HYDROCARBON POTENTIAL OF MIDDLE JURASSIC COALY AND LACUSTRINE AND UPPER JURASSIC – LOWERMOST CRETACEOUS MARINE SOURCE ROCKS IN THE SØGNE BASIN, NORTH SEA
H.I. Petersen1*, A.C. Holme2, E. Thomsen1, M. F. Whitaker2, T. Brekke3, J.A. Bojesen-Koefoed1, K.H. Hansen2 and B.T. Larsen2
1Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
2Det Norske Oljeselskap ASA, Støperigate 2, Oslo, Norway.
3Brekke Chemo, Fjellvegen 18, 6801 Førde, Norway.
*corresponding author: email@example.com
The Søgne Basin in the Danish-Norwegian Central Graben is unique as, so far, it has only been proven to contain commercial volumes of hydrocarbons derived from Middle Jurassic coaly source rocks. Exploration in the basin is subject to the identification of good quality and mature Middle Jurassic coaly and lacustrine and Upper Jurassic – lowermost Cretaceous marine source rocks and their distribution. The present study examines source rock data of 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 of the Middle Jurassic coaly strata located at ~3800 m and spanning at least ~650 m, and (ii) the oil window of the Upper Jurassic – lowermost Cretaceous marine mudstones located at ~3250 m and spanning ~650 m. In addition, a Middle Jurassic lacustrine oil window should possibly be included. The Middle Jurassic coaly strata are thermally mature in the southern part and probably also in the northern part of the Søgne Basin, 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 of the basin. In the remainder of the basin, the 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 Søgne Basin; in addition, palynofacies demonstrates 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 remains, however, speculative.
Middle Jurassic kitchen areas may be present in an elongated depression in the northern part of the Søgne Basin and in restricted areas in the south. The Upper Jurassic – lowermost Cretaceous mudstones are thermally mature in the central, western and northern parts of the basin and 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 mg HC/g TOC, and the present-day source rock quality is for the best samples fair to good. In the southern and probably the majority of the northern parts of the Søgne Basin, the mudstones are most likely gas-prone, whereas they may be gas/oil-prone in the central part of the basin. A narrow elongated zone in the northern part of the basin may be oil-prone. The marine mudstones are, however, volumetrically more significant than the Middle Jurassic strata. Possible Upper Jurassic – lowermost Cretaceous kitchen areas are today restricted to the central part of the Søgne Basin and the elongated depression in the northern part of the basin.
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