5:15 PM - 5:30 PM
[MIS24-05] Report on the drilling results of gas chimneys by R/V Hakurei (HR14) in the eastern margin of the Japan Sea
Keywords:shallow methane hydrate, Japan Sea, gas chimney, drilling
Two neighboring sites are selected to drill on a small mound of Tobishima West by the results of the SBP in order to characterize the gas chimney structure; the one shows a clear stratified structure (RC1408) and the other shows a sonic blanking zone that is called a gas chimney structure (RC1407). The drilling results show the stratified part bears no methane hydrate as deep as around 48 m and consists of alternated thinly laminated and bioturbated layers that is the typical sediments of the Japan Sea. On the other hand, the gas chimney structure is found to be composed of silty clay with methane hydrate and carbonate nodules.
The occurrence and the vertical distribution of methane hydrate in the gas chimney structure show characteristic features. The surface methane hydrate obtained was subdivided into massive, granular, platy and veined types by the occurrence in the cores. Massive and granular types are common in the shallower horizons, while the platy and veined types develop well in the much deeper horizons. Methane hydrate was abundant in the shallower horizons than 40 m to 60 m in two long cores. Gas is also found as well as solid methane hydrate within both methane hydrate and mud.
A model of the subsurface distribution of the shallow gas hydrate and gas in the gas chimney structure is proposed by the drilling results. The proposed model of the gas chimney structure of the shallow gas hydrate, although, has a weak point that it is based only on the three different drilling sites and the lateral changes in the occurrence and amount of methane hydrate within each gas chimney structure is not clarified yet.
Observed vertical distribution of the types of methane hydrate denies a concept that a constant and continuous supply of gas in a stable stability zone of methane hydrate as long as a few hundreds of thousands of years. Instead a catastrophic methane event that occurred in recent years such as the last glacial maximum should have caused the distribution pattern of the types of methane hydrate observed.
We express sincere gratitude to all the persons in JOGMEC who engaged to operate R/V"Hakurei". This research is a part of METI's project entitled "FY2014 Promoting research and development of methane hydrate".