Investigation of the origin of shallow gas in outer Moray Firth open blocks 15/20c and 15/25d

Abstract

This report briefly describes the evidence for the origin of shallow gas in Outer Moray Firth open blocks 15/20c and 15/25d. Sea floor pockmarks are known to occur within these blocks, and they indicate the seepage of gas from shallow levels into the local water column. An environmental concern is that any industry activity in these blocks must not plumb into any component of the system that is sustaining the gas seepages at sea bed. The conclusions of this study are: 1. Interpretation of the available BGS shallow seismic data and commercial site investigation data shows that gas is seeping from sea bed in three large active pockmark complexes in approximately 150 m or more water depth: the Challenger Pockmark Complex in the north of block 15/25d, the Scanner Pockmark Complex in the south of block 15/25d and the Scotia Pockmark which is adjacent to the north-east of the Scanner Pockmark Complex. 2. A review of the peer-reviewed scientific publications indicates that the majority of the arguments based on isotope analyses of gas and authigenic carbonate are for a predominantly biological origin for the gas seeping from the active pockmarks. However, biogenic isotopic signatures in the Gulf of Mexico and the North Sea are thought to have been generated when thermogenic hydrocarbons in shallow sediments were re-cycled by bacteria to produce ‘secondary’ methane with an identical isotopic signature to biogenic methane. Thus, the isotopic data derived from the Scanner Pockmark Complex do not provide a secure basis for determining whether the gas escaping from the pockmarks in block 15/25 is primarily biogenic or thermogenic in origin. 3. Interpretations completed during this project indicate that gas seeping to sea bed in the largest pockmarks is reservoired within the uppermost part of the Aberdeen Ground Formation, where it is preserved between buried sub-glacial channels. The gas seepages at sea bed are fed from an almost continuous blanket of buried gas-charged sediments situated between the sub-glacial channel margins at approximately 280-300 ms two-way time (around 120 m below sea bed) in the northern part of block 15/25d. 4. An empirical conclusion from distribution patterns observed in the interval between sea bed and 400 ms two-way time 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. Dry well 15/25b-1A, located immediately to the north of the Scanner Pockmark Complex, appears to have been drilled on the margin of the shallow gas reservoir. A recommendation is that future development operations should not disturb the shallow gas reservoir. 5. All of the hydrocarbon discoveries made within and around the study area are oil. The nearest Upper Jurassic Kimmeridge Clay Formation principal thermogenic gas kitchen lies some 30 km to the south-east, in the Fisher Bank Basin. Gas has been proved to have migrated up into Mid-Eocene sandstones (e.g. Alba Field) in that basin. Although it is possible that further vertical migration might have been achieved through minor faults and fractures in the Late Eocene to Pliocene, no evidence was observed in the 3D data to connect the location of the active pockmarks with supply from the thermogenic gas kitchen. For example, no gas chimneys have been observed within the Tertiary section on 3D seismic data across the study area.. Although no major faults have been found in 3D seismic data that transect the entire Eocene to Pliocene interval, minor polygonal faulting has been observed within the upper part of the Hordaland Group