MICROSCALE TRACE ELEMENT MAPPING OF MINERALIZED FRACTURES USING CRYOGENIC LA-ICP-MS

Geologic fluid flow is controlled by the hydraulic properties of the rock matrix as well as the properties of any deformation features. Fracturing is a commonly observed and pervasive deformation feature found in sedimentary basins. Depending on their characteristics (e.g. length, aperture, orientation, openness), fractures may play an important role in the migration of geological fluids on a variety of scales (micro- to macro- scale). Because vein filling minerals (e.g. calcite, halite, quartz) incorporate the chemical composition of pore fluids during mineralization, they provide a geochemical archive of pore fluid chemistry at the time of mineralization. The mineral character of healed fractures (veins) (i.e. grain size, crystal character), their chemical composition, and fluid inclusions record the physicochemical history of fractures conditions, changing deformation features, the evolution of crustal, and the volume of migrating fluids in the subsurface. Despite the potential insights gained from understanding vein chemistry, there is a surprisingly small volume of published data on the elemental composition of mineralized veins, specifically in unconventional hydrocarbon plays.

We hypothesize that this may result from a lack of suitable analytical methods for accurately analyzing trace element chemistry on the micro-scale within organic-rich samples. Herein we present the development and validation of cryogenic laser ablation ICP-MS as an analytical tool for “mapping” the trace element geochemistry of mineralized veins and fluid inclusions in organic-rich black shales. This technique allows us to both map and quantify the trace element concentrations on a micro-scale with a resolution of ~10 micron. We apply this technique to a complex set of mineralized fractures within the Marcellus shale of the Appalachian Plateau to evaluate the historical geochemical conditions and pulses of fluid flow within mineralized veins.