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

Abstract

This study re-examines large and deep U-shape reflections (2-4 km wide, 100-200 m deep) within the Upper Cretaceous-Danian Chalk Group in the inverted Roar Basin of in the Danish North Sea, previously interpreted as a moat associated with a contour-parallel current system and/or erosive channels by gravity-driven turbidites. Improved 3D seismic data quality and seismic interpretation techniques helped to identify overlooked reflection terminations that suggest that rather than a linear depression, the U-shape 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 shows 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 to Lower Cretaceous strata. In areas where Zechstein salt is present, the leakage root lies at salt welds, causing the formation of focussed 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-grabens, and gas chimneys are characterized seismically as broad vertical dim zones up to 10 km wide. Campanian inversion tectonics caused fault reactivation and several 100s of meters uplift of the Roar Basin, which created an instability of the trapped gas-bearing fluids. Gentle fluid venting through observed pipes caused sediment entrainment, which could be carried away by bottom current activity, causing localized zones of non-deposition and 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 already occurred on the Late Cretaceous seafloor from deeper source rocks that are at present overmature.

The occurrence of a palaeo‐lithification front in an overpressured basin (Chalk Group)—Timing from clumped isotopes and seismic stratigraphic analysis

“…The significance of the burial model for the North Sea Basin is fourfold: (1) the current porosity bipartition was established in the end of the Oligocene to Middle Miocene, during which several source rocks could expel biogenic gas or thermogenic hydrocarbons (Kubala et al, 2003; Smit et al, 2021); (2) it predicts the occurrence of a palaeo‐lithification front within the Chalk Group in the overpressured North Sea Basin and not an ongoing mechanical/chemical compaction of the chalk as proposed by Japsen et al (2011); (3) The prediction of a regional lithification front is of importance for building velocity models and depth‐conversion of seismic data for areas with more sparse datasets (such as nearshore and onshore Denmark). Lastly, it shows an integrative workflow that may be applied in other overpressured basins for other lithologies.…”

“…First, the burial model suggests that the bipartition of the Chalk Group was established as several source rocks entered the hydrocarbon generation window and thus source, reservoir and seal were in place (Smit et al, 2021). The occurrence of low‐porous (0%–10%) chalk below RPM acted as a seal underneath the reservoir in the upper part of the Chalk Group, and structural deformation of this lower seal must have been critical to create fluid flow pathways since fracture permeability is several orders of magnitude higher than matrix permeability (especially for low‐porosity chalks; Caine et al, 1996; Sagi et al, 2016).…”

“…The suggested burial model thus shows that overpressure started to form at different times within the basin, preserving porosity. A complication is that at the same time as the compaction was ongoing, the organic‐rich Farsund Formation occurred within a temperature range for peak biogenic gas production, and deeper sources entered the thermogenic hydrocarbon generation window (Kubala et al, 2003; Smit et al, 2021). Therefore, there was an interplay between the build‐up of pore fluids (compaction‐driven and hydrocarbons) on the one side, and polygonal faulting of the Palaeocene—Middle Miocene clays potentially enabling those fluids to escape and strata to compact.…”

APPLICATION OF MUD GAS DATA AND LEAKAGE PHENOMENA TO EVALUATE SEAL INTEGRITY OF POTENTIAL CO2 STORAGE SITES: A STUDY OF CHALK STRUCTURES IN THE DANISH CENTRAL GRABEN, NORTH SEA

“…9). The HARPs were therefore interpreted to indicate MDAC beds, and stable carbon isotope (δ 13 C) values of -40 ‰ to -20 ‰ for these carbonates were interpreted to suggest a mainly thermogenic origin for the methane (Smit et al, 2021). However, Burns (1998) showed that MDACs with δ 13 C values of -40 ‰ were sourced from biogenic gas, and therefore some mix with biogenic gas cannot be ruled out.…”

“…2). In both the Rogaland and Hordaland Groups, carbonate stringers have been identified in many intervals in well reports and logs and are interpreted as MDACs, for which the source of methane may have been biogenic, thermogenic or a mixture of the two (Smit et al, 2021). The overburden is constituted by the Nordland Group comprising middle Miocene -Pliocene marine mudstones with subordinate silts and fine sands.…”