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Using Noble Gases to Identify the Relationship Between Deformational Features and the Pathways of Geological Fluid Flow

Abstract

There is complex association between the tectonics of fold-thrust belts, the deformation of foreland basins, and the generation and migration of hydrocarbons and other geologic fluids in the subsurface. Accurately characterizing the relationship between these factors, specifically understanding how deformation features will regulate hydrocarbon migration, is critical to predicting the economic success of conventional and unconventional energy plays. The Marcellus Formation, located in the Appalachian plateau region, is the foreland of the archetypal Appalachian fold-thrust belt and contains various deformation features including folding, faulting, and fractures. As a result, this area provides an opportune location to examine the relationship between deformation and geological fluid flow. Noble gas geochemistry provides an underutilized geochemical technique to evaluate the fundamental properties of hydrocarbon gas migration. Herein, we employ noble gas geochemistry to examine the history of geological fluid migration in a highly deformed structural complex near the Marcellus outcrop in upstate New York. Specifically, we measure noble gas (helium, neon, argon) and hydrocarbon gas (methane, ethane, propane) chemistry of fluid inclusions and rock samples collected from various types of deformation features in the quarry. These include samples collected a) along a set of low angle thrust faults; b) around the limbs and hinges of anticlinal folds; and c) near zones of varying fracture intensity across the Seneca Stone Quarry. The noble gas isotopes can inform us about the history of hydrocarbon and crustal fluid migration because the initial 4He/21Ne from radiogenic/nucleogenic production is fixed and the initial 4He/40Ar can be calculated by measuring U, Th, and K concentrations. Although the initial radiogenic production rates are fixed, the rate of diffusional release of each radiogenic isotope (i.e., 4He*, 21Ne*, and 40Ar*) from mineral grains in the rock matrix varies as a function of mass. As a result, measuring the 4He/21Ne or 4He/40Ar in fluids or rocks provides information on porosity, temperature, and the rate and volume of fluid flow. Preliminary results suggest that the isotopic ratios of noble gases record variable extents of crustal degassing according to the type of deformation feature.