Iceberg ploughmarks illuminated by shallow gas in the central North Sea

Abstract

a b s t r a c tConventional 3D seismic data provides direct evidence for glacial influence during the early Pleistocene sedimentation in the Central North Sea. We identify iceberg ploughmarks as dim linear to curve-linear features in three early Pleistocene horizons that have high reflection amplitude compared to adjacent horizons. The anomalous horizons are interpreted to be reflections from thin sandy layers saturated with gas. The gas acts as a contrast liquid illuminating the thin sand layers. The reason for this is the difference in acoustic properties between water and gas saturated sand layers. The combination of thin bed effects and shallow gas makes the iceberg ploughmarks easily detectable as dim features in seismic reflection amplitude maps. Our interpretation is based on analysis of real seismic data, well logs and modeled seismic response. The methods we use include interpretation of horizons followed by extraction of reflection amplitudes, well log analysis, pre-stack amplitude versus offset analysis of high resolution 2D seismic data and time-lapse analysis of seismic. Seismic modeling is performed to study interactions between thin sand beds, shallow gas and iceberg ploughmarks. A new trapping mechanism for shallow gas is presented and seismic modeling of this trap strengthens our interpretation. The trap is created by iceberg ploughmarks in sandy layers that are covered by finer and less permeable sediments. For this area we find that conventional seismic interpretation is superior to the much used method of studying seismic time-slices for detection of iceberg ploughmarks, both with respect to time and detectability. This study shows that the interpreter should look for high amplitude horizons with amplitude variations laterally when trying to detect iceberg ploughmarks.

Aspect ratio histograms of 3D ellipsoids and 2D ellipses — Analytical relations and numerical examples

“…Another, similar, example can be found in a recent paper by Haavik and Landrø, 2014 (see Figure 19 modified from Haavik and Landrø, 2014). This example shows two iceberg plough marks that are hardly visible on the 2D seismic cross section but are clearly mapped from the 3D amplitude map extracted from the top sand reflection.…”

“…(Left) The 2D seismic cross section showing a seismic reflection at approximately 600 ms, where two iceberg plough marks are identified by two red arrows. (Right) The same plough marks are identified by two white arrows on the seismic amplitude map, which is extracted from the 3D seismic cube (figure modified from Haavik and Landrø, 2014). The 2D seismic line is marked as a solid white line (AB) on the amplitude map (right).…”

Monitoring Changes in the Overburden – Some Observations from Using Various 2D and 3D Seismic Time Lapse Data Sets

“…IFEMA Madrid, Spain, 1-4 June 2015 Figure 2 shows and example of gas that is trapped in a thin sand layer (at approcimately 600 m depth) between the two sand layers discussed in the previous section. A detailed analysis of these ice berg ploughmarks (Haavik and Landrø, 2014) shows that the dominant direction of these lineaments is North-South. The thin sand layer dips towards the West, and hence the ice scours cut the sand layer perpendicular to the dip direction, which is ideal for creating traps for gas accumulation.…”

“…Later, it was proposed by Landrø et al (2014) to use time lapse refraction analysis as an early warning system exploiting permanent reservoir monitoring systems. Haavik and Landrø (2014) used both 2D and 3D seismic data from the area close to the 2/4-14-well to analyse shallow gas and pointed out that ice berg plough marks might act as an additional trap mechanism for shallow gas. In this paper we will show some examples demonstrating the importance of detailed mapping of the overburden, and also show some examples of long-term migration of gas in the overburden.…”

Long-term fluid expulsion revealed by carbonate crusts and pockmarks connected to subsurface gas anomalies and palaeo-channels in the central North Sea

“…Fluid flow, shallow acoustic anomalies and geochemical characteristics of gases in different parts of the North Sea have been investigated in detail by many geophysical and geochemical studies inferring localised fluid flow models for specific areas (Rise et al 1999;Mazzini et al 2003;Fichler et al 2005;Schroot et al 2005;Hübscher and Borowski 2006;Hovland 2007;Andresen et al 2008;von Deimling et al 2011;Haavik and Landrø 2014;Karstens and Berndt 2015;Crémière et al 2016). Numerous pockmarks were found in different parts of the North Sea (Hovland and Sommerville 1985;Rise et al 1999;Hovland 2007;Gafeira 2012) that have been related to shallow gas availability and fluid migration pathways (Gafeira 2012).…”