Joint Interpretation of Electromagnetic and Seismic Data to Determine Gas and Methane Hydrate Distribution within the Nyegga CNE03 Seep.

Abstract:

The presence of the Nyegga pockmark field within the gas hydrate stability field on the Norwegian continental slope points to possible methane release through cold seeps. It is, however, not clear how the methane may migrate through the still existing hydrate stability zone as the gas should form new hydrate further up in the section. In an effort to better understand the physical processes and quantify the hydrate/gas distribution in the area, GEOMAR and the University of Southampton collected different types of electromagnetic data sets with different depth ranges and resolution in a region, which has been extensively covered by seismic investigation.

The presence of hydrate within seafloor sediments causes an increase in both the bulk modulus, and hence the seismic velocity, and in the electrical resistivity. The presence of gas decreases seismic velocity but increases electrical resistivity.

Both seismic and controlled source electromagnetic methods can and have been used to estimate hydrate and gas content. Since seismic velocity and the electrical resistivity are sensitive also to a range of other parameters such as lithology, porosity and pore fluid salinity, single rock property parameter investigations are hampered by many uncertainties. However, since the sensitivities are different for the two properties, determination of both velocity and resistivity for the same volume of sea bed provides a powerful tool for distinguishing the contribution of hydrate or gas from that of other parameters.

In this work we report on a new type of 3D electromagnetic high resolution tomography experiment which was focused on the CNE03 chimney in the centre of the region. The data indicates the presence of a highly resistive zone, where seismic travel time tomography has shown the presence of gas hydrates within the chimney. The distribution of methane hydrate concentrations are derived based on a joint interpretation of seismic velocity and electrical resistivities.