Petroleum Geology
Petroleum Geology
Petroleum Geology
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Petroleum geology
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Litho- and Biostratigraphy, Palaeogeography of the Upper Permian Zechstein of the Central
European Basin View project
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subsidence
Subsidence of the surface affects the surface water runoff
and the groundwater table, and is widely felt to be a se-
rious issue. Mining activities, including the production of
oil or gas, can cause such subsidence. Other processes that
can result in subsidence include tectonics, compaction
of near-surface layers and (man-induced) lowering of the
groundwater table. Subsidence caused by oil or gas pro-
duction is monitored at regular intervals and the environ-
Fig. 5. Licence situation for hydrocarbon exploration and mental implications are evaluated. Prior to the start of pro-
production in the year 2005. duction, sometimes even in the exploration phase, predic-
tions of future subsidence are issued for all onshore oil
73 109 m3 , while the Dutch gas consumption is some and gas elds.
50 109 m3 per year. The Netherlands is consequently a At the surface above the Groningen eld, which has
net gas exporter, with annual export of some 30 109 m3 been in production from 1963 onwards, to date a maxi-
and import of 10 109 m3 . The annual oil production of mum subsidence of some 26 cm has occurred in the cen-
almost 9.9 million barrels (1.53 106 m3 ) from 13 elds tre of a bowl with a diameter of more than 40 km. The
is relatively modest. In the year 2005, the total hydrocar- maximum subsidence after gas production is expected to
bon production provided the Dutch State with an income be less than 45 cm (NAM, 2000).
of 7.5 billion (Ministry of Economic Affairs, 2006). In At the surface above the Ameland eld, i.e. on the is-
order to save gas reserves for future generations, the State land of Ameland, in the Waddenzee area and in the adja-
has set a ceiling for yearly domestic gas production. In the cent coastal zone of the North Sea, the maximum subsi-
same light there are tax incentives for the development of dence after 15 years of gas production is ca. 25 cm. How-
gas reserves in small elds (kleine velden beleid = small ever, most of the subsidence in the Waddenzee as well
elds policy). The small elds are allowed to produce at as in the marsh areas on the island has been compen-
their optimal production rates, while the large Groningen sated by natural sedimentation. As a result the impact
eld is used to provide extra production capacity as swing on the environment has been negligible and the eco-
producer during winter. With the ongoing production and logical value of the area has not been affected (Eysink
corresponding pressure decline in this eld, it will become et al., 2000). All new gas elds to be developed in
increasingly difcult to full this role during extreme cold- the area would be smaller and cause less subsidence,
turities vary signicantly from top to bottom. Secondary 1999). In most places these source rocks became over-
source rocks for gas occur in basal Namurian organic- cooked during deep pre-Kimmerian burial. Nevertheless,
rich shales (Lokhorst, 1998; NITG, 1998; Gerling et al., the Namurian is thought to have contributed signicantly
tents, but also due to increased CO2 percentages. In ad- Dutch Central Graben they do not exceed 2%. The Wer-
dition, many Zechstein gas accumulations in the eastern kendam eld forms an exception with 70% of CO2 , prob-
Netherlands are sour, and contain up to several tens of ably related to local Jurassic volcanism. The gases in the
percent of H2 S (e.g. Vlagtwedde: 45%). This is a result of Triassic are mostly wet, with wetness ratios of 25 to 50. In
thermochemical sulphate reduction which strongly accel- the Dutch Central Graben dry gas also occurs, with ratios
erates at temperatures above ca. 120 C (Orr, 1977). Con- of over 500. The De Wijk eld also contains dry gas.
sequently, signicant quantities of sour gas occur only The gas in Jurassic and Cretaceous reservoirs contains
where the reservoir is buried sufciently deep, and where 5 to 25% of nitrogen, and around 1% of CO2 . The gas
anhydrite, required for sulphur supply, is present. How- is wet, with wetness ratios from 25 to 50. It is probably
ever, the complex interplay of all controls on this reaction mainly sourced from the Carboniferous, with contribu-
and other H2 S-generating processes, such as generation tions in places from Jurassic coaly sequences, and possi-
from carbonate source rocks and biochemical sulphate re- bly also from highly mature Posidonia Shale source rocks
duction, hampers accurate prediction of H2 S concentra- (De Jager et al., 1996).
tions in undrilled prospects. The gas in Tertiary and Quaternary sands in the north-
The gas in Triassic reservoirs generally contains less ern offshore differs markedly from the other gases. It is
than 5% of nitrogen. Higher concentrations occur in the composed almost exclusively of methane. Methane carbon
Broad Fourteens Basin (locally more than 20%), and also isotope data indicate a bacterial origin.
in the Dutch Central Graben (up to 10%). Anomalously
high amounts of nitrogen are found locally in the east Oil: source rocks and generation
Netherlands (around 40% in Sleen and Roswinkel). Nor- Unlike the areas to the north of the Dutch sector of the
mally, CO2 concentrations are less than 1%, while in the North Sea, the main source rock for oil in the Nether-
formed in most cases by Triassic shales, and where the sive reservoir package. To the north this package breaks
Triassic is truncated at the Base Cretaceous Unconfor- up into several thinner sandstone units, such as the Vol-
mity, by Lower Cretaceous shales. The gas comes mainly priehausen and Detfurth sandstones, which are separated
from Westphalian source rocks and occurs in a variety of by clay- and siltstones that form regional seals (Ames &
trap styles. Where the Zechstein salt is present, as in the Farfan, 1996; Geluk, 1999).
northern sector of the Dutch subsurface, it forms a bar- In the West Netherlands Basin, the exploration for gas
rier preventing Westphalian gas from reaching the Trias- in the Triassic really started only in 1982. Currently nine
sic reservoirs. Only where this regional seal is breached, gas elds are producing, and several others remain un-
by salt withdrawal or faulting, can Westphalian gas reach developed. The typical trap in this play consists of a Late
the Triassic reservoirs. In the West Netherlands Basin no Jurassic horst block in which the reservoir is sealed verti-
Zechstein salt is present, and there the Triassic sandstones cally by Upper Triassic evaporitic shales, and laterally by
form the rst well-sealed reservoir above the Westphalian. Upper Triassic to Lower Jurassic shales (Fig. 18; De Jager
The Triassic sandstones were deposited in uvial and et al., 1996). Lateral seal risks exist where fault-throws are
eolian settings. They were derived from the south, and so large that cross-fault juxtaposition is against the sandy
to the north progressively more shales are present. In Deland sequence. The best reservoirs occur along the
the south, the Main Buntsandstein forms a thick mas- south-western basin margin, where excellent gas produc-
Fig. 19. Trapping styles in the Triassic of the northern Cretaceous Unconformity, Lower Cretaceous marine shales
offshore. Most gas (dark green) is trapped in four-way may provide the seal. Salt-plugged reservoirs may provide
dip-closed structures above salt swells or in turtle-back lateral stratigraphic seals.
structures. Where the Triassic is truncated at the Base
tested from the Maastrichtian and Danian parts of the until 1988 before well A12-3 tested potentially economic
Chalk Group in the IJsselmonde structure. production rates from the shallow gas discovered by well
Although the reservoir characteristics of the Chalk A12-1 in silty sands in the topsets of Plio-Pleistocene pro-
Group are generally only poor to fair, they are compara- grading shelf sequences. Further shallow gas accumula-
ble to those of the chalk in the Danish and Norwegian tions were discovered in B10, B13 and B16-1. Sand produc-
sectors of the North Sea. The lack of success in the Chalk tion from these unconsolidated reservoirs poses a major
in the Netherlands is possibly related to the relatively shal- development problem.
low depth of burial and to a limited charge. The shallow
burial has resulted in a reduced sealing capacity of the Future potential
Lower Tertiary above the somewhat overpressured chalk
reservoirs, causing leaky traps. Proven plays
The Netherlands must qualify as a mature hydrocarbon
Tertiary and Quaternary plays province. With most of the current licences covered with
In the IJsselmonde structure in the West Netherlands 3D seismic data, and with many wells drilled, the main
Basin, small amounts of gas have been tested from the prospective fairways have already been well explored. Yet,
Lower Tertiary Basal Dongen Sand, which lies some 30 m new discoveries continue to be made, and a plot of cu-
above the base of the Lower North Sea Group, and has mulative volumes of gas found over time does not show
average porosities ranging from 34 to 39%. In the east indications of a reduced exploration efciency or cream-
Netherlands, the gas trapped in the Basal Dongen Tufte ing (Fig. 22). The continued steady pace of new discov-
in the De Wijk eld has not been developed because of eries over the last two decades certainly results in part
anticipated subsidence problems during production. from the application of seismic 3D technology, which has
The main Tertiary-Quaternary gas accumulations occur revealed traps that hitherto went unnoticed. Related ad-
in the A and B blocks of the northern offshore at depths vances in seismic processing, such as pre-stack depth mi-
of 400 to 700 m. Strong amplitude anomalies and deeper gration, have more recently further improved the accuracy
pull-downs on seismic proles clearly indicate the pres- of predictive subsurface models. In addition, geological
ence of gas in subtle structures of only several tens of me- concepts continue to be improved, resulting in the iden-
tres height. Most of the shallow gas accumulations are as- tication of new play opportunities and trapping styles.
sociated with gas chimneys, indicating leakage. Although The future potential for new gas discoveries in the Nether-
the gas-bearing sandstones had been seen on seismic, and lands, based on identied prospects in proven plays, as re-
were encountered in wells with deeper objectives, it took ported by the Ministry of Economic Affairs (2006), is esti-
mated to be between 180 and 440 109 m3 of gas. These fractured platform-edge carbonates sealed by Namurian
future reserves represent the addition of risked volumes shales that may contain source rocks. Wells have indicated
of identied prospects in proven plays above a cut-off vol- good porosities and permeabilities (Darcy scale) in north-
ume of 0.5 109 m3 of gas onshore and 2.0 109 m3 ern Belgium, where gas is stored in Dinantian rocks near
offshore. These numbers do not include the unidentied Loenhout, east of Antwerp.
potential that may result from identication of new plays In the northern offshore, the mixed clastic and carbon-
and prospects, for example through the application of new ate sequences of the Dinantian and Namurian Yoredale
technologies or extensions to proven play areas. Formation, sealed by intraformational marine shales,
In a densely explored area such as the Netherlands, on- form a speculative play. Charge could come from Visean
going exploration continues to lead to new insights. The coals or marine Namurian shales. There is a high risk,
great variety of structural styles and potential reservoir- however, that these potential source rocks are post-mature.
seal pairs, only improves the chances for unidentied Intra-Westphalian gas accumulations have to rely on
plays and prospects to occur. New technological develop- intra-formational seals and on lateral seals by favourable
ments are likely to provide increased accuracy to the pre- juxtaposition across faults. That this may work is proven
dictions and a better understanding of play and prospect by a small and uneconomic gas discovery in block Q13.
risks. A break-through in the area of improved produc- Also elsewhere in the Netherlands, intra-Westphalian
tion of gas from tight reservoirs may not only allow the shales occasionally seal small gas columns. Although no
exploitation of many accumulations that are currently un- regional Westphalian seals have been identied, the ma-
economic, but will also broaden the scope for exploration. rine bands, which form laterally extensive but thin shale
Similarly, a break-through in the area of permeability pre- intervals, could prove to be effective as intra-formational
diction (e.g. from seismic data) would result in the de- seals.
risking of prospects that are currently highly risky. And, Gas accumulations may be present in Rotliegend sand-
last but not least, the lowering of economic thresholds or stones along the northern margin of the Southern Per-
cheaper drilling and production technology may render mian Basin in the A and B quadrants. Prospectivity in
presently non-commercial accumulations economic in the this area relies on nding structurally valid and well-sealed
future. traps. A risk is the absence of Westphalian source rocks.
However, Namurian shales and possibly Visean coals dis-
New or speculative plays play source-rock quality in the nearby German and British
In most of the Netherlands the Devonian occurs deeper wells.
than 5 or 6 km, and is a speculative objective at best. Di- Elsewhere, remaining Rotliegend potential is likely to
nantian carbonates on the northern ank of the London- be present in down-faulted traps relying on sealing faults.
Brabant Massif and in well Winterswijk-1 in the east A technological break-through in the production of gas
Netherlands have been found tight and water-bearing. from tight reservoirs (permeability < 1 mD) would open
Nevertheless, some potential remains in karstied and up additional potential.