the North Sea remain undiscovered for so long, especially considering it lies in the area of the first ever exploration licence awarded on the Norwegian Continental Shelf (NCS). Yet it evaded discovery until 2010, even then coming as a big surprise to the majority of the oil industry. Four blocks on the Utsira High, including Block 16/2 which
contains Johan Sverdrup, were included in the first exploration licence PL 001, awarded to Esso Exploration and Production Norway in 1965. Exxon was interested in this area since its geologists found it natural to start searching for oil and gas where hydrocarbons could have migrated from deeper source rocks up towards a ‘high’, before eventually being trapped in structural and stratigraphic traps. A number of fields have been discovered in this region, including Balder, Sleipner, Grane, Jotun and Ringhorne. Why did Exxon, and subsequently other oil companies,
not recognise the huge potential of the Utsira High on the spot where Johan Sverdrup is located? Norsk Hydro and Elf in their first well 16/3-2 on the NCS in 1976 in PL 007 drilled only 400m off the Johan Sverdrup structure, targeting both Palaeocene and Jurassic sandstone reservoirs of good quality in a presumed stratigraphic trap – but without hitting oil. On the web page of the Norwegian Petroleum Directorate
it explains, with reference to this well: “Tere were no sands in the Palaeocene and the Cretaceous chalk was tight. A 20m-thick immature Draupne shale was encountered at 1,955m. Te well then encountered a 31m-thick late Jurassic sandstone from 1,975m to 2,006m. Below this sandstone was a 9m-thick layer of weathered basement overlying the solid granite. Te well proved to be water wet all through, and no shows were recorded. Tree cores were cut. Core 1 gave no recovery, while core 2 recovered 3.5m core from the interval 1,998m to 2,000.6m in the Late Jurassic sand. Core 3 was cut from 2,017.5m to 2,019m in basement rock… Te well was permanently abandoned on 8 March 1976 as a dry well.” Two simple reasons lie behind this apparent oversight. Firstly, the Johan Sverdrup field is located almost 40 km from
the ‘kitchen’ of the oil – the closest mature source rocks are in the Viking Graben – from which it is separated by the solid granite of the Utsira basement ridge, seen as an oil migration barrier. Most geologists therefore did not seriously believe that oil could have migrated from the deep source rocks into
Figure 1.20: An oily core sample from the Johan Sverdrup field. What sets the reservoir on Johan Sverdrup apart from many others is the coarse grain size, which results in large pores and exceptional flow properties.
Edvard Grieg
Migration routes PL625 PL779 Edvard Grieg 16/1-14 PL673 16/1-8 16/1-12 16/2-5 PL338 Luno II 16/4-6 PL778 PL410 PL359 PL265 16/3-7 16/2-11 16/3-2 16/3-5 16/1-15 16/1-13 Johan Sverdrup PL501 PL546
PL544 16/1-8 Luno II BCU 16/1-13 16/1-15 Johan Sverdrup
0
10 km
Middle Jurassic
Inconformity
BJU
16/4-6
16/1-14
16/1-12
16/2-11
16/3-5
16/3-7
Figure 1.19: Cores from the Edvard Grieg and Johan Sverdrup fields. Wells are cored through the reservoir section for direct analysis of reservoir properties, including age dating, log calibrations, and analysis of the depositional system of the reservoir sands. The upper right figure displays three possible migration routes for oil. The closest mature source rocks are located in the Viking Graben. The green arrows represent possible migration of oil from the Viking Graben around the Utsira High on the north or south side or through weathered basement, via Edvard Grieg South. A production licence (PL) is a concession which grants exclusive rights to conduct exploration drilling and production of oil and gas within a delimited area on the NCS. (Rønnevik et al., 2017. Lundin.)
sandstone reservoirs of Jurassic age on the basement high. Te oil either had to take the long journey around the ridge where it potentially could migrate into reservoir structures, or find a shortcut through the granite. Both routes were considered extraordinarily challenging journeys for migrating oil. Secondly, seismic data quality in the area had been
limited, as shown in the example from the Luno discovery in Figure 1.21. It is very hard to identify the structure above the basement, and also to identify the top of the reservoir. On new broadband seismic data we clearly see a triangle-shaped sedimentary infill on top of the basement. Te Edvard Grieg field is located in this triangle. Let’s discuss this in more detail. Te Edvard Grieg reservoir is a Triassic aeolian and alluvial
sandstone and conglomerate reservoir. In Edvard Grieg South, Lundin discovered oil in the weathered basement rock. Te Edvard Grieg discovery opened up an undersaturated oil system on the flank of a saturated, older hydrocarbon system proven on the crestal part of the southern Utsira High. Te discovery of the relatively recent migration of the undersaturated oil system also increased exploration focus on the eastern side of the Utsira High.
15
Viking Grabben
Anette Westgard/Statoil
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