Wattenberg and Spindle Fields--Paleostructural and Stratigraphic Traps, Denver Basin, Colorado: ABSTRACT

Abstract

The most important mineral resource activity in Colorado during the past decade has been the discovery and development of the Wattenberg gas field and the shallow overlying Spindle oil and gas field. Located north of Denver near the axis of the Denver basin, Wattenberg is estimated to have reserves of 1.3 Tcf of gas in the tight J sandstone (delta front) reservoir over an area of 600,000 acres (240,000 ha.), at depths of 7,600 to 8,400 ft (2,316 to 2,560 m). Spindle field, in the south-west part of the Wattenberg field, produces from two marine sandstone bar complexes (Terry and Hygiene Sandstones) in the middle part of the Pierre Shale. From an area of 30,000 acres (12,000 ha.), total production is in excess of 28,000,000 bbls of oil and 100 Bcf of gas at depths of 4,0 0 to 5,000 ft (1,219 to 1,524 m). Although both fields are regarded as stratigraphic traps, paleostructural analysis of the area clearly shows that during middle Cretaceous the fields were located on an ancient structural high that was subsequently downwarped into the present low structural setting. Evidence for recurrent movement on the paleohigh are unconformities at the top of the J sandstone and at the base and top of the Niobrara Formation, and also thinning of shale intervals and localization of marine sand bars within the Pierre Shale. The outline of the Wattenberg paleostructure is best shown by the area of truncation by erosion of the upper chalk of the Niobrara Formation over an area 10 mi wide × 50 mi long (16.1 km × 80.5 km). The east-west trend of the paleostructure changes to northeast and extends for more than 100 mi (16.1 km) into western Nebraska. Three other similar paleostructural trends can be mapped in the northern Denver basin. Knowledge of paleostructural control on reservoir facies and petroleum migration provides new ideas for petroleum exploration in Cretaceous rocks and in the deeper Paleozoic section of the Denver basin. End_of_Article - Last_Page 801------------