UNDERSTANDING THE SOURCE OF NEAR-SURFACE LIGHT HYDROCARBONS

Buoyant fluid compositions evolve during migration through geologic media. These alteration processes are relevant to a wide range of applications including Carbon Capture and Storage (CCS) and oil and gas exploration. Our research investigates changes in light hydrocarbons (methane through butane) with the goals of evaluating near-surface anomalies and assessing whether or not fluids originated from deeper geological units or from other sources closer to the surface. Whether fluids originate from infrastructure leakage, seepage, or natural near-surface processes has significant implications for identifying deeper commercial deposits or potential reservoir leakage. To achieve these goals, an extensive geochemical sampling and analysis program was undertaken in soil gas at a CO₂-EOR site, including stable and radiogenic radioactive isotopes of CO₂ and CH₄, light hydrocarbon concentrations, noble gases, and perfluorocarbon and sulfur hexafluoride tracers. At a separate offshore site, hydrocarbon compositions were measured on dropcores taken above shallow seismic anomalies. Results showed fluid anomalies at both sites originated from natural biological processes rather than leakage or seepage. Radiocarbon isotopes were the most useful indicator of fluid source. The data collection and analysis in this study can be applied to understand the sources of other near-surface anomalies.