Reports: DNI250999-DNI2: The Control of Dissolved Organic Matter Compositions on the Production of Biogenic Methane in Unconventional Gas Reservoirs

Michael Formolo, University of Tulsa

Dissolved organic matter (DOM) is one of the most complex carbon pools on Earth.  The molecular characteristics of DOM remain unresolved in unconventional shale gas reserves.  The growing economic and societal importance of shale gas necessitates a better understanding of the subsurface geochemical processes driving biogenic gas formation. This research is focused on exploring how subsurface formation water chemistry links host lithologies to resident microbial communities that generate shale gas.  The two main objectives of this study are to address if the (1) intrinsic shale characteristics and formation water chemistry influence the bioavailability and composition of the dissolved organic matter and/or (2) the composition of DOM promotes or inhibits biogenic gas production in shale reservoirs. 

Our study area, the Devonian Antrim Shale Formation in the Michigan Basin, is one of the most economically important shale gas reservoirs in North America.  The geological and geochemical conditions in the Michigan Basin provide a natural laboratory to study DOM as it relates to biogenic gas production.  We anticipate that results from this study can be applied to other economically important unconventional gas reserves while providing valuable information regarding deep subsurface carbon cycling, microbiology, and gas generation.

The research has been subdivided into two phases.  The first segment involved field sampling during the summer of 2012.  The objectives of our sampling were to test our newly developed field sampling equipment and to acquire preliminary samples to measure the trace metal and dissolved organic carbon (DOC) concentrations.  In preparation for this sampling, M.Sc. student Claire Astore and I had to overcome the logistics of sampling water from gas wells while keeping the formation water samples free of oxygen, free of organic contamination, and quickly acidified to maintain in-situ chemical signatures.  After testing our equipment we successfully sampled five gas-producing wells in the biogenic gas reservoir.  These wells represent the spectrum of production characteristics important to this study. 

During the initial phase of field sampling M.Sc. student Claire Astore also data-mined well production and inorganic geochemical data from previous publications and researchers.  Collaborator Dr. Anna Martini (Amherst College) generously provided Antrim water chemistry data from her extensive research in the region.  Integrating these historical records with our new data provides a protracted view into the geochemical controls and variations during biogenic gas production in an unconventional shale reservoir

Dissolved organic carbon concentrations were determined from the formation water samples collected during the 2012 field work.  Our molecular characterization of the DOC pool requires a sufficient concentration which we had to confirm in the formation waters.  We were able to prove our DOC concentrations will be adequate to characterize the organic matter during the second phase of the proposed research.

In addition to the DOC concentration data our preliminary data includes a suite of dissolved trace metal concentrations.  These data show variability among the five sample sites.  For many of the trace metals measured there are statistically significant relationships with overall production, while others show no apparent correlations.

During the second phase of the research we will prepare and characterize the DOM samples using ultra-high resolution mass spectrometry via the Fourier transform-ion resonance technique along with traditional gas- and liquid-chromatography.  This work will be done in collaboration with Dr. Thorsten Dittmar from the Max Planck Institute for Marine Geochemistry.  Linking this high resolution molecular characterization technique with the trace metal and DOC formation water geochemistry will provide information regarding the subsurface controls of biogenic gas production.

Funding from this research has provided financial support for M.Sc. Claire Astore.  She has presented, or will soon present, results at the AAPG Rocky Mountain Rendezvous and the Integrated Petroleum Environmental Consortium (IPEC) conferences.  For the IPEC conference she was invited as a presenter.  In addition, this work has been received with great interest by both the industry and academia, leading to new and exciting collaborations.