Insights from the Devils Fence Anticline: implications for the Lombard Thrust and thin-skinned deformation in the Helena Salient, Montana

Abstract

The Helena salient and features within have been studied for decades in the context of oil and gas exploration as well as in studying the timing and development of salients within the North American Cordillera. The Lombard thrust, a major thrust in the salient, has a convoluted history of varied structural interpretations and shortening estimates which are critical to estimating hydrocarbon reserves. One of the most prominent features within the Lombard thrust is the Devils Fence anticline. Motivated by a Lombard overthrust hypothesis, drilling began at the Devils Fence anticline with the hopes of encountering Paleozoic hydrocarbon reservoirs beneath the thrust sheet. The drilling resulted in a dry hole, which left geologists puzzled as to what strata had been penetrated within the well, and if the Lombard thrust was breached at all. The initial interpretation of the well log suggested that the Phanerozoic strata was encountered beneath the Lombard overthrust and that reservoir rock may be present further below. Others disputed that the homogeneity of the well log suggested that only Mesoproterozoic hanging wall rocks had been drilled. My work resolves this outstanding controversy to constrain the structure of the Lombard thrust. Whole rock geochemistry, petrographic, and scanning electron microprobe analysis were employed to identify the lithologies present within the well. Drill cuttings samples received from the Montana Board of Oil and Gas were compared to outcrop samples across the salient. Petrographic analysis revealed carbonate shales with bedded, laminar sulfides indicative of the Newland Formation at ~8000 ft, and whole rock geochemistry and petrographic analysis from ~10,000 feet to the base of the well revealed a complexly deformed sequence as evidenced by a sudden transition into a complexly deformed propylitic alteration zone throughout the interval which likely affected RockEval organic maturation analyses. A minimum of 25 miles (~40 km) of shortening on the Lombard overthrust is required based on restorable cross section estimates. Hydrocarbon reserves beneath the Lombard thrust sheet are unlikely due to the unknown extent of the propylitic zone which likely baked any hydrocarbons present which further complicates exploration in the region.