Methane hydrate exploration on the mid Chilean coast: A geochemical and geophysical survey

Methane hydrate exploration was conducted off the Mid-Chilean margin, coast of Chile, west of Concepción, during October 2004. The goal of the expedition was to survey regions of possible hydrate accumulations by integrating data from seismic surveys, geochemical analysis of porewater samples from piston cores, and heatflow probing. Piston cores were collected at 13 sites along a NE–SW transect, which ran between 36′10.38S, 73′35.72W and 36′12.50S, 73′3976W. This sample pattern was based on a previous deep towed acoustics geophysics system (DTAGS) seismic survey. A correlation was evident between the seismic, geochemical and heatflow data, however, some non-related data patterns indicate the necessity for combination of the survey parameters. Baseline heatflow values were approximately 40 mW m− 2, while values more than three times background were observed over an apparent gas wipeout zone in the DTAGS line. Sulfate and methane profiles from core porewaters were analyzed to identify the depth where sulfate and methane approached minimum concentrations. This depth, known as the sulfate–methane interface (SMI), is used to estimate the gas flux from deeper sediment methane. Along the DTAGS seismic line the SMI ranged from 33.3 to 555 cm below seafloor (cmbsf) and the vertical sulfate diffusion was from − 19.1 to − 362.0 mmol m− 2 yr− 1. In the study region, measurement of the shallow SMI and vertical sulfate diffusion, coupled with the seismic survey and heatflow provides a more thorough indication of the presence of deep sediment methane gas pockets. This survey provides data to assist in the selection of deep drilling locations for hydrate exploration.