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Abstract

A field of giant pockmarks was discovered at the base of the Upper Cretaceous Chalk unit in the westernmost Lower Saxony Basin in The Netherlands. 3D seismic and well data show that mostly circular, 300–850 m-wide and 10–50 m-deep, pockmarks formed at the top of the Lower Cretaceous Upper Holland Marl Formation, which overlies oil- and gas-filled Lower Cretaceous sandstone reservoirs in the vicinity of the study area. Based on our interpretations, we present a scenario of early gas generation in Carboniferous coals and a localized migration of the gas from its original subsalt reservoirs through a salt weld in the Zechstein evaporites into the shallow Cretaceous sandstone reservoirs and the fine-grained marl above. Diapiring salt walls thereby limited the gas migration and trapping to a 150 km2-sized basin. A sea-level drawdown during Base Chalk formation possibly led to excess pore pressure in the reservoir and the breaching of the seal close to the seafloor, which caused a short-lived expulsion of the gas and pockmark formation. While hydrocarbon generation, migration and trapping are common processes in this region, gas escaping at the seafloor with pockmark generation appears to be a rather rare and complex phenomenon. In general, the presence of pockmarks associated with salt welds may be used to constrain the timing and migration pathway of hydrocarbons from subsalt into shallower reservoir levels. Both features may imply a general reservoir potential for regions where suitable source rocks are missing in the post-salt succession.

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