Characterization and Probable Origin of Fractures in Florence Oil Field, Fremont County, Colorado: ABSTRACT

Abstract

The Florence oil field has yielded over 15 million bbl since the drilling of the first well in 1876. The oil is produced from fractures in the Rusty and Tepee members of the Upper Cretaceous Pierre Shale at depths from 1,300 to 3,100 ft (396-945 m). The fractures occur as vertical to near-vertical, parallel to subparallel, discrete joint sets with northwest-southeast and northeast-southwest orientations. The fracture openings vary from hairline to over 1 ft (0.305 m), with vertical and lateral extents up to 1,000 ft (305 m). A recent directional well demonstrates a fracture frequency of 50 to 100 ft (15-30 m). An analysis of production statistics through the year 1908 indicates that most of the 9 million bbl produced to that date was from 44 wells with an average per-well recovery in excess of 100,000 bbl. Nearly 70% of these producers lie within the most densely drilled portions of the 13,000-acre (5,260 ha) field. Whereas other oil-producing fracture shale reservoirs in the Rocky Mountain region are related to abrupt flexures, with tension fractures formed on the convex side of the flexure, the Florence field fractures occur on the gently westward-dipping (3{degree}5{degree}) eastern flank of the asymmetric Canon City-Florence basin.more » Price proposed a model for the development of post-compressional shear joints on the trailing limbs of folds. The conditions for this model can be satisfied by the pre-Laramide to late Eocene geologic history of this basin.« less