Byong-Jae Ryu, Korea Institute of Geoscience and Mineral Resources (Republic of Korea)
Michael Riedel, McGill University (Canada)
Ji-Hoon Kim, Korea Institute of Geoscience and Mineral Resources (Republic of Korea)
Roy D. Hyndman, Geological Survey of Canada (Canada)
Young-Joo Lee, Korea Institute of Geoscience and Mineral Resources (Republic of Korea)
Five piston cores collected in water depths from 1197 m to 2175 m, and 2404 line kilometers of single- and multi-channel reflection seismic lines have been analyzed to examine the potential for generation and presence of hydrocarbons in shallow sediments of the eastern deep-water Ulleung Basin, East Sea of Korea. The heat-flow measurement and core analysis revealed high heat flow and total organic carbon (TOC) contents which favor hydrocarbon generation. Sedimentation rates measured using the cores of the adjacent area indicate a good condition for organic diagenesis to occur. In case that similar TOC content and sedimentation rate also occur at a deeper sedimentary interval, substantial amounts of hydrocarbon gas could be generated. The generated and migrated hydrocarbon gas would be changed into natural gas hydrate (NGH) within the NGH stability zone of the shallow sediments in the study area. The cores recovered from the southern study area showed high residual hydrocarbon gas concentrations containing mainly biogenic methane up to 126.18 ml/l of sediment. In these cores, cracks developed parallel to the bedding were well observed. The cracks could be formed by expansion of in-situ free gas upon core recovery. However, due to the in-situ high pressure and low temperature conditions for the cores, gas dissociated from in-situ NGH is also expected. A number of chimney structures of reduced sediment layering reflectivity were well observed on seismic profiles. They are identified as vertical to sub-vertical features caused by the upward migration of pore fluid or gas, and containing of free gas or NGH. Often, they are associated with apparent velocity pull-up structures that are interpreted to be the result of high-velocity NGH. The NGH discoveries from the chimney structures of the adjacent area strongly support this interpretation. The chimney structures were mostly observed in the central part of the study area. Bottom simulating reflector (BSR) was the most commonly used seismic indicator of the presence of marine NGH. However, today, it is widely accepted that the presence or absence of a BSR provide little information on the distribution and concentration of NGH. BSRs were mainly identified in the southeastern study area. Seismic indicators related to the hydrocarbons such as chimney structure, pockmark, BSR, etc. were hardly detected in the northeastern study area. It may be caused by the lower amounts of methane flux that are assessed by analyses of sulfate methane interfaces.