Abstract

Naturally fractured reservoirs have become an important target for petroleum exploration, specifically with increasing use of horizontal drilling and multistage hydraulic fracture stimulation. In the Denver Basin, the Niobrara Formation is a resource play consisting of chalks and marls in which natural fractures aid in hydrocarbon deliverability within the reservoir units. Several models have been put forth in the literature to understand the fractures including: Laramide tectonics, Neogene extensional tectonics, solution of evaporites, hydrocarbon generation, and regional stress patterns. This study examines the controls on natural fractures that exist within the different units of the Niobrara Formation and how the fractures can aid in hydrocarbon production.

From outcrop analysis of jointing, the ideal σ₁ results indicate a sub-horizontal Laramide compression with an average attitude of 14/071. Separated calcite-filled walls are evidence for reactivation of previously induced shear fractures. Two main joint sets were observed and measured throughout the outcrop study areas with J₁ systematic joints averaging 248/76 and later J₂ joints averaging 162/75. Vertical stylolites formed perpendicular to J₁ joints with an average stylolitic pole to plane of 11/077.

Data from the bulk mineralogy, total organic content, fracture mapping, and geological characterization of the Niobrara productive units within the study interval indicate that the chalk units have a relatively high ratio of calcite to siliciclastics, low TOC wt. %, and low average fracture spacing, in comparison to the marls.

The structural events that caused the rock failure within the Niobrara Formation are based on syn-Laramide compression and later post-Laramide de-pressuring and unroofing.