Page 80 - An Atlas of Carboniferous Basin Evolution in Northern England

fairway
Play
analysis
59
accumulations in all hydrocarbon provinces. In the Carboniferous of northern
England there are several key factors that controlled trap-charge timing
relationships and these also affected the distribution and quality of the main
reservoirs.
Post-rift Carboniferous factors

Two key factors critically affect the potential of the Carboniferous plays at this
time.

Restricted distribution of basinal shales. Gross depositional facies maps
demonstrate the volumetric importance of the early Namurian pro-delta
mudstones as source rocks to an effective source kitchen. This generally limits
the play fairway to within a few kilometres of the early Carboniferous
depocentres. In the East Midlands the maximum horizontal migration distance
from this source kitchen is 12 km; the average is 5 km.

Silesian depocentres: impact ofpre- Variscan burial. Fraser et al. (1990) analysed
maturity data from over 40 wells and suggested that the top and the base of the
oil window would be 1900 m and 3600 m respectively, with maximum gas
generation at 3400 ± 200m. Thus the pro-delta mudstones located in the
central Pennine Basin and the northwest England province were probably
mature and producing hydrocarbons during the late Carboniferous (Fig. 32),
thereby reducing their potential to generate oil following Variscan trap
formation. Dinantian and Lower Namurian reservoir quality would also have
been adversely affected at this time.

End Carboniferous Variscan inversion

Variscan inversion has been identified as the dominant trap-forming event in
northern England (Fraser & Gawthorpe 1990). Variscan inversion also led to
the 'freezing' of oil generation from source rocks throughout the province. The
hydrocarbon system was essentially reset at this time, necessitating a second,
Mesozoic, phase of burial and hydrocarbon generation post-dating the
formation of Variscan traps. Areas where significant post-Carboniferous
hydrocarbon generation occurred are highlighted in Figures 58 and 59.
Furthermore, the main clastic delta play was locally removed by end-
Carboniferous erosion related to Variscan inversion (see Fig. 41). The areas
where significant erosion of the main clastic delta play occurred include the
Derbyshire Dome, parts of the Bowland Basin and extending along the
Pennine-Dent line and the Cleveland Basin. In the Northumberland Trough
removal of the clastic delta play is only partial.

Permo-Triassic extension

The presence of major Permo-Triassic basins on the western side of the Pennine
High substantially downgraded the hydrocarbon potential of this area for two
Fig. 50. Summary maps illustrating the advance of the major deltaic system during the Dinantian and early Namurian and the associated link to the age and distribution of pro-delta main reasons. Firstly, in the major Permo-Triassic depocentres (Cheshire Basin
source rocks (after Fraser & Gawthorpe 1990).
and parts of Manx-Furness), maximum burial cut-offs for both reservoir
quality and oil generation were exceeded (Figs 51 and 58). Secondly, pervasive
throughout
repeated
rifting
earlier
the
fragmented
in the East Midlands. The structure is that of a ramp anticline formed on a Trap-hydrocarbon charge timing relationships and reactivated trap-bounding faults. Permo-Triassic trap modifications at traps
Examples of
and
this
shallow detachment within the Dinantian. The Calow Field is a small gas time occur in the Formby-Fylde area (onshore Manx-Furness Basin) and in
accumulation (0.5 billion cubic feet) reservoired in channel and mouth bar An understanding of the relationship between the timing of trap formation and parts of the Cheshire Basin.
sandstones in sequence LClc. hydrocarbon charge is critical to predicting the distribution of oil and gas Gas generation commenced in the major depocentres towards the end of the