FLUID-ROCK REACTIONS RELATED TO DEFORMATION WITHIN ORGANIC-AND VANADIUM-RICH UNITS OF THE PHOSPHORIA FORMATION, SOUTHEAST IDAHO

Black mudstones and shales within the Permian Phosphoria Formation are prolific source rocks for oil that has travelled 100’s km in carrier beds of central Wyoming. Phosphatic and vanadium-rich beds of the Meade Peak Member Historical mining exploration occurred locally throughout the Idaho-Utah-Wyoming fold-and-thrust belt since the 1900’s.

The Paris Thrust is the oldest fault in the fold-and-thrust belt sequence, with evidence for reactivation and incremental fluid expulsion followed by periods of inactivity. However, relatively little data regarding deformation of the Phosphoria Formation exists within this portion of the thrust belt. We examine outcrop and new core samples from a depth of ~ 1700 - 2500 ft from the footwall of the Paris thrust. We focus on the meso- to sub-micron scale analysis of chemical and physical processes with the aim of providing insight into the spatial and temporal evolution of the migration of fluids, variations in pore fluid pressures, and fluid-rock reactions during episodic deformation and slip in fine-grained source rocks.

Petrographic/SEM micro-scale observations show evidence for mass transfer and migration including compaction, intragranular and transgranular fracture mechanisms, pressure solution seams coated with bitumen. Hydrobreccias (evidence for natural hydro-fracture), numerous cross-cutting carbonate vein systems with hydrocarbon inclusions, and highly-reflective V-bearing slip-surfaces are also observed. Whole-rock X-Ray mineralogical and XRF-geochemical composition studies provide the framework for alteration and textural analyses associated with deformation. Preliminary Total Organic Carbon values measured range from 1.6 up to 34 wt %; with variable δC¹³/δC¹² (0‰ -12‰) values and δO¹⁸/δO¹⁶ (-4‰ to -16‰) carbonate vein fillings in core and outcrop samples.

If fluid flow and thermally driven processes occurred episodically, and as a result of deformation, we hypothesize that it can be deciphered through detailed geochemical and microstructural studies of the fracture and vein systems dissecting the source rocks. Studies at the proposed scales are critical to determining how we develop migration pathways at the initial stages of motion.