49 point on the basis of wireline logs alone, but it is Reservoir potential: Unknown. defined by the occurrence of marls and limestones, or mudstones with reddish coloration above the boun­ Sealing potential: Fair, but this cannot yet be evalu­ dary. ated further. Distribution: The Formation is present in the Danish 0-1, A-2, G-1, M-1, M-8, E-1, E-3, and 1-1 wells. It is 3.6 Late Cretaceous widely distributed in the central North Sea outside and structural highs, in the southern part of the Danish­ Danian limestone Norwegian Basin, and south of the Ringk0bing-Fyn High. By Kirsten Lieberkind, Inger Bang, Naja Mikkelsen & Erik Nygaard Geological age: On the basis of foraminiferal faunas, At the termination of the Early Cretaceous period, the the Formation is invariably dated Albian and occa­ sea transgressed large earlier land areas. The trans­ sionally also referred to parts of the Aptian (? Late gression, the reduced relief of the continents, the Aptian). generally diminished tectonic activity, the climatic Depositional environment: Marine shelf. change, and the enormous production of calcareous nannoplankton in the oceans profoundly changed the Source rock potential: Poor. The content of organic gross facies pattern in the North Sea region. The material is too low to give any source rock potential. elastic sedimentation was replaced during Late Cre- FORMAL CHALK FORMATIONS CH RONOSTRATIGRAPHIC AGE DANISH CHALK UNITS ( Deegan & Scull, 1977) ( Central Graben l EKOFISK FORMATION DANIAN CHALK -6 UNIT ~----------- -------------- --- CHALK - 5 UNIT MAASTRICHTIAN TOR FORMATION --- ~------------- CHALK -4 UNIT -------,------- 0... I ~ ----- :::) CAMPANIAN 0... 0 :::) 0:: I 0 C) I 0:: C) CHALK :IC FLOUNDER - 3 UNIT/ ....I I ~------------- :IC <( ....I J: FORMATION I <( u J: I SANTONIAN u I HOD FORMATION - --- - I I ~------------- I CONIACIAN V HEERING CHALK-2 UNIT I FORMATION I -------------- I TURONIAN PLENUS MARL FORMATION HIDRA FORMATION CENOMANIAN CHALK-1 UNIT DGU 1981 Fig. 25: A correlation between Chalk Units in the Danish Central Graben, the chronostratigraphic time table, and the formal Chalk Formations established by Deegan & Scull (1977) for the Northern and Central North Sea area. 50 taceous time by offshore relatively deep water carbo­ Reference sections: A thickness estimate based on a nate sedimentation. compilation of thicknesses of units from a number of This resulted in the deposition of a chalk sequence wells in the type area suggests a thickness in the order reaching a thickness of 1-2 km in the Danish Subbasin of 1500 m. and the North Sea area. A similar estimate for the Danish Central Graben is 1000 m, but the figures show a wide variety. Chalk Group (Deegan & Scull 1977) Lithology: The Chalk interval in the North Sea is The chalk interval in the North Sea has been described represented by white limestone which shows wide as the Chalk Group by Deegan & Scull (1977). variations in induration, colour, chert, clay, shale, and glauconite content. The term 'chalk' is used here as in Type section: The type area of the Chalk Group is the Scholle (1977a). North Sea basin. A formal type section has not yet been established. Log characteristics: A number of formal chalk forma- CENTRAL GRABEN 0 6 10 15 Zll HKm DGU 1981 \____. \ -------- . -------- . -------- . 0 "' 0 •\{ f > Thickness of Chalk Units. Legend: ~ location of well 0 g } th.cikne,ss nf unit(l:5'0m) S5"ll)'\.,.--+o-.\..J.-.-~U------+-----~,-----,d•s-J0· 4)\~ .)j, II 2 3 1.. s 6 Chalk Unit V _/ Fig. 26: a) Thicknesses 0f the six Chalk Units illustrating the development of the Ohalk Group in relation to time and location in the Graben ara, 51 tions have been established in the Norwegian sector by Boundaries: The Chalk Group in the type area overlies Deegan & Scull (1977). the Early Cretaceous calcareous mudstone of the In the Danish sector of the Central Graben, a total Cromer Knoll Group and, in the D anish sector, the number of six log units have been established (see R0dby Formation or older deposits. The boundary in below). These units are mainly defined by gamma ray both cases is only distinct if a hiatus is present. and sonic velocity responses. The units are labelled The upper boundary to the overlying elastic sedi­ Chalk-I Unit to Chalk-6 Unit, where the first r epre­ ments of the Tertiary may b sharp or gradual through sents the older sequence and the Chalk-6 Unit the the calcareous 'North Sea Marl' unit. younger. The above mentioned units have not been given the Distribution: The Group has a wide distribution in the rank of formal formations, as work remains before the central North Sea. It is present in the southern part of definitions can be verified. The correlation between the Viking Graben, in southeast England, in the Du­ the formal chalk formations of Deegan & Scull (1977) tch and German parts of the North Sea, and in the and the proposed Danish chalk units (fig. 25) is D anish onshore and offshore areas. At least a part of therefore tentative. --4.)) ----- 1 I / : .,;:/:;..____ +-----+-I ----+--------.----1 •••,o· CENTRAL GRABEN ? 0 5 10 15 20 25 Km /I I DGU 1981 < -- ~- _____ .,, ____ _--- - -S.31- - --- ------5.3.2---- 1 /. ,2 I : I 1 I I ' I I I /. I -,\ : I /~-------'------45.2----,-----+----t------S.3 -----r---l ./t /1 . I \ I \ . ---- -----~L----- -----=--- -~--T---- - 1 \ I I I_"'- - - I - \ I I lo \ 5 '- I I I I \ I '- I 6 I 2 I \ I \ I \ 0 I \ I I I -- - -- 6.32--- -~•12------ -- - - - 621-- ---\ +---- .t,-1- ---------6,]1 ----- ., _______ . \ I I ' 1 I I ' I I I I I I I \ 0 8-------- \ I I 3 I I !, - ~ / : \ I I ~ 6.2r-----+- --4--f----"<--+----6.3------;-----, tjooo \ CHALK GROUP .\ I ,,.... I \ I Deposit zones 1-6 ---- - -- - -------6.3,l. -- -\~""~ -~-•~· ---- ~ l \ 0 I \ 5 ~o (\ I \ I I . I <\ 3 \ I I ss•,o ·\----\+l--o-l- 1- 4----\-\ ~ -'19.__:o~,---l-~-=--~-+----/---:,•' ~---~,~--- -':,~~" _/ ~ .._/ I . I ~~ - Fig. 26: b) Areal distribution of the chalk deposit zones I to 6 deduced from the chalk depositions. 52 the Chalk Group has been recognized in all wells from Reservoir rock potential: The primary porosity and the Danish Central Graben. Based on the presence/ fractures of the Chalk Group in general makes it a fair absence and thicknesses of the single chalk units (figs. reservoir rock despite its low matrix permeability. Oil 27 to 29), and the generalized isopach map (fig. 16) a and gas have been located within the Chalk Group and set of six deposit zones is suggested (fig. 26). Wells production has been started in a few fields of the situated within the same zone show a gross similarity Danish part of the North Sea. in the chalk development (fig. 31). Sealing potential: Clay lamina, stylolites, and diagene­ Geological age: The dating of the Chalk Group in the tic horizons within the Chalk Group represent sealing North Sea is essentially based on micropalaeontologi­ possibilities. However, the Chalk Group in general is cal studies of foraminifera. The Chalk Group is re­ considered to have a poor sealing potential. ferred to the stratigraphic interval from base Late Cretaceous to top Danian. From palaeontological studies it is evident that Danian limestone is present in nearly all Central Chalk-1 Unit (informal name) Graben wells. Chalk of Maastrichtian age is also Type section: The Danish Adda-1 well (7420-7512' recorded in all wells. Chalk of Campanian age is b.KB), thickness 28 m. present in most wells, whereas the older chalk of Cenomanian to Santonian age has a rather scattered Reference section: The Danish 0-1 well (7500-7580' occurrence in the area. b.KB), thickness 22 m. In the Danish Central Graben, the Chalk-I Unit is Depositional environment: Open marine conditions also present in the E-1 well, the data of which are given dominated during the deposition of the Chalk Group. in chapter 8. The general picture of the chalk sedimentation, The Unit may furthermore be present in the A-2, B- however, seems to indicate rather disordered pre­ 1, and H-1 wells, where the sections are very con­ Maastrichtian deposits in the Central Graben. The densed. distribution pattern of these deposits is affected by folding, salt movements, and non-deposition or ero­ Tentative correlations: The Chalk-I Unit may be cor­ sion, especially in the southern part. The Maastrich­ related with the Hidra Formation. tian-Danian chalk deposits tend to be less affected by these processes, and they unconformably overlie the Thickness: The Chalk-I Unit is only recognized with older chalk units on most piercement structures. certainty in three wells in the middle and southern The chalk sequence is often bounded by two uncon­ part of Central Graben. The maximum thickness of formities - a minor at the Cretaceous-Tertiary boun­ the Unit is recorded in Adda-1 (28 m) (fig. 27). dary and a major at the base of the chalk. The unconformity at the base of the chalk is diachronous. Lithology: The Chalk-I Unit is dominated by hard to However, it clearly indicates a conspicuous change in moderately hard chalk. The colour is white to light the depositional environment between the chalk se­ grey and occasionally slightly pink. The chalk is often quence and the older sediments. The unconformity is interbedded with greyish and light green marl and pronounced in the northern part of the Danish Central shale. Thin beds of calcisiltite may be present (0-1) Graben and less striking in the middle and southern and in places the chalk is slightly dolomitized (Adda- part. The most complete chalk sequences are recorded 1). in deposit zones 2 and 3 (fig.