This project was intended to test the usefulness of shallow geochemistry and heat flow data acquisition as an aid in the delineation of deeper, concentrated gas hydrate accumulations. NRL scientists conducted geochemical and heat flow surveys of selected sites in Alaminos Canyon Block 818, to support 1) DOE/ Chevron Joint Industry Project (JIP) on subsurface hydrates in the Gulf of Mexico and 2) NRL's Advanced Research Initiative on shallow sediment methane seeps. Geochemical data coupled with heat flow probe data were used to estimate regional variations in vertical methane flux through shallow seafloor sediments and to characterize the origin of the gas. Sites were selected based on a review of inline and crossline seismic profiles provided by WesternGeco Inc.
Marine Biogeochemistry Section, Naval Research Laboratory, Washington, D.C. 20375
Currently, preliminary evaluation of deep hydrate occurrences is accomplished using seismic surveys to define zones with strong bottom simulating reflectors (BSRs). Recent NRL studies of the mid-Chilean Margin and Atwater Valley in the Gulf of Mexico combined seismic data with heat flow data and pore water geochemical profiles to provide a more thorough evaluation of gas hydrate deposits. Conflicting geochemical and seismic data over strong seismic blanking regions indicate a need to integrate these parameters for deep sediment gas hydrate evaluation.
The biogeochemical cycles in shallow sediment over methane hydrate beds are active in response to upward vertical methane diffusion and advection from deep sediments, as well as the downward diffusion of sulfate from seawater. The interface between upward methane diffusion and downward sulfate diffusion is known as the sulfate methane interface (SMI). At the SMI anaerobic oxidation of methane (AOM) occurs, resulting in oxidation of methane and reduction of sulfate. These coupled metabolic processes are commonly carried out by a synthropic consortium of archaea and bacteria. AOM is the dominant process responsible for methane oxidation in oceanic sediments around the world.
This expedition to Alaminos Canyon was designed to combine porewater geochemical studies with thermal and seismic studies to assess potential deep sediment gas and gas hydrate distribution at and around seafloor seeps. Alaminos Canyon seismic data were provided by WesternGeco to assist in the selection of porewater geochemistry and heat flow sampling sites.
This project was expected to result in improved characterization of the gas hydrate system present at Alaminos Canyon, Block 818. The water sampling, sediment coring, and thermometry from this expedition were intended to provide valuable guidance in generating a JIP drilling plan.
Twenty piston cores were retrieved, and approximately 35 heat flow measurements were made. Geochemical and heat flow data were used to interpret the spatial variation in vertical methane fluxes in Alaminos Canyon Block 818.
Twenty piston cores were retrieved during 22 deployments. Core lengths ranged from 2.76 to 7.51 m. Onboard geochemical analysis included sediment (headspace) methane, and porewater sulfate, chloride, sulfide, and dissolved inorganic carbon concentrations. Detailed descriptions were logged for each core that noted visually recognizable variations in core composition, specifically the noting the presence of clay, silt and sand layers. Stable carbon isotope ratios, sediment porosity, and other sediment chemical parameters were measure at the onshore laboratory. Initial data interpretation used porewater CH4 and SO4-2 profiles to estimate CH4 fluxes and suggest relative rates of uncoupled organoclastic SO4-2 reduction (SR) and SR coupled to the anaerobic oxidation of methane (AOM). Sediment headspace methane gas concentrations varied by 7 orders of magnitude within all samples. In general, lower concentrations were found in the shallow core subsamples and higher concentrations in deeper sections. Core taken in the northeastern region of the study contained the highest CH4 concentrations with a range from the limits of detection (LTD) to 21.3 mM. One of these cores contained visible gas hydrates. Porewater chloride concentrations were generally near the background value of open ocean seawater (559 mM). The core containing hydrate was observed to have substantially lower chloride concentrations with a range of 390 mM to 502 mM. Porewater sulfate profiles were used to estimate the SMI depth for predictions of vertical methane fluxes through the study site. The SMI depths through the coring region ranged from 1793 cm to 308 cm. With key shallow regions corresponding to the high porewater methane concentrations.
Alaminos Canyon heat flow data ranged from 36.8 mW m-2 to 54.6 mW m-2. These data showed similar trends to the porewater shallow SMI with two distinct regions with higher thermal gradients. Higher heat flow values corresponded to the regions with shallow SMI. However, the range in heat flow data in this regions low relative to the data collected on Atwater Valley in the Gulf of Mexico where values were near 40 mW m-2 and maximum data up to 160 mW m-2 .
Geochemical and heat flow data were used to interpret the spatial variation in vertical methane fluxes in Alaminos Canyon Block 818. Goals of this fieldwork were to provide a hydrate pre-drilling data base, further develop calibration of shallow geochemical interpretation of deep seismic surveys of hydrate deposits, and continue basic research on the biogeochemical influence shallow sediment methane cycling. The following points outline the results of the current data set and plans for further data interpretation.
Preliminary estimates of SMI depths for Alaminos Canyon are generally deeper than data collected in the Gulf of Mexico, mid Chilean Margin and off the coast of New Zealand. The water column depth at these other location ranged from 1100 to 1400 meters, while this study site was 2900 to 3000 meters. Further data interpretation of upward methane and downward sulfate gradients will consider the influence of water column depth pressure on the hydrate stability and vertical methane flux. This factor needs calibration for geochemical data interpretation between different coastal regions. Shallow SMI data were observed in regions with elevated heat flow and seismic signatures that indicated possible vertical fluid and/or gas fluxes. Sediment porosity will be analyzed to provide calculations of the vertical methane flux rates through the study region.
Heat flow ranged from 36.8 mW m-2 to 54.6 mW m-2. These values were in the same range as low background data collected on Atwater Valley in the Gulf of Mexico. Regions with high vertical fluxes up to 160 mW m-2 in Atwater Valley were observed on top of a mound where high vertical methane fluxes were believed to result from deep sediment hydrate instability created by salt layers below the methane hydrate stability zone. Lower heat flow data on Atwater Valley was observed across a region off the mound with deep level BSR and deeper SMIs.
The field work was completed and a Cruise Report [PDF] was submitted in December, 2007. Subsequent data analysis of samples collected during the cruise included:
Other U.S. Federal Government and international contribution: $330,000 (Naval Research Laboratory)