Promote 2014 Lower Cretaceous deep-water sand plays, UK Central Graben

Promote
Lower Cretaceous deep-water
sand plays, UK Central Graben
1. Abstract
12
SVG
NBG
FB
JH
FMH
ECG
Jo
WG
SHB
Up to the present, exploration of the UK Lower Cretaceous deep-water
sandstone play has been confined largely to the Moray Firth basins. The
Lower Cretaceous of the Central Graben has been modelled as
predominantly shale-prone, and hence unattractive to exploration. There is a
growing realisation that this may not be the case.
Of the hundreds of wells drilled in the Central Graben area that targeted
deeper Jurassic-Triassic reservoirs, virtually all have been located on the
flanks of the graben, or on intra-graben highs. However, 131 of these wells
have proved sandstones or traces of sandstones within the Lower
Cretaceous outside the established fairways, giving grounds for optimism
that more substantial deep-water sandstone developments may be present
within the graben depocentres.
South Viking Graben
North Buchan Graben
Fisher Bank Basin
Jaeren High
Forties-Montrose High
East Central Graben
Josephine High
Witch Ground Graben
South Halibut Basin
13
United Kingdom
14
Scapa
Inner
Moray
Firth
Ground
Spur
Highlander
Platform / high
SVG
Basin / graben
Intra-basinal
high
Limit of 3D data
interpreted
16
WG
Halibut Horst
18
Scotland
15 Fladen
SHB
Outer
Moray
Firth
NBG
2014
Field/discovery
50 km
Britannia
FB
S Buchan
Basin
19
20
JH
21
22
FMH
ECG
Northern
North
Sea
Atlantic
Margin
Interpretation of more than 31,000 km2 of 3D seismic data, in combination
with data from 1065 wells, forms the basis for revised palaeogeographic
maps presented here.
Central
North
Sea
Twenty-six leads have been identified within the Lower Cretaceous
depocentres; most of these are located within stratigraphic traps in
interpreted detached basin-floor fans.
Forth
Approaches
Basin
25
26
West
Central
Graben
Devil’s
Hole
Horst
27
Jo
28
West
Central
Platform
Southern
North
Sea
29
30
Auk
High
Scotland
UK
E
Inner Moray Firth
Outer Moray Firth
K55
Rodby Formation
K50
Carrack Formation
K38
127 K30 K36
K34
K32
E
K20
132
RYAZ.
VAL.
L
K14
K10
E
K12
Coracle sst
Coracle
Sandstone
Member
Punt
Punt sst
Sandstone
Member
Sloop Sst
Member
Wick Sandstone Formation
L
K40
121
CROMER KNOLL GROUP
HAUT.
APTIAN
BAR.
L
M
E
Captain
Sandstone
Member
K45
E
Britannia
Sandstone Fm
Yawl Sst
Member
Scapa
Sandstone
Member
Valhall Formation
136
L
E
142
Part of
J70
SEISMIC
PICK
PLAY
Oil field reservoir
Gas/condensate
field reservoir
Chalk Group
112
L
KEY
Central Graben
FB
APTIAN-ALBIAN
Sequence
stratigraphy
98
K60
GENERALISED
LITHOSTRATIGRAPHY
LATEST RYAZANIAN-BARREMIAN
ALBIAN
CEN
L
M
AGE (my)
STAGE
Figure 1 Late Jurassic-Early Cretaceous structural framework of the
UK Central North Sea, location of Lower Cretaceous fields and
discoveries, and extent of seismic dataset used
Oil and gas/
condensate
field reservoir
TOP
L. CRET
Gas shows
FB
Fischschiefer Bed
FISCHSCHIEFER
Sandstone
Potential sandstone
indicated by trace
or minor sandstone
occurrences
2. Introduction
Lower Cretaceous fields and discoveries of the UK Continental Shelf (UKCS)
are almost exclusively limited so far to deep-water sandstone reservoirs in the
Moray Firth area of the UK Central North Sea (Figs. 1 & 2). As of end 2011, 29
fields and discoveries had been found in the Moray Firth area.
Some of the most important Lower Cretaceous fields were discovered by
chance during drilling to deeper Jurassic targets e.g Britannia, Scapa,
Highlander. Each of these discoveries is located within a low or syncline, and
has a strong stratigraphic trapping component. 78% of UKCS Lower
Cretaceous fields and discoveries are located within stratigraphic or
combination structural/stratigraphic traps. The limited lateral and vertical
distribution of coarse clastics within the Lower Cretaceous section gives
considerable opportunity for full or partial stratigraphic entrapment.
This poster aims to highlight the exploration potential of Lower Cretaceous
deep-water sandstones within the UK Central Graben; it revises and updates
Milton-Worssell et al. (2006). Our study comprised interpretation of 3D seismic
data to provide a regional framework and review of all wells in the area. These
data were then appraised in the context of the palaeogeographic development
of the UK Central Graben and used as a basis for lead identification.
BASE
CRET
Kimmeridge Clay Formation
3. Established fairways
Figure 2 Stratigraphic summary chart of the UK Central North Sea Lower Cretaceous
(chronostratigraphy and sequence stratigraphy after Copestake et al, 2003), also showing
occurrence of hydrocarbons
3.1 Latest Ryazanian-Barremian play
Deep-marine sandstones derived from the Scottish Highlands and Shetland
Platform to the west and north-west were deposited in the Moray Firth basins, along
with sandstones derived locally from intra-basinal highs such as the Halibut Horst.
These sandstones were predominantly developed in hanging-wall slope-apron fans;
however, incised narrow linear channel complexes have been identified in the Inner
Moray Firth, extending from shoreface to basin, as an integral part of a mini-basin fill
and spill process (Argent et al. 2000).
Lower Cretaceous shallow shelf sandstones occur locally on the West Central Shelf,
and Copestake et al. (2003) recognised a significant development of latest
Ryazanian to Early Valanginian deep-marine sandstones in the West Central
Graben to the east of the Auk High.
Two Lower Cretaceous plays are recognised: Latest Ryazanian-Barremian and
Aptian-Albian (Fig. 2). The Fischschiefer Bed is one of the few regionally
correlatable marker beds within the Lower Cretaceous. It represents a major
flooding event during the early Aptian, and we have used this horizon as a proxy
for the boundary between the two plays since it is stratigraphically very close to
the Barremian-Aptian boundary (Fig. 3).
SW
NE
Top Chalk
TWT
(secs)
Top Lower
Cretaceous
3.5
Fischschiefer
3.1 Aptian-Albian play
The Aptian-Albian palaeogeography is broadly similar to the Latest RyazanianBarremian, with major sediment input from the Shetland Platform to the north-west.
In addition to slope-apron type filling of the proximal Moray Firth depocentres, a long
distance transport route developed through a narrow channel belt from the Captain
Field area, extending south of the Halibut Horst, and into the South Buchan Basin.
Overlapping slope-apron fans in the Britannia Field area were derived from clastics
shed southward off the Fladen Ground Spur. Oakman (2005) suggested that the
Aptian-Albian play is more areally extensive.
3.0
4.0
Base Cretaceous
unconformity
2 km
4.5
Figure 3 Seismic section across the West Central Graben showing the Fischschiefer horizon
within the Lower Cretaceous. Seismic data courtesy of CGGVeritas.
Promote
Lower Cretaceous deep-water
sand plays, UK Central Graben
United Kingdom
2014
4. Lower Cretaceous sandstones in the UK Central Graben
Since seismic imaging of Lower Cretaceous sandstones in the North Sea is commonly poor (Law et al. 2000), a robust
depositional model must be developed from well and other data, or more sophisticated seismic techniques employed.
In the UK Central Graben, Morgan et al. (2002) and Morgan and Went (2004) showed that anomalous AVO effects can
be recognised from long-offset (6 km) 3D seismic data within channel-like features, and lobate, fan-like bodies in the
Lower Cretaceous, which can be implied to represent the presence of sandstones.
Comparison of the total Lower Cretaceous isochron map, derived from
seismic interpretation, with the distribution of Lower Cretaceous well
penetrations shows almost all of the wells are located outside of the
depocentres (Fig. 4). Well 29/1c-4 is the only well to have drilled a
major depocentre, but it proved 1940 m (6366 ft) of Lower Cretaceous
marine mudstones. All of the other UK Central Graben Lower
Cretaceous depocentres remain untested by drilling, and may contain
significant deep-water sandstone developments.
Our study has utilised well data and conventional 3D seismic data to develop a model of possible sandstone
distribution and geometry within the UK Central Graben.
Using more than 31,000 km2 of 3D seismic data across the UK Central Graben and adjacent areas (Fig. 1), the top and
base of the Lower Cretaceous have been interpreted along with the Fischschiefer horizon (Fig. 3). Isochron maps
have been generated from these horizons for the entire Lower Cretaceous, and for the pre-and post-Fischschiefer
components of the Lower Cretaceous.
4.1 Latest Ryazanian-Barremian play
A total of 104 wells within the study area found sandstone in the latest
Ryazanian-Barremian interval (outside the proven fairway) (Fig. 9), of
which 34 wells contain sandstone beds in excess of 3m (10 ft) thick,
and 70 wells have less significant sandstone developments.
Of the 1065 wells investigated within the study area, 769 proved the presence of Lower Cretaceous beds.
194 wells in our study area found traces or significant developments of sandstone within the Lower Cretaceous section
(Figs. 4, 5 & 6). 131 of these wells are located outside the established fairways, that is, in an area previously deemed
to be almost entirely mud-prone, and 6 of these record gas or oil shows.
13
14
14
15
Substantial thicknesses of latest Ryazanian-Barremian deep-marine
sandstones have yet to be proved in the UK Central Graben. Most of
the recorded minor sandstones occur predominantly within a basal unit
of latest Ryazanian-Valanginian age, comprising beds up to 10 m (30
ft) thick e.g well 22/19a-3 (Fig. 5), but are commonly less than 3 m (10
ft) thick, e.g well 30/11b-1 (Fig. 5). Such minor sandstone occurrences
are quite widely distributed across the Central Graben area.
Two-way time
thickness
16
Sandstones within the Lower Cretaceous cannot be imaged directly
with conventional seismic data due to the low acoustic impedance
contrast between the sandstones and overlying shales (Garrett et al.
2000, Law et al. 2000). However, possible sandstone bodies in the
Latest Ryazanian-Barremian play may be postulated through
geological modelling, utilising the isochron map for the base
Cretaceous to Fischschiefer interval in combination with well data, and
assessment of basin margin and intrabasinal sediment source areas.
0-200 ms
200-400 ms
400-600 ms
21/6-1
600-800 ms
800-1000 ms
21/15-2
19
21
20
The postulated areas of deep-water mass-flow sandstones within the
UK Central Graben illustrated in Figures 7 and 10 are highly
speculative, and require fuller investigation with more advanced
seismic techniques.
>1000 ms
21/15a-6
22/11-4
22
21/20a-1
Several areas are believed to have been emergent:
Limit of 3D seismic
data interpreted
Wells
Lower Cretaceous
absent
Lower Cretaceous
claystone~marl~
limestone
Lower Cretaceous
sandstone
Lower Cretaceous
21/6-1
sandstone outside
proven fairway with
gas/oil shows
The eastern margin of the West Central Shelf was emergent at
the Auk High and probably also along strike along a 125 km
strip (Fig. 7), and formed a potentially substantial sediment
source area complicated by the eruption of basalts on the high
during the Hauterivian (Trewin & Bramwell 1991, Trewin et al.
2003).
29/2b-5
The Forties-Montrose and Josephine highs were probably
emergent also, along with parts of the Jaeren High (Fig. 7).
29/1c-4
28
A smaller intra-basin high located in the south-western part of
Quadrant 22 is interpreted to have been emergent also, and to
have been a potential clastic sediment source
29
30
4.2 Aptian-Albian play
Shallow shelf
sandstones (Devil’s
Hole Sandstone
Member)
A total of 161 wells in the study area found sandstones in the AptianAlbian interval, 111 of which are located in the north within the
established Aptian-Albian play. Of the remaining 50 wells outside the
proven fairway, 13 contain sandstone beds in excess of 3 m thick, 34
contain less significant sandstone developments, and 3 found
conglomerates with basalt clasts on the Auk High (Fig. 9) that Trewin &
Bramwell (1991) and Trewin et al. (2003) have determined to be of
Aptian-Albian age. These conglomerates are interpreted to have been
derived from a Hauterivian basalt flow of localised extent on the Auk
High (Trewin & Bramwell 1991, Trewin et al. 2003).
Lower Cretaceous
basalt conglomerate
50 km
Figure 4 Lower Cretaceous well penetrations in the UK Central Graben area, also showing the
isochron of the Lower Cretaceous interval. Note zero limit of isochron is limit of seismic resolution
of Lower Cretaceous, not the absolute limit of the Lower Cretaceous.
22/19a-3
140
30/11b-1
Sonic
40 0
Gamma
100
140
21/15a-2
Sonic
40
13500
0
Gamma
150
140
29/5a-5
40
Sonic
Upper
Cretaceous
0
Gamma
100
190
Sonic
90
14000
M. Alb
13500
Upper
Jurassic
13700
Aptian
Barremian
11600
11700
Upper Jurassic
Sandstone
Mudstone
Sandstone
Mudstone
Limestone
Cored interval
Limestone
Gas shows
13800
Depths are in feet below KB
Upper Jurassic
Figure 5 Selected well penetrations of Lower Cretaceous sandstones,
Latest Ryazanian-Barremian play, UK Central Graben.
?
Aptian-Early Albian
?
Depths are in feet below KB
Late Ryaz.-Early Aptian
L. RyazE. Val
13600
Barremian to Aptian
13400
Hauterivian
13300
E. Val.
L. Valanginian
E. Hauterivian
Gamma
100
Late Ryazanian
0
14100
14200
Fischschiefer
14300
Figure 8 Selected well penetrations of Lower Cretaceous
sandstones, Aptian-Albian play, UK Central Graben.
Promote
Lower Cretaceous deep-water
sand plays, UK Central Graben
++ +
13
14 +
HH
+++
+ +
There are fewer well penetrations to date of Aptian-Albian deep marine
sandstones than of the older Lower Cretaceous sandstones in the Central
Graben; however, those Aptian-Albian sandstones found are geographically
widely spread across the Central Graben area (Fig. 11). Example sections
from wells 21/15a-2 and 29/5a-5 are illustrated in Figure 8; well 21/15a-2
penetrated 10 m gross sandstone of probable Aptian age with gas shows, and
well 29/5a-5 proved 6 m gross Aptian-Albian sandstone.
+
+
+
+++ + +
+
+
+
+
In common with the Latest Ryazanian-Barremian play, we have assessed the
potential for unidentified Aptian-Albian sandstone bodies through basin
analysis, utilising an isochron map for the Fischschiefer to top Lower
Cretaceous interval in combination with well data, and evaluation of basin
margin and intra-basinal sediment source areas (Fig. 11). The areas
postulated for Aptian-Albian deep-water mass-flow sandstones within the UK
Central Graben as illustrated in Figures 10 and 13 are highly speculative, and
require comprehensive investigation with more advanced seismic techniques
than were available to us.
++
19
20
+
15+++ + + 16
++ +
+ +
21
+
++
+
+
+
+
+
+
FMH
+
Late Ryaz.-Barr.
field / discovery
19
ine
TWT thickness of
Fischschiefer to top
Lower Cretaceous
+
+
+
AH
150-200 ms
JH
200-250 ms
250-300 ms
>300 ms
AH
FBB
FGS
FMH
HH
JH
Jo
MT
SBB
Auk High
Fisher Bank Basin
Fladen Ground Spur
Forties-Montrose High
Halibut Horst
Jaeren High
Josephine High
Marnock Terrace
South Buchan Basin
28
29
30/16-F37
West
Central
Shelf
+
The Latest Ryazanian to Barremian basins of the Central Graben
are largely separated from those of the Outer Moray Firth to the
north by E-W to SW-NE trending palaeohighs (Figs. 7 & 9):
The Marnock Terrace forms a SW-NE trending structural high
connecting the Forties-Montrose High to the Jaeren High
(Fig. 9, Line A). Very thin (sub-seismic resolution)
developments of poorly dated Lower Cretaceous beds have
been proved locally by wells on this high.
The western arm of the Forties-Montrose High forms an E-W
trending structural high (Fig. 9, Line B). This high appears to
have formed an almost complete barrier during the Latest
Ryazanian-Barremian. Thus, Latest Ryazanian-Barremian
mass-flow sediments within the Central Graben are likely to
be entirely separate from those proven within the Outer
Moray Firth area.
+
+
AH
Basalt-clast
conglomerates
on Auk High
Axis of buried high
+
These highs appear to have remained significant during AptianAlbian times (Figs. 8 & 9), and the possibility of contiguous AptianAlbian mass-flow sandstones extending southward into the Central
Graben depocentres is considered to be remote.
Interpreted sandstone developments
Aptian-Albian
field / discovery
30
100-150 ms
+
Limit of established
Aptian-Albian fairway
++
50-100 ms
29/5a-5
Sand transport
route
30/11b-1
Figure 7 Well penetrations of sandstone, and interpreted possible
sandstone developments within the latest Ryazanian-Barremian, also
showing isochron map of the near equivalent seismic interval, base
Cretaceous to Fischschiefer. Note zero limit of isochron is limit of seismic
resolution, not absolute limit of unit.
Jo
Emergent high potential sediment
source area
B
0-50 ms
Limit of 3D data
interpreted
Sandstone
(beds > 3m)
+
Fig 9
Line A
Basalts erupted on
Auk High during
Hauterivian
23
Trace sandstone
(beds < 3m)
Auk High
Fisher Bank Basin
Fladen Ground Spur
Forties-Montrose High
Halibut Horst
Jaeren High
Josephine High
Marnock Terrace
South Buchan Basin
+
25 km
Potential
source
area
Jo
9L
West
Central
Shelf
Axis of high (possible
barrier between basins)
+
+
+++
++
++
22/19a-3
29
Limit of established
Late Ryaz.-Barr. fairway
+ 16
+ 15
+
+ ++
+
+
++ SBB +
+
+
+
+
+ +
21/15a-2
FMH
+
?
MT
+21 +
+
20
22
+
AH
FBB
FGS
FMH
HH
JH
Jo
MT
SBB
+
Sand transport route
13
800-1000 ms
28
Emergent high potential sediment
source area
+
600-800 ms
JH
23
+
+
Interpreted sandstone developments
Coarse clastics derived from the UK landmass may have been deposited
locally as shallow shelf sandstones on the West Central Platform. Very long
transport routes would be required for any such sandstones to have fed
across the platform into the West Central Graben.
FGS
400-600 ms
MT
22
Fig
Limit of 3D data
interpreted
Sandstone
(beds > 3m)
Trace sandstone
(beds < 3m)
200-400 ms
++
+
? +
+
The Forties-Montrose High and part of the Josephine High remained
emergent, along with the north-western part of the Jaeren High (Fig.
11).
+
+
FBB
+
+
+
Only two small areas on the eastern margin of the West Central Shelf
are likely to have remained emergent (Fig. 11) and potentially
supplying coarse clastics to the West Central Graben.
14
HH
0-200 ms
+
Aptian-Albian sediments extend more continuously across the study area,
indicating a degree of drowning of previously emergent highs (Fig. 11).
+
TWT thickness of
base Cretaceous to
Fischschiefer
FGS
++
+
+ + SBB
+ ++
2014
United Kingdom
30
25 km
Figure 8 Well penetrations of sandstone and interpreted possible sandstone developments within the AptianAlbian, also showing isochron map of the near equivalent seismic interval, Fischschiefer to top Lower Cretaceous.
Note zero limit of isochron is limit of seismic resolution, not absolute limit of unit.
Fisher
Bank
NW Basin
Marnock
Terrace
East Central
Graben
Tertiary
Upper Cretaceous
Lower Cretaceous
South
Western arm of
Buchan Forties-Montrose
Basin
High
West Central
Graben
NW
SE
3
TWT
(secs)
Lower Cretaceous
4
Jurassic
SE
Tertiary
Upper Cretaceous
Triassic and
Upper Permian
3
TWT
(secs)
4
Upper Jurassic
5
Lower Permian
and older
Line A
5
Fischschiefer
Triassic and
Upper Permian
Lower Permian
and older
6
6
10 km
7
Line B
10 km
Figure 9 Regional geoseismic lines showing structural highs dividing the Lower Cretaceous basins of the East Central Graben and West
Central Graben from the Fisher Bank and South Buchan basins respectively. Location of lines shown on Fig. 7.
7
Promote
Lower Cretaceous deep-water
sand plays, UK Central Graben
Shetland
Platform
?
Scapa, Claymore
12
13
14
United Kingdom
50 km
Highlander
Fladen
15 Ground
16
MORAY FIRTH
Spur
Halibut Horst
18
22
Blake
FortiesMontrose
High
19
20
21
Scotland
East
Central
Graben
West
Central
Shelf
25
26
West
Central
Graben
27
28
29
Devil’s Hole
Sandstone
Member
Latest Ryazanian-Barremian
Mass flow sandstone
(modified after
Copestake et al.
2003)
Shallow marine
shelf / slope
Deep marine
basin
30
Hinterland / intra-basinal high
(erosion or non-deposition)
Field / discovery
Possible mass
flow sandstone
(this study)
Shallow shelf
sandstone
Auk
High
Possible sediment
transport route
Figure 10 Post-analysis: possible distribution of sandstones within the Latest Ryazanian-Barremian play of the UK
Central North Sea (modified after Copestake et al. 2003)
Shetland
Platform
12
Captain
13
14
Saltire
15
MORAY
Halibut Horst
FIRTH
18
Blake
Goldeneye, Hannay
Fladen
Ground
Spur
16
Witch
Ground
Graben
South
Buchan
Basin
19
50 km
Britannia
Fisher
Bank
Basin
Brodgar
FMH
20
Scotland
21
22
Cromarty, Atlantic
East
Central
Graben
West
Central
Shelf
25
26
27
West
Central
Graben
28
29
Auk
High
30
Aptian-Albian play
Key as above,
except for right
Proven mass flow
sandstone (2003)
Possible mass flow
sandstone (after
Oakman 2005)
Figure 11 Post-analysis: possible distribution of sandstones within the Aptian-Albian play of the UK Central North
Sea as identified in 2003 (after Copestake et al. 2003), by Oakman (2005), and by this study
2014
Promote
Lower Cretaceous deep-water
sand plays, UK Central Graben
United Kingdom
SW
5. Leads
2014
NE
TWT
(secs)
Top Chalk
Top Lower
Cretaceous
Twenty-six leads have been identified from our analysis of Lower Cretaceous deepwater sandstone distribution in the UK Central Graben, all of which have a strong
stratigraphic component. Trapping styles are as follows:
3.0
Hanging-wall fans located adjacent to active or residual fault scarps (Latest
Ryazanian-Barremian leads only)
3.5
Fischschiefer
On-slope, ponded mini-basin fill (Latest Ryazanian-Barremian leads only)
Detached basin-floor fans (Latest Ryazanian-Barremian and Aptian-Albian
leads)
Potential
sandstone shed
off intra-basin high
Two Latest Ryazanian-Barremian leads have been mapped on either side of an
intra-basinal high within relatively minor hanging-wall basins in the West Central
Graben (Fig. 12). Regional tilting up to the west provides the optimum trap
configuration of up-dip stratigraphic pinch-out, and avoids the potentially
problematic issue of lateral seal risk where these postulated Lower Cretaceous
sandstones may be in cross-fault contact eastwards with Jurassic to Triassic
sandstones of the footwall.
Base Cretaceous
unconformity
Top Triassic
4.5
1 km
Basin fill and spill processes were recognised by Argent et al. (2000) within the Late
Ryazanian-Valanginian Punt Sandstone Member of the Inner Moray Firth, whereby
sand-rich sediments accumulated within the first basin encountered, and only
spread to neighbouring basins once the first basin was filled to spill point, from when
the feeder channel was able to incise and cross inter-basinal highs. Several minibasins have been identified on the margins of the UK Central Graben, within which
the Latest Ryazanian-Barremian sequence is isolated from the main basin in the
graben, but the Aptian-Albian sequence continues across the intervening high. The
smaller lead on the western side of Figure 13 comprises the ponded fill of a basinmargin mini-basin, and may have bottom seal risk if the pre-Cretaceous section
comprises Jurassic or Triassic sandstones.
WSW
4.0
Figure 12 Seismic section across the West Central Graben showing leads within the
Latest Ryazanian-Barremian play. MC3D MegaSurvey seismic data courtesy of PGS.
SE
NW
Top Lower
Cretaceous
3.0
TWT
(secs)
Top Chalk
3.5
ENE
Fischschiefer
TWT
(s)
Top Chalk Gp
Top
Rotliegend
3.0
Possible
sandstone within
mini-basin
4.0
Potential detached
basin-floor
fan sandstone
Fischschiefer
2 km
Base Cretaceous
unconformity
Base Chalk Gp
4.5
Figure 13 Seismic section across the East Central Graben showing leads within the
Latest Ryazanian-Barremian play fairway. MC3D MegaSurvey seismic data courtesy of
3.5
Base
Cretaceous
1 km
Seismic data courtesy of CGGVeritas
Figure 14 Seismic section from the West Central Graben. The target reservoirs are
conjectural basin-floor sandstones within the Latest Ryazanian-Barremian play and
the Aptian-Albian play.
In the established Lower Cretaceous fairway of the Moray Firth basins, detached basin slope to
basin floor channel and fan-lobe sands are recognised as the most important play type,
especially within the Aptian-Albian play (Oakman 2005). In the UK Central Graben, the majority
of the mapped leads are located within detached basin floor fans, with either full stratigraphic
pinch-out trapping, or combination stratigraphic pinch-out and down-dip closure trapping.
Detached basin floor fan traps characterise all of the Aptian-Albian leads recognised in this
study.
Figure 13 illustrates an example of a potential detached basin-floor fan within the Latest
Ryazanian-Barremian play; this lead requires up-dip stratigraphic pinch-out at the proximal end
of the fan.
6. Summary
References
Copestake, P, Sims, A.P, Crittenden, S., Hamar, G.P., Ineson, J.R., Rose, P.T. and Tringham, M.F. 2003. Lower
Cretaceous. In: Evans, D., Graham, C., Armour, A., and Bathurst, P. (eds and coordinators).The
Millennium Atlas: petroleum geology of the central and northern North Sea. Geological Society, London,
191-211.
Garrett, S.W., Atherton, T., & Hurst, A. 2000. Lower Cretaceous deep-water sandstone reservoirs of the UK
Central North Sea. Petroleum Geoscience, 6, 231-240.
Law, A., Raymond, A., White, G., Atkinson, A., Clifton, M., Atherton, T., Dawes, I., Robertson, E., Melvin, A., &
Brayley, S. 2000. The Kopervik fairway, Moray Firth, UK. Petroleum Geoscience, 6, 265-274.
Milton-Worssell, R.M., Stoker, S.J. & Cavill, J.E. 2006. Lower Cretaceous deep-water sandstone plays in the UK
Central Graben. In: Allen, M.R., Goffey, G.P., Morgan, R.K. & Walker, I.M. (eds) The Deliberate Search for
the Stratigraphic Trap. Geological Society London, Special Publications, 254, 169-186.
Morgan, R, Toothill, S & Fogg, A. 2002. What remains to be found in the Central Graben, North Sea? A role for
long(er) offset seismic data. Abstracts CD-ROM, PETEX 2002.
Morgan, R & Went, D. 2004. The seismic characterisation of lithology using AVO: a window onto Lower
Cretaceous sands and their trapping potential in the Central Graben. Abstracts, The Deliberate Search for
the Stratigraphic Trap, Geological Society, London.
Oakman, C. 2005. The Early Cretaceous Aptian Play of the Central and Northern North Sea - Atlantean drainage
models and enhanced hydrocarbon potential. In: Doré, A.G. & Vining, B.A. (eds) Petroleum Geology:
North-West Europe and Global Perspectives-Proceedings of the 6th Petroleum Geology Conference,
Geological Society, London, 187-198.
Trewin, N.H. & Bramwell, M.G. 1991. The Auk Field, Block 30/16, UK North Sea. In: Abbotts, I.L. (ed) United
Kingdom Oil and Gas Fields 25 Years Commemorative Volume, Geological Society, London, Memoirs, 14,
227-236.
Trewin, N.H., Fryberger, S.G. & Kreutz, H. 2003. The Auk Field, Block 30/16, UK North Sea. In: Gluyas, J.G. and
Hichens, H.M. (eds). United Kingdom Oil and Gas Fields Commemorative Millennium Volume, Geological
Society, London, Memoirs, 20, 485-498.
The material presented in this document is for information only. Whilst every effort
has been made to ensure that the information is accurate, it does not constitute
legal, technical or professional advice.
For more information contact:
Richard Milton-Worssell
4.0
PGS.
Email: [email protected]
A regional appraisal of the Lower Cretaceous strata in the UK Central Graben gives grounds to
support the possibility that deep-water sandstones could be important future exploration
targets.
The Lower Cretaceous depocentres of the UK Central Graben are largely untested;
however, 131 wells found traces or significant developments of Lower Cretaceous
sandstone within the study area outside the established fairways.
The Lower Cretaceous sandstone plays within the Central Graben are likely to be entirely
separate from the established fairways to the north. E-W to SW-NE trending palaeohighs
may have acted as barriers to southward sediment transport.
Predicted sandstone developments in both the Latest Ryazanian-Barremian and AptianAlbian plays are predominantly detached slope-apron and basin-floor fans. they are most
likely to have been derived from local sediment sources around the margins of the Central
Graben.
The potential for stratigraphic entrapment of hydrocarbons within postulated detached
deep-water sandstones in the Lower Cretaceous depocentres of the East Central Graben
and West Central Graben is high. Example leads are relatively high risk, primarily in terms
of reservoir presence, but also in terms of trap definition since the predicted sand bodies
and their lateral distribution cannot be discerned directly on seismic data.
This preliminary study has identified 26 leads in total; they are quite deeply buried (33505850 m / 11000-19000 ft), and are located within, or on the flanks of deep graben where
the effects of overpressure are likely to be seen.