*2.1. Ice Thickness*

Glacial isostasy is calculated based on the palaeo ice extent (from dated marginal moraines) and models of the ice thicknesses. Observational data increasingly constrain the extent of the Quaternary ice sheets. The thickness and volume of these ice sheets are much harder to reconstruct and generally need to be inferred from indirect evidence and modeling.

Several models of ice thickness of LGM and the late-glacial deglaciation are published; the loading scenarios are produced using different methods and sets of constraints. Some of them consider thermomechanical coupling (e.g., [37]). A number of models of the LGM ice thickness are presented in [38]. We follow the work of [36]; their LGM extent model is shown in Figure 1.

**Figure 1.** Extent (based on [36]) and maximum thickness model for the Last Glacial Maximum (LGM), from [3]. The production licenses (from www.npd.no) in the Norwegian continental shelf are shown by grey polygons.

### *2.2. Glacial Isostasy and Tilting of Reservoirs*

The Earth's response to deglaciation shows that the lithosphere acts as an elastic shell overlying a viscous mantle. If a load is applied to the elastic lithosphere, part of the applied load will be supported by the elastic lithosphere, and part by buoyant forces of the mantle underneath, acting through the lithosphere.

In the literature, there is disagreement on the elastic thickness of the lithosphere; the elastic thickness of the lithosphere of Fennoscandia in GIA (Glacial Isostatic Adjustment) modeling varies from 30 to 160 km [39]. Our modeling of the post-glacial isostatic response, calibrated with the observations onshore Norway, gave best fit with an elastic thickness of 30 km; see [34]. Calibration with the Barents Sea gave similar results; see [40]. The method used in the isostatic calculations is described in [41] and in the Appendix A. The spatial resolution is 10 km.

There will be significant subsidence of potential sedimentary basins formerly covered by ice sheets. Sedimentary basins that are located in the periphery of the former ice sheet will be notably affected by tilting of the Earth's surface as the subsidence (or uplift after the melting) is gradually decreasing towards the periphery of the former ice sheet. Therefore, the largest tilts of the Earth's surface are found in the more peripheral areas of former ice sheets. This is clearly seen along the entire Norwegian coast and in the Barents Sea (Figure 2). Sedimentary basins located in the peripheral areas in the SW Barents Sea could be tilted by up to 2.7 m/km due to the LGM ice sheet (Figure 2), and even more in the previous greater glaciations [3]. The ice sheets will act as seesaw during the total glaciation period (Quaternary); the Earth will subside under the ice sheets and rise again in interglacial periods.

**Figure 2.** *Cont.*

**Figure 2.** Calculated tilts (m/km) due to glacial isostasy for the LGM ice sheet (based on [3]). The white box in **a**) marks the Norwegian part of the Barents Sea that is open for petroleum exploration (production licenses are marked in grey). The tilts of this area are shown in **b**).

This process is, of course, important for migration modelling. Figure 3 shows how a simple structure behaves during one glacial cycle. It is assumed, for illustration purposes, that the structure is filled with hydrocarbons to the spill point at the onset of glaciation. When the lithosphere and the structure subside due to the glacial load, the reservoir is tilted hydrocarbons start to spill out of the structure. This goes on until the maximum tilting is reached (Figure 3, upper right). The closure volume will be reduced.

When the lithosphere and structure are uplifted due to deglaciation, a new hydrocarbon-water contact (HCWC) will be established (Figure 3, bottom). Paleo-HCWCs can theoretically be found at the spill point level for the positions before and after deglaciation. Residual oil can be found in the water zone down to the original spill point level. In a gas-filled reservoir with an oil leg originally, residual oil can also be found in the gas-filled part, limited by the uppermost HCWC.

The effect of the glacially induced hydrocarbon migration is determined by several factors:


The isostatic response on the Pleistocene sediment redistribution and ice loading has been evaluated for the Bjørnøyrenna Fault Complex in the Barents Sea [11]. The isostatic impact on hydrocarbon trap capacity changes and hydrocarbon maximum spillage was assessed in that study and the conclusion was that tilting together with gas volume expansion might have been responsible for some part of the hydrocarbon loss during the Cenozoic.

**Figure 3.** Illustration of tilting during one glacial cycle (redrawn from [5]). **Upper left** shows the situation prior to tilting; **upper right** during tilting. **Lower panel** shows the situation after deglaciation. "Spill point" is the structurally lowest point in a hydrocarbon trap that can retain hydrocarbons. Once a trap has been filled to its spill point, further storage or retention of hydrocarbons will not occur. The illustration assumes the trap filled to the spill point. HCWC is the hydrocarbon-water contact.
