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doi:10.2204/iodp.sp.337.2010

Introduction

Marine subsurface hydrocarbon reservoirs and the associated microbial life in continental margin sediments are among the least characterized Earth systems that can be accessed by scientific ocean drilling. Our knowledge of the biological and abiotic processes associated with hydrocarbon production is limited because of the highly limited opportunities to conduct scientific ocean drilling initiatives using deep-riser coring in natural gas and oil fields. A number of fundamental questions regarding deep subseafloor hydrocarbon systems remain unanswered. For example:

• What role does subsurface microbial life play for the formation of hydrocarbon reservoirs?

• Do the deeply buried hydrocarbon reservoirs such as methane hydrates and terrestrial coalbeds act as geobiological reactors that sustain life by releasing nutrients and carbon substrates?

• Do the conversion and transport of hydrocarbons and other reduced compounds influence biomass, diversity, activity, and functionality of deep subseafloor microbial populations?

• What are the fluxes of both thermogenically and biologically produced organic compounds, and how important are these for the carbon budgets in the shallower subsurface and the ocean?

To address these important scientific questions, Expedition 337 will drill and study the hydrocarbon system associated with a deeply buried coalbed off the Shimokita Peninsula in the northwestern Pacific by coring to 2200 meters below seafloor (mbsf) using the riser drilling system of the Chikyu (Fig. F1). Based on the existing seismic profiles as well as previous natural gas drilling surveys, it has been demonstrated that thick Eocene to Cretaceous lignite layers (~60% total organic carbon [TOC], ~100 m in thickness) are present under an Oligocene unconformity layer, hosting large quantities of coalbed methane (CBM) with a production rate of ~210 × 10³ m³/day (Osawa et al., 2002). During the Chikyu shakedown cruise (Expedition CK06-06) in 2006, methane hydrates were observed in porous ash and sand layers of shallow subsurface sediments to 350 mbsf. In the recovered sediments, counts of living microbes exceeded 10⁷ cells/cm³ and were thus highly abundant compared to other continental margin locations (Morono et al., 2009) (Fig. F2). This project aims to extend the drilling depth to 2200 mbsf and explore the microbial ecosystem associated with the deep coalbed in oceanic sediments, a habitat that has not been accessed by previous scientific ocean drilling. Hence, the study area is an ideal natural subsurface laboratory to study the deep carbon cycle and the deep biosphere.

Deep subseafloor life in continental margin sediments

Subseafloor sediments harbor a remarkably sized microbial biosphere that constitutes ~10%–30% of the total living biomass on our planet (Parkes et al., 1994, 2000; Whitman et al., 1998; Lipp et al., 2008; D'Hondt et al., 2009). To date, microbial cells have been observed in sediments ranging in age to the Cretaceous and in subsurface depth to 1626 mbsf (Newfoundland Margin, Ocean Drilling Program [ODP] Leg 210; Roussel et al., 2008). Diagenetic models of pore water chemical constituents as well as ¹⁴C- and ³⁵S-radiotracer incubation experiments showed that metabolic activities of deep subseafloor microbes are extremely low because of the low supply of energy-rich substrates (D'Hondt et al., 2002, 2004) but are often stimulated at geochemical and/or lithologic interfaces such as porous ash layers and sulfate-methane transition (SMT) zones (Inagaki et al., 2003; Parkes et al., 2005; Biddle et al., 2006; Sørensen and Teske 2006). The metabolic activities of subseafloor microbial communities are controlled by the flux of bio-available electron donors and/or acceptors, some of which are derived either from the overlying seawater by photosynthetic primary productions (D'Hondt et al., 2004, 2009; Lipp et al., 2008) or from crustal fluids underlying sedimentary habitat (Cowen et al., 2003; Nakagawa et al., 2006; Engelen et al., 2008). Fluid flow regimes in the subseafloor environment control availability of energy to microbial life. Hence, the geologic and sedimentological characteristics represent crucial factors controlling habitability of the deep subsurface. Culture-independent molecular ecological surveys of 16S ribosomal ribonucleic acid (rRNA) gene fragments reveal that the microbial communities in continental margin sediments are predominantly composed of species lacking cultivated relatives, such as the bacterial members within the candidate division JS1, Chloroflexi, and Planctomycetes, as well as the archaeal members within the Deep-Sea Archaeal Group, the Miscellaneous Crenarchaeotic Group, and the South African Gold Mine Euryarchaeotic Group (e.g., Inagaki et al., 2003, 2006b; Inagaki and Nakagawa, 2008; Inagaki, 2010). The carbon isotopic analysis of intact polar lipids (IPLs) and fluorescence in situ hybridization (FISH)-stained cells suggest that sizeable populations of heterotrophic Archaea significantly contribute to microbial biomass in organic-rich sediments, even at the SMT zone where the occurrence of anaerobic oxidation of methane (AOM) mediated by methanotrophic archaea and sulfate-reducing bacteria takes place (Biddle et al., 2006). Quantitative analysis of the IPLs extracted from sediments (>1 mbsf) at a variety of oceanographic settings reveal that ~87% of IPLs are archaeal, suggesting that the previous deoxyribonucleic acid (DNA)-based polymerase chain reaction (PCR) experiments had significant biases affecting the extraction and quantification (Lipp et al., 2008; Teske and Sørensen, 2008). Despite the significance of heterotrophic microbes in biogeochemical cycling within the continental margin sediments, the metabolic characteristics of organic matter degradation and fluxes of secondary metabolites remain largely unknown (e.g., Hinrichs et al., 2006).

Coal diagenesis: microbiological significance for biogeochemical cycles

Within the generally energy-starved deep subseafloor biosphere, lignite coal is a conceivable source of nutrients and energy for deep microbial communities. Microbiological and geochemical studies of terrestrial coal deposits and subsurface aquifers suggest that microorganisms play important ecological roles in coal diagenesis, resulting in substantial quantities of CBM as a terminal product (Brown et al., 1999; Detmers et al., 2001; Fry et al., 2009; Krüger et al., 2008; Shimizu et al., 2007; Strapoc et al., 2008) (Fig. F3). The microbial communities in terrestrial coaly habitats are phylogenetically highly diverse with relatively low cell density of <10⁶ cells/cm³. For example, methane-producing archaea (i.e., methanogens) such as the genera Methanoculleus, Methanobacterium, Methanolobus, and Methanosarcina, as well as some potential acetate-producing bacteria (i.e., acetogens), such as Acetobacterium, were dominant in a deep borehole aquifer directly connected to coal deposits in Hokkaido Island, Japan (Shimizu et al., 2007). Using incubation tracer experiments and the FISH technique, active methanogenesis was found to occur even in a highly altered graphite deposit (Krüger et al., 2008). Very recently, Fry et al. (2009) reported sizable cultivatable populations of potential sulfate-reducing bacteria, methanogens, acetogens, and lignite-utilizing heterotrophs in the uplifted coaly sediments of northern New Zealand based on results from the most probable number cultivation method. Metabolic activities were stimulated at the geologic interfaces between coal and sand/silt layers as reported from other terrestrial deep subsurface black shales (e.g., Krumholz et al., 1997), and the concentrations of organic acids in the coal layers were higher than in normal deposits, consistent with the co-occurrence of coal diagenesis and microbial processes.

Despite the microbiological and (bio)geochemical significance of coaly deposits for the global carbon cycle, there have been no studies of coal layers that are deeply buried in the subseafloor, mainly because of the safety regulations applied to hydrocarbon gas–related hazards during riserless drilling. In continental margin sediments, large quantities of gaseous hydrocarbons, as well as H₂, CO, CO₂, organic acids, aromatic compounds, NH₄, N₂, sulfur compounds, etc., are potentially generated by thermogenic and/or biogenic degradation processes from the coaly deposit. All of these compounds are potential nutrient and energy sources that support redox reactions mediated by the deep subseafloor microbial communities. Hence, coalbeds and their active microbial life may influence the upward transport of dissolved gases and organic matter in geofluids as well as the accumulation of gas hydrates in overlying sediments. In this regard, the connectivity between deep subsurface microbial activity and the formation and deposition of gas hydrates is a frontier research theme in geobiology and biogeochemistry that can only be studied by a dedicated initiative as Expedition 337.

Exploring the feasibility of CO₂ sequestration in deep offshore geological repositories

To date, CO₂ capture and sequestration (CCS) into deep subsurface environments such as oil, gas, and porous aquifers is considered as a solution for reducing the emission of substantial amounts of anthropogenic CO₂ and preventing dangerous consequences of the anticipated future climate change. CCS offshore deep subseafloor environments has a number of advantages, including a positive risk assessment compared to shallow-water CCS (Schrag, 2009; House et al., 2006). It has been predicted that CCS can potentially reduce future world emissions from fuel energy by 20% (Dooley et al., 2006). However, the behavior and stability of CO₂ as well as its chemical reactions in deep subsurface repositories are still largely uncertain.

In this project, we will address the multiple scientific issues of the geological CO₂ sequestration through ex situ experimentations using cored materials, tackling the following fundamental questions:

• How does liquid or supercritical CO₂ spatially penetrate into the various lithostratigraphic settings?

• How does CO₂ react with minerals, organic matter, and life in the deep subsurface?

• What are the impacts of long-term CO₂ storage on biogeochemical carbon cycling and the subsurface biosphere on different time scales?

Conducting various multidisciplinary ex situ experimental studies using cored materials as well as in situ logging characterizations of the deep riser hole, Expedition 337 will significantly expand our knowledge of the coalbed subseafloor hydrocarbon system, including the physicochemical and biological factors that determine the potential for CO₂ sequestration.

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