Downloaded from http://sp.lyellcollection.org/ by guest on October 10, 2021 Development of N.W. Europe's Southern Permian Gas Basin K. W. Glennie Shell U.K. Exploration and Production SUMMARY: The Southern Permian Basin contains over 4.1 × 1015 m 3 of recoverable non-associated gas, most of which is found in Early Permian Rotliegend sandstones. The most important source of this gas is Carboniferous coal deposited in a proto-Tethys foredeep basin. Where the coals were overlain by Rotliegend desert sands and by Late Permian Zechstein halite, the scene was set for creating important reservoirs for gas. Deformation of the reservoir and seal to create a trap, and the generation and migration of Carboniferous gas, resulted from the structural history of the area, which was shaped in part by underlying crustal blocks and zones of crustal weakness. Much of this history was related to events most clearly expressed beyond the limits of N.W. Europe in the North Atlantic and Tethys: (1) E.-W. tension, which gave rise to the oblique-slip-induced subsidence of many sub- basins within the Southern Permian Basin, caused the consecutive creation of the Viking-Central graben system and Rockall Trough, and ended with the crustal spreading of the Atlantic Ocean; and (2) the early Mesozoic opening of Tethys, its Cretaceous closure and ensuing Alpine orogeny, which induced inversion in many areas of the Southen Permian Basin that are now gas bearing. Introduction beyond the limits of N.W. Europe, which were the cause of three major mountain-building North-West Europe's Southern Permian Basin episodes that affected the area to a greater or contains over 4.1 × 1015 m 3 of recoverable lesser extent, the Caledonian, Variscan and reserves of non-associated gas, most of which is Alpine orogenies. found in Early Permian Rotliegend sandstones, with minor quantities in reservoirs of Late There have always been slow changes in global Permian, Early Triassic, Cretaceous and mean surface temperature that were related to Tertiary ages (Fig. 1). By far the most important changes in the location and the relative propor- source of this gas is the underlying coals and tions of land and sea, and in the freedom of carbonaceous shales of the Carboniferous Coal movement of oceanic currents. Superimposed Measures. The maturation of this source rock on these have been long-term changes in climate and the migration and entrapment of the that were to effect the post-Caledonian deposi- generated gas is intimately connected with the tional environments of the North Sea area. structural and depositional history of this part These climatic changes were brought about by of Europe. the slow, apparently passive, drift of the North The Southern Permian Basin is both under- American-Eurasian land mass from a location lain and surrounded by an assortment of rocks, that was south of the Equator during the both sedimentary and crystalline, whose origins Devonian, that straddled the Equator during the stretch back to the early Palaeozoic and beyond. Carboniferous, and that today have their During the course of the basin's long history, separate locations centred some 50°-55 ° north vertical and horizontal movement of the older of the Equator (see e.g. Habicht 1979). These cratonic blocks continually influenced the struc- climatic changes had no direct effect on the tural and depositional history of their sedi- cratonic structural development of the Southern mentary cover as well as that of their sur- Permian Basin, but without those changes there roundings. Because many of these ancient would have been an entirely different set of blocks were not firmly welded together, the hydrocarbon source-rock, reservoir and seal zones that separated them were inherited as lines parameters to deal with, especially with respect of weakness that were activated repeatedly to the rocks of terrestrial and shallow-marine during the later history of the area in response to environments of deposition. changes in tensional, compressive and tangential It is clear from the foregoing that no sedimen- forces. Many of these forces were themselves tary basin should be studied in isolation, but induced by changes in crustal (plate) geometry rather should be considered in the context of its From BROOKS, J., GOFF, J. C. & VAN HOORN, B. (eds), 1986, Habitat of Palaeozoic Gas in N. W. Europe, Geological Society Special Publication No. 23, pp. 3-22. Downloaded from http://sp.lyellcollection.org/ by guest on October 10, 2021 4 K.W. Glennie / DISTRIBUTION OF PALEOZOIC GAS IN SOUTHERN PERMIAN BASIN OF N W EUROPE I RESERVOIR FIELDS CRETACEOUS K TRIASSIC T PERMIAN : ZECHSTEIN z : ROTLIEGEND CARBONIFEROUS c SIGNIFICANT GAS DISCOVERY • T~ Esmond T ° ~T z c L %wos, so,o C.B•C ~-p O~, Barque~ :~Viking ee'~KlO ~ • Ameland o Clipper~ ~:~Q~nde. ee eC Valiant %~- ~T~T ~e~ • ~ lezu'dwal• ..;~Groningen . L .... S.... • ~P6 ~'~T~T ~T. Z =~e .. 01 1OOL 2OOkml T Bergen• e Wijk .e_~Coevorden e j ~ 52. ~ oo ~o FIc. 1. Distribution of gas fields in the Southern Permian Basin that are known or presumed to contain gas of Carboniferous origin; the reservoirs, however, are of varying age. The reservoir age in many of the single well discoveries has not been published. surroundings. The Southern Permian Basin is seaway (Tornquist Sea) trending roughly no exception for, as its name implies, its origin is W.N.W.-E.S.E. between southern Baltica and associated, at least in time, with that of its the Gondwana-derived microcontinents of northern partner, the Northern Permian Basin. London-Brabant, Armorica-Bohemia and Both basins have, to a greater or lesser extent, Silesia (Cocks and Fortey 1982; Ziegler 1982). been affected by plate-tectonic events that resul- For the most part, the sediments of the Torn- ted in the opening and closing of the southern quist Sea are deeply buried beneath a cover of ocean Tethys, and in the long period of gesta- Devonian, Carboniferous and younger sedi- tion that eventually led to the opening of the mentary rocks (Fig. 3). Along the southern edge North Atlantic Ocean (Fig. 2); and both basins of Fenno-Scandia in Poland and northern are associated with rocks that are considerably Germany (Flensburg), however, well data indi- older than the basins themselves. cate that there is a change from crystalline base- ment in the northeast, through a (protected?) zone of unmetamorphosed Lower Palaeozoic Early Palaeozoic Origins sediments to a zone in which the sediments are During the early Palaeozoic, the major slightly metamorphosed (Frost et. al., 1981). A S.W.-N.E. trending Iapetus Ocean separated the little further to the north in southern Sweden, a old cratonic shield areas of Laurentia and Cambrian source rock, the Alum Shale, overlies Baltica (Harland and Gayer 1972). Late Silurian the Precambrian basement and is still not closure of the Iapetus Ocean gave rise to the mature for oil (Cornford 1984). If hyrdocarbon Important Caledonian mountain system, sub- source rocks are present within the zone of slight merged relics of which extend beneath the metamorphism mentioned above, like the post- northern North Sea between North Britain and mature source rocks of the Lake District Norway (Fig. 3). Long before this major (Parnell 1982), they will probably have a rank orogeny, however, there existed a deep-marine approaching that of graphite. Downloaded from http://sp.lyellcollection.org/ by guest on October 10, 2021 Ha. REGIONAL EVENTS NORTH SEA PERMIAN BASINS ML PERIODS TETNYS-RELATED ATLANTIC-RELATED NORTHERN SOUl'HERN 0 ......... 0 Hiccene REGIONAL SUBSIDENCE OVER GIRABEN SYSTEM SPREADING Oligocene ALPINE OF PRESENT HID ATLANTIC RIDGE Eocene I OROGENY Plateau ba.~lts Zechstein diapirism Palecc~ne-- - PLATE COLLISION SPREADING OF ROCKALL TROUGH Renewed faulting Inversk~ in of i Inversion of NW.SE trending Late EXTENDING NORTH TO GRADUAL Rotation NORWAY-GREENLAND SEA Central Danish : sub-basins CLOSURE of A Graben i Embayment J~ OF Iberia I SUBSIDENCE Extrusives I TETHYS ~i I SEA-FLOOR SPREADING IN in I Early CENTRAL Danish [ I IBERIA - NEWFOUNDLAND I GRABEN Embayrnent Zechstein diapirism ONSET OF SEA-FLOOR SPREADING LATE CIMMERIAN UNCONFORMITY Indefatigable erosion IN CENTRAL ATLANTIC DOMAL COLLAPSE & HAIN PHASE Rapid Sole Pit subsidence Late I OF GRABEN FORHATION u SEA-FLOOR SPREADING A IN TETHYS 1 RIFT & WRENCH TECTONICS Mid I ~e A DOMING AND I I LIMITED VOLCANIC ACTIVITY Early I 2~ I I -2~ I RIFTING IN Earliest Zechstein diapirism CENTRAL ATLANTIC 400m Triassic in RIFTING PHASE Polish Trough HARDIEGSEN UNCONFORMIrrY I DEVELOPMENT OF N.W. EUROPEAN I BASINIGRABEN SYSTEM Late I I Zechstein ,',~,~:n t | of sub-seal~vel I~slns Early I LATE HERCYNIAN I SUBSIDENCE OF SOUTHERN & NORTHEI~N PERHIAN BASINS BEGAN WRENCH TECTONICS Stephanian EARLY COLLAPSE OF VARISCAN 1 300- [ Extrusion of I- Rotllegend volcamia began .300 FOLD BELT IN EUROPE D Rifting in Right-lateral faulting: inver~on of Sole Pit Basin Westphal. Norway-Greenland Sea 250Om U. Carbemiferous A VARISCAN OROGENY Namurian T I VARISCAN FOREDEEP PLATE COLLISION INITIATION OF 1 NORTH ATLANTIC Dinantian FRACTURE PATTERN STEP-WISE CLOSURE I BACK-ARC RIFTING Z Probable strike-slip < Late OF PROTIO TETHYS I 2 movement of Great Glen Fault Marine Ur~,,sto~ in i & Extention in N. Atlantic Auk & Argyll I ~.~ Hid I Early I Volcanics in S. Scotland i Granites in Lake District I df~mAI I=IqU~IbIIALII ~f~.glklY FIG. 2. A simplified evolution of the Tethys and Atlantic oceans tentatively related to some post-Caledonian structural events within the Northern and Southern Permian basins (Modified from Glennie, 1984b. Table 1). Downloaded from http://sp.lyellcollection.org/ by guest on October 10, 2021 6 K. IV. Glennie During the late Cambrian-early Ordovician Collision between Gondwana and laurasia took closure of the Tornquist Sea, the sedimentary fill place south of Armorica in the Late Visean and was deformed and consolidated into continental marked the start of the Variscan orogeny (Fig.