The Mjّlnir marine impact crater porosity anomaly

We have developed a quantitative model for the porosity change induced by the Mjølnir impact using density and seismic traveltime distributions, and postimpact sediment deformation. Compared with the surrounding undisturbed platform sediments, the porosity increased immediately after impact up to 6.3% in the collapsed and brecciated crater periphery, whereas porosity decreased by 1% in the dense central uplift. Following impact, the crater was progressively buried by a ∼2–2.5 km thick overburden, which caused radially varying differential compaction among the disrupted sedimentary rocks. This compaction has significantly affected the postimpact crater evolution, and decreased the porosity anomaly to values of 2.5% in the periphery and −2.2% in the center. High-resolution seismic profiles reveal possible gas-related seismic-amplitude anomalies spatially related to the 13–33 km3 excess pore space in impact-deformed strata along the crater periphery. Potential plays may include Lower to Middle Jurassic sands structurally trapped by impact-generated tilted fault blocks, and the synimpact, extensively fractured, allochthonous breccia. However, the crater is a very high-risk hydrocarbon prospect, because reservoir properties may have been destroyed by the deep Mesozoic burial followed by large-scale Cenozoic uplift and erosion. Nonetheless, the Mjølnir porosity anomaly provides new data about the postimpact crater deformation. Similar data are almost entirely absent from the terrestrial impact record.