HYDROCARBON POTENTIAL OF MIDDLE JURASSIC COALY AND LACUSTRINE AND UPPER JURASSIC - LOWERMOST CRETACEOUS MARINE SOURCE ROCKS IN THE SOGNE BASIN, NORTH SEA

The Sogne 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 similar to 3800 m and spanning at least similar to 650 m; and (ii) the oil window for Upper Jurassic - lowermost Cretaceous marine mudstones, located at similar to 3250 m and spanning similar to 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 Sogne Basin and probably also in the north, whereas they are largely immature in the central part of the basin. HI(max) values of the Middle Jurassic coals range from similar to 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 Sogne Basin / southernmost Steinbit Terrace encountered Middle Jurassic organic-rich lacustrine mudstones with sapropelic kerogen, high HI values reaching 770 mg HC/g TOC and E(a)-distributions characterised by a single dominant E(a)-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 Sogne Basin, however, remains speculative. Middle Jurassic kitchen areas may be present in an elongated palaeo-depression in the northern part of the Sogne 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 mg HC/g TOC, and the present-day source rock quality is for the best samples fair to good. In the south and probably also in most of the northern part of the Sogne 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 Sogne Basin and the elongated palaeo-depression in the north.