Advances in 3D Imaging, Spectroscopy, and Stable Isotope Geochemistry to Constrain the Evolution of Liquids-Rich Shale-Gas Systems

Robert C. Burruss
U.S. Geological Survey, Reston, VA

The transition from liquid-rich hydrocarbons to non-associated dry gas in shale gas systems, especially in cases where separation between producing wells may be only a few kilometers, is not well understood. We are analyzing produced fluids (gas and oil) and fluids trapped as inclusions in diagenetic and fracture-filling minerals in shale-gas and tight-gas systems to understand the processes that control the evolution of hydrocarbons during the deepest stages of burial in these systems. Inclusions with hydrocarbons provide a record composition, temperature, and pressure of fluid migration during diagenesis and deformation. However, in deeply buried foreland basin fold and thrust terrains such as the Appalachian basin, individual mineral grains may contain multiple, overlapping fluid inclusion assemblages (FIAs) with distinct compositions generated during an extended history of burial, deformation, and uplift events.

By unraveling the sequence of events and comparing the compositions of inclusions to produced fluids, the geologic processes that produced the fluids recovered in economic shale gas systems can be identified. We are using 3D imaging and spectroscopic methods developed for biomedical research (coherent anti-Stokes Raman scattering, CARS; second harmonic generation, SHG; and two-photon excitation fluorescence, TPEF) combined with conventional methods of inclusion analysis (petrography, microthermometry, and Raman spectroscopy) to provide unprecedented views of fluid generation and migration in the shale-gas systems. For example, gases and solid organic matter in individual inclusions along intersecting microfractures within a single crystal of fracture-filling quartz within subsurface core show distinct compositions consistent with 10’s of kilometer migration of late-stage methane from the deep basin documented in inclusions in samples from outcrop.

The inclusions with the wettest gases also contain solids that with “graphitic” Raman spectra. In 3D TPEF images of the wet gas inclusions both the “graphitic” solids and the gas phase are weakly fluorescent, consistent with the presence of traces of aromatic hydrocarbons derived from hydrocarbon cracking. The fluid inclusion observations support previously published interpretations of hydrocarbon cracking and gas migration and mixing in the Appalachian basin based on compound-specific analysis of δ¹³C and δ²H in produced gases.

AAPG Search and Discovery Article #90186 © AAPG Geoscience Technology Workshop, Hydrocarbon Charge Considerations in Liquid-Rich Unconventional Petroleum Systems, November 5, 2013, Vancouver, BC, Canada