Abstract

Over 26,000,000 BO (4.13×10⁶ m³) have been produced from the fractured Upper Cretaceous Niobrara section in northwestern Colorado. Fracture intensity and permeability in the Niobrara is controlled by tectonic setting, calcite mineralization, and Niobrara lithology. Most producing Niobrara fields in northwestern Colorado are intimately associated with Laramide-age anticlinal and monoclinal folds. Detailed field studies indicate that significant tectonic fracturing developed during this folding phase, but that a second and perhaps more important tectonic element was the generation of post-Laramide fractures during mid-Tertiary extensional tectonism.

Productive fractures within the Niobrara Formation are mineralized with calcite, which partially filled the fractures prior to oil migration. Five crystallographic forms of calcite have been identified within the fractures. From carbon and oxygen isotope ratios, fluid inclusion studies, and cross-cutting relationships in both surface outcrops and subsurface drill cuttings and cores, the age sequence of the five crystal forms has been determined. Early (Stage I) fractures, formed during Laramide folding, tend to be completely or largely plugged and filled by solid vein calcite. In contrast, the best fracture porosity occurs in later (Stage II) extension fractures, or along zones of rejuvenation of pre-existing fractures. In these later extensional fractures, the calcite cements tend to line, but not fill, the fracture openings. Calcite-lined fractures are also often found in surface fractures in younger, brittle beds overlying or adjacent to producing Niobrara fields.

The Niobrara Formation, which is primarily a calcareous shale or marl in the study area, ranges in thickness from 1,000 to 1,400 ft (305 to 428 m) and is sealed between two plastic non-calcareous shales: the overlying Mancos and the underlying Carlile. Enhanced fracturing occurs in several discrete zones or benches of the Niobrara that are more calcareous or siltier and sandier than underlying and overlying units. The Niobrara is its own source rock and has no bottom water. Niobrara oil is low in sulphur and of high gravity (35-40 API), and sells for a premium price.

Detailed studies that include the Buck Peak and Rangely fields indicate that although tectonic fractures generated during Laramide compression may be significant, enhanced fracturing occurs along and adjacent to post-Laramide normal faults and zones of extensional tectonism.

Exploration for Niobrara fields involves the identification of good fracture fairways (sweet spots) without regard to structural closure. Application of concepts developed from studies of existing fields, combined with a variety of surface and subsurface exploration methods, should result in the discovery and development of significant additional oil reserves in the fractured Niobrara of northwestern Colorado.