OR/15/072 Introduction

Gent, C M A1, Stewart, M A1, Evans, D J1, Lamb, R2, Alcalde, J3, 4, Heinemann, N4 and Akhurst, M C1. 2015. A summary of the methodology for the seismic stratigraphic interpretation for the 'GlaciStore' bid to IODP. (Energy and Marine Geoscience Programme) British Geological Survey Internal Report, OR/15/072. 1  British Geological Survey    2  University of Manchester    3  University of Aberdeen     4  University of Edinburgh

Significant oil and gas accumulations and discoveries are found in the Central to Northern North Sea in both the UK and Norwegian sectors. These are usually hosted in strata no younger than Palaeocene in age. Although the younger strata may be considered immature for oil and gas generation, the thick succession has an important role in Carbon Capture and Storage (CCS), as demonstrated by the Pliocene-aged Utsira Sands utilised by the Sleipner CCS operation (e.g. Arts et al., 2004; Chadwick et al., 2004, 2008; Bickle et al., 2007; Hermanrud et al., 2009). To be able to accurately assess and model potential storage sites, the Cenozoic mid Palaeogene to Quaternary overburden strata need to be well characterised and constrained. However, in the past the thick (locally >1000 m) heterogeneous Neogene and Quaternary succession has been regularly neglected by oil and gas companies targeting deeper reservoir horizons. The Central to Northern North Sea region is also particularly interesting and in many ways unique in that the Quaternary succession forming the cap rock to anticipated CO₂ storage horizons, comprises sequences deposited during, and as a result of, major glacial cycles.

Characterisation of the subsurface was required to inform the GlaciStore IODP proposal, not only of the target Quaternary overburden succession but also strata from the Balder Formation of Early/Mid Eocene (Palaeogene) age and younger and which contain potential CO₂ storage horizons. During the initial stages of the seismic interpretation the top of a large prograding unit originally classified as Pliocene in age (the so-called ‘Top Pliocene Prograding Unit’, or ‘TPPU’) was taken as the near-top of the Neogene succession. However, detailed seismic interpretation elsewhere to the south suggests strata giving rise to these reflections are younger than previously thought and are now classified as Quaternary in age (Head et al., 2008). The focus of the seismic interpretation was therefore re-defined, with two groups proceeding to interpret what are here termed the ‘shallower seismic’ and the ‘deeper seismic’.

Proposal to the IODP

Drilling sites were selected to address the scientific objectives in the ‘GlaciStore’ proposal to IODP within the UK and Norwegian North Sea sectors. The scientific objectives proposed are to:

• Establish a depositional and chronological framework for the multiple cycles of glacial advance and retreat in the centre of the North Sea;
• Investigate temporal changes in depositional environment and pore water geochemistry of the stratigraphic units;
• Determine the measurable impact on geomechanical properties caused by glacial loading and unloading.

The seismic interpretations summarised here within the UK sector of the North Sea informed two phases of investigation for a pre-proposal and a full proposal submitted to the IODP. The initial site selection was presented in a pre-proposal and submitted in March 2014. The pre- proposal was reviewed and IODP invited the ‘GlaciStore’ proponents to submit a full proposal in April 2015. The subsequent detailed investigation refined understanding of the proposed sites, informed adjustment of site positions to better address the scientific objectives, where possible, or to ensure the proposed sites met with safety requirements of IODP. A total of 13 sites were investigated across the UK and Norwegian North Sea sectors. The selected sites were prioritised as only a maximum of four would be drilled should the full proposal be accepted by IODP. Detailed investigations of all the proposed sites were submitted in the full proposal, retained as contingency options to meet practical constraints during drilling, if needed.

‘Shallower seismic’ interpretation

The ‘shallower seismic’ section is here defined as all strata down to the top Utsira Sands (Mio-Pliocene in age). Therefore this interpretation is of the latest Neogene and Quaternary succession in the study area, which, in this region comprises strata deposited during major glacial cycles. Areas of focus include, major horizons and sequence boundaries, surface and near-surface glacial tunnel valleys and evidence for shallow gas in either chimney form (represented on seismic reflection data as areas of poor data quality with loss of reflection continuity and amplitude), or give rise to ‘bright’ reflections, with specific interest locally around potential drill sites.

‘Deeper seismic’ interpretation

The initial ‘deeper seismic’ interpretation was of Mid-Quaternary to top Oligocene horizons. Later interpretation follows the reclassification of the overburden succession (Head et al., 2008) with the focus of the deeper interpretation mainly on the base of the Utsira Sands (Mio-Pliocene in age) to the top Balder formation (Early/Mid-Eocene). All major seismic horizons or ‘events’ are interpreted laterally as far as possible across the regional CNST82RE survey, from the UK sector into the Norwegian sector of the Central North Sea. Particular interest is paid to a series of depositional units prograding from the UK sector to the south-east.

The aim of interpreting the deeper, mid Eocene-Quaternary section is to provide an understanding of the major structural controls on the distribution and stratigraphy of the Cenozoic overburden succession and thus potential controls on the nature and distribution of the Quaternary succession.

The ‘deeper seismic’ interpretation sets the regional structural and stratigraphical framework for the sites in the proposal for scientific drilling and as required by IODP.