Determining the Origin of Shallow Gas in the Dutch North Sea Using Gas Chimney Detection - Implications for Deep Exploration

Shallow gas in the Dutch sector of the North Sea has largely been encountered in marginal marine to continental deposits of the Plio-Pleistocene Eridanos delta. However, the origin of this shallow gas is not well understood. A gas chimney detection project was undertaken in a 3D seismic survey to assess the reliability of the suspected chimneys, determine from which interval they originated (a potential biogenic or thermogenic source rock) and how they were linked to the shallow gas occurrences. By understanding the hydrocarbon migration pathways we should be able to delineate deep prospective traps and high grade additional shallow gas leads. Gas chimneys were detected using a supervised neural network trained on reliable examples of gas chimneys. Results of the study indicate that chimneys providing charge to the shallow gas sands are generally reliable. The chimneys originate from a Carboniferous gas-prone source rock interval, and can be directly linked to the shallow gas sands. Gas migrated vertically from the Paleozoic interval through faults in the Upper Permian Salt and were focused over salt cored anticlines. We observe gas clouds which closely follow the stacked channels, suggesting that at least some of the shallow reservoirs may be fully saturated with gas.