Page 83 - An Atlas of Carboniferous Basin Evolution in Northern England

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Chapter
(Glennie & Boegner 1981). This had essentially two effects. Traps were
disrupted due to localized fault movements and successive phases of basin
inversion during the late Cretaceous-Tertiary. In addition, localized deep
burial, principally as a product of early Cretaceous rifting in northern
Cleveland and the Southern North Sea (Fig. 58), took source rocks into the gas
generation window (Fig. 59) with consequent flushing of liquid hydrocarbons
from existing traps.

Cenozoic uplift and erosion

From the end of late Cretaceous chalk deposition onwards, the northern
England basins experienced strong uplift and erosion, with a consequent
'freezing' of hydrocarbon generation. Offshore in the Southern North Sea,
basins have experienced continued subsidence and hydrocarbon generation to
the present day. A regional heating event which has elevated present day
geothermal gradients in parts of northwest England and the East Midlands
may have been associated with early Tertiary volcanism in the North Atlantic
province. However, these elevated heat flows are unlikely to have significantly
retriggered generation from source rocks in these areas, which were already in
an advanced stage of maturity. Regional estimates of uplift and erosion range
from 1100-3000 m in the northwest England to 1000-2000 m in the East
Midlands, which imposed a regional easterly tilt to eastern England of 1-2°
(Fraser et al. 1990) (Fig. 60). The effects of uplift and tilting on existing
accumulations are clearly demonstrated in many of the East Midlands fields,
where palaeo oil-water contacts can be related back to pre-tilt closures.

Play assessment: summary

An assessment of the main Carboniferous plays in northern England is
illustrated in Figures 61 and 62 (also see Figs 48 and 52). From the description
of the hydrocarbon system described here, it is clear that the geological history
of the East Midlands, compared to the rest of northern England, is most
favourable for the development of an oil province. There are several critical
factors that combine to achieve this.

(1) The East Midlands includes several isolated early Carboniferous rift
basins containing thick basal Namurian source rocks, that became
mature for oil generation in the Mesozoic, after Variscan trap formation.
(2) There is an abundance of reservoir-seal pairs providing reservoir-seal
Fig. 53. Composite 2D seismic line and interpreted geological cross section across the Gainsborough Trough illustrating the location and geometry of the Beckingham/Gainsborough
Field (see Fig. 11 for location). 'back-up' with burial depths over most of the area shallower than the
critical threshold for an effective reservoir.
(3) The area was tectonically quiescent following Variscan trap formation
with passive burial accompanied by a mild easterly tilting.
Triassic, but trap modification and repeated faulting of marine band seals burial left only minor onshore areas in the south of the Formby-Fylde area
make prospects in basins lying in NW England unsuitable for gas retention. above the oil maximum for basal Namurian shales. Gas generation from The presence of gas within the Carboniferous is likely to be more extensive,
With continued gas generation, flushing of oil accumulations becomes an Westphalian coals, where preserved following Variscan uplift and erosion, was particularly in NE England and the offshore Southern North Sea (e.g. Leeder
additional risk. In contrast, basins lying east of the Pennine High underwent widespread, particularly in the Cheshire Basin (Fig. 59). & Hardman 1990). However, gas potential may be hampered by poor reservoir
gradual burial at this time, with renewed oil generation in the basinal areas and Prior to early Jurassic times the Carboniferous basins in NE England and quality and, particularly in NW England, by contemporaneous trap reactiva-
little trap modification. East Midlands (and their offshore extensions into the Southern North Sea) had tion and the increased need for an effective sealing facies.
undergone remarkably similar burial and thermal histories. Both areas had In most other areas in northern England, several factors have contributed to
been strongly structured in late Variscan times, forming an array of anticlinal the lack of success in the Carboniferous play fairways.
Jurassic-Cretaceous subsidence and local inversion and fault-related traps, and had been subject to gentle regional subsidence
during the Permo-Triassic. The contrast between basins in northeast England • Excessive burial of Dinantian and Namurian sediments has resulted in loss
The later Mesozoic history of northwest England is highly conjectural but it and the East Midlands commenced in early Jurassic times and continued into of reservoir quality in both the syn-rift elastics and clastic delta play
has been suggested from fission track analysis that some 1-1.5 km of Jurassic the Tertiary as the Cleveland and Stainmore basins became involved in rifting fairways below burial depths of 2500 m (e.g. Cheshire Basin).
and Cretaceous sediments were deposited (Lewis et al. 1992). This continued and inversion related to the Sole Pit fault system in the Southern North Sea • The carbonate platform margin play forms narrow zones around