norway in the crowded North Sea. Australia and East Timor have a unitization agreement. So do Nigeria and Equatorial Guinea.

But the U.S. and Mexico have long skirted the topic, given their prickly history with oil.