Hydrocarbon Potential of Middle Jurassic Coaly and Lacustrine and Upper Jurassic – Lowermost Cretaceous Marine Source Rocks in the Søgne Basin, North Sea

Abstract

The Søgne Basin in the Danish‐Norwegian Central Graben is unique in the North Sea because it has been proven to contain commercial volumes of hydrocarbons derived only from Middle Jurassic coaly source rocks. Exploration here relies on the identification of good quality, mature Middle Jurassic coaly and lacustrine source rocks and Upper Jurassic – lowermost Cretaceous marine source rocks. The present study examines source rock data from almost 900 Middle Jurassic and Upper Jurassic – lowermost Cretaceous samples from 21 wells together with 286 vitrinite reflectance data from 14 wells. The kerogen composition and kinetics for bulk petroleum formation of three Middle Jurassic lacustrine samples were also determined. Differences in kerogen composition between the coaly and marine source rocks result in two principal oil windows: (i) the effective oil window for Middle Jurassic coaly strata, located at ∼3800 m and spanning at least ∼650 m; and (ii) the oil window for Upper Jurassic – lowermost Cretaceous marine mudstones, located at ∼3250 m and spanning ∼650 m. A possible third oil window may relate to Middle Jurassic lacustrine deposits. Middle Jurassic coaly strata are thermally mature in the southern part of the Søgne Basin and probably also in the north, whereas they are largely immature in the central part of the basin. HImax values of the Middle Jurassic coals range from ∼150–280 mg HC/g TOC indicating that they are gas‐prone to gas/oil‐prone. The overall source rock quality of the Middle Jurassic coaly rocks is fair to good, although a relatively large number of the samples are of poor source rock quality. At the present day, Middle Jurassic oil‐prone or gas/oil‐prone rocks occur in the southern part of the basin and possibly in a narrow zone in the northern part. In the remainder of the basin, these deposits are considered to be gas‐prone or are absent. Wells in the northernmost part of the Søgne Basin / southernmost Steinbit Terrace encountered Middle Jurassic organic–rich lacustrine mudstones with sapropelic kerogen, high HI values reaching 770 mg HC/g TOC and Ea‐distributions characterised by a single dominant Ea‐peak. The presence of lacustrine mudstones is also suggested by a limited number of samples with HI values above 300 mg HC/g TOC in the southern part of the basin; in addition, palynofacies demonstrate a progressive increase in the abundance and areal extent of lacustrine and brackish open water conditions during Callovian times. A regional presence of oil‐prone Middle Jurassic lacustrine source rocks in the Søgne Basin, however, remains speculative. Middle Jurassic kitchen areas may be present in an elongated palaeo‐depression in the northern part of the Søgne Basin and in restricted areas in the south. Upper Jurassic – lowermost Cretaceous mudstones are thermally mature in the central, western and northern parts of the basin; they are immature in the eastern part towards the Coffee Soil Fault, and overmature in the southernmost part. Only a minor proportion of the mudstones have HI values >300 m...

Oil Family Typing, Biodegradation and Source Rock Affinity of Liquid Petroleum in the Danish North Sea

“…2b). The Harald and Trym gas/ condensate fields and the Lulita oilfield in the Danish-Norwegian Søgne Basin are assumed to have been charged from these coaly units (Petersen et al, 1996;2000, 2011Petersen and Brekke, 2001).…”

“…1b), and potentially also Carboniferous deposits; but these have not been proven to be effective source rocks in the Danish Central Graben, although fluids charged from Lola/ Fjerritslev Formation marine shales may be difficult to distinguish from those derived from the Farsund Formation. Source rock data suggest that the Lola Formation may locally possess some generation potential for oil/gas (Petersen et al, 2011).…”

“…This liquid petroleum is therefore interpreted to have been charged from the coaly Middle Jurassic Bryne/ Lulu Formations, and they are typed as oil family 6(D/E-F). The presence of this oil family indicates that the Middle Jurassic terrigenous/coaly petroleum system is active in the Danish Central Graben in areas such as Elly and Alma which are outside the Søgne Basin, where it is well documented to have charged the Harald, Lulita and Trym fields (Petersen et al, 1996;2000;2011). Generation from organofacies D/E yields overall higher API fluids with low sulphur content.…”

Provenance of gravity-flow sandstones from the Upper Jurassic–Lower Cretaceous Farsund Formation, Danish Central Graben, North Sea

“…The gravity-flow sandstones are potential oil and gas reservoirs, as they were deposited within the Farsund Formation. The Farsund Formation is equivalent to the Kimmeridge Formation, which is the main source rock in the Central Graben and is described from other parts of the North Sea Basin area (Petersen et al, 2011(Petersen et al, , 2010. In order to evaluate the oil and gas potential of these gravity-flow sandstones, it is important to understand their distribution through time.…”

A Review of the Coaly Source Rocks and Generated Petroleums in the Danish North Sea: An Underexplored Middle Jurassic Petroleum System?

“…Petroleum System) ( Fig. 1b) (Petersen et al, , 2000(Petersen et al, , 2011(Petersen et al, , 2016Petersen and Brekke, 2001;Odden, 1999;Carr and Petersen, 2004). In contrast to the oils generated from the marine shales, these coaly sourced hydrocarbons are recognised by a typical terrigenous geochemical composition such as Pr/Ph ratios generally >3, sterane C 27 /C 29 ratios around unity or less, waxy n-alkane distributions, and relatively heavy stable carbon isotopes.…”