RADIOGENIC NOBLE GASES TRACK NATURAL GAS MIGRATION ALONG FAULTS AND FRACTURES IN BLACK SHALES

There is an inextricable link between the tectonics of mountain belts, rock deformation, and the generation and migration of fluids in sedimentary basins. Understanding the relationship between these processes is critical to understanding regional fluid migration of brines and the subsequent emplacement and entrapment of hydrocarbons and CO₂. Despite the critical influence of these processes, many questions about the mechanisms and pathways of fluid flow through fractures and faults, the timing of fluid migration, and the role of basin evolution (e.g., basin subsidence vs. inversion) remain unanswered.

The inert nature and distinct isotopic compositions of noble gases make them ideal, but underutilized, tracers of subsurface fluid migration. Because the production of radiogenic noble gases can be predicted by measuring the relative abundance of parent isotopes (i.e., U, Th, K), the distribution of radiogenic noble gases in crustal rocks and fluids can provide important insight into the behavior of fluids in the crust. For example, the production of ⁴He/²¹Ne* is fixed at ~⁴He/²¹Ne*=22x10⁶ in the crust. Nonetheless, the preferential release of each radiogenic isotope (i.e., ⁴He*, ²¹Ne*) from mineral grains in the rock matrix can vary as a function of temperature, mineral phase, and extent of fluid migration. As a result, measured ⁴He/²¹Ne* values in fluids or rocks provide information on fluid-rock interactions, temperature, and the rate and volume of fluid flow.

Here, we present He and Ne isotopic compositions of quartz grains, produced gases, and fluid inclusions from samples collected near zones of deformation (e.g., a thrust fault, a fold, and well-characterized fracture sets) and from drill-cuttings from producing wells in the Marcellus Shale (Appalachian Basin, USA). Preliminary results suggest that this technique can determine the percentage of gas-in-place, identify localized zones of gas accumulation and loss, and evaluate the relative influence that specific crustal deformation features have on fluid migration throughout the evolution of a basin.