Scientific objectives and hypotheses

During Expedition 337, extending the riser drilling/coring depth at Site C9001 is planned to 2200 mbsf, where the terrigenous Eocene coalbed (lignite) is situated beneath the overlying marine sedimentary realm (Fig. F5). In addition, using Hybrid-PCS on the Chikyu, the shallow sedimentary sections to the maximum depth of 365 mbsf will be retrieved under in situ pressure condition, including methane hydrate–bearing sediments (Fig. F6).

The proposed drilling exploration of the deep hydrocarbon system off Shimokita provides the unique opportunity to examine geobiological and diagenetic processes at interfaces between marine and terrigenous sediment and coal formation in deeply buried strata. No microbial life or its activities have been documented to date at the targeted burial depths and environments. Expedition 337 will be driven by three overarching testable hypotheses:

  1. The deeply buried Eocene coalbed acts as a geobiological reactor that releases dissolved organic compounds such as methane, acetate, and other substances.

  2. The conversion and transport of coalbed-derived organic substances influence microbial and diagenetic processes in the overlying, shallower strata.

  3. The subsurface coalbed has the potential to serve as a cap rock for potential CO2 sequestration and can support biological conversion of CO2 into biomass and organic compounds even at high CO2 concentrations.

The following operational objectives, to be addressed during Expedition 337, will be tied to the above hypotheses and guide our research strategy:

  • Constrain the impact of a thermally immature coalbed on the diagenetic and microbial processes at great burial depths;

  • Quantify the upward fluxes of dissolved organic compounds, such as gaseous hydrocarbons and volatile fatty acids, out of the coalbed and evaluate their impact on microbial processes in shallower strata; and

  • Test whether distinct active microbial communities inhabit the deeply buried coalbed, the overlying sediments of terrigenous origin, and the even shallower marine sediments and how they respond to high CO2 concentrations and low pH.

We will address the following set of specific research questions:

  • What is the ecological and biogeochemical relevance of deeply buried lignite in the natural hydrocarbon system offshore the Shimokita Peninsula?

  • What are the fluxes of both thermogenically and biologically produced methane and other diagenetic products, such as organic acids, into shallower strata and how important are these for the carbon budget?

  • How does coal diagenesis affect subseafloor microbial biomass, diversity, and metabolic activities?

  • Does the presence of the low-maturity coalbed stimulate heterotrophic and autotrophic microbial communities?

  • What is the natural flux of CO2 and CH4 from the coalbed hydrocarbon system, and what is the potential for CO2 sequestration in the Shimokita system?

  • How does excess CO2 react with minerals and organic matter in the drilled formation, how will this change the physical and chemical characteristics, and how will it affect the microbial communities?

  • What is the paleoenvironmental information recorded at Site C9001?

  • What is the extent of subseafloor life and the biosphere?

During Expedition 337, we will meet these objectives by: (1) spot coring marine (Pliocene to Oligocene) and terrestrial (Eocene) sediments, which include unconformity layers as well as coal-tuff-sand layers; (2) wireline logging of various geophysical and geochemical properties in situ; (3) sampling of in situ pristine formation fluids using wireline sampling tool; and (4) undertaking extensive microbiological, biogeochemical, geological, and geophysical analyses of the cores and borehole logging data.

This project will expand our knowledge of geobiological and biogeochemical properties in the coalbed hydrocarbon system. Similar coaly environments are widely distributed along the western coast of the Pacific Ocean, and hence our results will be of great societal relevance. Because the effect of high CO2 concentrations and the associated decrease in pH under conditions of CO2 sequestration into the deep coal/sand-layers is one of the primary objectives to be addressed, the shore-based laboratory experiments will include quantitative evaluation and modeling of fluid flow and biological systems in the subseafloor environment, including their response to high CO2 concentrations and low pH. These applied scientific aspects will add an important new component to IODP.