Stratigraphy and Petroleum Potential of Dakota Group (Cretaceous), Western Denver Basin, Colorado: ABSTRACT

Abstract

The Dakota Group is the most important stratigraphic unit for petroleum production in central and eastern Colorado. In outcrop sections along the Front Range the Dakota is dominantly sandstone. However, minor shale and siltstone strata, and inorganic and organic sedimentary structures provide the basis for subdivision of the group into a lower Lytle Formation and an upper South Platte Formation. The South Platte Formation shows a uniform westward thinning from an average of about 250 ft along the eastern margin of the Front Range to 140 ft along the western margin in South Park. Recognition of 3 genetic units within the Dakota Group permits correlation from the more shaly sections in the eastern area to the conglomeratic sandstone sections in South Park. Genetic unit A is a fluvial channel-floodplain unit corresponding to the Lytle Formation. Genetic unit B is the lower South Platte and represents a widespread submergence of the depositional basin with encroachment of marine and brackish water into central Colorado. The marine shales of the Skull Creek, Thermopolis, Kiowa, and Glencairn Formations were deposited in eastern Colorado during this submergence. Trace fossils are used as criteria to recognize the marine and brackish-water environments of genetic unit B in areas of the southern Front Range where the Dakota is dominantly sandstone of a fluvial-estuar -deltaic origin. Genetic unit C, including the upper South Platte, the Muddy sandstone, and the J sandstone of the subsurface, records a widespread regression of the shoreline by delta progradation and associated fluvial-channel deposition. An area of maximum channel development, accompanied by erosion of underlying marine shale, extends eastward across South Park into the Denver basin between Denver and Colorado Springs. Thirty feet of oil-saturated sandstone occurs in the upper part of genetic unit C at Turkey Creek, 10 mi southwest of Denver. The saturated sandstone is associated with an isopach thin suggesting deposition in a fluvial-estuary channel that may have as much as 100 ft of associated scour. All 3 genetic units in the southern Front Range area show strong influence of a nor h-, northeast-, east-flowing stream system that probably persisted throughout Dakota deposition. The depositional framework for the Dakota Group, as inferred from the outcrop studies, suggests excellent petroleum potential from stratigraphic traps in the western Denver basin. The area with maximum potential lies between the old producing trends northeast of Denver and the outcrop section on the west at depths from 7,000 to 11,000 ft. End_of_Article - Last_Page 544------------