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.

As a result, instead of the 600,000 barrels a day from the Tengiz field that the planners had envisioned by now, Chevron has been limited to pumping about 420,000 barrels through the C.P.C. pipeline to the Black Sea — the nearest entry point to international sea lanes. And Chevron has held off on further production investment that would raise the daily total to about a million barrels. (By comparison, the Gulf of Mexico’s daily output is about 1.5 million barrels.)

For now, some of the Tengiz oil that cannot be accommodated by the pipeline moves via a costly bucket brigade of ships on the Caspian and overland railway tankers to the Black Sea. The effort has required Chevron to become Kazakhstan’s largest railroad operator.

“If Chevron had our way and everything worked beautifully, we would have C.P.C. expanded five years ago,” said Guy Hollingsworth, managing director for Chevron in Europe and Asia, referring to the pipeline.

But Chevron does not decide. As the pipeline’s controlling partner, Russia has declined to expand it while trying to line up investors and international rights-of-way for a second, separate pipeline that would provide the next leg of the oil’s journey by an overland link between the Black Sea and the Mediterranean.

Besides further controlling the transport of oil in the region, Russia is seeking to bypass shipping through the Bosporus Straits in Turkey, the typical passage out of the Black Sea, which is a potential bottleneck already operating at full capacity for oil tankers. Russian pipeline negotiations have long been led by the former president and now prime minister, Vladimir V. Putin, who has taken a keen personal interest in Eurasian energy politics.

The standoff over the C.P.C. expansion is a reminder that while environmental concerns pose a big risk to oil production in the United States and its waters, global politics can pose their own business risks to the industry.

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