Investigation of the origin of shallow gas in Outer Moray Firth open blocks 15/20c 15/25d (seabed-400 milliseconds two way time)

摘要

1. Interpretations of the BGS shallow seismic data and commercial site investigation data show that gas is seeping from seabed in three large active pockmark complexes in approximately 150m or more water depth. The Challenger pockmark complex is in the north of block 15 / 25d, the Scanner pockmark complex is in the south of block 15 / 25d and the Scotia pockmark complex is adjacent and northeast of the Scanner pockmark.ud2. A review of the peer-reviewed scientific publications indicates that the majority of the arguments, based on isotopic analyses, are for a predominantly biological origin for the gas seeping from the active pockmarks. There is not, however, a secure scientific consensus as to whether there is a primary origin for the gas. Thus the possibilities are that the gas originates from a shallow biogenic source, a deep thermogenic source or from mixtures of these sources.ud3. Interpretations undertaken for this project indicate that gas seeping to seabed in the largest pockmarks is sited above the shoulders of buried sub-glacial channels. The gas seepages are fed from a laterally almost continuous blanket of buried gas-charged sediments situated between the sub-glacial channel margins at a depth interval of approximately 280-300ms two-way time (down to approximately120m below seabed).ud4. An empirical conclusion is that loss of shallow gas from the gas-charged interval at approximately 280-300ms two-way time will cut off the supply of shallow gas to the active pockmarks.ud5. The regional unconformity at the Crenulate Rreflector is the focus for shallow gas accumulation and it is a significant conduit for shallow gas ascending from depth to the east of the study area and into the study area.