Tectonic and Sedimentation Model For Morrow Sandstone Deposition, Sorrento Field Area Denver Basin, Colorado

Pennsylvanian Morrow sandstones are oil and gas productive throughout a large area in southeast Colorado. The Sorrento field is a recent major Morrow discovery with recoverable reserves estimated at over 10 million barrels of oil from an area of 3200 acres (1295 ha) at depths of 5400 to 5600 ft (1646 to 1707 m). Minor production also occurs from the Mississippian Spergen Formation, the Mississippian Saint Louis Formation, and the Pennsylvanian Marmaton Group. On the basis of subsurface mapping, productive Morrow sandstones are interpreted to be fluvial valley-fill deposits, consisting mainly at channel sandstones. These deposits are encased in marine shale and range in thickness from O to 55 ft (O to 16.7 m); net pay ranges from 5 to 30 ft (1.5 to 9.1 m). Porosities average 19% and permeabilities range from 1 to 4000 md. Isopach maps of the Morrow and associated stratigraphic intervals indicate that paleostructure influenced Morrow depositional patterns. Morrow channel sandstones accumulated in paleostructural low areas created by movements on basement fault blocks. Structural nosing is present in the same location and trend as the Morrow channels, indicating structural inversion. Analyses of stratigraphic intervals above the Morrow indicate that the structural inversion occurred during the Early and Middle Pennsylvanian. The field is regarded as a combination structural-stratigraphic trap. Knowledge of paleostructural control on reservoir facies provides a new idea for exploration for Morrow reservoirs in southeast Colorado.