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Carboniferous- Total Description and Assessment Results Summary

By Donald L. Gautier U.S. Geological Survey Bulletin 2211

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey

U.S. Geological Survey, Reston, Virginia: 2003

Charles G. Groat, Director

For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225

For more information about the USGS and its products: Telephone: 1-888-ASK-USGS

Although this report is in the public domain, it contains copyrighted materials that are noted in the text. Permission to reproduce those items must be secured from the individual copyright owners. iii

Contents Foreword ...... 1 Abstract ...... 2 Introduction ...... 2 Province Geology and Petroleum Occurrence ...... 2 Province Boundaries ...... 2 Structural-Tectonic Controls on Sedimentary Sequences ...... 2 Petroleum System Summary ...... 12 History of Exploration and Production ...... 12 Total Petroleum System: -Rotliegend (403601) ...... 14 Source Rocks ...... 14 Organic Geochemistry of Kerogen and ...... 14 Burial History, Gas Generation, and Migration ...... 14 Reservoirs ...... 16 Seals ...... 17 Traps and Timing ...... 17 Assessment Units ...... 17 Assessment Unit: Southern Basin-Offshore (40360103) ...... 17 Description ...... 17 Offshore Exploration Plays and Their Resource Potential ...... 17 The Rotliegend Play ...... 17 Play Description ...... 17 Exploration Potential of the Rotliegend Play ...... 18 Carboniferous Play ...... 18 Play Description ...... 18 Exploration Potential ...... 18 Play ...... 18 Play Description ...... 18 Exploration Potential ...... 18 Plays ...... 18 Play Description ...... 18 Exploration Potential ...... 18 Assessment Unit: Southern - Onshore (40360102) ...... 19 Description ...... 19 Exploration Plays and their Resource Potential ...... 19 The Rotliegend Sandstone Play ...... 19 Play Description ...... 19 Exploration Potential of the Rotliegend Play ...... 19 Carboniferous Play ...... 19 Play Description ...... 19 Exploration Potential ...... 19 Zechstein Play ...... 19 Play Description ...... 19 Exploration Potential ...... 19 Mesozoic Plays ...... 20 iv

Play Description ...... 20 Exploration Potential ...... 20 Assessment Unit: Southern Permian Basin-U.K. Onshore (40360101) ...... 20 Description ...... 20 Carboniferous Play ...... 20 Play Description ...... 20 Resource Potential ...... 20 Undiscovered Gas Resources of the Carboniferous-Rotliegend Total Petroleum System ...... 20 References Cited ...... 22

Figures

1. Provinces, coastlines, and national boundaries associated with the Carboniferous- Rotliegend Total Petroleum System ...... 3 2. Prominent oil and gas fields of the southern and adjacent onshore areas ...... 4 3. Carboniferous-Rotliegend Total Petroleum System boundary, locations of oil and gas fields, and U.S. Geological Survey World Energy Project province boundaries ...... 5 4. Southern Permian Basin-Offshore Assessment Unit boundary ...... 6 5. Southern Permian Basin-Europe Onshore Assessment Unit boundary ...... 7 6. Southern Permian Basin-U.K. Onshore Assessment Unit boundary ...... 8 7. Some structural features of the southern North Sea and adjacent areas ...... 9 8. Events chart for the Carboniferous-Rotliegend Total Petroleum System ...... 10 9. Late Carboniferous Variscan orogenic front, and the distribution of Permian Rotliegend sedimentary facies and Zechstein halite deposits ...... 11 10. Stratigraphic column of the southern Permian Basin ...... 13 11. Comparison of published burial curves for Carboniferous-age rocks in and around the southern North Sea ...... 15

Table

1. Carboniferous-Rotliegend Total Petroleum System (403601), Assessment Results Summary—Allocated Resources ...... 21 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

By Donald L. Gautier

This report was prepared as part of the World Energy history, and risk to be assessed individually. Assessment units Project of the U.S. Geological Survey. In the project, the world are considered to be: (1) established units if they contain more was divided into 8 regions and 937 geologic provinces, and than 13 fields, (2) frontier units if they contain 1 – 13 fields, or the provinces were then ranked according to the discovered (3) hypothetical units if there are no discovered fields. oil and gas volumes within each (Klett and others, 1997). Of A graphical depiction of the elements of a total petroleum these, 76 priority provinces (exclusive of the U.S. and chosen system is provided in the form of an events chart, which shows for their high ranking) and 26 emerging (boutique) provinces (exclusive of the U.S. and chosen for their anticipated petro- the time of deposition of essential rock units and the timing of leum richness or special regional economic importance) were processes, such as trap formation, necessary for the accumula- selected for appraisal of oil and gas resources. The petroleum tion of hydrocarbons. geology of these priority and boutique provinces is described A unique eight-digit numeric code identifies each AU in this of reports. A detailed report containing the with respect to the region, province, and TPS. For example, assessment results is available separately (U.S. Geological the Southern Permian Basin-Offshore AU was assigned num- Survey Assessment Team, 2000). ber 40360103. The first digit is the region number, indicating The purpose of the World Energy Project is to aid in the AU is in Europe (Region 4). The next three digits uniquely assessing the quantities of oil, gas, and liquids that identify the province; in this case the Anglo-Dutch Basin is have the potential to be added to reserves within the next 30 . These potential resources reside either in undiscovered number 036. The following two digits, 01, refer to the TPS, fields whose sizes exceed the stated minimum field-size (in and the final two digits are the unique AU number (03 in this this case one million barrels of oil equivalent) or they occur as example). potential reserve growth of fields already discovered. The codes for the regions and provinces were established The total petroleum system (TPS) is the basic geologic and listed by Klett and others (1997); provinces for Europe unit of the World Energy Project. The TPS includes all geneti- (Region 4) are shown and listed in greater detail in USGS cally related shows and accumulations of petroleum (discov- Digital Data Series DDS-60, the principal publication of the ered and undiscovered) that (1) have been generated by a pod World Petroleum Assessment 2000 (U.S. Geological Survey or by closely related pods of mature , and (2) exist Assessment Team, 2000). Oil and gas reserves quoted in this within a limited mappable geologic space. The petroleum sys- tem concept is modified from Magoon and Dow (1994). The report are derived from Petroleum Exploration and Production minimum petroleum system is defined as that part of a total database (Petroconsultants, 1996) and other area reports from petroleum system encompassing discovered shows and accu- Petroconsultants, Inc., unless otherwise noted. mulations together with the geologic space in which the vari- Figures in this report that show boundaries of the total ous essential elements of the system (source, reservoir, seal, petroleum systems, assessment units, and pods of active and overburden rocks) have been proved by these discoveries. source rocks were compiled using geographic information An assessment unit (AU) is a mappable part of a total system (GIS) software. Political boundaries and cartographic petroleum system in which discovered and undiscovered fields representations were taken, with permission, from Environ- constitute a single relatively homogenous population such mental Systems Research Institute’s ArcWorld 1:3 million that the chosen methodology of resource assessment based on estimation of the number and sizes of undiscovered fields is digital coverage (1992), have no political significance, and are applicable. A TPS might equate to a single AU or, if necessary, displayed for general reference only. Oil and gas field center a TPS may be subdivided into two or more AUs such that each points, shown on these figures, are reproduced, with permis- is sufficiently homogeneous in terms of geology, exploration sion, from Petroconsultants (1996). 2 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

Abstract accounts for approximately 12 TCF of produced and proven gas reserves. The Anglo-Dutch Basin and the Northwest German The Northwest German Basin and the Anglo-Dutch Basin Basin are two of the 76 priority basins assessed by the U.S. were combined for the purposes of the petroleum resource Geological Survey World Energy Project. The basins were assessment. This lumping was considered justifiable because assessed together because most of the resources occur within a the basins are contiguous and because most of the natural gas single petroleum system (the Carboniferous-Rotliegend Total in these provinces has been generated and entrapped within Petroleum System) that transcends the combined Anglo-Dutch a single total petroleum system (TPS), referred to as the Car- Basin and Northwest German Basin boundary. The juxtaposi- boniferous-Rotliegend TPS. Geographically, it occupies most tion of thermally mature coals and carbonaceous shales of of the Anglo-Dutch Basin and the Northwest German Basin as the Carboniferous (source rock), well as parts of the adjacent North Sea and Polish-Ger- of the Rotliegend sedimentary systems (reservoir rock), and man Basin provinces (fig. 3). The Carboniferous-Rotliegend the Zechstein evaporites (seal) define the total petroleum was the only TPS assessed in the Anglo-Dutch and Northwest system (TPS). Three assessment units were defined, based German Basins, even though other geologically interesting but upon technological and geographic (rather than geologi- less economically important petroleum systems are present cal) criteria, that subdivide the Carboniferous-Rotliegend (Kockel, Wehner and Gerling, 1994; Fraser and others, 1990). Total Petroleum System. These assessment units are (1) the The Carboniferous-Rotliegend TPS was subdivided into three Southern Permian Basin-Offshore Europe Assessment Unit, assessment units, based largely on economics, accessibility, (2) the Southern Permian Basin Onshore Europe Assessment and exploration history; these are Southern North Sea Off- Unit, and (3) the Southern Permian Basin Onshore United shore (AU40360103, fig. 4), Europe Onshore (AU40360102, Kingdom Assessment Unit. Although the Carboniferous-Rot- fig. 5), and United Kingdom Onshore (AU40360101, fig. 6). liegend Total Petroleum System is one of the most intensely explored volumes of rock in the world, potential remains for undiscovered resources. Undiscovered conventional resources Province Geology and Petroleum associated with the TPS range from 22 to 184 million barrels of oil, and from 3.6 to 14.9 trillion cubic feet of natural gas. Occurrence Of these amounts, approximately 62 million barrels of oil and 13 trillion cubic feet of gas are expected in offshore areas, and Province Boundaries 26 million barrels of oil and 1.9 trillion cubic feet of gas are predicted in onshore areas. The Anglo-Dutch Basin and the Northwest German Basin geologic provinces are contiguous regions (fig. 1) that include onshore and offshore areas of , , , , United Kingdom, and the . The Anglo- Introduction Dutch Basin contains a number of smaller sub-basins, includ- ing the Sole Pit Basin (also referred to as the Sole Pit High, as The Northwest German Basin and the Anglo-Dutch Basin the basin was the site of a tectonic inversion), East Midlands of northern Europe (U.S. Geological Survey World Energy Shelf, West Netherlands, Broad Fourteens, and Central Nether- Project provinces 4036 and 4035, respectively) are two of the lands sub-basins (fig. 7). The Northwest German Basin is also 76 World Energy Project priority provinces. The Northwest a compound basin, including the Lower Saxony Basin (fig. 7). German Basin is number 21 and the Anglo-Dutch Basin is The two provinces are bounded on the southwest and south by number 39 in the World Energy Project global ranking (Klett the London-Brabant Platform and the Rhenish Massif, on the and others, 1997). These basins share common boundaries north and northwest by the Mid-North Sea High, the Central with the North Sea Graben Province (number 4025), which Graben, the Ringkobing-Fyn High, and the Horn Graben (fig. ranks number 8 in the world. Taken together, these three prov- 7). The provinces are bounded on the east by a structurally inces account for the vast majority of oil and gas resources positive saddle near the Polish-German border, on trend with known in Western Europe. The location and boundaries of the northeast-striking Hesse Depression (Ziegler, 1990), which these provinces are shown in figure 1. is not shown on figure 7. In contrast to the oil-prone North Sea Graben, the Anglo-Dutch and Northwest German Basins are gas-prone. The gas field, located just onshore in the Nether- Structural-Tectonic Controls on Sedimentary lands (fig. 2), is the largest hydrocarbon accumulation in the Sequences Northwest German Basin and one of the largest gas fields in the world. Groningen contains approximately 100 trillion The elements giving rise to petroleum generation and cubic feet (TCF) of produced or proved reserves of natural accumulation in the Carboniferous-Rotliegend TPS are largely gas. The largest field in the Anglo-Dutch Basin is the Leman the result of structural and sedimentary events that took place field, in the United Kingdom continental shelf (fig. 2), which during Carboniferous and Permian time (fig. 8). Particu- Province Geology and Petroleum Occurrence 3

0° 5°E 10°E

North Sea

DENMARK

4025 Copenhagen 55°N 4036 4035

Hamburg

Bremen 4033

Amsterdam UNITED KINGDOM NETHERLANDS

Dortmund London Antwerpen GERMANY Brussels BELGIUM Bonn 4039 50°N

Paris

0 250 KILOMETERS

EXPLANATION Northwest German Basin geologic province 40

4036 Anglo-Dutch Basin geologic province 4036 4035 Other geologic province boundary

National boundary

Oil field centerpoint

Gas field centerpoint

Index Map

Figure 1. Provinces, coastlines, and national boundaries associated with the Carboniferous-Rotliegend Total Petroleum System (403601). 4 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary g Frankfurt am Main gas field Groningen Duisbur Amsterdam E ° 5 135 KILOMETERS Antwerpen 0 Lille Sole Leman st We Clipper ° 0 TION Oil field Gas field London EXPLANA Prominent oil and gas fields of the southern North Sea adjacent onshore areas. Sheffield N N ° ° Figure 2. 50 55 2°W 0° 2°E 4°E 6°E 8°E 10°E 12°E NORTH SEA DENMARK North 4028 GRABEN Sea Odense 4025 403601 ANGLO-DUTCH BASIN 4036 NORTHWEST GERMAN BASIN 4035 54°N Leeds Hamburg POLISH- Sheffield GERMAN Bremen BASIN 4033

NETHERLANDS Birmingham Amsterdam 4037 The Hague 52°N UNITED KINGDOM Rotterdam 4038 London Duisburg Dortmund Essen Antwerpen Gent Koln Bruxelles Bonn GERMANY Lille BELGIUM Liege 4039 5 Province Geology and Petroleum Occurrence 0 150 KILOMETERS Robertson projection, Central meridian 0

EXPLANATION 4037 Geologic province code and boundary Oil field centerpoint

Total petroleum system Gas field centerpoint

National border

Figure 3. Carboniferous-Rotliegend Total Petroleum System with center point locations of oil and gas fields and U.S. Geological Survey World Energy Project province boundaries. 6 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary 0 E ° 12 4033 g Robinson Projection, Central meridian Odense E ° 0 1 Hambur ERMANY G Bremen ENMARK D ° E 8 4038 4035 Oil field centerpoint Oil field centerpoint Gas field 40360103 E 150 KILOMETERS ° 6 ETHERLANDS N TION Amsterdam Rotterdam EXPLANA The Hague 0 E ° 4 Assessment unit boundary National border Geologic province code andboundary province code Geologic 4037 ° E 2 Sea 4037 North 4036 ° 0 4028 Leeds Sheffield UNITED KINGDOM mingham Southern Permian Basin-Offshore Assessment Unit (40360103) boundary. Bir W ° 2 N N ° ° Figure 4. 54 52 Province Geology and Petroleum Occurrence 7

4°E 6°E 8°E 10°E 12°E

DENMARK

North Odense Sea

4035

54°N

Hamburg 40360102 4033 LOWER Bremen SAXONY BASI 4036 N Amsterdam

The Hague NETHERLANDS 52°N Rotterdam 4038 Duisburg Dortmund 4036 Antwerpen Essen LONDON-BRA Gent PLA TFOR BAN Koln 4046 Bruxelles M Bonn GERMANY Liege BELGIUM 4037 0 100 KILOMETERS Robertson projection, Central meridian 0

EXPLANATION 4037 Geologic province code and boundary Oil field centerpoint

Assessment unit 40360102 Gas field centerpoint

National border

Figure 5. Southern Permian Basin-Europe Onshore Assessment Unit (40360102) boundary. larly important is the juxtaposition of the western part of the cynian Basin) was first closed and then deformed into a series Southern Rotliegend sedimentary basin and related Zechstein of complex thrust/nappe pairs. The northernmost (and latest) evaporite deposits on the Late Carboniferous Variscan foreland Variscan deformation occurred during the Late Carboniferous basin (fig. 9). and was soon followed by eruption of the Lower Rotliegend Volcanics (Ziegler, 1990). Tectonic relaxation of the foreland The Variscan foreland is a downward flexure, caused and Late Carboniferous tectonic inversion are represented in by tectonic loading north of the active Variscan orogenic belt the stratigraphic record by a major at the base of that formed as African collided with European the Upper Rotliegend (Glennie, 1998; Gast, 1993) (fig. 10). Laurasia during the final stages (Visean to Westphalian) of the The rapidly subsiding Variscan foreland accommodated assembly of the supercontinent. The earliest Variscan as much as 9,000 m of nonmarine sedimentary rocks of West- deformation probably occurred to the south of the foreland in phalian age, collectively called the Coal Measures (Collinson the Late . Deformation progressed northward during and others, 1993), that accumulated at tropical latitudes with the Early Carboniferous as a back-arc seaway (the Rheno-Her- luxuriant vegetation in a variety of depositional settings within 8 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

1°W 0° 55°N

North Sea

4036

UNITED KINGDOM

54°N

40360101 Leeds

Sheffield

53°N

4037

0 50 KILOMETERS

EXPLANATION 4037 Geologic province code and boundary Oil field centerpoint

Assessment unit boundary Gas field centerpoint

Figure 6. Southern Permian Basin-U.K. Onshore Assessment Unit (40360101) boundary. extensive fluvial systems. In northwestern continental Europe North Sea, fluvial systems were southward flowing (Ramsbot- and the southern North Sea, these fluvial systems generally tom and others, 1978; Ziegler, 1990). Coals and carbonaceous flowed northward, whereas in the British Isles and the central shales in the fluvial sequences are thought to be the source Province Geology and Petroleum Occurrence 9

H

G I

H ER gas field gas

Groningen BASIN

N

135 KILOMETERS Y

F G

R LOW

A

N B

E

N R

O H

SAXONY

MASSIF

RHENISH

G

N

I B

0

O

K NDS NDS

G N

I WEST R

E CENTRAL ° 5

N NETHERLA NETHERLA E B A R G TEENS BROAD L A R FOUR T N E C

M R O F T A L P

T HELF N

H SOLEHIGH PIT A

B G I EASTDS S A

H R

B A - E ° S N

0 MIDLAN

O H

T D R N

O O

L N

D

I M TION EXPLANA Basin/graben boundary Fault Shallow Paleozoic basement W Some structural features of the southern North Sea and adjacent areas. ° 5 N N ° ° 50 55 Figure 7.

10 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

ACCUMULATION

- N

PETROLEUM MIGRATION- E VERBURDEN ROCK SYSTEM EVENTS ROCK UNIT RESERVOIR ROCK

O

PRESERVATION CRITICAL MOMENT TRAP FORMATION SOURCE ROCK SEAL ROCK SCALE GEOLOGIC TIM GENERATIO 0 NEOGENE MIO. 24 EOC. 50 PAL. 65 L 100 E

150 144 L M E 200 206 L M 248 E 250 L PERMIAN E 290 300 L M 323 E L 350 E 363 L DEVONIAN M 400 E 408 L E 439 450 L M E 500 510 L M 550 E Petroleum System. Events chart for the Carboniferous-Rotliegend Total 570 600 Figure 8. 0° 4°E 8°E

DENMARK

54°N

NORTHWEST GERMAN BASIN

ANGLO-DUTCH BASIN

NETHERLANDS 52°N UNITED KINGDOM rvneGooyadPtoemOcrec 11 Province Geology and Petroleum Occurrence

GERMANY BELGIUM

50 0 50 100 150 200 250 MILES

50 0 50 100 150 200 250 300 350 KILOMETERS

EXPLANATION National boundary Sand Sand and conglomerate Halite and shale Front of Variscan deformation

Sand and shale Sulphate and shale Zechstein halite Geologic province boundary

Figure 9. Late Carboniferous Variscan orogenic front, and the distribution of Permian Rotliegend sedimentary facies and Zechstein halite deposits (after Ziegler, 1990). 12 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary rocks for natural gas in the Carboniferous-Rotliegend TPS marine water, as might result from the presence of a sill that (Cornford, 1998). separated the evaporite basin from its water source. Close Permian sedimentary rocks record profound changes in examination of various Zechstein cores and outcrops seems climate, vegetation, and sedimentary processes in the Variscan to indicate episodes of partial to complete desiccation of foreland. A decrease in the rate of erosion of the adjacent the Zechstein basin (Tucker, 1991; Kiersnowski and others, highlands and northward drift of Pangaea into arid latitudes 1995). resulted in declining sediment supply and continuous deserti- Carbonate rocks form platform and slope facies that fication (Ziegler, 1990). In the Southern Permian Basin, which bound the margins of the first three evaporite cycles (Z1–Z3). trends east-west between the northeast coast of across Such carbonates can locally form reservoir rocks, but they do the southern North Sea and into northernmost Germany and not serve as seals to gas accumulations in underlying Rotlieg- southern Denmark, continental desert sedimentary sequences, end reservoirs. Some basin-center carbonate rocks, however, collectively constituting the Rotliegend, were deposited at the have low and permeabilities and are effective parts site of the old foreland basin (Glennie, 1998). Alluvial fans of the seal-rock system. Thick halite beds are prominent in the and braid-plain deposits that accumulated near the Variscan second Zechstein cycle (the Z2 beds), which, in addition to front grade northward into eolian and fluvial (wadi) facies, providing tight seals, are the main salts involved in the diapiric and, farther north, to sabkha and desert-lake evaporites and movements that have influenced oil entrapment in the southern shales such as the Silverpit Claystone Formation (fig. 10) and central North Sea (Schultz-Ela and Jackson, 1996). (George and Berry, 1997). The end of Permian glaciation caused global sea level rise and marine transgression across the Rotliegend desert. The Petroleum System Summary Zechstein sea expanded into the Southern Permian Basin from Over a wide area of the southern North Sea, and adja- the north, probably flowing along pre-existing Proto-Atlantic cent onshore areas of Europe and England, hydrocarbons are and Proto-North Sea fracture systems. The basin is believed predominantly found associated with the same source rocks to have lain below sea level prior to the Permian Zechstein (Carboniferous Coal Measures), reservoir rocks (Permian transgression, and the scouring, reworking, and soft-sediment Rotliegend sandstones), and evaporite seals (Zechstein) that deformation seen in the uppermost Rotliegend is consistent combine to form the Carboniferous-Rotliegend TPS. The with speedy filling of the basin (Glennie, 1990; 1998). Organic southern boundary of the TPS is determined by the northern carbon-rich mud of the (Copper Shale) draped edge of the old Variscan deformed zone. The northern, eastern, desert sediments and basal Zechstein lag deposits. High evapo- and western boundaries are near the approximate limit of the transpiration rates in conjunction with restricted flow caused evaporite facies of the Zechstein (fig. 9). This single petroleum persistent evaporite accumulation. The resulting Zechstein system accounts for most hydrocarbon accumulations in this consists of thick carbonate and evaporite rocks that are area and forms the basis for resource assessment. largely of Late Permian age (Taylor, 1998). The Zechstein is present throughout the southern North Sea and onshore areas of northwestern Europe, especially in England, Denmark, History of Hydrocarbon Exploration and the Netherlands, Germany, , and Lithuania. Zechstein Production evaporites occupy two sub-basins, the southern one of which coincides with the boundaries of the Carboniferous-Rotliegend The geographic area encompassed by the Carbonifer- TPS, wherein the Zechstein is more than 2,000 m thick. ous-Rotliegend TPS became the site of one of the world’s first In the southern North Sea and on the continent, the oil developments when production began in Lower Saxony in Zechstein is characterized by four major (Z1, Z2, Z3, Z4) and 1858 (Kockel and others, 1994). Although this development one or more minor cycles of evaporite deposition. The ideal established hydrocarbon production in the area, the Lower cycle, reflecting increasing evaporation and salinity through Saxony oil is not geologically part of the Carboniferous-Rot- time, consists of (1) a thin basal clastic deposit, (2) a middle liegend TPS but instead was generated from Mesozoic source , dolomite, anhydrite, and halite sequence, and (3) an rocks such as the Posidonia Shale (not shown in figure 10). upper unit of bittern salts of magnesium and potassium. Each The defining event in the development of the Carbonifer- successive cycle is more evaporitic than the previous one, and ous-Rotliegend TPS occurred in 1959, when a wildcat well there is an inverse correspondence between areal distribution (the Slochtern-1) discovered a gas-bearing Rotliegend sand- of evaporite facies and intensity of evaporation (Tucker, 1991); stone 180 m thick. Seismic exploration and additional drilling for example, carbonate rocks are much more extensive than identified a large, gas-bearing structure (Stauble and Milius, halite or bittern salts. 1970) that became known as the Groningen field (figs. 2, 7). A deep basin model has been popular among It was initially estimated to contain about 58 trillion cubic studying the German Zechstein for many years, but lithologic feet (TCF) of natural gas, but subsequent work has increased and stratigraphic relations in the southern North Sea Basin the estimated recoverable resources to about 100 TCF (Glen- seem to require additional constraints. In particular, the large nie, 1997a). The discovery of this prolific gas field changed volumes of evaporites indicate continual or episodic influx of the energy landscape of Europe; in a region where every town Province Geology and Petroleum Occurrence 13

SW NE Sole Pit Mid-North Sea Central Graben Midland Shelf AGE High High (Dutch Sector) ROCK UNIT

MID TERTIARY NORTH INVERSION EUSTATIC FALL IN SEA LEVEL SEA GROUP CENOZOIC 50 LATE CRETACEOUS INVERSION INVERSION LATE GROUP

100 Red

ACEOUS Chalk HOLLAND VLIELAND

CRET EARLY Speeton Clay DELFLAND

SCRUFF (Extensional thermal uplift) 150 LATE LATE CIMMERIAN TECTONISM Humber Group WERKEN- DAM West MIDDLE INDE-CLEAVER Sole Group Ma BANK HIGH JURASSIC EARLY 200 Winterton

LATE Hais- Graben - Flankfault borough Group MUSCHEL- MIDDLE KALK TRIASSIC EARLY Bactron BUNTER 250 Group ZECHSTEIN Leman Sandstone Silverpit Clst./Ten Boer Fms. LATE U. ROTLIEGEND

SAALIAN UNCONFORMITY PERMIAN EARLY

300 POST-VARISCAN BARREN RED INVERSION MEASURES COAL MEASURES

Dinantian NOT PENETRATED CARBONIFEROUS LIMESTONE 350 CARBONIFEROUS EXPLANATION

Sandstone Dolomitic Volcanics

Chalk Evaporites Unconformity

Limestone Shale

Figure 10. Stratigraphic column of the southern Permian Basin (modified from Glennie, 1997a). 14 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary previously relied on coal, or had its local town-gas facility, gas about 3 percent of the total stratigraphic sequences (Lutz and was delivered via pipeline from the Groningen field. others, 1975), and carbonaceous shales account for a much Original work by K.W. Glennie and others at Royal larger percentage (Cornford, 1998). Dutch Shell (Glennie, 1970, 1972) led to the interpretation of the Rotliegend reservoirs as eolian deposits and identified the source rock as Carboniferous Coal Measures. The investiga- Organic Geochemistry of Kerogen and tions also indicated the possible existence of gas accumula- Hydrocarbons tions offshore, with the result that exploratory drilling in the United Kingdom sector of the North Sea was undertaken in Westphalian coals and carbonaceous shales are domi- 1964. After a few unsuccessful wells, the West Sole gas field nated by type III kerogen, which is rich in vitrinite derived (fig. 2) was discovered in late 1965, confirming the facies from terrestrial plant tissues, and in charcoal (inertinite) model and exploration concept. This initial discovery was (Cornford, 1998). Such humic kerogen displays hydrogen/ followed closely by a string of successes that pursued the carbon ratios of 1.0 to 0.5 and oxygen/carbon ratios of 0.4 Carboniferous-Rotliegend play model through large structural to 0.02, both of which decline as time-temperature exposure closures in the southern North Sea (Brennand and others, increases (Hunt, 1995). 1998). According to Cornford (1998) and Bernard and Cooper By 1969, the large discoveries saturated the United King- (1983), relatively little information concerning gas composi- dom gas market, which at that time was managed as a monop- tion in the southern North Sea has been publicly released. In oly by the British Gas Council (later British Gas), and led to any case, correlation of natural gases to particular source rocks a dramatic downturn in drilling during the 1970s. Exploration is difficult. Generally, gases of the Carboniferous-Rotliegend companies then turned north to explore for oil in the central TPS are reported to be sweet, dry gas with carbon isotopic and northern North Sea. However, in 1981, fearing a national compositions consistent with derivation from strata of the Coal gas shortage, British Gas raised the price for new gas, starting Measures (-22 to -28o/oo, PDB for gases, compared with -22 a renewed pattern of discovery and development that continues to -25o/oo, PDB for kerogen). Regional trends in carbon diox- to the present. As of 1997, 290 gas fields had been discovered ide and nitrogen content have been reported (Boigk and Stahl, offshore in the southern North Sea, accounting for approxi- 1970; Boigk, 1981; Kettel, 1982, 1989; Krooss and others, mately 75 TCF of recoverable natural gas and more than 400 1995); the most significant trend being an increase in nitrogen million barrels of oil (MMBO). Onshore in the Netherlands content eastward from the northern Netherlands and Gronin- and Germany, more than 229 gas fields had been discovered gen, where nitrogen content is about 4 percent and about 15 accounting for about 150 TCF of gas, as well as some oil and percent, respectively, into Germany and Poland where nitrogen natural gas liquids. is so abundant that the gas is considered uneconomic. Kettel (1982, 1989) and Krooss and others (1993, 1995) have argued that such a trend reflects the increasing contribution of late- gas generation from the most thermally mature terrig- Total Petroleum System: enous source rocks. Carboniferous-Rotliegend (403601) Burial History, Gas Generation, and Migration Source Rocks Carboniferous strata in the undeformed foreland basin Coals and carbonaceous shales of the Westphalian-age north of the Variscan highlands emerged from the Paleozoic (Carboniferous) Coal Measures (fig. 10) are believed to be the without having been buried sufficiently for maximum gas principal source rocks for gas accumulations in the southern generation (fig. 11). Within the deformed zone, south of the North Sea and adjacent areas (fig. 2) (Cornford, 1998). By Variscan front (fig. 9), most Carboniferous rocks have been even conservative estimates, the generative capacity of the metamorphosed to at least greenschist facies, and kerogen Coal Measures is considered to far exceed the amount of gas has been overly mature with respect to maximum gas genera- presently discovered (Barnard and Cooper, 1983; Cornford, tion since the Late Carboniferous. Onshore in northwestern 1998). Europe, rank of Westphalian coal ranges from subbituminous Although complex in detail, the Coal Measures in general to meta-anthracite, corresponding to vitrinite reflectance val- consist of upward-coarsening deltaic rocks that are overlain by ues of 1 to 3 percent (Cornford, 1998). fluvial deposits and associated coals. The fluvial rocks include North of the Variscan front (fig. 9), post-Paleozoic burial conglomeratic, complexly stacked sandstones attributed to of Coal Measures strata has occurred to various degrees from deposition in major braided stream systems. Within the bounds the Triassic to the present day. However, the burial history has of the Carboniferous-Rotliegend TPS, the Coal Measures are differed considerably from place to place (fig. 11). Several probably 1,000 to 3,000 m thick (Leeder and Hardman, 1990), areas of post-Carboniferous gas generation have been identi- although reliable estimates are difficult to make. Elsewhere the fied based upon lithology, kerogen content, and reconstructed Carboniferous can be much thicker. Coal seams account for burial and time-temperature history. In the southern North Sea Total Petroleum System: Carboniferous-Rotliegend (403601) 15

CARBONIFEROUS JURASSIC CRETACEOUS TERTIARY TRIASSIC PERMIAN

TIME (MILLIONS OF YEARS BEFORE PRESENT) 300 200 100 0

0

Broad Fourteens Basin {

NE SW 2 Sole Pit High

4 DEPTH (Km) TYPE lll KEROGEN

Central North 6 Graben German Basin

8 Horn Graben

Figure 11. Comparison of published burial curves for Carboniferous-age rocks in and around the southern North Sea (after Cornford, 1998). these areas include the Sole Pit High, the West Netherlands when they had reached depths as great as 4,000 m (Cornford, and Broad Fourteens Basins, and northern Germany-southern 1998). The burial depths are indicated by stratigraphic recon- Denmark (Fig. 7). struction, sonic velocity, fission track analysis, and measured vitrinite reflectance values ranging from about 1 percent near In the Sole Pit High, the Coal Measures were rapidly bur- the basin margin to nearly 3 percent near the basin axis (Corn- ied from the time of deposition until the end of the Cretaceous, 16 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary ford, 1998; Glennie and Boegner, 1981). These data have been (U.S. Geological Survey Assessment Team, 2000) of this area, interpreted to indicate that at least one and possibly two epi- at least the oil-prone shales are considered to constitute a sepa- sodes of gas generation and migration from the basin center to rate petroleum system. basin-flanking traps occurred between and Late Cretaceous time. Thinning and onlap of the Chalk Group onto the Sole Pit High (fig. 7) indicate a Late Cretaceous inversion Reservoirs from a structural low to a structural high. This inversion is Sandstones of the Rotliegend are the predominant res- believed to have caused remigration of gas, making structural ervoirs of the Carboniferous-Rotliegend TPS, accounting for interpretation of gas accumulations and thermal maturity more than 85 percent of the discovered gas accumulations in difficult (Green and others, 1995). Another inversion of the the offshore and virtually all of the gas in the giant Groningen Sole Pit Basin during the Tertiary evidently resulted in erosion field onshore in the Netherlands. In addition, these sandstones or non-deposition of pre- rocks (Glennie, 1997a; have significant potential for undiscovered small accumula- Alberts and Underhill, 1991). Similar two-stage inversions tions offshore and for further growth of reserves in existing also occurred in the West Netherlands Basin and in the Broad Fourteens Basin, although the Late Cretaceous inversion was fields. However, much of the undiscovered natural gas in the relatively stronger in those basins than in the Sole Pit High, southern North Sea may occur in sandstones of the Coal Mea- where the Tertiary inversion seems to have had greater effect sures, adjacent to the carbonaceous source rocks, and in the (Cornford, 1998). overlying Bunter Sandstone (fig. 10). Areas of deep burial and subsequent inversion, such Based upon the work of Glennie (1970, 1972) and as the Sole Pit High and the West Netherlands Basin, are many others, the Rotliegend sandstones have been attributed probably the sites of gas generation for the gas fields in the to deposition in a variety of arid, terrestrial environments. southern North Sea. In the Coal Measures, most gas genera- Dominant among these are ephemeral fluvial (wadi) systems, tion occurred at vitrinite reflectance values of 1 to 3 percent, various types of eolian deposits, desert-lake environments, corresponding to the in the Sole Pit High and adjacent sabkhas. According to Glennie (1998), in the and Broad Fourteens Basin (fig. 11) and only slightly later in southern North Sea and adjacent onshore areas of the Neth- the Central Netherlands Basin (Cornford, 1998). The timing erlands, Rotliegend facies representing these depositional of generation is estimated from burial history modeling and environments are systematically arranged areally from south to from K/Ar ages of diagenetic illite in gas-bearing sandstone north and stratigraphically from bottom to top. The lowest and reservoirs of the Rotliegend (Hamilton and others, 1989). In southernmost sandstones and gravels of wadi and eolian origin the vicinity of the Sole Pit, gas migration is believed to have give way northward and upward to successive accumulations occurred after 158±18.6 Ma (Robinson and others, 1993). In of eolian sandstone, sabkha deposits, and desert-lake sedimen- the Broad Fourteens, gas entered the reservoirs after 140 Ma. tary sequences. Most of the natural gas entrapped onshore, especially that Reservoir quality is strongly influenced by the deposi- of the Groningen field, is believed to have been generated in tional environments of the Rotliegend. Most commercial gas northern Germany and southern Denmark, but possibly also accumulations are found in eolian dune deposits, with some just offshore in the West Netherlands Basin (fig. 7). Additional production coming from fluvial facies. Those sandstones onshore gas generation may have taken place as a result of with the cleanest, best at the time of deposition, and contact metamorphism (Glennie, 1986). K/Ar studies of authi- those sandstones deposited away from the water table tend genic illite in the Rotliegend reservoirs of the Groningen field to display the best preservation of porosity and permeability. indicate the structure was progressively filled over a period Eolian facies of all types generally make better reservoirs than of about 30 million years. Natural gas entrapped onshore in do interdune deposits, laterally adjacent wadi and sheet flood the southern United Kingdom was probably also generated deposits, or any other fluvial, alluvial, or sabkha rocks. The from Carboniferous strata, although specific source rocks and best reservoirs are thus commonly found in the middle of the generation sites are not well known. Rotliegend, within which eolian facies predominate (George In the Carboniferous strata of northeastern England, the and Berry, 1993; 1997). However, considerable variation in coals and shales constitute two distinct source rock compo- reservoir quality exists within the eolian facies. Compared nents, with the coals containing approximately 60 percent to horizontally stratified, finely laminated ripple strata of the total organic carbon (TOC) and type III kerogen, whereas the dune bases and tops, the more steeply dipping foreset beds, shales have approximately 1 percent TOC and mixed type II consisting of avalanche and grainfall strata, tend to be initially and type III kerogen. Marine shales range from less than one less compacted and to be less affected by burial percent to several percent TOC, mostly in type II kerogen. Oil (Glennie and others, 1978). accumulations are directly related to the presence of prodelta Diagenesis is a problem for reservoir quality in the Rotli- marine shales with type II kerogen. Source rocks became egend. Fluvial sandstones are commonly so extensively altered mature for oil and other liquids as early as the Late Carbon- by diagenetic processes as to render them noncommercial for iferous in some areas. Although these diverse Carboniferous hydrocarbon production (Glennie and others, 1978). Effects of source rocks were included in the World Energy Assessment compaction, pressure solution, and precipitation of chlorite all Assessment Unit: Southern Permian Basin-Offshore (40360103) 17 negatively impact the porosity and permeability of Rotliegend Assessment Units reservoirs. However, the most damaging diagenetic effect is the precipitation of fibrous illite in sandstones that had already The close relations and similar distribution patterns of undergone significant porosity reduction (Ziegler, 1992). Suc- Carboniferous source rocks, Rotliegend sandstone reservoirs, cessful gas production of illitized reservoirs is only possible and Zechstein evaporites in gas fields led to the decision to where there has been natural fracturing, such as the inversion consider the voluminous gas accumulations of the southern fractures observed at Clipper or West Sole fields (fig. 2), or North Sea and adjacent onshore areas of Europe as forming where and expensive reservoir engineering a single petroleum system—the Carboniferous-Rotliegend is employed (Glennie, 1998). TPS (AU403601, fig. 3). Division into assessment units was, however, based upon technological and geographic criteria rather than upon geological distinction. Three AUs were Seals identified: (1) the Southern Permian Basin-Offshore Europe (AU40360103, fig. 4), (2) the Southern Permian Basin- Evaporite deposits of the sabkha and desert-lake Onshore Europe (AU40360102, fig. 5), and (3) Southern sequences of the Rotliegend may seal gas accumulations in Permian Basin Onshore United Kingdom (AU40360101, fig. underlying Carboniferous reservoirs, but the Zechstein salt 6). Within the AUs, various geologic settings of hydrocarbon is the seal for virtually all gas accumulations in Rotliegend accumulation, informally referred to here as exploration plays, reservoirs themselves. The near-perfect seal of the Zechstein is were considered for resource assessment purposes. the main reason for the existence of the large gas fields of the Carboniferous-Rotliegend TPS. Radiometric ages of diage- netic illite in the reservoirs show that the Zechstein evaporites have effectively sealed gas in structural and stratigraphic traps Assessment Unit: Southern Permian for approximately 150 million years (Lee and others, 1985; Hamilton and others, 1989; Robinson and others, 1993). These Basin-Offshore (40360103) observations are consistent with Klemme’s (1983) assertion that giant gas fields are in large structures that have highly Description effective seals. The Southern Permian Basin-Offshore AU coincides with the extent of the Carboniferous-Rotliegend TPS in the offshore Traps and Timing area of the southern North Sea between England and the European continent (fig. 4). It is bounded on the south by the Gas accumulations in Carboniferous reservoirs depend London Brabant Platform and on the north by the mid-North upon traps developed (1) wholly within the Carboniferous, (2) Sea high (fig. 7). Northwestern and southeastern boundaries, by lateral facies changes from sandstone or gravel to mudstone respectively, are the coastlines of England and the European which occurs within the thick sequences of fluvially domi- continent (figs. 1, 7). nated rocks, and (3) by being sealed beneath the desert-lake evaporites of the Rotliegend (for example, Silverpit Claystone Formation). Gas-bearing structures found to date consist of Offshore Exploration Plays and Their Resource Variscan-age folds and various – Early Creta- Potential ceous and Late Cretaceous – early Tertiary uplifts. However, intraformational stratigraphic traps could also have developed The Rotliegend Sandstone Play where thermally mature coals or carbonaceous shales are in contact with the reservoirs. Play Description Gas accumulations in Upper Permian Rotliegend strata All of AU40360103 is predominantly the Rotliegend depend upon the juxtaposition of mature Carboniferous source sandstone play, which has been the objective of intensive rocks, the Rotliegend sandstone or conglomeratic reservoir exploration since the beginning of offshore drilling. Over most rocks, and the Zechstein evaporite seals. Effective gas trapping of the area, the Coal Measures have provided abundant hydro- structures formed after deposition of the evaporite, but were carbon charge in all settings and retain generative capacity of insufficient magnitude to breach the seal. These gas-filled even today. Likewise, Zechstein seal exists in almost all areas structures date from the Late Jurassic to and seal integrity has been preserved, even in areas of intense and from the Late Cretaceous to early Tertiary. In addition, deformation. Failures of charge are most commonly related transpression in the Tertiary caused localized basin inversions to timing of structural inversion relative to gas generation or that also resulted in the development of gas-bearing structures. due to leakage in those few areas where the seals are thin. Seal Diapiric movements of the Zechstein salt at various times in quality also is locally inadequate where carbonate rocks sup- the burial history have also been responsible for primary gas- plant evaporite beds as a result of facies variation. Exploration bearing structures and traps for remigrated gas. uncertainties arise from poor reservoir quality where either 18 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary the best eolian reservoir facies is absent, or where depth and Zechstein Play facies-related diagenetic effects have reduced reservoir poros- ity and permeability. Identification of structural closures can Play Description be difficult, as Rotliegend closures are small compared to the Shelf-edge and slope carbonate facies of the Zechstein thick and lithologically heterogeneous overburden. are potential exploration targets. Oil-prone source rocks, such as the Kupferschiefer Shale, could provide a local charge, but Exploration Potential of the Rotliegend Play the most likely potential would be realized where Zechstein The Rotliegend play is presently highly mature. Years of carbonates are charged from the Carboniferous Coal Measures intense and expensive exploration drilling and a dense seismic underlying the Rotliegend. Except for Z-1 carbonates, which grid appear to have identified most of the significant footwall are not separated from the Rotliegend by salt, this scenario closures. Nevertheless, discovery of progressively smaller requires that the near-perfect Zechstein seal be breached. fields continues and potential remains in limited hanging-wall traps, in known closures previously judged too small to war- Exploration Potential rant development, and in subtle stratigraphic traps that have Although oil, gas, and condensate are produced from the thus far avoided detection. Zechstein in Germany, Poland, and the Netherlands, where the source rock is the Stinkschiefer Shale, the Zechstein has never Carboniferous Play yielded much gas in offshore areas. Nevertheless, moderate to small stratigraphic traps for gas and possibly for Zechstein oil Play Description may be expected. Loss of reservoir quality in the carbonate Comparatively few wells have been drilled and few fields facies adjacent to the evaporites, however, may substantially have been found in the Carboniferous, but the stratigraphic reduce the economic potential of these small traps. interval remains prospective because of its substantial gas generative capacity and complex lithologic variations. Various Mesozoic Plays sandstones in valley fill sequences should be of acceptable res- ervoir quality, but the stratigraphic complexities that character- Play Description ize this thick section of fluvio-deltaic rocks make for problems Gas accumulations sourced from the Coal Measures have in the location of exploration targets. Compared to the quartz- ose Rotliegend reservoirs, the sandstones of the Carboniferous been discovered within the Triassic Bunter and Hewett Sand- are mineralogically immature. Consequently, loss of reservoir stones (fig. 10) in fault-bounded . Other possibili- quality from burial-related compaction and diagenesis may ties for undiscovered accumulations include stratigraphically be the limiting factor in successful development of reservoir entrapped gas within the Bunter, as well as within the overly- targets within the Carboniferous. So far, all Carboniferous ing Lower Cretaceous sandstones. discoveries have been in two structural settings—the Variscan folds and the post-Paleozoic structures reactivated during tec- Exploration Potential tonic inversion. Traps are expected to result from lateral and Obvious structures including Bunter and younger reser- vertical permeability barriers at the margins of discontinuous voirs have been tested, which leaves relatively subtle strati- sandstones. The best seals are probably the Permian desert- graphic or structural traps as remaining exploration targets. lake evaporites such as the overlying Rotliegend Silverpit An obvious negative aspect of this play is the Zechstein and Claystone Formation (fig. 10). Rotliegend seals, which are as effective in shielding these sandstones from a gas charge as they are at preventing leakage Exploration Potential from Rotliegend and Carboniferous reservoirs. Potential exists The Carboniferous strata have considerable exploration for discovery of additional small accumulations, possibly in potential in offshore settings, but technical problems involved association with other traps. with exploration have so far discouraged extensive drilling. The lateral and vertical variability of sedimentary facies, the difficulties in discerning gas-saturated rocks within the Assessment Unit: Southern Permian complex lithologies, and the paucity of reliable stratigraphic markers offshore combine to make fields in the Carboniferous Basin-Europe Onshore (40360102) difficult to exploit given current exploration technology and economics. Nevertheless, the Carboniferous is probably the Description most prospective stratigraphic interval remaining in the south- ern North Sea. Success will require the ability to overcome The Carboniferous-Rotliegend TPS and the Southern seismic imaging problems in order to identify thick channel Permian Basin-Europe Onshore AU coincide with the extent sandstones that are gas-charged. of thermally mature Westphalian (Coal Measures) source Assessment Unit: Southern Permian Basin-Europe Onshore (40360102) 19 rocks and related gas and liquid accumulations in the onshore end. In this setting, accumulations could exist in structural area on the European continent adjacent to the southern North closures. Accumulations may also occur as stratigraphically Sea (fig. 5). Assessment Unit 40360102 is located mainly entrapped gas where lithologic boundaries within the Carbon- in the Netherlands and Germany, but also includes parts of iferous provide the trapping mechanism. Possible examples Belgium and Denmark. It contains several sub-basins, includ- include intraformational traps formed where Carbonifer- ing onshore areas of the West Netherlands Basin and the ous sandstones are enclosed by fine-grained rocks or where Lower Saxony sub-basins (fig. 7). The AU is bounded on the unconformable contacts exist, such as at the overlying Saalian southwest and south by the London-Brabant Platform and by unconformity (fig. 10). the Rhenish Massif, on the north and northwest by the North Sea and the Trans-European fault, and on the east, near the Exploration Potential German border, by a structurally positive saddle on trend with the northeast-striking Hesse Depression (Ziegler, 1990). The Although most of the obvious larger structures have been boundary separating AU 4060103 and AU 4030102 is placed tested, the Carboniferous remains comparatively unexplored. at the coastline because of the distinct differences in tech- Successful exploration will probably require both significantly nology, economics, and regulations encountered in onshore improved seismic resolution and more reliable sedimentologic versus offshore settings. Because the two assessment units and stratigraphic models. This play and its offshore counter- share most formation boundaries, structural properties, and part probably hold the greatest potential for new gas discover- geologic history, the following description of exploration plays ies in the Southern Permian Basin, some of which could be in the onshore assessment unit is necessarily abbreviated and large fields. However, burial-related diagenetic loss of reser- somewhat redundant with play descriptions from the offshore voir quality in the highly feldspathic, lithic-rich Carboniferous assessment unit. sandstones is commonly severe, and lack of adequate porosity and permeability may ultimately limit resource potential. Exploration Plays and their Resource Potential Zechstein Play The Rotliegend Sandstone Play Play Description Play Description Although the Zechstein is mainly valued as a seal, shelf The prolific source rock capacity of the Carboniferous and slope carbonate facies might represent an exploration Coal Measures is present throughout the AU and can be relied target. Traps would be provided by Zechstein evaporites or upon to provide adequate hydrocarbon charge to fill structures, by permeability barriers caused by lithologic discontinuities including fields as large as the giant , to within the carbonate sequences. The play requires reservoir- their spill-point (Lutz and others, 1975). Similarly, the effec- quality carbonate rocks, which have generally been limited to tive seal of the overlying Zechstein carbonate-evaporite com- oncolitic and oolitic shelf margin facies of the Zechstein. Frac- plex is present throughout the AU. All discovered Rotliegend tured reservoirs may also provide attractive targets (Cooke- fields are underlain by coals and carbonaceous shales of the Yarborough, 1994). Coal Measures and overlain by Zechstein evaporites. Exploration Potential Exploration Potential of the Rotliegend Play According to Glennie (1997a), the Zechstein was the The Rotliegend sandstone play has been extensively original exploration target when Groningen was discovered. developed and is in a mature state of exploration. Potential for Oil-prone shales are present in the Zechstein, but the most new discoveries is mainly in small structural traps, some of likely potential would be where Zechstein carbonates have which have already been identified during exploratory studies been charged by the Coal Measures underlying the Rotliegend. but not drilled, and in subtle stratigraphic traps. This model is risky for all but Z-1 carbonates because effective charge requires the Zechstein seal to have been breached. Carboniferous Play Mesozoic Plays Play Description Play Description The prolific gas-prone source rocks of the Carboniferous Coal Measures, the abundance of closely associated channel Gas accumulations, sourced from the Coal Measures, sandstones and related valley-fill complexes, and the presence have been discovered within the Triassic Bunter and Hewett of shales and evaporitic rocks of the overlying Rotliegend, Sandstones in fault-bounded anticlines in the southern North indicate that the Carboniferous should be an important explo- Sea. Other possibilities include stratigraphically entrapped gas ration target. Traps are to be expected where the Carboniferous within the Bunter and within the overlying Lower Cretaceous is overlain by fine-grained, desert-lake facies of the Rotlieg- sandstones. 20 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

Exploration Potential Resource Potential Most structures with Bunter and younger reservoirs have Hydrocarbon accumulations have been known from been tested. This leaves subtle stratigraphic traps and small England’s Carboniferous strata since before the Second structures within which Triassic reservoirs are in direct contact with pre-salt Carboniferous strata. World War. In spite of relatively mature exploration and small sizes of discovered fields, the area continues to be the target of a rather low but persistent level of exploration interest, Assessment Unit: Southern Permian largely because of the inexpensive exploration and develop- Basin-U.K. Onshore (40360101) ment programs needed to bring discoveries to market. Future discoveries will probably continue to consist of small, shallow Description accumulations.

The total petroleum system (403601) and corresponding assessment unit (40360101) coincide with the extent of ther- Undiscovered Gas Resources of mally mature Carboniferous source rocks and related gas and oil accumulations in the onshore area of the Southern Permian the Carboniferous-Rotliegend Total Basin in the East Midlands of England (fig. 6). Within this assessment unit only a single exploration play is defined. Petroleum System

The recent USGS World Petroleum Assessment (U.S. Carboniferous Play Geological Survey Assessment Team, 2000) included an evaluation of undiscovered resources in the three assessment Play Description units of the Carboniferous-Rotliegend TPS in and adjacent For purposes of the World Energy Project, the resource to the southern North Sea. The results of this assessment are potential in the U.K. onshore part of the Carboniferous-Rotli- summarized in table 1 and reported in USGS Digital Data egend TPS is considered to consist mainly of reserve growth Series DDS-60 (U.S. Geological Survey Assessment Team, and undiscovered accumulations within and in close associa- 2000). Undiscovered resources associated with the Carbonif- tion with the Carboniferous strata of part of northeast England. erous-Rotliegend TPS in the European and United Kingdom Source rocks include marine shales of age and deltaic shales and carbonaceous shales and coals, onshore areas are estimated to range from 23 to 148 MMBO mainly of Namurian and Westphalian (Carboniferous) age. and from 5.0 to 21.6 TCF of natural gas, plus gas associated Organic matter in these diverse source rocks ranges from small with undiscovered oil and quantities of natural gas liquids. volumes of hydrogen-rich type II kerogen related to marine Undiscovered resources in the onshore part of the United deposition, to carbonaceous shales and coals of the deltaic Kingdom are considered to be relatively minor. Offshore sequences. Distribution of fields in this well-explored province undiscovered oil resources are estimated to range from a mini- depends critically on the burial depth of good quality type II mum of 13 MMBO to about 133 MMBO. Undiscovered natu- organic matter. Correlation of depth to thermal maturity of organic ral gas resources are estimated to range from 2.8 TCF to more matter indicates relatively short distances of migration along than 27.6 TCF, not including additional resources of natural pathways provided mainly by porous and permeable sand- gas liquids and associated and dissolved gas. Thus, estimated stones, as well as along fractures in fine-grained sedimentary mean values of undiscovered resources in the onshore and off- rocks. Most reservoir rocks are of Carboniferous age, but shore areas of the Anglo-Dutch Basin and adjacent parts of the also include the , Waulsortian mounds and Northwest German Basin provinces are about 144 MMBO and other carbonates of Dinantian age, deltaic and fluvial sand- more than 26 TCF of natural gas. These estimates are compa- stones associated with the Coal Measures, and Permian red rable to those reported by the USGS in 1997 (mean = 24 TCF) beds. Local lithologic variations provide a variety of complex seals, including calcareous marine shales resting above deltaic (Masters and others, 1997). In addition to the undiscovered sandstones and lithologic variations within the sandstone-shale resources, significant volumes of gas will probably be added sequences of the Carboniferous. to reserves as a result of growth of existing fields. Table 1. Carboniferous-Rotliegend Total Petroleum System (403601), Assessment Results Summary—Allocated Resources. [MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids; MFS, minimum field size assessed (MMBO or BCFG); Prob., probability of at least one field equal to or greater than the MFS. Results shown are fully risked estimates. For gas fields, all liquids are included under the NGL (natural gas liquids) category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. Shading indicates not applicable.] nicvrdGsRsucso h abnfru-oleedTtlPtoemSse 21 Undiscovered Gas Resources of the Carboniferous-Rotliegend Total Petroleum System 22 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

References Cited George, G.T., and Berry, J.K., 1997, Permian Upper Rotli- egend synsedimentary tectonics, basin development and palaeogeography of the southern North Sea, in Ziegler, P., Alberts, M.A., and Underhill, J.R., 1991, The effect of Tertiary Turner, P., and Daines, S.R., eds., of the structuration on Permian gas prospectivity, Cleaver bank southern North Sea: Geological Society of London Special area, southern North Sea, UK, in Spencer, A.M., ed., Gen- Publication 123, p. 31–61. eration, accumulation, and production of Europe’s hydrocar- bons: European Association of Geoscientists and Engineers Glennie, K.W., 1970, Desert sedimentary environments: Special Publication, Oxford, Oxford University Press, p. Developments in Sedimentology 14, Amsterdam, Elsevier, 161–173. 222 p.

Barnard, P.C., and Cooper, B.S., 1983, A review of geochemi- ———1972, Permian Rotliegendes of north-west Europe cal data related to the north-west European gas province, in interpreted in light of modern desert sedimentation studies: Brooks, J., ed., Petroleum geochemistry and exploration of American Association of Petroleum Geologists Bulletin, v. Europe: Geological Society of London Special Publication 56, p. 1048–1071. 12, p. 19–33. ———1986, Development of NW Europe’s Southern Permian Boigk, H., 1981, Erdol und Erdgas in der Bundesrepublik gas basin, in Brooks, J., Goff, J., and van Hoorn, B., eds., Deutschland: Stuttgart, Ferdinand Enke Verlag, 330 p. Habitat of Palaeozoic gas in northwest Europe: Geological Society of London Special Publication 23, p. 3–22. Boigk, H., and Stahl, W., 1970, Zum Problem der Entstehun Nordwestdeutcher Erdgaslagerstratten: Erdol Kohle, Erd- ———1990, Lower Permian-Rotliegend, in Glennie, K.W., ed., Introduction to the petroleum : gas, and Petrochemiie, v. 23, p. 104–111. Oxford, Blackwell Scientific Publications, p. 120–149. Brennand, T.P., Van Hoorn, B., James, K.H., and Glennie, ———1997a, History of exploration in the southern North K.W., 1998, Historical review of North Sea exploration, Sea, in Ziegler, Karen, Turner, Peter, and Daines, Stephen in Glennie, K.W., ed., Petroleum geology of the North R., eds., Petroleum geology of the southern North Sea— Sea—Basic concepts and recent advances: London, Black- Future potential: Geological Society of London Special well Science Publishers, p. 1–41. Publication 123, p. 5–16. Collinson, J.D., Jones, C.M., Blackbourn, G.A., Besly, B.M., ———1997b, Recent advances in understanding the southern Archard, G.M., and McMahon, A.H., 1993, Carboniferous North Sea Basin—A summary, in Ziegler, Karen, Turner, depositional systems of the southern North Sea, in Parker, Peter, and Daines, S. R., eds., Petroleum geology of the J.R., ed., Petroleum geology of northwest Europe: Proceed- southern North Sea—Future potential: Geological Society ings of 4th Conference, Geological Society, London, p. of London Special Publication 123, p. 17–29. 677–687. ———1998, Lower Permian-Rotliegend, in Glennie, K.W., Cooke-Yarborough, P., 1994, Analysis of fractures yields ed., Petroleum geology of the North Sea—Basic concepts improved gas production from Zechstein carbonates, Hewett and recent advances: London, Blackwell Scientific Publish- field, U.K. Continental Shelf: First Break, v. 12, p. 243–252. ers, p. 137–173. Cornford, C., 1998, Source rocks and hydrocarbons of the Glennie, K.W., and Boegner, P., 1981, Sole Pit inversion North Sea, in Glennie, K.W., ed., Petroleum geology of the tectonics, in Illing, L.V., and Hobson, G.P., eds., Petroleum North Sea—Basic concepts and recent advances: London, geology of the Continental Shelf of north-west Europe: Blackwell Scientific Publishers, p. 376–462. London, Institute of Petroleum, p. 110–120. Fraser, A.J., Nash, D.F., Steele, R.P., and Ebdon, C.C., 1990, Glennie, K.W., Mudd, G.C., and Nagtegaal, P.J.C., 1978, A regional assessment of the intra-Carboniferous play of Depositional environment and diagenesis of Permian Rotli- northern England, in Brooks, Jim, ed., Classic petroleum egendes sandstones in Leman Bank and Sole Pit areas of the provinces: Geological Society of London Special Publica- UK southern North Sea: Journal of the Geological Society tion 50, p. 417–440. of London, v. 135, p. 25–34. Gast, R.E., 1993, Sequenzanalyse von aoelischen Abfolgen Green, P.F., Duddy, I.R., and Bray, R.J., 1995, Applications im Rotliegenden underen Verzahnung mit Kukstgensedi- of thermal history reconstructions in inverted basins, in menten: Geologische Jahrbuch A 131, p. 117–139. Buchanan, J.G. and Buchanan, P.G., eds., Basin inversion: Geological Society of London Special Publication 88, p. George, G.T., and Berry, J.K., 1993, A new palaeogeographic 149–166. and depositional model for the Upper Rotliegend, offshore The Netherlands: First Break, v. 12, p. 147–158. References Cited 23

Hamilton, P.J., Kelly, S., and Fallick, A.E., 1989, K-Ar dating Leeder, M.R., and Hardman, M., 1990, Carboniferous geology of illite in hydrocarbon reservoirs: Clay , v. 24, p. of the southern North Sea Basin and controls on hydro- 215–232. carbon prospectivity, in Hardman, R.F.P., and Brooks, J., eds., Tectonic events responsible for Britain’s oil and gas Hunt, J.M., 1995, Petroleum geochemistry and geology, (2d reserves: Geological Society of London Special Publication ed.): New York, W.H. Freeman and Company, 743 p. 55, p. 87–105. Kettel, D., 1982, Norddeutsche Erdgase—Stickstoff Gehald und Isotopenvariationen als Reife-und Faziesindkatoren: Lutz, M., Kaaschister, J.P.H., and Van Wijhe, D.H., 1975, Erdol und Kohle, Erdgas, Petrochemie mit Brenstoff-Che- Geological factors controlling Rotliegend gas accumula- mie, v. 35, p. 557–559. tions in the Mid-European Basin: Proceedings, 9th World Petroleum Congress, v. 22, p. 93–97. ———1989, Upper Carboniferous source rocks north and south of the Variscan front, northwest and central Europe: Masters, C.D., Root, D.H., and Turner, R.M., 1997, World Marine and Petroleum Geology, v. 6, p. 170–181. resource statistics geared for electronic access: Oil and Gas Journal, v. 95, no. 41, p. 98–104. Kiersnowski, H., Paul, J., Peryt, T., and Smith, D.B., 1995, Facies, paleogeography and sedimentary history of the Magoon, L. B., and Dow, W. G., 1994, The petroleum system, southern Permian Basin in Europe, in Scholle, P.A., Peryt, in Magoon, Leslie B., and Dow, Wallace G., The petroleum T.M., and Ulmer-Scholle, D.S., eds., The Permian of system—From source to trap: American Association of Northern Pangaea, vol. 1, paleogeography, paleoclimates, Petroleum Geologists Memoir 60, p. 3–24. stratigraphy: Berlin, Springer-Verlag, p. 119–136. Petroconsultants, 1996, Petroleum exploration and production Klemme, H.D., 1983, The geologic setting of giant gas fields, database: Houston, Texas, Petroconsultants, Inc. [Database in Delahaye C., and Grenon M., eds., Conventional and currently available from IHS Energy Group, 5 Inverness unconventional world natural gas resources: Laxenburg, Way East, D205, Englewood, CO 80112] International Institute for Applied Systems Analysis, p. 133–160. Ramsbottom, W.H.C., Claver, M.A., Eagar, R.M.C., Hodson, Klett, T.R., Ahlbrandt, T.S.J., and Schmoker, J.W., 1997, F., Holliday, D.W., Stubblefield, C.J., and Wilson, R.B., Ranking of the world’s oil and gas provinces by known 1978, (Upper Carboniferous): Geological Society petroleum volumes: U.S. Geological Survey Open-File of London Special Publication 10, 82 p. Report 97-463, one CD-ROM. Robinson, A.G., Coleman, M.L., and Gluyas, J.G., 1993, The Kockel, Franz, Wehner, Hermann, and Gerling, Peter, 1994, age of illite cement growth, Village Fields area, southern Petroleum systems of the Lower Saxony Basin, Germany, in North Sea—Evidence from K-Ar ages and 18O/16O ratios: Magoon, L. B., and Dow, W. G., eds., 1994, The petroleum American Association of Petroleum Geologists Bulletin, v. system—From source to trap: American Association of 77, p. 68–81. Petroleum Geologists Memoir 60, p. 573–586. Schultz-Ela, D.D., and Jackson, M.P.A., 1996, Relation of Krooss, B.M., Leythaeuser, D., and Lillack, H., 1993, Nitro- subsalt structures to suprasalt structures during extension: gen-rich natural gases—Qualitative and quantitative aspects American Association of Petroleum Geologists Bulletin v. of natural gas accumulation in reservoirs: Erdol Kohle-Erd- 80, p. 1896–1924. gas-Petrochem, v. 46, p. 271–276. Stauble, A.J., and Milius, G., 1970, Geology of Groningen gas Krooss, B., Littke, R., Muller, B., Frielingsdorf, J., field, Netherlands, in Michael T. Halbouty, ed., Geology of Schwochau, K., and Idiz, R. F., 1995, Generation of nitro- giant petroleum fields: American Association of Petroleum gen and methane from sedimentary organic matter—Impli- Geologists Memoir 14, Tulsa, p. 359–369. cations on the dynamics of natural gas accumulations: Chemical Geology, v. 126, p. 291–318. Taylor, J.C.M., 1998, Upper Permian-Zechstein, in Glennie, Lee, M., Aronson, J.L., and Savin, S.M., 1985, K/Ar dating of K.W., ed., 1998, Petroleum geology of the North Sea— time of gas emplacement in Rotliegendes Sandstone, Neth- Basic concepts and recent advances: London, Blackwell erlands: American Association of Petroleum Geologists Scientific Publishers, p. 174–211. Bulletin, v. 9, p. 1381–1385. Tucker, M.E., 1991, Sequence stratigraphy of carbonate-evap- orite basins—Models and application to the Upper Permian (Zechstein) of northeast England and adjoining North Sea: Journal of the Geological Society of London, v. 148, p. 1019–1036. 24 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary

U.S. Geological Survey Assessment Team, 2000, U.S. Geolog- ical Survey World Petroleum Assessment 2000—Descrip- tion and results: U.S. Geological Survey Digital Data Series DDS-60, four CD-ROMs.

Ziegler, Karen, 1992, Authigenic illites in the Rotliegend Leman Sandstone, southern North Sea—K/Ar dating, oxygen and hydrogen isotopes [abs.]: BSRG, 31st Annual Meeting, Southampton, U.K., p. 23.

Ziegler, P.A., 1990, Geological atlas of Western and Central Europe (2d ed.): Shell International Petroleum Maatschap- pij, 238 p., 52 enclosures.