Page 76 - An Atlas of Carboniferous Basin Evolution in Northern England

Play
fairway analysis
55
1994; Fig. 15). Turbidite sandstones sourced from the main delta systems to the
north and the Wales-London-Brabant Massif in the south also form potential
reservoirs in pro-delta settings (e.g. Longnor Sandstone; Fig. 37). In particular,
basin floor turbidites associated with bypass across remnant steep half graben
basin margins represent the most favourable turbidite targets. Outcrop
examples of this type of bypass turbidite system include the Pendle Grit and
equivalents in NW England (Fig. 33), the Mam Tor Sandstone and Shale Grit
in the Edale Gulf (Fig. 37), and the Ashover/Roaches Grit equivalents in north
Staffordshire (Fig. 38).
Additional important reservoirs are provided by southerly derived,
quartzose fluvial sandbodies which form valleys incised into Dinantian
carbonate platforms and abandoned delta plains during major lowstands.
These are particularly well developed in the late Brigantian following the EC6
regional uplift and inversion event (e.g. Fig. 33). Oil-bearing sandstones of this
age have been encountered in both Welton and Eakring/Dukes Wood (Figs 2,
12, 54 and 55), with flow rates of 570 bbls of oil per day from Welton-A4.
However, problems exist in predicting the location of the palaeovalleys, and
the location of the individual channels within them, because these sandbodies
are generally below seismic resolution.
In addition to reservoir distribution, post-depositional modification of
porosity and permeability (poroperm) during burial diagenesis is a major
factor influencing reservoir quality. Based on over 1.5 km of core data from the
East Midlands, relationships between log and core porosities, and porosities
and permeabilities, permit the prediction of porosity in different facies to
within one or two porosity units. For example, within Namurian fluvial
channel sandstones the porosity cut-off for a commercially viable reservoir is
estimated at 10%, corresponding to a permeability of c. 1 mD. This cut-off
suggests that beyond a maximum depth of burial of 2550 ± 200 m reservoir
quality is likely to be poor (Fig. 51). Therefore reservoir quality is likely to be
significantly downgraded in the main depocentres in Cheshire and NW
England/Irish Sea, and in the Cleveland Basin (Fig. 51) because of excessive
burial.
Regional seal is provided by marine bands deposited during maximum
flooding events. The marine bands have been shown to be adequate seals for oil
accumulations, but are unreliable for gas (e.g. the Calow Field, Fraser et al
1990). In the pro-delta environment, marine mudstones provide excellent seals
for turbidite sandstone reservoirs.
The richest hydrocarbon source rocks identified are interpreted as distal pro-
delta mudstones that were deposited in advance of the southerly prograding
Fig. 47. Sequence architecture of the northern margin of the Widmerpool Gulf showing development of the EC6 hanging-wall dip-slope carbonate platform margin and associated delta systems (Fig. 50). In the East Midlands the pro-delta source rocks are
clinoforms and reflector terminations. Note onlapping early LC1 a succession, composed of pro-delta mudstone source rocks.
well developed in the basinal areas, but are poorly represented on the East
Midlands Platform. The pro-delta source facies are predominantly oil prone.
Biomarker studies and carbon isotope analyses clearly indicate a mixed
and
development of local highs (e.g. footwalls to intrabasin faults). The fades is not Clastic delta play (Fig. 49) marine/terrestrial derived kerogen (Fraser et al. 1990). Both the delta-top pro-
across and interfinger with the distal
turbidite reservoir systems prograded
identifiable on seismic and strong emphasis would have to be placed on delta source rocks forming hydrocarbon migration pathways. In addition,
mapping facies-structural relationships around the basin margins. The fairway This play is by far the most important in the Carboniferous of northern delta-top coal swamps developed regionally during the Westphalian (LC2) over
is likely to be most prospective in the Widmerpool Gulf and Gainsborough England and contains the majority of the reserves discovered to date in the northern England and the Southern North Sea. These provide gas and
Trough. province (Table 2), with reservoirs developed in both delta-top/shallow marine occasional oil prone source rocks which are likely to be in good communica-
and deep water turbidite facies (Fig. 49). In the southerly prograding delta tion with the delta-top channel and mouth bar reservoirs.
Shelf carbonates. Shelf carbonates in the Dinantian preserve little primary systems, channel and mouth bar sandstones form the main producing reservoir Assessment of reservoir distribution and quality, together with source rock
porosity and only patchy secondary porosity is developed. In addition, the facies, with channel sandbodies exhibiting the more favourable reservoir distribution and the amount of Mesozoic burial, allows the main prospective
carbonate shelf areas are largely beyond the limits of hydrocarbon migration characteristics. Antecedent rift bathymetry exerted a significant control on the areas for the clastic delta play to be identified (Fig. 52). In particular the East
from basinal source kitchens and thus are unlikely to have received a distribution of reservoir sandstones (e.g. Figs. 34 and 38), with delta-top and Midlands, with limited Mesozoic burial and good communication between
hydrocarbon charge. Nevertheless, fractured shelf carbonates associated with mouth bar reservoirs best developed where they axially infill remnant rift source and reservoir is highlighted as the main oil-prone area for this play
Variscan folding have produced some 32 000 bbls (50 000 bbls reserves in place) bathymetry (Fig. 50). Major lowstand incised valley fills form particularly (Fraser et al. 1990; Fraser & Gawthorpe 1990).
of oil in Hardstoft in the East Midlands (Fig. 11; Table 2) indicating the thick, but often laterally restricted, fluvial reservoirs (e.g. the Rough Rock
potential for reservoir development in this play. along the northern margin of the Widmerpool Gulf; Church & Gawthorpe