An integrated subsurface analysis of clastic remobilization and injection; a case study from the Oligocene succession of the eastern North Sea

Abstract

The North Sea giant sand injectite province (NSGSIP) is the global type area for large-scale sandstone intrusion complexes. Despite decades of research on the NSGSIP, this paper presents the first detailed case study in which all aspects of the intrusion process have been constrained, including fluid and sediment sources, injection timing and driving mechanisms. The study describes and analyses high-amplitude discordant amplitude anomalies within the Oligocene succession in the eastern North Sea, which are interpreted as large-scale brine-saturated sand injectites. Potential feeder conduits extending from the top of the Paleocene Lista Formation to the base of the injectites indicate that the source sand was located within the Lista Formation; possibly deposited in a distinct valley cut into the top of the Chalk Group. The geometry of the observed injectites ranges from a basal sill with wings to V-shaped and conical; their dimensions range from 300 to 3700 m in width and up to 150 m in height. In all cases, a significant deformation of the overburden is observed. The study area is located in the Ringkøbing-Fyn High area above the basement high separating two smaller Paleozoic half-grabens. During the Oligocene, rapid and significant differential loading occurred. We interpret that the injectites formed due to remobilization of the source sand facilitated by overpressure caused by differential loading combined with a possible influx of fluids from the deeper succession. The case study has with its assessment of the full injection system, implications for the understanding of subsurface remobilization processes and furthermore for oil and gas exploration in the eastern North Sea.

Geometry and lithofacies of coarse-grained injectites and extrudites in a late Pliocene trench-slope basin on the southern Boso Peninsula, Japan

“…The schematic summary diagram of Fig. 13 does not illustrate multiple and conical vents, fracture-related faults, stepped and staggered sills, and/or collapsed structures, which have been observed in large-scale seismic data and small-scale analog experiments (e.g., Duranti, 2007;Rodrigues et al, 2009;Ross et al, 2011;Hurst et al, 2011;Andersen and Clausen, 2014), because the present outcrop example dose not contain these structures possibly reflecting a limited outcrop belt compared with seismic-scale examples. The present example of extrudites is underlain by thick muddy deposits, and a cone-shaped extrudite is considered to have formed, because the top layer had a high cohesive strength (Nichols et al, 1994).…”