SEEP - Building a SEabed Environmental baseline for Platform abandonment

As societies gradually shift from oil and gas to renewable energy, many offshore wells will be plugged and abandoned, while some will be transformed to facilitate carbon storage (CCS). In order to mitigate any methane leakage associated with abandonment, it is necessary to understand whether the leakage has a natural or anthropogenic origin. During exploration and production, emphasis is almost exclusively on the reservoir, while post abandonment monitoring programs generally focus on the water column. Thus, only little attention has been given to the shallow subsurface, how it has been influenced by hydrocarbon production, and to which degree the geology facilitates or inhibits fluid migration. In order to monitor and evaluate future leakage of hydrocarbons to the marine environment, it is crucial to understand the natural seepage through the seabed both locally at platforms and regionally. For CCS, understanding the shallow subsurface is equally important as monitoring cannot be confined to the water column alone. Mapping potential migration paths is thus necessary in order to mitigate any leakage.

 The aim of the SEEP project is to develop a Danish North Sea baseline for methane seepage in the shallow subsurface, near oil and gas platforms and in areas without any hydrocarbon production. Applying such a baseline will facilitate identification of anthropogenic seepage and thus help recognize the potential local environmental impact associated with abandonment.

Using newly collected geophysical data, we have categorized various types of shallow methane seeps, and placed them in a geological context. By combining the shallow seismic data with deep industry seismic data, we have identified potential sources and paths for thermogenic methane migrating from reservoir depths.  The geophysical data is then integrated with results from sediment core analysis. These include Facies analysis of cores, dating of sediments, benthic faunal variations between core sites, geochemistry of bivalves and foraminifera, studies of the chemical and isotopic composition of the dissolved gas in the pore water, as well as the community-composition of gas-degrading bacteria.

This integrated approach will provide a solid model for gas distribution, frequency and origin, as well as impact on the environment in the shallow subsurface.    

Here we present the different tools that are the foundation of such a baseline, including results and examples from geophysical mapping (both multibeam echosounder and seismic data), biostratigraphy, geochemistry and geomicrobiology.