关键词: 杭州湾, 全新世, 生物气, 地质特征

Abstract: From the study of depositional setting of Holocene ultra shallow biogas,sedimentary facies and its distribution at Hangzhou Bay area are analyzed,and the forming and evolution regularity of the sedimentary facies in close relationship with space time distribution of the biogas in the former river valley are pointed out.Jiazao biogas pool which is the most typical is analyzed and then some typical features of ultra shallow biogas pool are discussed.The study shows that the river downcutted and then the Qiantangjiang old channel with depth of 40 100 meters was formed during the last Glaciation at low sea level,and the old channel was infilled and covered during sea level rising in post glacial period.Muddy sediments of flood plain facies and shallow sea facies are not only gas source rocks,but also cap rocks,the latter may be regional cap rocks.Sandy lenticular body of flood plain facies distributing in the old channel is the sole gas bearing reservoir which has industrial value.The gas pool is of lithologic trap and of authigenic and autostoring and syngenetic type.On shallow S wave seismic profiles,top interface of gas bearing sand layer shows stronger reflecting boundary,and this reflection wave will sharply decline when gas bearing sand layer pinches out in transverse.Thus,gas bearing sand body can be outlined based on this result.

Key words: Hangzhou Bay, Holography, biogenic gas, geologic feature