More than 1.0Bbo and 3.70Tcf (650MMboe) of gas have been produced from the Denver Basin that encompasses the area east of the Front Range in Colorado and Wyoming. Of the more than 1,500 oil and gas fields found in the basin, Colorado's Florence field is the oldest, discovered in 1881. It is said to be the longest continuously working field in the United States.
The Dakota Hogback offers a rare opportunity to peek into the strata that source and produce much of the oil and gas in the Denver and other nearby basins. Dinosaur tracks and fossils, views of Denver and historic Golden, hiking the ridge, and eye-catching rock formations are a few of the by-lines.
In the long, distant past, most of Colorado had been reduced to a plain underlain by one to two billion year old metamorphic and granitic rock. The erosion of these Precambrian and possibly younger rocks left over one billion years of geologic history missing in the Dinosaur Ridge area, nearly a quarter of our planet's age. This gap in the geologic record is known as the Great Unconformity.
Around 300 million years ago (Late Carboniferous), a continent to continent collision occurred associated with the assembly of the Gondwana supercontinent. This collision formed uplifts from what is now Arkansas to Utah. The two largest ranges formed in Colorado becoming the Ancestral Rocky Mountains. An eastern range was pushed up near where the Front Range stand today was called Frontrangia.
These mountains were eroded, shedding terrigenous debris in all directions as a complex of shallow braided channels deposited on alluvial fans adjacent to the uplift and directly over the Precambrian basement. The rocks were later uplifted and tilted as a result of the Laramide orogeny starting about 70 million years ago. Subsequent erosion has formed the beautiful landscapes that make Red Rocks Park and the Fountain Formation a major attraction.
Following deposition of the Fountain Formation, Permian and Triassic times were arid with the deposition of desert dune and fluvial sands of the Lyons Formations and muddy coastal plain redbeds of the Lykins and Ralston Creek formations.
Major climatic change came to the area with the Mid-Jurassic breakup of the Pangea supercontinent. A wet, temperate climate returned to the area of Dinosaur Ridge promoting lush vegetation and supporting a growing population of dinosaurs. During the Jurassic, sandstones, red and gray shales, and thin gray limestones accumulated at the edge of a vast inland sea that covered much of the Midwest and extended north into Canada.
Dinosaur fossils were found in these rocks near the small, historic town of Morrison in 1877. This location at the south end of Dinosaur Ridge lent the name, Morrison Formation, to this Jurassic strata that covers over 1 million km² in 13 midwestern and western states and equivalents in Canada. Dinosaur wise, the Morrison is some of the most fossiliferous on Earth. Other famous fossil locations include Dinosaur National Monument near Vernal, Utah, Como Bluff, Wyoming, and outcrops near Canon City and Grand Junction, Colorado. It is also a major source of uranium with mining occurring on the Colorado Plateau in Arizona, New Mexico, Colorado, and Utah.
Covering such a large area, the Morrison landscape was varied. The area north of present day Montana was close to the coast of the Jurassic sea. Coal beds were deposited in this area where the environment was much wetter and the landscape was covered with swamps and abundant vegetation. In areas such as western Colorado, the climate was much dryer and most of the plant and animal fossils are found in river and lake deposits.
The Dakota sandstones
Deposited along the shoreline of the Western Interior Seaway that extended from the present day Gulf of Mexico to the Arctic, Early Cretaceous sandstone units of the Dakota Group form the backbone of the dominate hogback that parallels the Front Range. Sandstones deposited along this seaway are the primary reservoir rocks for the Denver Basin. Most production has been from Muddy (J) Sandstone and the (D) Sandstone. A walk up the trail at Dinosaur Ridge shows many of the depositional features of the Dakota as well as some interesting surprises.
The Wattenberg field north of Denver is the largest and most continuous-type gas accumulation in the Denver Basin. The primary reservoir is the Muddy (J) Sandstone which can be seen in outcrop along the Dakota Hogback. (Continuous-type accumulations include coal bed gas, low-permeability gas, and fractured shales and are not significantly affected by hydrodynamics like most conventional accumulations.) The U.S. Geological Survey estimates the ultimate recovery from current wells will be 1.27Tcf (230MMboe) of gas with a potential 1.09 Tcf (200MMboe) that could be added over the next 30 years.
Denver Basin petroleum system
All petroleum systems need a source for the hydrocarbons and several ‘hot' or very organic-rich zones in shale units are found directly above the reservoir sandstones of the Dakota Group. Exposures can be found in the low-lying hills to the east of the Dakota Hogback.
The organic-rich layers occur in the Graneros, Greenhorn, Niobrara, and Pierre Formations. Widespread marine condensed sections were deposited in the lower 275 m of the Upper Cretaceous with about 150 m of this interval containing 1 to 6 percent total organic carbon.
The Denver Basin is a structural basin where, after deposition and burial, the rock units were downwarped into structural lows. This deformation resulted from the Laramide orogeny, occurring between 71 and 50 Ma. The source rocks near the center of the basin generated large volumes of oil and gas where the greatest heat flux was concentrated. Hydrocarbons migrate laterally and updip into a variety of stratigraphic, anticlinal, and fault traps. Oil generation occurred early in the basin's deformation and increasing burial temperatures may have generated the thermogenic gas produced in the Wattenberg field today.
Now we have gone nearly full circle. From the times ancient seas flooded the region depositing some of the key ingredients found in the Denver Basin petroleum system to the dinosaurs that roamed along the rivers and shorelines. The Rockies would rise in the background, tilting the strata of the basin that would help ‘cook' the carbon-rich source rocks and trap hydrocarbons. Along the western edge of the basin, erosion would eventually leave the impressive landforms near the Front Range and expose the wonders of the Dinosaur Ridge we see today.