Regional stratigraphy and source rock potential of the Devonian-Mississippian Cottonwood Canyon member of the Madison limestone, northwest Wyoming

Abstract

The Cottonwood Canyon Member of the Madison Limestone was deposited mostly in a narrow, northeast-trending fairway in northwest Wyoming and southern Montana during two separate transgressions of the Madison seaway in the Late Devonian (Famennian) and Early Mississippian (Kinderhookian). It consists of up to 80 ft of black and gray silty mudstones, dolomitic siltstones, and silty dolostones deposited in a deep to shallow subtidal carbonate ramp setting. It is time-equivalent to the Englewood Formation of eastern Wyoming and South Dakota and is partially time-equivalent and lithologically similar to several units in the Rocky Mountain region including the Bakken Formation, Exshaw Formation, Sappington Member of the Three Forks Formation, and Leatham Formation. It was deposited at a time of widespread source rock deposition and displays some of the characteristics of the more well-known Devonian-Mississippian source rocks. Dark mudstones of the Cottonwood Canyon Member, although they are somewhat diluted by dolomite and quartzofeldspathic silt, contain sufficient organic matter to be effective source rocks. Lab results show that their TOC levels are good to very good (1.5–2.3 wt. %) and that they contain mixed Type II and Type III kerogen with minor Type IV. Pyrolysis data from previous work indicates a high generative potential (10.3 gal HC/ton, equivalent to a ~35 mg/g S2 on source rock analysis) and corresponding high TOC (5–15 wt.%). As evidenced by source rock analysis and regional burial history curves, the Cottonwood Canyon Member did not reach thermal maturity before the Laramide orogeny. Samples from outcrops, which were thrusted upward during Laramide deformation, have immature Tmax levels of approximately 425–430 ̊C. In basins that were formed at this time, including the Bighorn, Wind River, Green River, Fish Creek, Absaroka, and Younts, the Cottonwood Canyon Member resides at depths sufficient for generation of hydrocarbons. Subject to further investigations of fresh outcrop and/or subsurface samples, it is entirely possible that the Cottonwood Canyon Member is an effective source rock in these basins.