Large-scale development of the mid-Norwegian shelf over the last three million years and potential for hydrocarbon reservoirs in glacial sediments

Abstract

About 3 million years ago, major ice sheets developed over Scandinavia and began to deliver large volumes of sediment to the mid-Norwegian shelf. The shelf was built out in a prograding pattern towards the west, and more than 1000 m of sediments was deposited over large areas on the middle/outer shelf. The dominating large-scale depositional pattern is a series of prograding wedges and flat-lying, sheet-like units mainly of glacial origin. On top of these units are flat-lying till units deposited during the last few glaciations, commonly separated from the underlying units by one or several erosional unconformities. The lithology of these layers is generally fine-grained, mainly clay and silt, but with sporadic clasts up to boulder size. Based on regional and detailed bathymetry, the ice-flow pattern from the last glaciation has been reconstructed on the shelf. This involved a very dynamic ice sheet with fast-flowing ice streams in the transverse, cross-shelf troughs, separated by more passive ice domes on the intermediate shallow banks. The ice streams appear to follow the cross-shelf troughs from glaciation to glaciation, but occasionally they switched to new flow paths. The thick Quaternary sediments on the outer part of the mid-Norwegian shelf represent a challenge for hydrocarbon exploration. Several large slides have occurred from the shelf break and down the continental slope. During the last few years there has been an increased focus on investigating the Quaternary succession in order to search for hydrocarbon prospects. The potential for finding reservoirs in these sediments is discussed, and a few examples of gas discoveries are shown. The Peon gas field (c. 250 km2) is located in the glacially eroded Norwegian Channel above the Upper Regional Unconformity (URU) separating flat-lying glaciogenic sediments from dipping sedimentary units. The reservoir is developed in glaciofluvial sands a few hundred thousand years old, and sealed by flat-lying glaciomarine sediments and till units.

Glaciogenic reservoirs and hydrocarbon systems: an introduction

“…In contrast, the northern hemisphere only shows scattered glacial evidence in high latitude areas or areas of extreme altitude prior to the onset of the Pleistocene glaciations. Spectacular glacioterrestrial and glaciomarine successions occur across northern Europe, from the north European plains of Poland, Germany, Denmark, the Netherlands and eastern England (Ehlers et al 2011), and along the Atlantic continental margin (Dowdeswell et al 2002;Ottesen et al 2008Ottesen et al , 2012 through to the North Sea, where these glacial sequences are superbly imaged on highresolution 3D seismic data (e.g. Praeg 2003; Rise et al 2004;Graham et al 2010;Kristensen et al 2007;Buckley 2012).…”

“…Schack Pedersen 2012), allow sequence stratigraphic methodologies to be applied and the complex sequence and styles of glaciotectonic deformation to be determined in detail. Advancing ice during the Pleistocene was, as in large modern-day ice sheets, preferentially fluxed via ice streams (Dowdeswell et al 2002;Ottesen et al 2008Ottesen et al , 2012. The 'footprint' of one such palaeo-ice stream is clearly evident in the Norwegian Channel (Ottesen et al 2012), where a large volume of sediment was transported to the shelf break and deposited as a trough-mouth fan (Sejrup et al 2003;Ottesen et al 2008).…”

“…As the North Sea hydrocarbon province is maturing, gas-bearing glaciogenic deposits are currently being viewed as potential resources including, most notably, the giant Peon gas reservoir hosted in glaciogenic outwash fan sandstones and sealed by diamicton ( Fig. 10; Ottesen et al 2012). In Pleistocene deposits, gas-bearing sands are easily seen as high-amplitude reflections from the interface between overlying sealing rocks and the gas-bearing sands of low acoustic velocity and density.…”

“…In mature basins, such as the North Sea, rising oil and gas prices have increasingly focused exploration towards previously uneconomical hydrocarbon accumulations, including some that were previously considered shallow hazards, most notably the 1 TCF Peon gas field, which is hosted in a submarine glacial outwash fan of middle Pleistocene age ( Fig. 10; Petroleum Geo-Services 2010; Ottesen et al 2012). Similar shallow gas occurrences in glaciogenic strata are currently being studied for their hydrocarbon resource potential.…”

“…Glaciogenic deposits are largely preserved in formerly glaciated lowland areas, including land, lake and continental shelf areas, as well as adjacent continental slopes where large trough mouth fans constitute volumetrically important deposits from Pleistocene glaciations (Sejrup et al 2003;Ottesen et al 2012). In deep ocean settings, ice-rafted debris provides the most important physical record of the Pleistocene shelf glaciations (Hemming 2004).…”