海洋沉积环境中深层地下水溶性天然气开采引起的地面沉降

LAND SUBSIDENCE DUE TO WITHDRAWAL OF WATER SOLUBLE NATURAL GAS FROM DEEP MARINE SEDIMENTS

日本东京周围的关东地区的地层是由海洋环境经数十万乃至数百万年沉积而成。由于大量的海洋生物沉积于地下,经数十万乃至上百万年的地质作用,形成质量上乘的水溶性天然气与医药用碘。自20世纪二三十年代起在东京附近就有天然气的开采活动。自1956年起在东京东侧的日本千叶县房总半岛东部的九十九里平原地区,开始了大规模天然气与碘的开采活动。野外开采活动主要通过从地下500～2000m深处抽取地下水而获得。20世纪60年代在该地区发现有地面沉降,1969年起开始地面沉降的观测。在过去的35a中地面最大累积沉降达0.85m,发现沉降范围的面积近800km2。九十九里观光地的海岸线后退近20～50m。2002年的大范围的地面沉降达2cm。最后从日本关东地下水盆的向斜构成式构造出发,讨论分析减缓地面沉降的途径。

The formation of Kanto Plain around Tokyo Bay, Japan, is a sediment in the oceanic environment through several hundred thousand years or even million years＇ geological activity. Under this geological activity, Ocean Plankton was transformed into high quality water soluble natural gas and medicine iodine. Since 1930s, there were gas production activities near Tokyo. Large-scale production of gas and iodine in Kujyukuri area started from 1956. Production was conducted through withdrawal of groundwater from the depth of 500 to 2 000 m. Land subsidence was found since 1960s and large-scale land subsidence surveying was started in 1969. During past 35 years, the maximum accumulated subsidence was 0.85 m and subsided area was 800 km^2. The coastal line of Kujyukuri backed off 20 - 50 m from its position in 1960s. In 2002, there is about 20 mm subsidence per year yet. Finally this paper describes South Kanto gas field and subsidence in Boso area. Discussion is made on the possible countermeasures to mitigate the subsidence： （1） recharge of some percentage of extracted volume of groundwater. （2） reduction of the withdrawal volume of groundwater. Through analysis of the structure of Kanto Geologic Basin, it is pointed out that reduction of the withdrawal volume of groundwater is more economic and effective than the other methods.