News that three oil companies have discovered what might be an enormous reserve in the Gulf of Mexico comes at an auspicious moment. With Congress returning to Washington, Democrats planning to campaign on gas prices, and lawmakers of both parties dragging their feet in respect of allowing additional drilling, the time is apt for a clear-headed conversation about energy policy. Yesterday’s announcement is a great place to start.

Chevron and two partners said yesterday they had conducted test drilling on a site that could turn out to harbor between 3 billion and 15 billion barrels of oil and natural gas. If those estimates are correct, the 300-square-mile site could prove to be the largest domestic oil find in America in a generation, matching the output of Prudhoe Bay in Alaska. If the upper estimate is true, Chevron has just increased America’s domestic reserve by 50%. It will be several years before that production comes online in all its glory — analysts are predicting that the new supply will start hitting the world market some time in 2010 — but yesterday’s news helped sustain a slide in the price of oil futures that had already been underway.

This marks the second lesson in oil supply and demand in the past month. The shutdown of British Petroleum’s operations at Prudhoe Bay last month cut world production by only 400,000 barrels a day yet sent prices soaring to a record $78.44 a barrel. Meantime, the prospect of increased supply — some from sources that make sense only at higher oil prices, as our Youssef Ibrahim, a newspaperman particularly savvy about the petroleum beat, has noted — has led to a further drop in crude futures.

The find also raises hopes that even more oil may be lurking under the Gulf of Mexico. More than a fourth of America’s domestic production already comes from the Gulf, and no one knows how much more might be out there. The House has passed a bill that would effectively give individual states in the area an opportunity to decide how much exploration and drilling they want to allow off their coasts.

The Senate, however, has resisted, preferring a more restrictive bill that would allow new access to fields believed to contain only about 1.125 billion barrels. Chevron has already found, at a minimum, more than twice that in its new field. No one knows how much oil could be lurking under the 125-mile “protection zone” Florida politicians slipped into the Senate bill, not to mention the 80% of American coastline that’s currently off limits to exploration.

Even if the World’s Greatest Deliberative Body manages not to get the message on supply, senators can at least obtain a lesson in industry economics. Chevron’s discovery is the result of the kind of expensive deep-water exploration necessitated by current regulations that make it difficult or impossible to explore in shallower waters. While the average cost to drill a well offshore has been about $32 million, Chevron and its two partners reportedly have already spent about $100 million digging this well under 7,000 feet of water. They may have to spend as much as $1 billion constructing a rig to extract the oil for market.

Those numbers will bear remembering the next time any politician starts bloviating about “windfall profits.” One study prepared by Ernst & Young for an oil trade group, the American Petroleum Institute, found that between 1992 and 2005, the five largest oil companies re-invested an amount equal to 120% of profits. That re-investment includes costs like exploration and new drilling. Recent “windfalls” are merely a reward for earlier exploration undertaken when times were not as good for big oil.

Congress will have it in its power over the next month to decide whether Chevron’s big announcement will be an aberration or the “new normal.” If Americans are going to be able to look forward to many more price-depressing big finds, Congress needs to open up both the Gulf and the Arctic National Wildlife Refuge — which would produce as much each day as the new Gulf well is expected to — to drilling. This is for what “windfall profits” are needed. Chevron’s discovery suggests that Americans are getting their money’s worth from big oil.