Variation in Methane Hydrate Structure and Composition
Authors: Coffin R.B. , Grabowski K.S. , Linton J.A. , Thieu V. , Halpern Y. , Montano P.A. , Doctor R.D. 
Published: October 2008

Methane hydrates are now recognized to be present in substantial quantities along most ocean margins. Current estimates indicate that methane in hydrates exceeds fixed carbon in petroleum reserves by about a factor of three. This is a large potential source of energy, capable of lasting for centuries. In response, Japan, Korea, Norway, India and Canada are actively investigating the acquisition of methane from hydrates as an energy source. Their success could significantly restructure the global economy. While a large international focus is on methane hydrates as a source of energy, hydrates are also thought to influence ocean carbon cycling, global warming, and coastal sediment stability. Thus, methane hydrates are a significant emerging research issue. While the global distribution and quality of hydrate fields are certainly being pursued, this presentation describes more fundamental investigations of naturally formed methane hydrates. Specifically, the Naval Research Laboratory (NRL) has initiated research on the influence of hydrates on ocean floor geoacoustical and geotechnical properties. In addition, NRL and Argonne National Laboratory (ANL) have studied methane hydrates from different regions of the world ocean for variations in structure and composition. This presentation compares the methane hydrate structure, composition, and source of carbon for samples taken from the Norwegian-Greenland Sea and the Gulf of Mexico. Structural analysis is compared using x-ray diffraction. Composition is described from gas chromatography, and the carbon source is identified with carbon isotope analysis.

Introduction

Methane hydrates vastly exceeds other carbon reservoirs in the ocean1. Research demonstrates that both thermogenic and biological carbon sources contribute to the formation of the methane hydrates2,3. There is a large variation in the sources of methane between ecosystems. The carbon cycling that controls methane production in the ocean floor is related to a complex mixture of biological, chemical and physical processes. Physical factors on the ocean floor relate to motion of the continental plates resulting in expulsion of fluids and volatile compounds, flows of geothermal energy toward the sediment-ocean interface, cold pressure mediated seeps of reduced compounds, and high levels of land based carbon transport to the ocean floor. These processes result in a variety of chemical speciation through key elemental pools that enhance microbial activity. For example, ocean floor thermal seeps have a high flux of reduced compounds, e.g. CH4, H2S and NH4, that supports the chemoautotrophic population. These cycles serve as the base of the food chain where enzymatic oxidation results in fixation of CO2 into cellular biomass. In active regions this results in CO2 serving as the terminal electron acceptor and methane production and fixation in hydrates. The variation in thermogenic and biological methane sources influences the hydrate content and structure. The degree that these factors contribute to the hydrate stability and formation warrants further research. Results from this research topic will contribute to methane hydrate mining strategies, understanding of coastal stability and environmental health, analysis of ocean carbon cycling, and predication of global warming.