Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
권호 표지

Distribution of dissolved methane on the seafloor at the gas hydrate drilling sites, Ulleung Basin

울릉분지 가스하이드레이트 시추 지점의 해저면 해수메탄 분포

Kang, Nyeon-Keon;Hwang, In-Gul;Kwon, Young-In
강년건;황인걸;권영인

  • Published : 20081000

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Abstract

In 2007, gas hydrate drilling expedition was conducted at 5 gas hydrate potential sites in Ulleung Basin, East Sea, Korea. During the LWD(Logging While Drilling) phase, seafloor topography and distribution of methane concentration were investigated using depth and methane sensors, attached on the ROV(Remotely Operated Vehicle). Seafloor topography is represented by pockmarks and small mounds, along which high concentration of methane was detected. In the inner part of the pockmark, methane concentration is higher than that of the marginal part. Seepage of methane from deep and over-pressured sediments may be responsible for development of dome shape mounds and pockmark. During the drilling, methane concentration on the seafloor changed with drilling depth. These changes are thought to be related to the leakage of methane which is dissolved in the sediment pore and free gas zone below the BSR(Bottom Simulating Reflector), stimulated by the drilling.

2007년 동해 울릉 분지에 분포하는 5개 가스하이드레이트 유망지역에 대한 심부 시추가 수행되었다. 이 연구에서는 LWD(Logging While Drilling)동안 무인잠수정에 부착된 측정기를 이용하여 시추 예정지역의 해저지형과 해저면 메탄 농도 분포 특성을 조사하였다. 연구지역에서는 포크마크(pockmark)와 소규모 언덕지형이 나타난다. 연구지역의 해수 내 메탄 농도 분포를 측정한 결과 포크마크와 소규모 언덕지형을 따라 높은 메탄 농도 분포가 나타났다. 포크마크에서는 연변부 보다 함몰된 내부에서 상대적으로 높은 메탄 농도 분포를 보였다. 언덕지형에서는 언덕부와 인접한 곳에서 높은 메탄 농도가 나타났다. 이와 같은 지형은 퇴적물 내에 과압의 상태로 분포하는 메탄이 해수 중으로 누출되면서 해저면의 지형 변화를 가져와 포크마크 또는 소규모 언덕지형을 형성한 것으로 판단된다. 시추 동안 측정된 메탄 농도는 시추 깊이에 따라 변하는 양상을 보이며, 연구지역에서는 특히 BSR(Bottom Simulating Reflector) 예정 심도를 지난 이후 메탄 농도가 증가하는 경향이 나타난다. 이는 퇴적물 내 존재하던 메탄이나 BSR 하부의 자유 메탄가스가 시추로 인해 해저면으로 방출된 것으로 추정된다.

Keywords

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