ATYRAU, Kazakhstan  Even as the petroleum industry continues drilling in the Gulf of Mexico at considerable expense and risk, a single field here in Central Asia stands ready to produce two-thirds as much oil each day as the entire gulf does, with less danger to the environment.

But 30 years after its discovery, this field, known as the Tengiz, is still running at only about half speed. Blame geopolitics, not geology.

The problem with the Tengiz field, whose lead operator is the American company Chevron, is not a matter of extracting the oil. More than 100 working wells have already been successfully drilled into the scrub brush desert of western Kazakhstan, near the Caspian Sea.

The challenge is getting the oil to the market.

The Tengiz field, one of the world’s largest known petroleum reservoirs, is tied to a 935-mile pipeline to the Black Sea that the Russian government has declined for years to expand. That refusal has held even though Chevron is a minority partner in the Russian-led pipeline, the Caspian Pipeline Consortium, or C.P.C., which agreed a dozen years ago to more than double its carrying capacity when demand required.