DETERMINING THE SOURCE AND MIGRATION OF NATURAL GAS USING NOBLE GAS GEOCHEMISTRY

Over the last several years, interest in natural gas reservoirs in the Silurian (Medina and Clinton) and Devonian (Marcellus) black shales of the Northern Appalachian Basin has greatly increased. These gas reservoirs represent unconventional hydrocarbon resources because they do not consist of multiple, isolated, and productive fields, but instead consist of a continuous network of dispersed oil and natural gas. As a result, enhanced recovery zones are difficult to locate, particularly where fluids may have migrated over geological time. For example, in western NY, natural gas accumulations in these units constitute a mixed thermogenic gas. This gas mixture has previously been proposed to have originated from deeper source rocks and migrated up into Silurian sands and Devonian shales and mixing with in situ production (e.g. Jenden et al, 1993). Noble gas geochemistry provides promising techniques to evaluate hydrocarbon generation and migration processes, because noble gases are conservative tracers that can be used to identify source, migration mechanisms, and mixing with other geological fluids. We present the noble gas composition of natural gases from the northern part of the North Appalachian Basin (western New York) to evaluate the source of hydrocarbon generation, the style of fluid migration, and the degree of and potential for interaction with surface waters (drinking water).