Gas migration along fault systems and through the vadose zone in the Latera caldera (central Italy): Implications for CO 2 geological storage

Abstract

A clear and detailed understanding of gas migration mechanisms from depth to ground surface is fundamental to choose the best locations for CO 2 geological storage sites, to engineer them so that they do not leak, and to select the most appropriate monitoring strategy and tools to guarantee public safety. Natural test sites (or “natural analogues”) provide the best opportunity to study migration mechanisms, as they incorporate such issues as scale, long-time system evolution, and interacting variables that cannot be adequately addressed with laboratory studies or computer models. To this end the present work examines the migration to surface of deep, naturally produced CO 2 along various buried and exposed faults in the Latera caldera (central Italy) by integrating structural geology and near-surface gas geochemistry surveys. Results show how gas migration is channelled along discrete, high-permeability pathways within the faults, with release typically occurring from spatially restricted gas vents. Size, distribution, and strength of these vents appear to be controlled by the evolution and deformation style of the fault, which is in turn linked to the rheology of the lithological units cut by the fault. As such gas migration can change drastically along strike. Gas migration in the vadose zone around these vents is also discussed, focussing on how the physical–chemical characteristics of various species (CO 2, CH 4, and He) control their spatial distribution and eventual release to the atmosphere.