28 Boundaries: The lower boundary is primarily marked tance early in Permian. The initial phase of subsidence by the sonic velocity which is high and uniform in the was accompanied by extensive subaerial volcanism. underlying metamorphic greenstone, and relatively This was followed by a period of oxidation and erosion lower and nervous in the present series. The upper under desert conditions and deposition of red beds and boundary is between the present mainly greyish series sabkha sediments in the two Permian basins (fig. 19). and the overlying reddish series of assumed Rot­ These rocks are included in the Rotliegendes Group as liegendes age. The log motifs are not significant, but originally established by Werner (1786). Continuous the change from high gamma ray readings above to subsidence and transgression of the sea, but with a relatively lower below is used for location of the restricted connection to the ocean, lead to the forma­ boundary. tion of the evaporites of the Zechstein Group. Distribution: The Early Carboniferous deposits are probably present in major parts of the Danish Central Rotliegendes Group Graben, but due to lack of data (seismic mapping is Autunian critical), a delineation of the extension is not possible. Type section: For the present being the P-1 well, Geological age: Based on studies of the miospores, 10541-10865' b.KB may suffice as a reference for the part of the series is dated to Early Carboniferous, Late Rotliegendes volcanics and underlying sediments be­ Visean or Early Namurian (Bertelsen 1978). - The longing to the Autunian stage. However, it should be lower part of the series has not been dated, thus it may noted that this correlation is based purely on be of Carboniferous or Devonian age. lithostratigraphical evidence. Depositional environment: It is a marginal fluvially Thickness: The volcanic rocks and interbedded sedi­ dominated environment with minor coal swamps. The ments form a 99 m thick pile in the P-1 well, but in all occurrence of thin limestone beds with foraminifera, other wells of the Danish off shore sector, the base of ostracods, and crinoid fragments shows that short the volcanic rocks has not been penetrated. Therefore marine transgressions periodically flooded the area. only minimum thicknesses of 44 m in B-1, 148 m in W-1, 71 m in Q-1 can be given. The underlying sedi­ Source rock potential: Probably poor for oil since the ments in P-1 well comprises 33 m of redbrown sand­ Rotliegendes volcanism and/or depth of burial-tem­ stone, siltstone, claystone, and marlstone (Bertelsen perature increase may have led to overcooking. No 1978). studies have been carried out in the Danish region. Lithology: The lithology is summarized in fig. 18. The Reservoir potential: Limited reservoirs in form of volcanic sequences are classified as lava flows with porous sandstones may be expected. interbedded agglomerates, tuffs and detrital sedi­ ments. The lava flows are generally a few metres in Sealing potential: The series is probably not sealed, thickness or less, may be of the pahoehoe type, but being overlain by the Rotliegendes. The Carboniferous flows up to 7 m in thickness occur. Presumably some shale beds will possibly be able to act as seals. of the 'agglomerates' noted in the mudlog may origi­ nate from scoriaceous or vesicular top zones. The cores and cuttings indicate, together with the log data, especially gamma ray, the predominance of basalts; 3.2 Permian but more differentiated rocks occur in the P-1 and W-1 wells. From By Fritz Lyngsie Jacobsen & JfiJrgen Gutzon Larsen the latter well these rocks are described as andesites and rhyolites (mudlog). They may occur as lava flows and pyroclastic beds. Most of the rocks look In North-West Europe two mega-basins began their rather oxidized and they are often cut by secondary development during Late Carboniferous to Early Per­ veins. The interbedded sediments are mostly (silty) mian: The South Permian Basin stretching from east­ claystone but a 28 m thick metamorphosed limestone ern England into Poland , and the North Permian occurs in W-1. Basin reaching from Scotland into Denmark. These two basins were separated by the Mid North Sea High Log charactetistic: The volcanic rocks are charac­ and the Ringk0bing-Fyn High which came into exis- terized by cyclic variations in the sonic velocity, and 29 P-1 8-1 W-1 Q-1 CRETACEOUS ZECHSTEIN 10350 .... .....--- 13700 11283 -.·!:-~-:i:;:- 14385 -~- ..... 14400 z 10400 w . .•. ...... .....~. (/) vvvvv I .v.v.v.v. u =·.·.·.·. 13800 w 11400 - : : ...... N <'-· 10500 :: =:::2r": 14500 p::::i:: ..... 14515 13860 v-v-v vvvv 10541 vvxvv -v-v 13888 vvvvv 13900 V·-·V -V­ 11500 - : '. :'. :.r::r:. (/) V· · ·•V w 10600 :::=::: 14600 - v- z0 13952 w (!) (/) w w ::i 0 14000 z II::::: (/) 0 11600 w 0::: w 0 10700 (!) vvvvv z 14700 vvvv w w vvvvv ::i (!) vvvv ...... ·:.=. w vvvvv 0 V?V?V . ·-:-:-?<· ::i v?v? ::i 14745 vvvv 0::: x·:.·:: ...... 14100 v?v?v T.D. 0 V?V? 0 11700 - :7 7 7 0::: V ?°V ?'v 0::: 10800 h.-:i.-:i. V? V? •• o, - •• • ,o ••••• 10865 14200 V?V?V v?v? 11800 -··-··· V?V?V 10900 .......:-, ::, ::1. 14250 Sill? 11841 14300 11900 - : 11000 14352 11038 14375 VVV V 11085 11985 T.D. 11100 - 1'· RINGK0BING-FYN HIGH 11240 11259 Q-1 ~ 11300 • 11400 11464 T. □. Fig. 18: Lithological profiles of the Rotliegendes deposits. For legend, see fig. 3. 30 Central Graben - Danish section DGU Sept. 1981 Palinspastic profile for the Zechstein Southern Permion Basin Northern Permian Basin SE NE x x X X X X X A X X X x x Y __x ,X ~ X X X X y--.,.: · · 7 7 7 7 7 7 ~.J<...,t-0 .. .. ::~ 7 7 7 7 7 7 7 Z-4 ···-~ ........ 7 7 7 .....7 7 7 - 7 7 7 7 7 7 ~ x'x'Y'xx""' ,;.·· 7 7 7 7 7 7 7 k 7 k 7 k 7 k 7 k 7 k 7 ~ffl xx: x,s:~7 k 7 k 7 k 7 k 7 k 7 k 7 k 7 Z-3 ::ZZ: :rn:_ --ZZ-~J<(>(-//J"x ¥xx'XY'x xx ::ZZ:7:xxx:["°_~ a.. 7 7 7 z :::) 7 7 7 ~ '--2 "xV?x"x.Vx~7 7 7 7 7 7 7 7 0 7 k 7 k 7 k 7 k 7 k 7 k ~x,. Jy<JV<,?Y<x" .,.__,., k 7 k 7 k 7 k 7 k 7 k 7 k 7 k <t a:: A XY " ~AA A.,/-7,X,,...- c.!) - z ~ ....w a:: Ill :r: Z-2 LU u UJ a.. N Z-1 Fig. 19: Palinspastic profile of the Zechstein deposits. For legend, see fig. 3. by formation density logs with high values in the 2/7-2 well of the Norwegian sector (Dixon et al. 1981). massive besaltic lava flows, and lower values in the Unfortunately the ages of the volcanic rocks recorded interbedded deposits or porous top zones. The gamma within the Central Graben (P-1, Q-1, W-1) are uncer­ ray intensity is variable but generally relatively low tain as they are not overlain by dated, typical Zech­ especially in the massive basalt lava flow. Massive lava stein sediments (see fig. 18). In this connection it flows with high gamma ray intensities and lower should be mentioned that Triassic(?) and Jurassic densities may be rhyolites. The lithologies of the B-1, volcanics are known from the triple junction between W-1, Q-1, and P-1 wells based on the geophysical logs the Viking Graben and the Moray Firth Basin (Wood­ and the mudlog are shown in fig. 18. The differentiated hall & Knox 1979). rock types are not indicated as they await more detailed examination. They form a smaller portion of Petrography: So far only a few petrographic descrip­ the massive flows, but could be more frequent among tions are available. Dixon et al. (1981) recorded pla­ the intrabasaltic deposits. gioclase and scattered clinopyroxene and altered olivine phenocrysts in the B-1 basalts, which are simi­ Geological age: At present radiometric age determina­ lar to those of the C-1 and D-1 well of the North tions have been performed on the B-1 well, giving 212 Permian Basin. Altered plagioclase phenocrysts occur +I- 14 Ma (K/Ar by Larsen 1972) and on the W-1 in the cored basalt of W-1. Rhyolites have only been well, where a large spread is observed, 89-230 Ma described from the R-1, associated with the Horn (Geological Dating Services). This age is regarded as Graben. The secondary minerals of the basalts include updated, due to metamorphism, and the volcanics are among others: calcite, anhydrite, epidote, chlorite, overlain by Upper Permian sediments. Permian vol­ serpentine, haematite, goethite, and quartz, possibly canic rocks also occur in the North Permian Basin, i.e. originating from the thermal rise associated with the in the D-1 well, dated to 237 +I- 16 Ma; the C-1 well, volcanism. 281 +/- 8 Ma (Larsen 1972, Rasmussen 1974); the L-1 Chemical analyses of the basalts of B-1 show that well overlain by marginal Zechstein deposits; and the they can be classified as transitional olivine tholeiites 31 grading into hawaiites, typical for the Permian volca­ foldbelt to the south caused influx of fanglomerates nism north of the variscian fold belt. In contrast the and braided stream (wadi) deposits along the southern younger volcanic rocks in the region appear to be more margin of the basin. The desert sand dune deposits in alkaline: the ?Middle Triassic - Middle Jurassic, al­ the southern part of the basin are the main reservoirs kaline basalts of the east Moray Firth Basin (Woodhall of the Rotliegendes gas play.