Fracture pattern and stratigraphic analysis of the niobraraequivalent Cody Shale, Bighorn Basin, Wyoming

Abstract

Unconventional shale resources have become a significant focus of petroleum exploration and production in North America over the past few decades, and recent USGS assessments of unconventional targets include the Cody Formation of the Bighorn Basin, Wyoming. To further appraise the unconventional play potential of this unit, this study will characterize the fracture pattern, stratigraphy, and porosity controls of the Cody shale. The Cody Formation was deposited during the Late Cretaceous while the foreland range was inundated by the Western Interior Seaway. The Cody shale is composed of thousands of feet of grey to black shale and interbedded sandstone towards the top. As a result of the Laramide Orogeny, the area in what is today North-Central Wyoming and South-Central Montana was compressed into an intermontane basin known as the Bighorn Basin. This compression forced the Cody to depths sufficient to generate hydrocarbons from the organic sediment within its shale. Fractures left in rocks affected by the orogeny can not only be used to learn about the stresses experienced by the Bighorn Basin but can also be used to help extract hydrocarbons from its source rock. When drilling horizontal wells, fracture orientation and characteristics within the formation must be understood in order to return the best results. In order to do this, seven outcrop locations and cored Cody intervals from six wells have examined for fracture occurrence and pattern in the Bighorn Basin. Based off this data, a dominant fracture orientation was realized in the range of 45 to 55 degrees, with a secondary strike direction on the order or 315- 320 degrees. Throughout the outcrop study, a relationship between increased bedding thickness and a decrease in fracture occurrence has been established. After comparing fractures found in core to the corresponding depths in porosity logs, there was no evidence of increased porosity.