Carboniferous-Rotliegend Total Petroleum System 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 Carboniferous-Rotliegend Total Petroleum System 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 Charles G. Groat, Director
U.S. Geological Survey, Reston, Virginia: 2003
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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 Hydrocarbon Exploration and Production ...... 12 Total Petroleum System: Carboniferous-Rotliegend (403601) ...... 14 Source Rocks ...... 14 Organic Geochemistry of Kerogen and Hydrocarbons ...... 14 Burial History, Gas Generation, and Migration...... 14 Reservoirs ...... 16 Seals ...... 17 Traps and Timing ...... 17 Assessment Units ...... 17 Assessment Unit: Southern Permian Basin-Offshore (40360103) ...... 17 Description...... 17 Offshore Exploration Plays and Their Resource Potential...... 17 The Rotliegend Sandstone Play ...... 17 Play Description ...... 17 Exploration Potential of the Rotliegend Play ...... 18 Carboniferous Play ...... 18 Play Description ...... 18 Exploration Potential ...... 18 Zechstein Play ...... 18 Play Description ...... 18 Exploration Potential ...... 18 Mesozoic Plays ...... 18 Play Description ...... 18 Exploration Potential ...... 18 Assessment Unit: Southern Permian Basin-Europe 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 iv
Exploration Potential ...... 19 Mesozoic Plays ...... 20 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 North Sea 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
Foreword applicable. A TPS might equate to a single AU or, if necessary, a TPS may be subdivided into two or more AUs such that each This report was prepared as part of the World Energy is sufficiently homogeneous in terms of geology, exploration Project of the U.S. Geological Survey. In the project, the world history, and risk to be assessed individually. Assessment units was divided into 8 regions and 937 geologic provinces, and are considered to be: (1) established units if they contain more the provinces were then ranked according to the discovered than 13 fields, (2) frontier units if they contain 1 – 13 fields, or oil and gas volumes within each (Klett and others, 1997). Of (3) hypothetical units if there are no discovered fields. these, 76 priority provinces (exclusive of the U.S. and chosen A graphical depiction of the elements of a total petroleum for their high ranking) and 26 emerging (boutique) provinces system is provided in the form of an events chart, which shows (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 series 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 natural gas 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 number 036. The following two digits, 01, refer to the TPS, years. These potential resources reside either in undiscovered and the final two digits are the unique AU number (03 in this fields whose sizes exceed the stated minimum field-size (in example). this case one million barrels of oil equivalent) or they occur as The codes for the regions and provinces were established potential reserve growth of fields already discovered. and listed by Klett and others (1997); provinces for Europe The total petroleum system (TPS) is the basic geologic (Region 4) are shown and listed in greater detail in USGS unit of the World Energy Project. The TPS includes all geneti Digital Data Series DDS-60, the principal publication of the cally related shows and accumulations of petroleum (discov World Petroleum Assessment 2000 (U.S. Geological Survey ered and undiscovered) that (1) have been generated by a pod Assessment Team, 2000). Oil and gas reserves quoted in this or by closely related pods of mature source rock, and (2) exist report are derived from Petroleum Exploration and Production within a limited mappable geologic space. The petroleum sys database (Petroconsultants, 1996) and other area reports from tem concept is modified from Magoon and Dow (1994). The Petroconsultants, Inc., unless otherwise noted. minimum petroleum system is defined as that part of a total Figures in this report that show boundaries of the total petroleum system encompassing discovered shows and accu petroleum systems, assessment units, and pods of active mulations together with the geologic space in which the vari source rocks were compiled using geographic information ous essential elements of the system (source, reservoir, seal, system (GIS) software. Political boundaries and cartographic and overburden rocks) have been proved by these discoveries. representations were taken, with permission, from Environ An assessment unit (AU) is a mappable part of a total mental Systems Research Institute’s ArcWorld 1:3 million petroleum system in which discovered and undiscovered fields digital coverage (1992), have no political significance, and are constitute a single relatively homogenous population such displayed for general reference only. Oil and gas field center that the chosen methodology of resource assessment based on points, shown on these figures, are reproduced, with permis estimation of the number and sizes of undiscovered fields is 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 a Petroleum System) that transcends the combined Anglo-Dutch single total petroleum system (TPS), referred to as the Carbon- Basin and Northwest German Basin boundary. The juxtapo iferous-Rotliegend TPS. Geographically, it occupies most of sition of thermally mature coals and carbonaceous shales of the Anglo-Dutch Basin and the Northwest German Basin as the Carboniferous Coal Measures (source rock), sandstones well as parts of the adjacent North Sea Graben 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 geological) less economically important petroleum systems are present criteria, that subdivide the Carboniferous-Rotliegend Total (Kockel, Wehner and Gerling, 1994; Fraser and others, 1990). Petroleum System. These assessment units are (1) the South The Carboniferous-Rotliegend TPS was subdivided into three ern Permian Basin-Offshore Europe Assessment Unit, (2) the assessment units, based largely on economics, accessibility, Southern Permian Basin Onshore Europe Assessment Unit, and exploration history; these are Southern North Sea Off and (3) the Southern Permian Basin Onshore United Kingdom shore (AU40360103, fig. 4), Europe Onshore (AU40360102, Assessment Unit. Although the Carboniferous-Rotliegend fig. 5), and United Kingdom Onshore (AU40360101, fig. 6). Total Petroleum System is one of the most intensely explored volumes of rock in the world, potential remains for undiscov ered resources. Undiscovered conventional resources associ Province Geology and Petroleum ated 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. Of Occurrence these amounts, approximately 62 million barrels of oil and 13 trillion cubic feet of gas are expected in offshore areas, and 26 million barrels of oil and 1.9 trillion cubic feet of gas are Province Boundaries 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 Belgium, Denmark, France, Introduction Germany, United Kingdom, and the Netherlands. The Anglo- Dutch Basin contains a number of smaller sub-basins, includ The Northwest German Basin and the Anglo-Dutch Basin ing the Sole Pit Basin (also referred to as the Sole Pit High, as of northern Europe (U.S. Geological Survey World Energy the basin was the site of a tectonic inversion), East Midlands Project provinces 4036 and 4035, respectively) are two of the Shelf, West Netherlands, Broad Fourteens, and Central Nether 76 World Energy Project priority provinces. The Northwest lands sub-basins (fig. 7). The Northwest German Basin is also German Basin is number 21 and the Anglo-Dutch Basin is a compound basin, including the Lower Saxony Basin (fig. 7). number 39 in the World Energy Project global ranking (Klett The two provinces are bounded on the southwest and south by and others, 1997). These basins share common boundaries the London-Brabant Platform and the Rhenish Massif, on the with the North Sea Graben Province (number 4025), which north and northwest by the Mid-North Sea High, the Central ranks number 8 in the world. Taken together, these three prov Graben, the Ringkobing-Fyn High, and the Horn Graben (fig. inces account for the vast majority of oil and gas resources 7). The provinces are bounded on the east by a structurally known in Western Europe. The location and boundaries of positive saddle near the Polish-German border, on trend with these provinces are shown in figure 1. the northeast-striking Hesse Depression (Ziegler, 1990), which In contrast to the oil-prone North Sea Graben, the is not shown on figure 7. Anglo-Dutch and Northwest German Basins are gas-prone. The Groningen gas field, located just onshore in the Nether lands (fig. 2), is the largest hydrocarbon accumulation in the Structural-Tectonic Controls on Sedimentary Northwest German Basin and one of the largest gas fields in Sequences the world. Groningen contains approximately 100 trillion cubic feet (TCF) of produced or proved reserves of natural The elements giving rise to petroleum generation and gas. The largest field in the Anglo-Dutch Basin is the Leman accumulation in the Carboniferous-Rotliegend TPS are largely field, in the United Kingdom continental shelf (fig. 2), which the result of structural and sedimentary events that took place Province Geology and Petroleum Occurrence 3
0° 5°E10°E
North Sea
DENMARK
40254025 Copenhagen 55°N 40364036 40354035
Hamburg
Bremen 40334033
Amsterdam UNITED KINGDOM NETHERLANDS
Dortmund London Antwerpen GERMANY Brussels BELGIUM Bonn 40394039 50°N
Paris
0 250 KILOMETERS
EXPLANATION Northwest German Basin geologic province 4035
40364036 Anglo-Dutch Basin geologic province 4036 40354035 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 Frankfurt am Main gas field Groningen Duisburg Amsterdam 5 ° E 135 KILOMETERS Antwerpen 0 Lille Leman st Sole We Clipper 0 ° Oil field Gas field London EXPLANATION EXPLANATION Prominent oil and gas fields of the southern North Sea adjacent onshore areas. Sheffield Figure 2. 50 ° N 55 ° N 2°W0° 2°E4°E 6°E 8°E 10°E 12°E NORTH SEA DENMARK 4028 North 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 GeologyandPetroleumOccurrence 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 12 ° E 4033 Robinson Projection, Central meridian 0 Odense ° E 0 Hamburg GERMANY Bremen DENMARK 8 ° E1 4038 4035 Oil field centerpoint Gas field centerpoint 40360103 150 KILOMETERS 6 ° E NETHERLANDS Amsterdam Rotterdam EXPLANATION The Hague 0 ° E Assessment unit boundary National border Geologic province code and boundary 4037 2 ° E4 Sea 4037 North 4036 0 ° 4028 Leeds Sheffield UNITED KINGDOM Birmingham 2 ° W Figure 4. Southern Permian Basin-Offshore Assessment Unit (40360103) boundary. 54 ° N 52 ° N Province Geology and Petroleum Occurrence 7
4°E 6°E8°E10°E12°E
DENMARK
North Odense Sea
4035
54°N
Hamburg 40360102 4033 LOWER SAXONYBremen BASIN
4036
Amsterdam
The Hague NETHERLANDS 52°N Rotterdam 4038 Duisburg Dortmund 4036 Antwerpen Essen LONDON-BRABAN Gent PLATFORM Koln 4046 Bruxelles 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. during Carboniferous and Permian time (fig. 8). Particu- Early Carboniferous as a back-arc seaway (the Rheno-Her- 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 and was soon followed by eruption of the Lower Rotliegend basin (fig. 9). Volcanics (Ziegler, 1990). Tectonic relaxation of the foreland The Variscan foreland is a downward flexure, caused by and Late Carboniferous tectonic inversion are represented in tectonic loading north of the active Variscan orogenic belt that the stratigraphic record by a major unconformity at the base of formed as African Gondwana collided with European Laurasia the Upper Rotliegend (Glennie, 1998; Gast, 1993) (fig. 10). during the final stages (Visean to Westphalian) of the assembly The rapidly subsiding Variscan foreland accommodated of the Pangaea supercontinent. The earliest Variscan defor- as much as 9,000 m of nonmarine sedimentary rocks of mation probably occurred to the south of the foreland in the Westphalian age, collectively called the Coal Measures Late Devonian. Deformation progressed northward during the (Collinson and others, 1993), that accumulated at tropical 8 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary
1°W0° 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. latitudes with luxuriant vegetation in a variety of depositional fluvial systems generally flowed northward, whereas in the settings within extensive fluvial systems. In northwestern British Isles and the central North Sea, fluvial systems were continental Europe and the southern North Sea, these southward flowing (Ramsbottom and others, 1978; Ziegler, Province Geology and Petroleum Occurrence 9
H
G I
H
gas field field gas ER Groningen Groningen
N 135 KILOMETERS Y
F
G LOW
R
A
N N B
E
N N R
O H
SAXONY BASIN
MASSIF MASSIF
RHENISH RHENISH
G G
N N
I I B B
0
O O
K K
G G N
I WEST R CENTRAL 5 ° E
N NETHERLANDSNETHERLANDS E B A R G BROAD L A R FOURTEENS T N E C
M R O F T A L P
T
N
H SOLEHIGH PIT A
B G I EAST A
H R
B A - E
S N
0 ° MIDLANDS SHELF
O H
T D
R R N
O O O
L
N N
D D
I I M M EXPLANATION EXPLANATION Basin/graben boundary Fault Shallow Paleozoic basement Some structural features of the southern North Sea and adjacent areas. 5 ° W 50 ° N 55 ° N Figure 7.
10 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary
ACCUMULATION
PETROLEUM
MIGRATION ERATION SYSTEM EVENTS ROCK UNIT RESERVOIR ROCK
OVERBURDEN ROCK
PRESERVATION PRESERVATION CRITICAL MOMENT TRAP FORMATION SOURCE ROCK SEAL ROCK SCALE GEOLOGIC TIME GEN 0 PLIOCENE NEOGENE MIO. 24 OLIGOCENE EOC. 50 PALEOGENE PAL. 65 L 100 CRETACEOUS E
150 144 L M JURASSIC E 200 206 L TRIASSIC M 248 E 250 L PERMIAN E 290 300 L PENNSYLVANIAN M 323 E L MISSISSIPPIAN 350 E 363 L DEVONIAN M 400 E 408 L SILURIAN E 439 450 L ORDOVICIAN M E 500 510 L M CAMBRIAN Petroleum System. Events chart for the Carboniferous-Rotliegend Total 550 E 570 PRECAMBRIAN 600 Figure 8. Province Geology and Petroleum Occurrence 11 National boundary deformation Front of Variscan Geologic province boundary 250 MILES DENMARK 8 ° E 200 GERMANY 300 350 KILOMETERS 250 NORTHWEST GERMAN BASIN GERMAN BASIN NORTHWEST 150 150 Zechstein halite Halite and shale 200 00 1 150 150 EXPLANATION EXPLANATION 4 ° E 100 100 BELGIUM 50 50 0 NETHERLANDS 0 Sand and conglomerate Sulphate and shale 50 ANGLO-DUTCH BASIN BASIN ANGLO-DUTCH 50 0 ° Sand Sand and shale UNITED KINGDOM Late Carboniferous Variscan orogenic front, and the distribution of Permian Rotliegend sedimentary facies and Zechstein halite deposits (after orogenic front, and the distribution of Permian Rotliegend sedimentary facies Zechstein halite Late Carboniferous Variscan Figure 9. 1990). Ziegler, 54 ° N 52 ° N 12 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary
1990). Coals and carbonaceous shales in the fluvial sequences volumes of evaporites indicate continual or episodic influx of are thought to be the source rocks for natural gas in the marine water, as might result from the presence of a sill that Carboniferous-Rotliegend TPS (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 the foreland. A decrease in the rate of erosion of the adjacent Zechstein basin (Tucker, 1991; Kiersnowski and others, 1995). highlands and northward drift of Pangaea into arid latitudes Carbonate rocks form platform and slope facies that resulted in declining sediment supply and continuous desertifi bound the margins of the first three evaporite cycles (Z1–Z3). cation (Ziegler, 1990). In the Southern Permian Basin, which Such carbonates can locally form reservoir rocks, but they do trends east-west between the northeast coast of England across not serve as seals to gas accumulations in underlying Rotlieg the southern North Sea and into northernmost Germany and end reservoirs. Some basin-center carbonate rocks, however, southern Denmark, continental desert sedimentary sequences, have low porosities and permeabilities and are effective parts collectively constituting the Rotliegend, were deposited at the of the seal-rock system. Thick halite beds are prominent in site of the old foreland basin (Glennie, 1998). Alluvial fans the second Zechstein cycle (the Z2 beds), which, in addition to and braid-plain deposits that accumulated near the Variscan providing tight seals, are the main salts involved in the diapiric front grade northward into eolian and fluvial (wadi) facies, movements that have influenced oil entrapment in the southern and, farther north, to sabkha and desert-lake evaporites and and central North Sea (Schultz-Ela and Jackson, 1996). shales such as the Silverpit Claystone Formation (fig. 10) (George and Berry, 1997). The end of Permian glaciation caused global sea level rise Petroleum System Summary and marine transgression across the Rotliegend desert. The Over a wide area of the southern North Sea, and adja Zechstein sea expanded into the Southern Permian Basin from cent onshore areas of Europe and England, hydrocarbons are the north, probably flowing along pre-existing Proto-Atlantic predominantly found associated with the same source rocks and Proto-North Sea fracture systems. The basin is believed (Carboniferous Coal Measures), reservoir rocks (Permian Rot to have lain below sea level prior to the Permian Zechstein liegend sandstones), and evaporite seals (Zechstein) that com transgression, and the scouring, reworking, and soft-sediment bine to form the Carboniferous-Rotliegend TPS. The southern deformation seen in the uppermost Rotliegend is consistent boundary of the TPS is determined by the northern edge of the with speedy filling of the basin (Glennie, 1990; 1998). Organic old Variscan deformed zone. The northern, eastern, and west carbon-rich mud of the Kupferschiefer (Copper Shale) draped ern boundaries are near the approximate limit of the evaporite desert sediments and basal Zechstein lag deposits. High evapo facies of the Zechstein (fig. 9). This single petroleum system transpiration rates in conjunction with restricted flow caused accounts for most hydrocarbon accumulations in this area and persistent evaporite accumulation. The resulting Zechstein forms the basis for resource assessment. Group consists of thick carbonate and evaporite rocks that are largely of Late Permian age (Taylor, 1998). The Zechstein is present throughout the southern North Sea and onshore areas History of Hydrocarbon Exploration and of northwestern Europe, especially in England, Denmark, Production the Netherlands, Germany, Poland, and Lithuania. Zechstein evaporites occupy two sub-basins, the southern one of which The geographic area encompassed by the Carbonifer- coincides with the boundaries of the Carboniferous-Rotliegend ous-Rotliegend TPS became the site of one of the world’s first TPS, wherein the Zechstein is more than 2,000 m thick. oil developments when production began in Lower Saxony in In the southern North Sea and on the continent, the 1858 (Kockel and others, 1994). Although this development Zechstein is characterized by four major (Z1, Z2, Z3, Z4) and established hydrocarbon production in the area, the Lower one or more minor cycles of evaporite deposition. The ideal Saxony oil is not geologically part of the Carboniferous-Rot- cycle, reflecting increasing evaporation and salinity through liegend TPS but instead was generated from Mesozoic source time, consists of (1) a thin basal clastic deposit, (2) a middle rocks such as the Posidonia Shale (not shown in figure 10). limestone, dolomite, anhydrite, and halite sequence, and (3) an The defining event in the development of the Carbonifer- upper unit of bittern salts of magnesium and potassium. Each ous-Rotliegend TPS occurred in 1959, when a wildcat well successive cycle is more evaporitic than the previous one, and (the Slochtern-1) discovered a gas-bearing Rotliegend sand there is an inverse correspondence between areal distribution stone 180 m thick. Seismic exploration and additional drilling of evaporite facies and intensity of evaporation (Tucker, 1991); identified a large, gas-bearing structure (Stauble and Milius, for example, carbonate rocks are much more extensive than 1970) that became known as the Groningen field (figs. 2, 7). halite or bittern salts. It was initially estimated to contain about 58 trillion cubic feet A deep basin model has been popular among geologists (TCF) of natural gas, but subsequent work has increased the studying the German Zechstein for many years, but lithologic estimated recoverable resources to about 100 TCF (Glennie, and stratigraphic relations in the southern North Sea Basin 1997a). The discovery of this prolific gas field changed the seem to require additional constraints. In particular, the large 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
50 CENOZOIC LATE LATE CRETACEOUS CRETACEOUS INVERSION INVERSION CHALK LATE GROUP
100 Red Chalk HOLLAND VLIELAND
CRETACEOUS 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 LIAS GROUP 200 Winterton
LATE KEUPER Hais Graben - Flank fault 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 Namurian
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 although reliable estimates are difficult to make. Elsewhere was delivered via pipeline from the Groningen field. the Carboniferous can be much thicker. Coal seams account Original work by K.W. Glennie and others at Royal for about 3 percent of the total stratigraphic sequences (Lutz Dutch Shell (Glennie, 1970, 1972) led to the interpretation of and others, 1975), and carbonaceous shales account for a the Rotliegend reservoirs as eolian deposits and identified the much larger percentage (Cornford, 1998). source rock as Carboniferous Coal Measures. The investiga tions also indicated the possible existence of gas accumula tions offshore, with the result that exploratory drilling in the Organic Geochemistry of Kerogen and United Kingdom sector of the North Sea was undertaken in Hydrocarbons 1964. After a few unsuccessful wells, the West Sole gas field Westphalian coals and carbonaceous shales are domi (fig. 2) was discovered in late 1965, confirming the facies nated by type III kerogen, which is rich in vitrinite derived model and exploration concept. This initial discovery was from terrestrial plant tissues, and in charcoal (inertinite) followed closely by a string of successes that pursued the (Cornford, 1998). Such humic kerogen displays hydrogen/ Carboniferous-Rotliegend play model through large structural carbon ratios of 1.0 to 0.5 and oxygen/carbon ratios of 0.4 closures in the southern North Sea (Brennand and others, to 0.02, both of which decline as time-temperature exposure 1998). increases (Hunt, 1995). By 1969, the large discoveries saturated the United King According to Cornford (1998) and Bernard and Cooper dom gas market, which at that time was managed as a monop (1983), relatively little information concerning gas composi oly by the British Gas Council (later British Gas), and led to a tion in the southern North Sea has been publicly released. In dramatic downturn in drilling during the 1970s. Exploration any case, correlation of natural gases to particular source rocks companies then turned north to explore for oil in the central is difficult. Generally, gases of the Carboniferous-Rotliegend and northern North Sea. However, in 1981, fearing a national TPS are reported to be sweet, dry gas with carbon isotopic gas shortage, British Gas raised the price for new gas, starting compositions consistent with derivation from strata of the Coal a renewed pattern of discovery and development that continues Measures (-22 to -28o/oo, PDB for gases, compared with -22 to the present. As of 1997, 290 gas fields had been discovered to -25o/oo, PDB for kerogen). Regional trends in carbon diox offshore in the southern North Sea, accounting for approxi ide and nitrogen content have been reported (Boigk and Stahl, mately 75 TCF of recoverable natural gas and more than 400 1970; Boigk, 1981; Kettel, 1982, 1989; Krooss and others, million barrels of oil (MMBO). Onshore in the Netherlands 1995); the most significant trend being an increase in nitrogen and Germany, more than 229 gas fields had been discovered content eastward from the northern Netherlands and Gronin accounting for about 150 TCF of gas, as well as some oil and gen, where nitrogen content is about 4 percent and about 15 natural gas liquids. percent, respectively, into Germany and Poland where nitrogen is so abundant that the gas is considered uneconomic. Kettel (1982, 1989) and Krooss and others (1993, 1995) have argued Total Petroleum System: that such a trend reflects the increasing contribution of late- stage gas generation from the most thermally mature terrig Carboniferous-Rotliegend (403601) enous source rocks.
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 deposition in major braided stream systems. Within the bounds from the Triassic to the present day. However, the burial of the Carboniferous-Rotliegend TPS, the Coal Measures are history has differed considerably from place to place (fig. probably 1,000 to 3,000 m thick (Leeder and Hardman, 1990), 11). Several areas of post-Carboniferous gas generation have 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). been identified based upon lithology, kerogen content, and In the Sole Pit High, the Coal Measures were rapidly reconstructed burial and time-temperature history. In the buried from the time of deposition until the end of the southern North Sea these areas include the Sole Pit High, the Cretaceous, when they had reached depths as great as 4,000 West Netherlands and Broad Fourteens Basins, and northern m (Cornford, 1998). The burial depths are indicated by Germany-southern Denmark (Fig. 7). stratigraphic reconstruction, sonic velocity, fission track 16 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary analysis, and measured vitrinite reflectance values ranging as the Late Carboniferous in some areas. Although these from about 1 percent near the basin margin to nearly 3 percent diverse Carboniferous source rocks were included in the World near the basin axis (Cornford, 1998; Glennie and Boegner, Energy Assessment (U.S. Geological Survey Assessment 1981). These data have been interpreted to indicate that at least Team, 2000) of this area, at least the oil-prone shales are one and possibly two episodes of gas generation and migration considered to constitute a separate petroleum system. from the basin center to basin-flanking traps occurred between Early Jurassic and Late Cretaceous time. Thinning and onlap of the Chalk Group onto the Sole Pit High (fig. 7) indicate a Reservoirs Late Cretaceous inversion from a structural low to a structural Sandstones of the Rotliegend are the predominant res high. This inversion is believed to have caused remigration ervoirs of the Carboniferous-Rotliegend TPS, accounting for of gas, making structural interpretation of gas accumulations more than 85 percent of the discovered gas accumulations in and thermal maturity difficult (Green and others, 1995). the offshore and virtually all of the gas in the giant Groningen Another inversion of the Sole Pit Basin during the Tertiary field onshore in the Netherlands. In addition, these sandstones evidently resulted in erosion or non-deposition of pre-Miocene have significant potential for undiscovered small accumula rocks (Glennie, 1997a; Alberts and Underhill, 1991). Similar tions offshore and for further growth of reserves in existing two-stage inversions also occurred in the West Netherlands fields. However, much of the undiscovered natural gas in Basin and in the Broad Fourteens Basin, although the Late the southern North Sea may occur in sandstones of the Coal Cretaceous inversion was relatively stronger in those basins Measures, adjacent to the carbonaceous source rocks, and in than in the Sole Pit High, where the Tertiary inversion seems the overlying Bunter Sandstone (fig. 10). to have had greater effect (Cornford, 1998). 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 Middle Jurassic 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 Reservoir quality is strongly influenced by the deposi that of the Groningen field, is believed to have been gener tional environments of the Rotliegend. Most commercial gas ated in northern Germany and southern Denmark, but pos accumulations are found in eolian dune deposits, with some sibly also just offshore in the West Netherlands Basin (fig. production coming from fluvial facies. Those sandstones 7). Additional onshore gas generation may have taken place with the cleanest, best porosity at the time of deposition, and as a result of contact metamorphism (Glennie, 1986). K/Ar those sandstones deposited away from the water table tend studies of authigenic illite in the Rotliegend reservoirs of the to display the best preservation of porosity and permeability. Groningen field indicate the structure was progressively filled Eolian facies of all types generally make better reservoirs than over a period of about 30 million years. Natural gas entrapped do interdune deposits, laterally adjacent wadi and sheet flood onshore in the southern United Kingdom was probably also deposits, or any other fluvial, alluvial, or sabkha rocks. The generated from Carboniferous strata, although specific source best reservoirs are thus commonly found in the middle of the rocks and generation sites are not well known. Rotliegend, within which eolian facies predominate (George In the Carboniferous strata of northeastern England, and Berry, 1993; 1997). However, considerable variation in the coals and shales constitute two distinct source rock reservoir quality exists within the eolian facies. Compared components, with the coals containing approximately 60 to horizontally stratified, finely laminated ripple strata of the percent total organic carbon (TOC) and type III kerogen, dune bases and tops, the more steeply dipping foreset beds, whereas the shales have approximately 1 percent TOC and consisting of avalanche and grainfall strata, tend to be initially mixed type II and type III kerogen. Marine shales range less compacted and to be less affected by burial diagenesis from less than one percent to several percent TOC, mostly (Glennie and others, 1978). in type II kerogen. Oil accumulations are directly related to Diagenesis is a problem for reservoir quality in the presence of prodelta marine shales with type II kerogen. the Rotliegend. Fluvial sandstones are commonly so Source rocks became mature for oil and other liquids as early extensively altered by diagenetic processes as to render them Assessment Unit: Southern Permian Basin-Offshore (40360103) 17 noncommercial for hydrocarbon production (Glennie and times in the burial history have also been responsible for others, 1978). Effects of compaction, pressure solution, and primary gas-bearing structures and traps for remigrated gas. precipitation of chlorite all negatively impact the porosity and permeability of Rotliegend reservoirs. However, the most damaging diagenetic effect is the precipitation of fibrous Assessment Units illite in sandstones that had already undergone significant The close relations and similar distribution patterns of porosity reduction (Ziegler, 1992). Successful gas production Carboniferous source rocks, Rotliegend sandstone reservoirs, of illitized reservoirs is only possible where there has been and Zechstein evaporites in gas fields led to the decision to natural fracturing, such as the inversion fractures observed consider the voluminous gas accumulations of the southern at Clipper or West Sole fields (fig. 2), or where directional North Sea and adjacent onshore areas of Europe as forming drilling and expensive reservoir engineering is employed a single petroleum system—the Carboniferous-Rotliegend (Glennie, 1998). TPS (AU403601, fig. 3). Division into assessment units was, however, based upon technological and geographic criteria Seals rather than upon geological distinction. Three AUs were identified: (1) the Southern Permian Basin-Offshore Europe Evaporite deposits of the sabkha and desert-lake (AU40360103, fig. 4), (2) the Southern Permian Basin- sequences of the Rotliegend may seal gas accumulations in Onshore Europe (AU40360102, fig. 5), and (3) Southern underlying Carboniferous reservoirs, but the Zechstein salt Permian Basin Onshore United Kingdom (AU40360101, fig. is the seal for virtually all gas accumulations in Rotliegend 6). Within the AUs, various geologic settings of hydrocarbon reservoirs themselves. The near-perfect seal of the Zechstein is accumulation, informally referred to here as exploration plays, the main reason for the existence of the large gas fields of the were considered for resource assessment purposes. 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 effective seals. Description
Traps and Timing The Southern Permian Basin-Offshore AU coincides with the extent of the Carboniferous-Rotliegend TPS in the off Gas accumulations in Carboniferous reservoirs depend shore area of the southern North Sea between England and the upon traps developed (1) wholly within the Carboniferous, (2) European continent (fig. 4). It is bounded on the south by the by lateral facies changes from sandstone or gravel to mudstone London Brabant Platform and on the north by the mid-North which occurs within the thick sequences of fluvially domi Sea high (fig. 7). Northwestern and southeastern boundaries, nated rocks, and (3) by being sealed beneath the desert-lake respectively, are the coastlines of England and the European evaporites of the Rotliegend (for example, Silverpit Claystone continent (figs. 1, 7). Formation). Gas-bearing structures found to date consist of Variscan-age folds and various Late Jurassic – Early Creta Offshore Exploration Plays and Their Resource ceous and Late Cretaceous – early Tertiary uplifts. However, Potential intraformational stratigraphic traps could also have developed where thermally mature coals or carbonaceous shales are in contact with the reservoirs. Gas accumulations in Upper Permian Rotliegend strata The Rotliegend Sandstone Play depend upon the juxtaposition of mature Carboniferous source rocks, the Rotliegend sandstone or conglomeratic reservoir rocks, and the Zechstein evaporite seals. Effective gas Play Description trapping structures formed after deposition of the evaporite, All of AU40360103 is predominantly the Rotliegend but were of insufficient magnitude to breach the seal. These sandstone play, which has been the objective of intensive gas-filled structures date from the Late Jurassic to Early exploration since the beginning of offshore drilling. Over most Cretaceous and from the Late Cretaceous to early Tertiary. In of the area, the Coal Measures have provided abundant hydro addition, transpression in the Tertiary caused localized basin carbon charge in all settings and retain generative capacity inversions that also resulted in the development of gas-bearing even today. Likewise, Zechstein seal exists in almost all areas structures. Diapiric movements of the Zechstein salt at various and seal integrity has been preserved, even in areas of intense 18 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary deformation. Failures of charge are most commonly related markers offshore combine to make fields in the Carboniferous to timing of structural inversion relative to gas generation or difficult to exploit given current exploration technology and due to leakage in those few areas where the seals are thin. Seal economics. Nevertheless, the Carboniferous is probably the quality also is locally inadequate where carbonate rocks sup most prospective stratigraphic interval remaining in the south plant evaporite beds as a result of facies variation. Exploration ern North Sea. Success will require the ability to overcome uncertainties arise from poor reservoir quality where either seismic imaging problems in order to identify thick channel the best eolian reservoir facies is absent, or where depth and sandstones that are gas-charged. facies-related diagenetic effects have reduced reservoir poros ity and permeability. Identification of structural closures can be difficult, as Rotliegend closures are small compared to the Zechstein Play thick and lithologically heterogeneous overburden. Play Description Exploration Potential of the Rotliegend Play Shelf-edge and slope carbonate facies of the Zechstein The Rotliegend play is presently highly mature. Years of are potential exploration targets. Oil-prone source rocks, such intense and expensive exploration drilling and a dense seismic grid appear to have identified most of the significant footwall as the Kupferschiefer Shale, could provide a local charge, but closures. Nevertheless, discovery of progressively smaller the most likely potential would be realized where Zechstein fields continues and potential remains in limited hanging-wall carbonates are charged from the Carboniferous Coal Measures traps, in known closures previously judged too small to war underlying the Rotliegend. Except for Z-1 carbonates, which rant development, and in subtle stratigraphic traps that have are not separated from the Rotliegend by salt, this scenario thus far avoided detection. requires that the near-perfect Zechstein seal be breached.
Exploration Potential Carboniferous Play Although oil, gas, and condensate are produced from the Zechstein in Germany, Poland, and the Netherlands, where the Play Description source rock is the Stinkschiefer Shale, the Zechstein has never yielded much gas in offshore areas. Nevertheless, moderate to Comparatively few wells have been drilled and few fields small stratigraphic traps for gas and possibly for Zechstein oil have been found in the Carboniferous, but the stratigraphic interval remains prospective because of its substantial gas may be expected. Loss of reservoir quality in the carbonate generative capacity and complex lithologic variations. Various facies adjacent to the evaporites, however, may substantially sandstones in valley fill sequences should be of acceptable res reduce the economic potential of these small traps. ervoir quality, but the stratigraphic complexities that character ize this thick section of fluvio-deltaic rocks make for problems Mesozoic Plays in the location of exploration targets. Compared to the quartz ose Rotliegend reservoirs, the sandstones of the Carboniferous are mineralogically immature. Consequently, loss of reservoir Play Description quality from burial-related compaction and diagenesis may be the limiting factor in successful development of reservoir Gas accumulations sourced from the Coal Measures have targets within the Carboniferous. So far, all Carboniferous been discovered within the Triassic Bunter and Hewett Sand discoveries have been in two structural settings—the Variscan stones (fig. 10) in fault-bounded anticlines. Other possibili folds and the post-Paleozoic structures reactivated during tec ties for undiscovered accumulations include stratigraphically tonic inversion. Traps are expected to result from lateral and entrapped gas within the Bunter, as well as within the overly vertical permeability barriers at the margins of discontinuous ing Lower Cretaceous sandstones. sandstones. The best seals are probably the Permian desert- lake evaporites such as the overlying Rotliegend Silverpit Exploration Potential Claystone Formation (fig. 10). Obvious structures including Bunter and younger reser voirs have been tested, which leaves relatively subtle strati Exploration Potential graphic or structural traps as remaining exploration targets. The Carboniferous strata have considerable exploration An obvious negative aspect of this play is the Zechstein and potential in offshore settings, but technical problems involved Rotliegend seals, which are as effective in shielding these with exploration have so far discouraged extensive drilling. sandstones from a gas charge as they are at preventing leakage The lateral and vertical variability of sedimentary facies, from Rotliegend and Carboniferous reservoirs. Potential exists the difficulties in discerning gas-saturated rocks within the for discovery of additional small accumulations, possibly in complex lithologies, and the paucity of reliable stratigraphic association with other traps. Assessment Unit: Southern Permian Basin-Europe Onshore (40360102) 19
Assessment Unit: Southern Permian which have already been identified during exploratory studies but not drilled, and in subtle stratigraphic traps. Basin-Europe Onshore (40360102) Carboniferous Play Description Play Description The Carboniferous-Rotliegend TPS and the Southern The prolific gas-prone source rocks of the Carboniferous Permian Basin-Europe Onshore AU coincide with the extent Coal Measures, the abundance of closely associated channel of thermally mature Westphalian (Coal Measures) source sandstones and related valley-fill complexes, and the presence rocks and related gas and liquid accumulations in the onshore of shales and evaporitic rocks of the overlying Rotliegend, area on the European continent adjacent to the southern North indicate that the Carboniferous should be an important explo Sea (fig. 5). Assessment Unit 40360102 is located mainly ration target. Traps are to be expected where the Carboniferous in the Netherlands and Germany, but also includes parts is overlain by fine-grained, desert-lake facies of the Rotlieg of Belgium and Denmark. It contains several sub-basins, end. In this setting, accumulations could exist in structural including onshore areas of the West Netherlands Basin and the closures. Accumulations may also occur as stratigraphically Lower Saxony sub-basins (fig. 7). The AU is bounded on the entrapped gas where lithologic boundaries within the Carbon southwest and south by the London-Brabant Platform and by iferous provide the trapping mechanism. Possible examples the Rhenish Massif, on the north and northwest by the North include intraformational traps formed where Carbonifer Sea and the Trans-European fault, and on the east, near the ous sandstones are enclosed by fine-grained rocks or where German border, by a structurally positive saddle on trend with unconformable contacts exist, such as at the overlying Saalian the northeast-striking Hesse Depression (Ziegler, 1990). The unconformity (fig. 10). boundary separating AU 4060103 and AU 4030102 is placed at the coastline because of the distinct differences in tech Exploration Potential nology, economics, and regulations encountered in onshore versus offshore settings. Because the two assessment units Although most of the obvious larger structures have been share most formation boundaries, structural properties, and tested, the Carboniferous remains comparatively unexplored. geologic history, the following description of exploration plays Successful exploration will probably require both significantly in the onshore assessment unit is necessarily abbreviated and improved seismic resolution and more reliable sedimentologic somewhat redundant with play descriptions from the offshore and stratigraphic models. This play and its offshore counter assessment unit. part probably hold the greatest potential for new gas discover ies in the Southern Permian Basin, some of which could be large fields. However, burial-related diagenetic loss of reser Exploration Plays and their Resource Potential voir quality in the highly feldspathic, lithic-rich Carboniferous sandstones is commonly severe, and lack of adequate porosity and permeability may ultimately limit resource potential. The Rotliegend Sandstone Play Zechstein Play Play Description The prolific source rock capacity of the Carboniferous Play Description Coal Measures is present throughout the AU and can be relied Although the Zechstein is mainly valued as a seal, shelf upon to provide adequate hydrocarbon charge to fill structures, and slope carbonate facies might represent an exploration including fields as large as the giant Groningen gas field, to target. Traps would be provided by Zechstein evaporites or their spill-point (Lutz and others, 1975). Similarly, the effec by permeability barriers caused by lithologic discontinuities tive seal of the overlying Zechstein carbonate-evaporite com within the carbonate sequences. The play requires reservoir- plex is present throughout the AU. All discovered Rotliegend quality carbonate rocks, which have generally been limited fields are underlain by coals and carbonaceous shales of the to oncolitic and oolitic shelf margin facies of the Zechstein. Coal Measures and overlain by Zechstein evaporites. Fractured reservoirs may also provide attractive targets (Cooke-Yarborough, 1994). Exploration Potential of the Rotliegend Play The Rotliegend sandstone play has been extensively Exploration Potential developed and is in a mature state of exploration. Potential for According to Glennie (1997a), the Zechstein was the new discoveries is mainly in small structural traps, some of original exploration target when Groningen was discovered. 20 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary
Oil-prone shales are present in the Zechstein, but the most Organic matter in these diverse source rocks ranges from small likely potential would be where Zechstein carbonates have volumes of hydrogen-rich type II kerogen related to marine been charged by the Coal Measures underlying the Rotliegend. deposition, to carbonaceous shales and coals of the deltaic This model is risky for all but Z-1 carbonates because effective sequences. Distribution of fields in this well-explored province charge requires the Zechstein seal to have been breached. depends critically on the burial depth of good quality type II organic matter. Correlation of depth to thermal maturity of organic Mesozoic Plays matter indicates relatively short distances of migration along pathways provided mainly by porous and permeable sand stones, as well as along fractures in fine-grained sedimentary Play Description rocks. Most reservoir rocks are of Carboniferous age, but Gas accumulations, sourced from the Coal Measures, also include the Old Red Sandstone, Waulsortian mounds and have been discovered within the Triassic Bunter and Hewett other carbonates of Dinantian age, deltaic and fluvial sand Sandstones in fault-bounded anticlines in the southern North stones associated with the Coal Measures, and Permian red Sea. Other possibilities include stratigraphically entrapped gas beds. Local lithologic variations provide a variety of complex within the Bunter and within the overlying Lower Cretaceous seals, including calcareous marine shales resting above deltaic sandstones. sandstones and lithologic variations within the sandstone-shale sequences of the Carboniferous. Exploration Potential Most structures with Bunter and younger reservoirs have Resource Potential been tested. This leaves subtle stratigraphic traps and small structures within which Triassic reservoirs are in direct contact Hydrocarbon accumulations have been known from with pre-salt Carboniferous strata. England’s Carboniferous strata since before the Second 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, largely because of the inexpensive exploration and develop ment programs needed to bring discoveries to market. Future Assessment Unit: Southern Permian discoveries will probably continue to consist of small, shallow Basin-U.K. Onshore (40360101) accumulations.
Description Undiscovered Gas Resources of the Carboniferous-Rotliegend Total The total petroleum system (403601) and corresponding assessment unit (40360101) coincide with the extent of ther Petroleum System mally mature Carboniferous source rocks and related gas and oil accumulations in the onshore area of the Southern Permian The recent USGS World Petroleum Assessment (U.S. Basin in the East Midlands of England (fig. 6). Within this Geological Survey Assessment Team, 2000) included an evalu assessment unit only a single exploration play is defined. ation of undiscovered resources in the three assessment units of the Carboniferous-Rotliegend TPS in and adjacent to the southern North Sea. The results of this assessment are sum Carboniferous Play marized in table 1 and reported in USGS Digital Data Series DDS-60 (U.S. Geological Survey Assessment Team, 2000). Undiscovered resources associated with the Carboniferous- Play Description Rotliegend TPS in the European and United Kingdom onshore areas are estimated to range from 23 to 148 MMBO and For purposes of the World Energy Project, the resource from 5.0 to 21.6 TCF of natural gas, plus gas associated with potential in the U.K. onshore part of the Carboniferous-Rotli- undiscovered oil and quantities of natural gas liquids. Undis egend TPS is considered to consist mainly of reserve growth covered resources in the onshore part of the United Kingdom and undiscovered accumulations within and in close associa are considered to be relatively minor. Offshore undiscovered tion with the Carboniferous strata of part of northeast England. oil resources are estimated to range from a minimum of 13 Source rocks include calcareous marine shales of Dinantian MMBO to about 133 MMBO. Undiscovered natural gas age and deltaic shales and carbonaceous shales and coals, resources are estimated to range from 2.8 TCF to more than mainly of Namurian and Westphalian (Carboniferous) age. 27.6 TCF, not including additional resources of natural gas 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.] Undiscovered GasResourcesoftheCarboniferous-RotliegendTotal PetroleumSystem 21 22 Carboniferous-Rotliegend Total Petroleum System Description and Assessment Results Summary liquids and associated and dissolved gas. Thus, estimated Fraser, A.J., Nash, D.F., Steele, R.P., and Ebdon, C.C., 1990, mean values of undiscovered resources in the onshore and off A regional assessment of the intra-Carboniferous play of shore areas of the Anglo-Dutch Basin and adjacent parts of the northern England, in Brooks, Jim, ed., Classic petroleum Northwest German Basin provinces are about 144 MMBO and provinces: Geological Society of London Special Publica more than 26 TCF of natural gas. These estimates are compa tion 50, p. 417–440. rable to those reported by the USGS in 1997 (mean = 24 TCF) Gast, R.E., 1993, Sequenzanalyse von aoelischen Abfolgen (Masters and others, 1997). In addition to the undiscovered im Rotliegenden underen Verzahnung mit Kukstgensedi resources, significant volumes of gas will probably be added menten: Geologische Jahrbuch A 131, p. 117–139. to reserves as a result of growth of existing fields. George, G.T., and Berry, J.K., 1993, A new palaeogeographic and depositional model for the Upper Rotliegend, offshore References Cited The Netherlands: First Break, v. 12, p. 147–158. George, G.T., and Berry, J.K., 1997, Permian Upper Rotli egend synsedimentary tectonics, basin development and Alberts, M.A., and Underhill, J.R., 1991, The effect of Tertiary palaeogeography of the southern North Sea, in Ziegler, P., structuration on Permian gas prospectivity, Cleaver bank Turner, P., and Daines, S.R., eds., Petroleum geology of the area, southern North Sea, UK, in Spencer, A.M., ed., Gen southern North Sea: Geological Society of London Special eration, accumulation, and production of Europe’s hydrocar Publication 123, p. 31–61. bons: European Association of Geoscientists and Engineers Special Publication, Oxford, Oxford University Press, p. Glennie, K.W., 1970, Desert sedimentary environments: 161–173. Developments in Sedimentology 14, Amsterdam, Elsevier, 222 p. 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