Compositional changes of light hydrocarbons during migration through overburden : proxy for assessing potential leakage from Geological Carbon Storage
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Light hydrocarbon compositions evolve during migration through geologic media, but our understanding of geochemical alteration is limited because of the challenges with analyzing fluids in the sedimentary column. Understanding fluid evolution is timely because of the possibility of upward fluid migration from Geologic Carbon Storage (GCS) operations. The first goal of this research is to identify to what extent hydrocarbons migrate to shallower intervals. Addressing this goal is challenging because microbial hydrocarbon production commonly occurs in the near-surface. Light hydrocarbon compositions are investigated in soil gas above a hydrocarbon system and in offshore sediment above a gas chimney. In both cases, the fluid sources are interpreted as microbial in origin. However, these geochemical datasets are relevant to attributing future light hydrocarbon seeps and anomalies above GCS sites. The second goal is to quantify alteration processes when migration has occurred. I hypothesize that phase changes and sorption are the primary alteration processes. To test this hypothesis, I numerical simulation these processes to compare with field datasets that are interpreted as migration. The models indicate that sorption has the most significant influence on light hydrocarbons, although more lab work is warranted to improve these models. Forward models of CO₂ migration show that phase changes are important in attenuating CO₂ and can be identified with noble gas compositions. This conclusion may be valuable to determining the source of CO₂ anomalies above GCS sites.