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)

July 7. The Fountain Formation

When the Ancestral Rockies were uplifted, they immediately started to be eroded, just as happens to all mountains. Some of the Ancestral Rockies uplifts seem to have been pretty steep sided, high uplifts that dropped fairly precipitously down to the sea. A steep mountain front means that the sediments eroded will be dumped rapidly into basins along the mountains.  

The Flatirons west of Boulder, Colorado, made of Pennsylvanian Fountain Formation

Part of the sedimentary pile that eroded off the Ancestral Rockies in central Colorado is called the Fountain Formation. It was named for a creek near Colorado Springs, Colorado, and we can infer that the origin of the sediment was a nearby high mountain range from the composition and texture of the rocks. The Fountain Formation is mostly a pinkish sandstone with coarse quartz and feldspar grains. The quartz is no surprise, since quartz is highly resistant and often enough, it’s the only significant mineral that survives the sedimentary process. Feldspar is common in igneous rocks like granite, and it’s relatively unstable in the conditions that prevail on the earth’s surface, so it tends to weather quite easily, into tiny silt fragments or even down to the basic chemicals that make it up, potassium, aluminum, and silicon, plus others for some varieties of feldspar. So the presence of big chunks of feldspar in the rocks tells us they didn’t have much time to weather, to alter the feldspar. The sediment was dumped quickly. Coarse-grained feldspar-rich sandstones like the Fountain Formation are called arkose. The definition of arkose is that it contains at least 25% feldspar. Because of the feldspar, we know that the rocks that were eroding to make this sediment were probably granite or similar rocks, and that’s exactly what we find in the cores of the Ancestral Rockies uplifts. In many places, the Fountain Formation lies directly on top of the Precambrian rocks that were uplifted, and from which the sediment was eroded to make the Fountain. That’s a pretty profound unconformity, representing the erosion or non-deposition of about a billion and a half years of rocks between the two.

The fact that the grains are coarse, some of them large pebbles and cobbles, making the rock a conglomerate, tells us that the sediment wasn’t transported very far. If it had been, the transportation would have broken the rock down in to finer particles.

Lastly, the fact that the Fountain Formation is pink, and even red in many places, says there was plenty of iron in the rock that was oxidized to rusty iron oxide and that there wasn’t time to winnow the iron out of the sediment. Some of the color does come from the pink feldspar in the rock, but a lot comes from iron.

The Fountain Formation was laid down in vast alluvial fans by a multitude of streams eroding the eastern flank of the Colorado Ancestral Rockies. Today, the sandstones, arkoses, and conglomerates that resulted lie along the Front Range in Colorado and Wyoming, where they form such striking features as the Red Rocks at Morrison, the Flatirons near Boulder, and the Garden of the Gods near Colorado Springs. They’ve been tilted by later deformation that made the present-day Rocky Mountains, deformation that came about 200 million years after the Ancestral Rockies were uplifted and eroded to make the Fountain Formation. The Ancestral Rockies were pretty much eroded away long before the modern Rocky Mountains formed, but the fact that there are some close correlations in geographic position between the two says that they were both probably exploiting the same or related weak zones. In all likelihood, the breaks in the crust that produced the Ancestral Rockies when Gondwana and North America collided provided the structures that would be rejuvenated much later to make the present-day Rockies.

—Richard I. Gibson

Photo by Richard Gibson

References
Guide Book to the Geology of Red Rocks Park and Vicinity, by Andrew M. Taylor
Gibson's page
Mallory, W. W., 1958, Pennsylvanian coarse arkosic redbeds and associated mountains in Colorado, in Symposium on Pennsylvanian rocks of Colorado and adjacent areas: Rocky Mountain Assoc. Geol., p. 17-20.

July 6. The Ancestral Rockies

By Middle Pennsylvanian time, around 310 million years ago, the collision between Gondwana on the south and the combined North America-Eurasia on the north was well underway. It had probably started by late Mississippian time, and if you want to count the early episodes of island arcs and microcontinents colliding, you could say it was underway by the end of the Ordovician. But now we’re talking about the continents themselves. The big battering rams are finally coming together.

The main force of the continent-continent collision was felt in eastern and southern United States, where the Alleghenian and Ouachita Mountains were formed. But well into the continent, in the west, effects of the massive collision were seen too. This is analogous to the tectonic deformation thousands of kilometers into Asia that results from the collision of India and the southern edge of Eurasia.

It’s challenging to reconstruct the stress fields and other factors that led to the uplift of the Ancestral Rocky Mountains, but the rock record does clearly show where the uplifts were located. Mostly in Colorado, with a pair of long, sub-parallel mountain belts extending from southeast to northwest across the state, into parts of Utah and Wyoming. The southeastern ends of the ranges continued into northern New Mexico, and across the Oklahoma and Texas panhandles and on into southwestern Oklahoma.

Generally speaking the Ancestral Rockies were big block uplifts, bounded by faults that broke the interior of the continent as a distant result of the collision of Gondwana. The uplifts and the faults created high mountains with associated thick piles of sediment, and the breaking of the continent created weak zones in the crust that had impacts in the geologic development of this region right up to today.

There are other ideas about how the Ancestral Rockies formed, including the presence of a possible subduction zone in western New Mexico and Utah that might have caused compression to force the mountains up, as well as a possible persistent weak zone that may have run from southern Oklahoma northwest to Idaho and beyond. But most alternative ideas still rely on the Pennsylvanian collision of Gondwana to provide the impetus, the force, that broke what was fundamentally a pretty strong part of the ancient craton of North America.

—Richard I. Gibson

References
Ancestral Rockies  

Kluth & Coney 1981


Map

Drawing after Mallory, W. W., 1958, Pennsylvanian coarse arkosic redbeds and associated mountains in Colorado, in Symposium on Pennsylvanian rocks of Colorado and adjacent areas: Rocky Mountain Assoc. Geol., p. 17-20.