These days, the Rocky Mountain region is heralded for its profusion of unconventional gas reservoirs. But the region is also home to huge, aging oil fields. One of the largest is Rangely, a giant accumulation that sits in Rio Blanco County, Colorado, astride the Douglas Creek Arch, a broad structural rise that separates the prolific Piceance and Uinta basins. Chevron Corp. operates the massive field, the only one that achieves giant status in Colorado. It’s a classicâ€" tremendously thick, high-quality Weber sands drape across a gargantuan asymmetrical anticline. The Permian- to Pennsylvanian-age aoelian Weber is a reservoir engineer’s dream: Rangely’s oil column is 820 feet and its average reservoir thickness is 520 feet. The Weber, which is found at depths between 5,500 and 6,500 feet, holds original oil in place (OOIP) of 1.9 billion barrels. During some six decades of production, Rangely has produced 863 million barrels of oil and natural gas liquids (NGL). And it’s far from played out: as one of the largest and longest-running enhanced oil-recovery operations employing CO2, Rangely continues to make some 14,500 barrels of oil and 1,400 barrels of NGL per day. Indeed, the field has already produced nearly 100 million barrels of incremental oil and NGL from its CO2 flood, and Rangely has tens upon tens of millions of barrels to go. "Overall recovery, just on the plan that we are on now, will be 51% of the 1.9 billion barrels of OOIP," says Jeff Roedell, Chevron’s Rangely technical team leader. That will push Rangely’s cumulative recovery to 965 million barrels. "We are 35 million barrels away from being a billion-barrel field, which is an elite class. We are certainly looking for additional opportunities to take us over a billion." For more on this, see the December issue of Oil and Gas Investor. For a subscription, call 713-260-6441.

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