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

Abstract

Shallow gas in the Dutch sector of the North Sea has largely been encountered in marginal marine clastic deposits of the Plio-Pleistocene Eridanos delta. However, the origin of this shallow gas is not well understood. The shallow gas may originate solely from biogenic bacterial alteration. However, it is difficult to distinguish truly biogenic gas from thermogenic gas altered by shallow bacterial processes. Disturbed zones indicating potential gas chimneys were noted below the shallow gas anomalies. However, it was difficult without additional analysis to determine if this seismic response represented true vertical gas migration or was a seismic processing artifact. A gas chimney detection project was undertaken in the 3D seismic survey to assess the reliability of the suspected chimneys, determine from which interval they originated, 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. The resultant chimneys were validated based on a set of criteria used in many studies. 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 that can be directly linked to the shallow gas sands. Gas migrated vertically from the Paleozoic interval through faults in the Upper Permian Zechstein Salt. We observe gas clouds over some of the shallow channel reservoirs, indicating they are fully saturated with gas. The results of this study have three important implications for successful gas exploration in this sector of the North Sea. First, the study highlights deep vertical gas migration pathways which provide charge to prospective Triassic reservoir objectives. Second, the study highlights vertical gas migration pathways which provide preferential charge to shallow reservoirs. Third, the study provides clues about the top seal integrity of the shallow traps. Top seal integrity is a critical risk in the poorly consolidated shallow sands. Breached traps can still have strong AVO anomalies due to “fizz gas”. The morphology of gas chimneys above the trap (gas cloud or fault related) provides important clues about the top seal integrity.