A composite mud volcano system in the Chalk Group of the North Sea Central Graben

Abstract

The study describes a large (kilometre-scale) composite mud volcano system within the North Sea Chalk Group, containing both intrusion and extrusion features. The upper part of the system consists of three stacked constructive components. At the top, a Late Palaeocene mound-shaped extrusion with subtle onlaps is observed. Below, a Danian near-surface intrusion caused forced-folding of the c . 100 m thick overburden. This is underlain by a shallow intrusion. The lower part of the mud volcano system is defined by a depletion-related depression and the fluid conduit. Seismic data suggest that the source unit for the linked intrusion–extrusion is to be found either within a Lower Cretaceous diapiric siliciclastic structure or within the fine-grained carbonates of the Chalk Group. The latter is more probable, implying that chalk may generate hitherto undocumented intrusions and extrusions comparable with siliciclastic mud volcano systems. Remobilization is inferred to have been driven by fluidization caused by the influx of mainly pore water expelled from the Lower Cretaceous succession. A distinct depositional variation within the Chalk Group caused early diagenetic differences in porosity preservation and mechanical properties of the chalk units, allowing the remobilization of certain chalk units even at burial depths as great as 200 m.

Fluid flow features in hydrocarbon plumbing systems: What do they tell us about the basin evolution?

“…The chalk mud volcano system (Fig. 4) and the formation hereof (Andresen et al, 2010) furthermore indicate a significant late Cretaceous fluid migration phase and subsequent remobilization in the area of the observation.…”

“…Subsurface remobilization of non-clastic sediments such as chalk may produce mud-grade carbonate volcano systems (Fig. 3), which are generally forming under similar conditions as clastic mud volcano systems (Andresen et al, 2010). Mobilized salt and igneous intrusions will not be dealt with in this paper.…”

“…5 and 6). Four of the six case studies concern data from the Danish North Sea area (Table 2) and include the description and analysis of c. 30 Oligocene and c. 650 Miocene elongated pockmarks (Andresen et al, 2008), a mid-Miocene sand volcano (Andresen et al, 2009), a mud-grade carbonate volcano system (Andresen et al, 2010), and sandstone intrusions above the Ringkøbing-Fyn High (Andresen, 2011) (Fig. 4 and Table 2).…”

“…Extrusive fluid flow features such as mud, chalk and sand volcanoes in theory likewise record the timing to the paleo-surface onto which they occur. The extrusive nature can however be hard to establish on seismic data and the timing of formation should thus be supported by observations such as reflection terminations like onlaps and downlaps (Mitchum et al, 1977;Stewart and Davies, 2006;Hansen et al, 2005;Andresen et al, 2009Andresen et al, , 2010. Sandstone intrusions may also represent useful timing indicators if they are associated with forcedfolding of the overburden and onlaps (Shoulders and Cartwright, 2004;Frey-Martínez et al, 2007;Cartwright, 2010;Andresen, 2011).…”

“…The paper presents a qualitative integration of fluid flow features in basin analysis of the Danish North Sea area and of salt mini-basins offshore Angola. The integration is based on a synthesis of results from six case studies focusing on the detection, mapping and analysis of features related to focused fluid flow in these areas and carried out as part of the author's PhD study (Andresen et al, 2008(Andresen et al, , 2009(Andresen et al, , 2010Andresen, 2011;. The results of the six case studies accent discrete fluid flow features such as elongated pockmarks, focused fluid flow pipes, gas chimneys, bottom simulating reflections, direct hydrocarbon indicators and several observations without similar reports in literature; e.g.…”