Abstract

Field growth is a phenomenon where the estimates of known recovery tend to increase systematically over time. Generally, field growth is associated with the following: (1) boundaries of proved areas are extended by drilling; (2) new pay zones, pools, and reservoirs are found by drilling or recompletions; (3) infill wells or stimulation procedures; (4) improved drilling and completion costs; (5) and secondary or tertiary recovery. Items 1 through 4 occurred at Wattenberg, and item 5 is currently being tested. Unconventional fields experience field growth just like conventional fields. The overall economics of conventional and unconventional systems benefit from the field growth phenomenon.

The giant Wattenberg Field of Colorado was discovered in 1970 by Amoco Production Company with initial production from the Lower Cretaceous Muddy (J) Sandstone. Wattenberg straddles the Denver Basin synclinal axis and is regarded as a basin-center stratigraphic petroleum accumulation covering approximately 2600 mi². Subsequent to discovery, the number of producing layers and production grew to include five additional formations (Dakota Plainview, Codell Sandstone, Niobrara Formation, Terry and Hygiene sandstone members of the Pierre Shale). The Terry and Hygiene were first produced in 1971, the Codell in 1981, the Niobrara in 1985, and the Dakota Plainview in 1998. Production occurs from approximately 4000 to 8500 ft below the surface across the field. Reservoir quality in the various horizons is generally poor, which mandates hydraulic fracture stimulation for economic production.

The addition of multiple productive horizons in the field area has significantly added to the total reserve number. The field is ranked by the U.S. Energy Information Administration (EIA) based on reserves as the fourth-largest oil field and the ninth-largest gas field in the United States.

Original reserves were estimated to be 1.1 TCFG for the Muddy (J) Sandstone. Fifty years after discovery, the field is currently at peak production because of recent horizontal drilling activity in the Codell and Niobrara with cumulative production to date at 1060 MMBO and 9.4 TCFG from more than 40,000 wells. Estimated ultimate recovery is 1100 MMBO and 9.7 TCFG. This represents reserve growth of over 1000% from discovery.

Hydrocarbon source rocks in Wattenberg are the Skull Creek Shale, Graneros Shale, Greenhorn Limestone, Carlile Formation, Niobrara Formation, and Sharon Springs Member of the Pierre Shale. Total source rock thickness in Wattenberg is 200 to 250 ft (>2 wt.% TOC). The source beds are dominantly Type II with some Type III kerogen present.

The Wattenberg area is a “hot spot” or positive temperature anomaly. This is an important reason the area is so prolific. Temperature gradients range from 1.6°F to 1.8°F/100 ft on the edges of the field to about 2.8°F to 2.9°F/100 ft in high gas-oil ratio (GOR) areas. The temperature anomaly is related to where the Colorado Mineral Belt intersects the Denver Basin. The mineral belt is a northeast trending zone across Colorado of Late Cretaceous to early Tertiary mineralization. The mineralization is associated with high geothermal gradients and hot fluids. The high heat flow and hydrocarbon generation contribute to abnormal pressure in parts of the field.