A revised lithostratigraphic scheme for the Eocene to Pleistocene succession on the Norwegian continental shelf

Accepted manuscript

Introduction and background

A formal lithostratigraphic nomenclature of the pre-Permian to the upper Miocene/lower Pliocene for the central and northern North Sea was published by Deegan & Scull (1977). A revision of the Cretaceous and Cenozoic parts of the nomenclature was established by Isaksen & Tonstad (1989). Dalland et al. (1988) introduced a lithostratigraphic nomenclature for the Triassic to Quaternary of the mid- and northern Norwegian continental shelf. A “Lithostratigraphy of the Palaeogene–lower Neogene succession of the Danish North Sea” was published by Schiøler et al. (2007). Subsequently, Gradstein et al. (2010) have published an informal lithostratigraphic nomenclature for the Mesozoic and Cenozoic of the North Sea, Norwegian Sea, Barents Sea and Svalbard (stratigraphic database). However, they introduced no new formal lithostratigraphic units. The present publication presents a revision of the Eocene to Pleistocene lithostratigraphies of Isaksen & Tonstad (1989) and Dalland et al. (1988; see Figs. 1 and 2) for the Norwegian central and northern North Sea, the Norwegian Sea and the western part of the Barents Sea.

When drilling exploration wells, the petroleum companies commonly sample the Eocene to Pleistocene sediments with drill cuttings. Generally, the post-Eocene deposits have been far less sampled and investigated than the older sediments, which have been the main target for hydrocarbon exploration. The sampling programme is usually considerably less dense than in the deeper sections, e.g., every ten metres compared to every three metres in reservoirs. Only a small number of wells have been sampled with sidewall cores and short conventional cores, and the number of cored shallow stratigraphic boreholes covering the Eocene to Pleistocene is limited. The conventional cores are used mainly for geotechnical investigations. Contracted biostratigraphical consultants usually execute routine investigations. However, the samples are commonly investigated with large spacing and limited effort is put into the analyses. Mistakes and inaccuracies in the biostratigraphical analysis and age interpretations have led to errors in completion logs, final well reports, regional seismic mapping, scientific publications and in the stratigraphic nomenclatures. Figs. 3 & 4 show a compilation of maps based on NPD’s own re-dated/re-interpreted/re-correlated well data with maps based on well data obtained by the oil companies’ contracted consultants.

Historically, the errors in completion logs and final well reports have led to considerable confusion in our understanding of the post-Eocene stratigraphy and basin histories. Several scientific investigations focusing on regional seismic stratigraphy have tried to improve this situation, but some of the correlations have been hampered by the inaccurate/incorrect well data (Eidvin et al., 2013d). The need for well documented age-determinations and a good nomenclature is now increasing due to increased interest for caprock properties and use of shallow aquifers for CO2 injection as well as produced water and ditch cuttings injection. Shallow aquifers can also play an important role as secondary storage for deeper storage sites.

A number of biostratigraphic and strontium isotope stratigraphic (SIS) studies dealing with re-dating of exploration and production wells and boreholes combined with regional seismic interpretations have improved the situation. The NPD became early aware that there were some significant errors in the dating of the post-Eocene sediments executed by the oil companies’ contracted consultants. Consequently, in the late eighties the NPD started a re-study of the wells from the Senja Ridge in the western Barents Sea to clarify the age of the base of the large sediment fans on the margin, and by this indirectly date the uplift of the Barents Sea shelf. At that time, this issue was a widely debated topic. In the Senja Ridge wells, re-worked fossils created an especially large problem for the dating. The fauna of the fossils was completely dominated by Paleocene–Eocene forms, but also had a small percentage of Pleistocene forms. The first biostratigraphical consultants, who tried to date the wells, suggested that the Paleocene–Eocene forms were in situ and that the ‘young’ forms were from random caving. However, the stratigraphical workers at the NPD compared the occurrence of the “young” forms in several wells and found a pattern that repeated itself and could thus rule out the view that the Pleistocene forms were caved. As additional controls of the biostratigraphic correlations, they used strontium isotope analysis of the fossil tests, which at that time was a new stratigraphic tool (Eidvin et al., 1993, 2013d; Eidvin & Rundberg, 2016b) and the occurrences of icerafted debris in the well samples.

Errors and inaccuracies in the dating of post-Eocene successions were not limited to the succession on the western Barents Sea margin. The NPD proceeded to re-date post-Eocene sediments from other areas and has gradually built up a database of re-dated wells and boreholes from most basins around Norway, i.e., from Svalbard and the margin off Svalbard in the north to the Norwegian– Danish Basin including Denmark in the south (Figs. 1 & 2). So far, the NPD has analysed microfossils (calcareous foraminifera and Bolboforma) in over 2000 samples from more than 60 wells and boreholes. Strontium isotope analysis of fossil tests have been carried out in more than 1500 of the respective samples (Figs. 1 & 2; Eidvin et al., 2013d; Eidvin & Rundberg, 2016b).

On the Norwegian shelf, microfossils with calcareous tests occur basically in the post-Eocene parts of the Cenozoic deposits. Prior to the Oligocene, depositional depth was generally deeper than the calcium carbonate compensation depth and calcareous microfossils were usually not deposited or were dissolved (King, 1983). Consequently, it is mainly only from post-Eocene deposits that we were able to collect Sr isotope data. However, in well 7316/5–1 (Fig. 1B) in the Vestbakken Volcanic Province (western Barents Sea) we also recorded some calcareous benthic foraminifera in middle Eocene deposits and were able to collect Sr data (Eidvin et al., 1998b). The Sr data are important as they can verify the ages given by the biostratigraphical correlations and, in most cases, they help to increase the stratigraphic resolution.

The stratigraphic data are set in a regional context, using log correlations and seismic interpretations, in collaboration with researchers from the industry and academia. The results of these investigations are thoroughly documented in scientific papers and reports of which a majority are posted on the NPD's web pages. These include: Eidvin (2005, 2009, 2016a, b, c, 2018a, b, 2019), Eidvin & Nagy (1999), Eidvin & Rundberg (2001, 2007, 2016b), Eidvin & Øverland (2009), Eidvin et al. (1993, 1998a, b, c, 1999, 2000, 2007, 2013a, b, c, 2019, 2020), Jarsve et al. (2014), Riis et al. (2004) and Rundberg & Eidvin (2005, 2016a, b). Syntheses of most of these publications are published in the comprehensive interactive NPD Bulletin 10 (Eidvin et al., 2013d) and in the paper of Eidvin et al. (2014). These publications have been fundamental for the revised lithostratigraphic nomenclature presented herein. The post-Eocene strata from all the new supplementary reference wells have been reanalysed and re-interpreted by the NPD. The exception is well 7216/11–1 S (Fig. 1B; western Barents Sea) which is based on data from Ryseth et al. (2003). This well contains very few calcareous foraminifera in the Cenozoic succession and the chronostratigraphy is based on analyses of dinocysts. The stratigraphy in the other Eocene sections is based on data from the NPD factpages (npd.no/facts).