Oil-Impregnated Outcrops and Their Relationship to Petroleum Generation in Late Paleozoic Eagle Basin, Northwestern Colorado: ABSTRACT

Abstract

Oil-impregnated outcrops of Mississippian and Pennsylvanian rocks are present in widely separated areas of the Eagle basin. Along the northeast margin of the fault-bounded Eagle basin, oil-impregnated arkosic sandstones of the lower Minturn Formation (Desmoinesian) are interbedded with black, organic-rich shales that provided a local source for the oil. Algal mounds in the lower Minturn Formation in this area contain oil residue. Oil residue is also present in the Mississippian Leadville formation in several parts of the basin, both in paleokarst-related vugs and intergranular porosity in oolite grainstones. The Leadville formation was sourced by overlying organic-rich shales of the Belden Formation (Morrowan-Atokan). Under Pennsylvanian-Permian arkosic sandstones near the southwestern margin of the basin are not adjacent to potential source rocks. The Schoolhouse Tongue of the Weber Sandstone is heavily stained in many areas over its 50 mi of outcrop exposure. Numerous arkosic sandstones in the underlying Maroon Formation are also sained. Faults bounding intrabasinal horst blocks were conduits for upward oil migration, and the horsts may have provided the trap. Oil is also present in overlying Triassic and Jurassic sands in this area, supporting the theory of vertical migration up faults. Oil residue is also present in the Maroon Formationmore » 40 mi to the southeast in the Crystal River Valley. Secondary porosity from dissolution of calcite cement preceded oil emplacement. Beaching of normally red-brown sandstones is ubiquitous with the oil residue and probably represents reduction and partial removal of iron by acidic, reducing fluids generated from the Belden Shale.« less