Characterization and origin of large Campanian depressions within the Chalk Group of the Danish Central Graben – implications for hydrocarbon exploration and development

Abstract

AbstractThis study re-examines large and deep U-shaped reflections (2–4 km wide and 100–200 m deep) within the Upper Cretaceous–Danian Chalk Group in the inverted Roar Basin of the Danish North Sea, previously interpreted as a moat associated with a contour-parallel current system and/or erosive channels formed by gravity-driven turbidites. Improved 3D seismic data quality and seismic interpretation techniques helped to identify overlooked reflection terminations, which suggest that rather than a linear depression, the U-shaped reflections outline several bowl-shaped depressions. In addition, vertical high-amplitude columns and vertical discontinuity zones within and below the depressions were recognized and interpreted to indicate the presence of small fluid pipes, suggesting that the formation of the depressions is more complex. Carbon isotope analysis of high acoustic impedance beds within the underlying Lower Cretaceous chalk showed negative δ13C values down to −20‰, and are interpreted to indicate sediments influenced by methane-derived authigenic carbonates. Permo-Triassic half-grabens seem to have been a major source of gas-bearing fluids, as evidenced by hydrocarbon leakage phenomena within Triassic–Lower Cretaceous strata. In areas where Zechstein salt is present, the leakage root lies at salt welds, causing the formation of focused seismic reflection wipe-out and dim zones. In areas where salt was absent, the leakage root comprises a much more diffuse zone across the fault boundaries of the Permo-Triassic half-graben, and gas chimneys are characterized seismically as broad vertical dim zones up to 10 km wide. Campanian inversion tectonics caused fault reactivation and several hundreds of metres of uplift in the Roar Basin, which created an instability for the trapped gas-bearing fluids. Gentle fluid venting through observed pipes caused sediment suspension and entrainment, which could be carried away by bottom-current activity, causing localized zones of non-deposition and the formation of individual depressions. This model thus does not disregard the role of bottom-current activity in the formation of the depressions, yet it includes a fluid-venting element that fits better with the architecture and overall evidence for fluid-venting features in pre-chalk strata, as well as in the Chalk Group. Importantly, it shows that prior to the thermogenic maturation of the main source rock (i.e. the Bo Member of the Farsund Formation in the Late Miocene), fluid venting had already occurred on the Late Cretaceous seafloor from deeper source rocks that are at present overmature.