EVALUATING THE GEOLOGY AND GEOCHEMISTRY OF HELIUM, CO2, AND LOW-BTU HYDROCARBON RESERVOIRS IN THE CRUST (Invited Presentation)

Throughout the last several decades, low-BTU natural gas fields, including natural gases rich in CO₂, helium, and nitrogen, have become critically important economic natural gas deposits in the US and abroad. For example, the use of carbon dioxide for enhanced oil recovery (EOR) has revitalized U.S. domestic oil production, while investigations of their occurrence are important analogs for the potential storage of anthropogenic CO₂. Similarly, anthropogenic demands for helium in the medical, nanotechnology, and defense industries have increased precipitously in recent decades amid a decreased commitment to the US Strategic Helium Reserve. For these reasons, there has been a renewed interest in previously overlooked natural gas deposits that are rich in low-BTU natural gases, specifically CO₂ and helium in recent years.

Much of the previous work on low-BTU natural gases has established a relationship between the introduction of volcanic or geothermal fluids and the occurrence of natural gas reservoirs with elevated CO₂ and helium contents. The principal factors for predicting the discovery of these deposits, in addition to the availability of suitable seals and structural or stratigraphic traps, are establishing the relative timing of natural gas emplacement, the mechanisms and dynamics of fluid migration, and the associated compendium of gas-water-rock interactions that typically reduce the volume of CO₂ via trapping and increase the release of helium from minerals in the crust by metamorphism. Herein, we will review the state-of-the-art in gas compositional analysis and geochemistry of established and emerging low-BTU natural gas deposits in the continental US. We will focus on recent developments in noble gas and isotope measurements, interpretation, and numerical modelling that have helped constrain the source of low-BTU natural gases, the timing and mechanisms of natural gas migration from source to emplacement, and the interaction of gas with water and rocks in the crust. In combination with other traditional petroleum geology and structural/tectonic analysis, this data will provide a perspective on the search for these increasing economically relevant low-BTU natural gas reserves.