Introduction

Frontiers of knowledge and technology
Extensive petroleum exploration was taking place in the Barents Sea in the 1970's and the 1980's. This exploration led to the acquisition of abundant seismic reflection data and the drilling of more than 60 wells in the Norwegian Barents Sea and more than 30 wells in the Russian Barents Sea by the early 1990's. Large gas fields were soon discovered in the Norwegian Barents Sea (Hammerfest Basin) but later exploration has not been that successful. Only one area has been set into production (the Snøhvit field development), and commercial quantities of oil have recently been discovered in the Goliath field. In contrast, the exploration in the Russian sector has resulted in discoveries of several giant gas fields (e.g. Shtockman), and large oil fields in the Pechora Sea in the SE part of the Barents Sea. The reason for the different exploration success rates in the western and eastern Barents Sea is not well understood. The problems of finding commercial hydrocarbon deposits in the Norwegian Barents Sea have been attributed to glaciations and glacial erosion and uplift causing seal failure and expulsion of hydrocarbons from previously filled traps. There are major geological differences between the western and eastern Barents Sea. The western Barents Sea is characterized by broad platform areas cut by fairly narrow rifts. In contrast, the Eastern Barents Sea is a very broad and deep sag basin with little internal structuring. Currently, the nature and origin of these differences are not well documented and understood.

The effects of the Plio-Pleistocene glaciations
Fennoscandia has experienced major uplift in post-glacial time. The rate of uplift along the coasts is so high that its effects have been observed within one generation. In the northern Gulf of Bothnia uplift is occurring at 9 mm/yr; similar uplift rates are observed also on Svalbard. The uplift has been intensely studied and discussed since the 18th century and it is now widely accepted to be an isostatic response to the recent deglaciation.  In fact, our knowledge of the fluid properties of the Earth comes largely from its uplift response to load redistributions that occurred over the last ice age.

The elevation of past shorelines and the present rate of land uplift (e.g. Fig. 1b) constrain the fluid properties of the Earth and the elastic rigidity of the lithosphere (e.g. Cathles, 1975; Ekman, 1991, and many, many other references). Models of post-glacial rebound have generally assumed the uplift to be exclusively of glacial isostatic origin, but this is an assumption not fully justified by evidence. Mörner (1979) has long suggested that the observed post-glacial uplift consists of two components, one linear (neotectonic) and one exponential (post-glacial rebound). Fjeldskaar et al. (2000) found that there are areas where the measured present rate of uplift is greater than that predicted by regional glacial isostatic models. The difference is assumed to be a tectonic component in the post-glacial uplift. The largest differences are located in the mountainous areas onshore north Norway (North Scandes Dome) and northern part of west Norway (South Scandes Dome), the very areas most affected by the more long-term Neogene tectonic uplift.

Numerous glaciations preceding the last one (Fig. 1a), also caused rapid glacial erosion and subsequent differential uplift and tilting. It is commonly envisioned to have led to spillage of hydrocarbons, phase transition from oil to gas, expansion of gas, seal failure, cooling of source rocks, and slope instability. In addition to glacial erosion, repeated ice and sediment loading had great influence on hydrocarbon migration routes, the temperature history and maturation. Detailed control on the glacial erosion, sediment deposition and ice loads (i.e. the glacial history) is therefore an important and insufficiently utilized factor for identification of the remaining hydrocarbon resources in the Barents Sea.

It is widely accepted that the glacial erosion has been of major importance for the regional hydrocarbon evolution in the Barents Sea. Rapid glacial erosion in the Barents Sea area also affected local stress regimes. Changes in local stresses and associated fluid pressures in petroleum reservoirs generated by rapid glacial erosion are topics of great practical importance since sudden stress changes may reactivate or initiate faults and other fractures, allowing oil and gas to escape from reservoirs.

For estimation of the amount (and timing) of the erosion, well data are used for calibration (temperature and vitrinite reflectance). The calculated temperature history of a basin in the Barents Sea depends strongly on the amount of erosion, but also significantly on the thermal conductivity of the eroded overburden

For estimation of the amount (and timing) of the erosion, well data are used for calibration (temperature and vitrinite reflectance). The calculated temperature history of a basin in the Barents Sea depends strongly on the amount of erosion, but also significantly on the thermal conductivity of the eroded overburden. A better estimation of the thermal conductivity is therefore of utmost importance.