July 8. Rangely Oil & Gas Field

Each dot is a well in Rangely Oil Field, averaging about 6000 feet to the Weber Sandstone.
The squares are one mile on a side. After Dobbin, 1956 (USGS)

Rangely Field in northwestern Colorado is an elliptical dome about 11 miles long that contains oil and natural gas in the Upper Pennsylvanian Weber Sandstone. The Weber is mostly a river sand deposit, but some eolian, wind-borne, sand dunes are present as well, and they form some of the best oil reservoirs. Rangely is one of the largest oil fields in the United States, with cumulative production of about 900 million barrels of oil and 700 billion cubic feet of natural gas. That makes it about the 18th or 19th largest oil field in the U.S. in terms of total production. The dome, a structure like an inverted bowl, is caused by a large deep-seated fault on the southwestern flank of the structure. That fault which produced the fold or dome in the Weber Sandstone didn’t form until toward the end of the Cretaceous Period, 200 million years or more after the Weber was laid down. The fault was part of the Laramide Orogeny, and the anticlines and domes that Rangely is part of are essentially a buried extension of the Uinta Mountains of northeastern Utah. 

The rivers whose sand became the Weber Sandstone were flowing off the Uncompahgre Uplift, one of the high mountain ranges formed by the Ancestral Rockies uplifts. The dome makes a nice anticline that’s quite evident on the surface, so it was an early target for oil exploration, with the first discovery in 1933 by the California Company, which we know today as Chevron. It’s a pretty remote area, however, and production didn’t begin until after World War II, and the depth to the Weber is around 6,000 feet or more, which would be a pretty deep well in those days. Because it has been produced for so long, the easy-to-get oil has all been pumped out. In the late 1980s producers were working to get the last bits of oil out of the field by pumping carbon dioxide into the reservoir to force the oil out. During earlier water injection, in the 1960s, it was shown that the deep injection was causing small earthquakes in the Rangely area, some with magnitudes of 4, but mostly smaller.

With the ongoing CO2 injection, Rangely in 2011 was producing about 11,000 barrels per day from almost 1000 wells, which works out to about 11 barrels per day per well, just a bit above the US average oil well production. The CO2 injection has significantly increased the projected production of the field, which otherwise would probably have been half or less than the 11,000 barrels a day. And the CO2 injection does not appear to be causing any earthquakes. See below for a link to a report on the CO2 project.

—Richard I. Gibson

References:
Weber Sandstone 

Earthquakes 

Rangely today – CO2 injection project

Top oil Fields (US)

Drawing after Dobbin, 1956 (USGS)