Lower Cretaceous Aptian LST sands and distribution in N and C North Sea. Micropalaeontology, palynology, stratigraphy and prospectivity

1. Journal of Petroleum Geology, vol. 21(2), April 1998, pp. 187-211
THE DISTRIBUTION OF APTIAN SANDSTONES IN
THE CENTRAL AND NORTHERN NORTH SEA
(UK SECTOR) - A LOWSTAND SYSTEMS TRACT PLAY
Part 2: distribution and exploration strategy
S. Crittenden*, J. M. Cole** and M. J. Kirk***
In this paper, we discuss the distribution in the Central and Northern North Sea (UK
sector) of the Late Aptian sandstones r~f reservoir potential which are assigned to
members ofthe Sola Formation. An exploration strategyforthese sandstones is proposed,
based on sequence-stratigraphic and palaeogeographic models derived from the
examination ofnumerous wells. The sandstones were deposited by mass-flow processes
as a consequence of a major, tectonically-induced or enhanced, Late Aptian fall in
relative sea-level. The distribution ofthese lowstandsandstones, and thefacies developed,
was controlled by the structure and palaeogeography that existed in the study area both
before and after this sea-level fall.
The pattern offaulting in the study area during Sola Formation deposition is related
to Jurassic and older tectonism, which is associated with the development ofthe proto­
North Atlantic and the thermal subsidence ofthe North Sea rift system. Faults controlled
the areas ofsandstone provenance, and also determined the routes by which reworked
sediments were transported into depocentres.
We have mapped the distribution ofthese sandstones using both well and seismic data.
Maps offault pattern s, basinal and high areas, and facies distributions have been
generatedfor the top- Valhall Formation level (i.e. immediatelybeneath the Sola Formation),
and also for the Sola Formation itself. These maps were used to mode! the distribution
ofAptian lowstand sandstones andprospects in the UK Central and Northern North Sea.
This modelling exercise is ofrelevancef or the identification oflowstand sandstones with
reservoir potential elsewhere on the NE Atlantic margin.
* Consultant, Waye Cottage, Chagford, Devon, UK TQ13 8HN.
** Geochem Group Ltd, Chester Street, Chester CH4 8RD.
*** Stratasearcli Consulting, Inverurie. ASS! 8XE.

2. 188 The distribution ofAptian sandstones. Central/ Northern North Sea: part 2
INTRODUCTION
Sandstones of Aptian age in the Central and Northern North Sea (UK sector) tend to
be very localised in occurrence. Here, we discuss the distribution of Aptian sediments
in this area, focussing in particular on those sandstones which have reservoir potential.
The exploration strategy we propose is based on a sequence-stratigraphic model considered
in Part I of this paper (referred to hereafter as Crittenden et al. , 1997). Data from
numerous wells in the study area has been analysed. Throughout, we refer well depths
to the Rig Kelly Bushing in metres, although Imperial units are used for a number of
completion logs held by the UK Dept of Trade and Industry (DTI).
Aptian sediments form part of the late syn-rift succession in the North Sea. In the overall
transgressive Early to mid-Cretaceous interval, the Sola Formation represents a very significant
Late Aptian regressive phase which we interpret to have had a mainly tectonic cause. The
Sola Formation lies above the Valhall Formation and beneath the Redby Formation (see fig.
I in Crittenden et al., 1997); in the study area, it comprises dark claystones and sandstones
varying from a few metres thick to (exceptionally) over 350m in thickness where sand rich.
Late Aptian lowstand sandstones with reservoir potential occur in a number of fields
in the North Sea, and are sealed by Sola Formation claystones. These fields include
Britannia, Captain. Saltire, Andrew and Tiffany in the UK sector, Agat in the Norwegian
Northern North Sea (Norwegian Quadrant 35), and Victory west of Shetland (UK 207/1).
Hydrocarbon shows in Late Aptian sandstones have been recorded in numerous wells
elsewhere in the North Sea and the NE Atlantic margin. Aptian lowstand sandstone may
have reservoir potential over a wider area than that discussed in this paper, extending
perhaps from the Haltenbank area (off mid-Norway) SW to the Porcupine Basin, west
of Ireland. We have extended the search for Cretaceous lowstand sandstone reservoirs
throughout the en-echelon basin suite on the NE Atlantic margin as part of a wider study,
and have examined a large number of wells tied to seismic profiles. Of particular interest
in this context is the recent report of the discovery of a substantial gas accumulation in
thick Upper Cretaceous sandstones by a "deep-water" well drilled on the Nyk High, off
Mid-Norway.
This paper is intended to contribute to the search for Aptian lowstand sandstone
reservoirs in the NE Atlantic margin and the North Sea. However, we do not consider
other important parameters, such as seals, traps, source rocks, maturation and migration.
AN EXPLORATION STRATEGY FOR THE APTIAN SANDSTONES
Our discussion of Aptian sandstones with reservoir potential begins by considering the
palaeotopography and palaeogeography that existed at the onset of. and during. Aptian
times, and by constructing appropriate palaeogeographical maps and depositional models. ­Simplified maps resulting from the study ofan extensive well and seismic data-base were
published for the Lower Cretaceous of the southern and central Norwegian sectors of the
North Sea by Hesjedal and Hamar (1983). Although these maps were not presented in
a sequence-stratigraphic framework, they served to emphasise the reservoir and source­ •
rock potential of the Lower Cretaceous in this area.
Published maps for the North Sea (e.g. Cope et al.. 1992) indicate that the present-day
distribution of Cretaceous sediments is determined both by the erosional margin of the
interval, and also by the limits of seismic resolution and the amount of data available.
For example, Late Cretaceous and Tertiary erosion of the Jurassic and Lower Cretaceous
succession defines the margins of the Fladen Ground Spur. Therefore, although Lower
Cretaceous sediments may be absent from a particular area at the present day, it does not
of course follow that the area was a site of non-deposition during the Early Cretaceous.
An understanding of the areal extent of marine deposition during the Early Aptian
transgression is necessary in order to identify likely source areas for clastic sediments

3. ------
---
189S. Crittenden et al.
3°E
208
•
EARLY APTIAN
TRANSGRESSIVE
MAXIMUM
LEGEND
60'N _~.._....vs-ve ­
2l7 ·ICXrN1l
•... i.,.
- I
_ 1.lAl~ flrU.JS
....--- =..e-~
­-- -- ~ h
•
Thin venee r 01
shallow shelf
carbonates
and ctasncs
26 27 • 28
56"N l..- L -.JL ---l ---l --J.-......- - - ­
Fig. I. Map of the Central and Northern North Sea, showing the extent of sedimentation
during the Early Aptian (the V5 and V6 units of the Valhall Formation).

4. - -
190
.............
The distribution ofAptian sandstones. Central/ Northern North Sea: part 2
which were re-deposited during the Late Aptian regression. These source areas comprise
regions on the shelf and upper slope, on which coarse clastics accumulated during the
Early Aptian; at the same time, the V5 and V6 claystone units of the Val hall Formation
were being deposited in sediment-starved basinal areas. Fig. I is a palaeogeographic map
of the Central and Northern North Sea in the early Aptian, and is based on well-log and
seismic data. From this map, we can predict likely areas of shelf sand and carbonate
deposition, and also areas in which hemipelagic, carbonate-rich claystones were deposited.
The shelfal deposits were reworked during the Late Aptian regression. and, together with
eroded bedrock, were redeposited as the sandstones assigned to members of the Sola
Formation.
The structural configuration of the study area at the end of the Early Aptian transgression
is also important, because fault patterns and fault-block configurations control sediment
provenance, transport and deposition. Some of the fault patterns and structural lineaments
may have been established during the Palaeozoic, and may have had a controlling effect
on Jurassic as well as Lower Cretaceous sedimentation. For example, the present- day
coastline of Norway retlects deep-seated structural lineaments, which are thought to
have controlled Jurassic and Lower Cretaceous sediment transport routes from the
Scandinavian Massifacross the Norwegian shel f (Dore etal .• 1997). Deep-seated lineaments
controlling sediment transp0l1 routes in the UK sector of the Central and Northern North
Sea are less obvious. However, we propose that the Inner Moray Firth, which is bounded
to the west and north by the Helmsdale and Wick faults and by the Grampian uplands
to the south. intluenced the distribution of sediments derived from the Scottish Massif
during the Lower Cretaceous.
Fig. 2 shows the extent of marine conditions in the Central and Northern North Sea
following the Late Aptian regression, and illustrates the distribution of Sola Formation
claystones and sandstones. Both Figs 1 and 2 are derived from more detailed maps we
have constructed which integrate well and seismic data. Viewed together, Figs. 1 and 2
indicate where the Val hall Formation is truncated by erosion. where faults were active
at this time, and where depocentre margins are located. These factors assist in predicting
likely areas of Late Aptian mass-tlow deposition. In addition. these maps show areas of
non-deposition, and areas where the Valhall and Sola Formations were originally deposited
but have subsequently been partially or completely removed.
GENESIS OF LATE APTIAN SANDSTONES IN THE UK CENTRAL AND
NORTHERN NORTH SEA
During the Early Aptian transgression, the shallow shelves tlanking the North Sea
Central and Viking Graben rift system were sites either of non-deposition and erosion.
or slow deposition resulting in condensed (sometimes incomplete) hemipelagic carbonate-
rich sequences with local sandstones. These sediments contain reworked fossils derived
from Lower Cretaceous and older stratigraphic units. During the Late Aptian regression,
these relatively unconsolidated sediments (particularly the shelfal sands) were reworked
by slumping and gravity-tlow processes into deeper-water slope and basin environments.
Thegravity flows were triggered primarily by tectonic activity, causing uplift and shallowing
of the basin margins and intra-basin highs, and localised emergence and erosion.
Sedimentation continued during the ensuing Late Aptian sea-level lowstand and relative
rise, until the clastic source areas were submerged as the shorelines moved landward.
The location ofsediment entry points into the grabens was controlled by the intersection
offaults on the graben margins and by the graben-margin topography. Forexample, sediment
supply may have occurred at transferzones betweenen-echelon normal faults. The redeposited
sandstones rapidly change laterally into the relatively organic-rich marine claystones of the
Sola Formation, which constitutes a potential hydrocarbon source rock.
•
•

5. 191s. Critte nden et al.
208
1
LEGEND
AQ URE2
B M. . e UCHAN tKIfl$l
G," . GL.LH~ tfORST
A FI . ANDRt: W RlOOt:
E.QW . ENO Of WO~LO FAUL.T
_ ._IM FIWLT$
__ . ~ .1.....1cl'
....e -•.-.........
. ........ ........ ....................,in_
-
60"N
LATE APTIAN
REGRESSION
Sola FOfmallCJrl
d uyr;lone in
cepoccerres
(70' · 450')
59'·N r---J>,!*=-----.-+--+-r-r-+--+=-+--1Ift-=---'r----'~ Known Sola
Sand M~~r
Sola FOrR'allCl1
abl;.cnl/croo-d
•
57' N r----j-----l---~-.-II_:__~=----=fi~-.."....~-"-<h
Thin veneer of
Sola Formation
post Late Aplian
regression
26 • 2827
56' N L __.-L L ---l.. L --L=-...:..~---'
Fig. 2. Map of the Central and Northern North Sea, showing the extent of sedime ntation
during the Late Aptian (the Sola Formation).

6. 192 The distribution ofAptian sandstones. Central/ North ern North Sea: part 2
LEGEND
_ I,lAlN f AU.lS
.........- p,~ -...u
s.. c· ~rfWIll'I
. ....., .....• ' 9Iud«t"'~Il'I .. . 1
_ .- ...~ ...
19
FORTH
APPROACHES
BASIN (PART)
26 27 • 28
NORTHERN MARGIN OF
MID-NORTH SEA HIGH
Fig. 3a. Location map of the Central North Sea (UK Sector), showing structural elements
and oilfields referred to in the text.
DISTRIBUTION OF THE EARLY APTIAN VALHALL FORMATION
VSIV6 UNITS
The general distribution of Early Aptian sediments in the study area is shown in Fig. •
I.Hemipelagic calcareous-rich claystones assigned to the V5 and V6 units were deposited
in graben depocentres; on seismic sections, these units appear to be condensed. For
example, the V6 (6m) and V5 (8.2m) units are present but thin at the top of the Valhall
Formation in Well /6//3a-2z (located on Fig . 3b) in the South Viking Graben. In the
majority of the well s studied in the Banff and Buchan Grabens, however, the Valhall
Formation is thicker (e.g. 671-m thick in We1l20/3a-2: location on Fig. 3b) including
the V6 and V5 units. The presence of the V6 unit in a large number of wells both in the
Central and Viking Graben depocentres and on the flanking shelfal areas indicates that
the Early Aptian transgression was widespread. Shallow-water siliciclastic sediments
were therefore deposited over a wide area.
In the following section, we discuss the occurrence of the V5 and V6 units in wells
on the shelf areas flanking the depocentres. In most instances. these two units are too thin
to be distinguished on seismic profiles; hence, well data is invaluable. The areas considered
are the East Shetland Platform/ Fladen Ground Spur, Renee Ridge, Buchan Horst-Glenn
Horst, Halibut HorstlHalibut Platform. Western Platform. Eastern flank of the Central
and Viking Graben, and Forties-Montrose High (Fig. 3a).

7. 193S. Crittenden et al.
6 7 8 9 10
21 22 23 24 25
18 17 18 '9 20
23-2 . 24-'
o
l°E
o 0
12-1
o 28
0°
7- 1
10-1 6· 1 •
o 0
04-1
lOW
11 12 13 14 15
7
26 27 28 29 30
o
o
19
26
56°N L __-l.-.l..---l.....l..-l......L .-I. -L.:..-J-----'
Fig. 3b. Location map of the Central North Sea, showing wells referred to in the text.
.. East Shetland Platform and Fladen Ground Spur
The East Shetland Platform is a Mesozoic "high" which forms the western flank of the
Viking Graben and the East Shetland Basin (Fig. 1). It includes the poorly-studied East
Orkney, Dutch Bank and Unst Basins. which are intra-platformal areas offault-con trolled
subsidence in which Lower Cretaceous sediments are preserved (Fig. 3a). The SE extremity
of the East Shetland Platform is known as the Fladen Ground Spur. In the Witch Ground
Graben to the south and SW, Lower Cretaceous strata thin markedly towards the flanks
of the Spur, covering thin Permo-Triassic strata and onlapping thicker Carboniferous
units at the margin of the horst block. The core of the Spur comprises Devonian Old Red
Sandstones, from which all the younger Palaeozoic sediments have been stripped. Even
during the Late Cretaceous transgression. this palaeohigh was submerged beneath
comparatively shallow waters, and only accumulated a thin sequence of Upper Cretaceous
sediments. Thus, Well /5/20-1 records Maastrichtian chalk unconformably overlying
Devonian sediments. In Wells 16121([-6. 1612/([-13 and /6/21-5 on the southern extremity
of the Spur, Upper Cretaceous chalks unconformably overly a thin Valhall Formation
interval.
We propose. however, that during the Early Aptian, the Fladen Ground Spur comprised
either an emergent landmass, or more probably, a shallow-shelfsea in which siliciclastics
and carbonates were deposited. A relative high-stand depositional model for the southern

8. 194 The distribution ofAptian sandstones, CentralI Northern North Sea: part 2
Graben /basinul area v, Ith N
condensed irnerval of
hemipelagic claystones and
marts (the V5 & V6 units]
GRABEN INTERSECTION AREA
+A A • A' . ~:fl~' 2~~lh
A'
St"(I -ln~ I .------------------,
ValhoJll h llnl. lllOJI VS/V6 UlIla .
TSTIII~1
Fig. 4. Relative highstand depositional model for the Fladen Ground Spur during
deposition of the Early Aptian V5 and V6 units (Valhall Formation)
part of this region is illustrated in Fig. 4. The Fladen Ground Spur is a primary candidate
for sediment sourcing during the Late Aptian lowstand. Sediments may have been •
derived both from reworking of the Palaeozoic and Early Mesozoic sediments forming
the high itself, as well as from the overlying, Early Aptian shelfal sediments.
The western margin of the Fladen Ground Spur extends from quadrant 14 through the
northern and central parts ofquadrant 15, to the SW corner ofquadrant 16. Released well
data (15/18-1 and -2) indicate that little or no Lower Cretaceous sediments or sandstones
are present along this flank of the Spur, although this is probably a consequence of the
well locations.
The Early Aptian highstand also resulted in the deposition of a thin veneer of V5 and
V6 sediments on the Piper Shelf, as is seen in Wells 14/4-1,15//2-1,15/17-4 and 15/17-6.
Shelfal sands may also have accumulated on the Piper Shelf at this time.
The Renee Ridge (Fig. 5)
The Renee Ridge forms the southern boundary ofthe Witch Ground Graben. The northern
flank of the Ridge is, according to seismic evidence, onlapped across a series ofstepped fault
blocks by a diminishing thickness of Lower Cretaceous sediments; this also occurs on the
southern flank abutting the Central Graben. Indeed, existing LowerCretaceous sediments

9. 195s. Crittenden et ill.
' ., H,IUBVT [fORST ----.-
, . JOm d »,(
IIJrn s~
............,
, ppn" IIlo!1Ccxtcr unl Calcduru;m S.luth HaJINHGrarurc
SUJl.Ju:d v.dl number &.IhidnC'"(II
Sul", CLaY"l.IDP and ~
Arrro:u DIlte hmu of ..lIlUMn nc dc["l"llllln
and fWU'lf'C'd1ft' f~noo .a~
Knol..... SlI14 S,n"hlutll: lkpu'll ~
. 1f'IP1,_illll&c:' ckf'(~ltinnal OlIn
Fig. 5. Schematic palaeogeographic map of the Buchan Horst- Renee Ridge- Halibut Horst
area, illustrating probable transportation routes for shallow-shelf sandstones into
deeper-water depocentres.
•
almost overlap the Renee Ridge. and may have done so during the Early Aptian transgression.
They were subsequently stripped off during the Late Aptian regression and other post­
Early Cretaceous erosional events. Wells examined on the Renee Ridge have recorded
a thin veneer of Cromer Knoll Group sediments (e.g. Wells 15/26-1 and 15/27-2).
The Renee Ridge may have affected sediment deposition within the Witch Ground
Graben by acting as a barrier to the transport further southwards of sediments derived
from the Fladen Ground Spur. However, fault-controlled gulleys across the Renee Ridge
may have acted as sediment conduits, because some very thick Aptian sandstones are
present in wells just to the south and SE in blocks 21/1 and 21/2 (see Fig. 5).
The Ridge may have accumulated shallow-shelf sands which were reworked during
the Late Aptian sea-level low, and together with the products of erosion of the subcrop,
provided a minor proportion of the Late Aptian sandstones. However, the majority of the
sands were probably derived from the Fladen Ground Spur to the NE and the Halibut
Horst to the NW (Fig. 5).
The Buchan Horst - Glenn Horst (Fig. 5)
This structure is a tilted and eroded, generally east-west trending fault block composed
of Devonian and Carboniferous strata bounded by major faults to the north and south;
it is overlain unconformably by Lower Cretaceous to Recent rocks. It can be traced from
blocks 20/5 and 21/ I eastwards into block 21/2 (Fig. 5). Well data indicate that the Horst
was completely submerged during the Early Aptian transgression. It may have exerted some
l

10. 196 The distribution ofAptian sandstones. Central/ Northern North Sea: part 2
control on coarse- clastic deposition during the Late Aptian regression. The V6 and V5
units of the Valhall Formation are present in Wells 2111-1, Ia-8 and 1a-13 on the horst
flanks, and are overlain by the Sola Formation. In Well 21/1-6 in the centre of the horst,
the Sola Formation is thin and lies unconformably upon the Devonian Buchan Formation.
Well 2113-1a on the Glenn Horst penetrated 19m of the Redby Formation (dated here
as Late Albian) immediately overlying the volcanics ofthe Raden Group Rattray Formation.
In Well 21/3a-4 (illustrated in fig. 6, Crittenden et al., 1997), the Sola Formation Sand
Member contains feldspars which may have been derived from these volcanics.
The Halibut Horst and Halibut Platform
The Halibut Horst (Fig. 5) and Halibut Platform (Fig. 3a) are very shallow features,
on which little or no Lower Cretaceous sediments are preserved. However, the fault
complexes at their margins include downfaulted terraces on which Lower Cretaceous
sediments are present, including both shelfal and mass-flow sands (e.g. at the Captain
field). We suggest that the area formed a shallow shelf during the Early Aptian, and that
the Halibut Horst was a positive area. This horst acted as both an important source of . .
sediments during the Late Aptian lowstand, and also provided a topographic control on ~
sediment transport and deposition (Fig. 5). That the Horst is prospective is illustrated by
the occurrence of Late Aptian sands recorded in wells drilled in block 13/24 in the faulted
terraces on its southern flank.
The Western Platform (Aberdeen 1 Forth Approaches Basin)
Throughout the Mesozoic, the Aberdeen Basin and Western Platform (parts of the
western margin of the Central Graben) formed an uplifted area, on which a thin veneer
of Lower Cretaceous sediments was deposited. BGS borehole 81/40 partially cored a
Lower Cretaceous sequence of mudstones (Barremian to Albian) which, in this vicinity,
is approximately 200-m thick (according to seismic data). Lott et al. (1985) described
the stratigraphy ofthe section. The Early Aptian transgression is represented by planktonic
foraminiferal "floods" which we assign to the V6 unit (our KL6 Biozone; Zone 4 of
Banner et al., 1993 - see Crittenden et al. , 1997, p. 19). The area undoubtedly
accumulated shallow-shelf sands, claystones and carbonates during the Early Aptian,
and provided material during the Late Aptian lowstand for erosion and transport into
adjacent depocentres, such as the Banff-Buchen Graben and the Central Graben.
Elsewhere on the Western Platform, Lower Cretaceous sediments are either entirely
absent, as for example in Well 28/5-1 on the graben shoulder (where Upper Cretaceous
chalks lie unconformably upon the Kimmeridge Clay Formation); or are present as a thin
veneer, as in Well 28/5-2, where a thin Sola Formation claystone interval lies upon the •
V6 unit of the Valhall Formation. Wells 28/12-1 and 29/7-2 are typical of the Western
Platform. A very thin veneer of Lower Cretaceous sediments is present in both wells,
above the Triassic in the former and above the Kimmeridge Clay Formation in the latter,
beneath the Upper Cretaceous Chalk.
The Auk Horst (Fig. 3) on the western shoulder of the Central Graben is interpreted
to have been a "high" during the Early Cretaceous, and only local deposits of marls
(Wells 30/16-1 and -2), conglomerates (Aptian-Albian), carbonate breccias (Neocomian)
and volcanics (Hauterivian basalt) are preserved (Trewin and Bramwell, 199 I). In Wells
30/16-4, -5 and -9, Upper Cretaceous chalks rest upon well-cemented Rotliegend sandstones.
To the SE, the Auk Horst is offset to the east and forms the Argyll Ridge (Fig. 3a) , a
NW-SE trending Palaeozoic high, with the Chalk Group resting on Late Jurassic and
older sediments. However, in Well 30/24-4, approximately 13m of?AlbianlAptian marls
lie beneath the Chalk and above Jurassic argillaceous and glauconitic sediments of the
Duncan Sandstone Member. In addition, in Wells 30/24-5, -6 and -/2, the Lower
Cretaceous interval is thin and rests directly upon the Permo-Triassic.

11. 197S. Crittenden et al.
P1.£NUS .. ....
'"I-­
H3
HID'" I-­
H2
I-­
HI
A3
I--
RODeY R2
I-­
A'
53
-
SO..... S2
-
51
y,
-
vs
-y.
YAUW.L -
Y3
-
YO
-
Y'
N1519-13
DEPTHS THICKNESS
Z655m _......­........... ........­
' 3m
2677m ..........
) '3m
2600m .._....­
) 8m
,.­ ........­
Om
2703m ..........
j
IHIi> 38m
;J..~
27.'171
N15/12-1 N15/12·2
DEPTHS THICKNESS DEPTHS THICKNESS
2OS4m "'23m5m
2542 .5m
19 5m
..,.... --­ -­
' Sm
,...."
"'"22m ,......
37m
2630m
.....
2981171 ,..2m
I ' Sm 5m
2O'7.Sm ""m
2667 171
~
r)%
2Q87,Sm ,......
5.5171 3m
2903m ..12m
11
I 27.5m
H 13m
r! 2Ei ~5m
35m
3OO6m 2703m
N15/12-3
DEPTHS THICKNESS
27_ ........ ­.....mlm .. ........
2830m ..........
) 31m
2861171 ~'..r-..
r) Om
28_ '-'""-"""­
3m
286.... ..........
1085171
..nOm ..........
200m
2908m
N15/12-4
OEPTHS THICKNESS
...,-.,"'­
~%
27'2m
11m
2735m
25m
2750m
Om
2755m
2<lm
2' .....
N1616-1
DEPTHS nllCKNESS
...,-.,"'-f.A.-'""­
ABSENT ABSENT
--;;i~
~,,,,,-
~ l n l 2m ..........
IUm
cl784m ......... ­
Y.­ 11...
c 18Q6m _........
8m
ctiCMm ..-_..­
223m
elm.3m ..........
"mc fG-3l.4f1'1 ..........
3.' 171
c fG3&.8m --_....
BASE
NOT
STUDIED
I
Fig. 6. The Early Cretaceous stratigraphy of six wells in the Norwegian sector on the
eastern flank of the Central Graben. (Li ne of section is marked on Fig. 3a).
•
The Eastern Flanks of the Central and Viking Graben
The Jaeren High represents the eastern margin of the Central Graben in the UK sec tor.
The entire area is deepl y eroded, as exemplified by Well 22/ 10-1; here, Upper Cretaceous
chalks lie unconformably upon Triassic sedime ntary rocks. Sediments may have been
deposited on the high during the Early Aptian tran sgression, and were removed by
subsequent erosion. For example, the Maureen Shelf is a fault-bounded terrace on the
east flank of the Graben Intersection Area, bordered by the Vestland Arch/Jaeren High
to the east and limited to the NW , west and SW by the South Viking and SE Witch Ground
Grabens. It is referred to as the eastern part of the Andrew Ridge by various authors (e.g.
Bisewski . 1990).
.. Well 16/23-1 is located on the flank of the Maureen Shelf, on a terrace fault block
which is downthrown to the west. The well penetrated a thin Valhall Formation sec tion
(a relatively condensed interval with a number of hiatuses and non- sequences) overlain
by a thin and incomplete Sola Formation interv al; no sandstones are present in this well.
Thi s would be expected in a high are a subjec t to non-depositi on and/or subsequent
eros ion during the Late Apti an regre ssion. However. the V5 and V6 units deposited
during the Early Aptian transgression are present at the top of the Valhall Formation.
In well 16/28-3. located on an isolated high in the South Viking Graben adj acent to
the Maureen Shelf. UpperCretaceous chalks lie unconformably upon the Jurassic. Whether
sandstones and a complete Low er Cretaceous interval were or were not deposited on the
Maureen Shelf is not clear, as the area underwent pre- Senoni an erosion which was
associated with tectonic and halokinetic movem ents during the overall Late Cretaceous
tran sgression.
However, in wells in the Norwegian sector on the Vestland Arch (Sleipner Terrace of the
Utsira High) adjacent to the Viking Graben. a thin, highly-condensed Early Apti an interval

12. 198 The distribution ofAptian sandstones. Central/ Northern North Sea: pan 2
(V5 and V6 units) is present (Fig . 6), and provides further evidence of the widespread nature
of the transgression.
Forties - Montrose High
This high is 50-miles long, and extends from block 21/1 0 in the north to block 22/24
in the south. It trends approximately NNW - SSE and is bounded by normal faults; it was
a dominant positive feature from the Late Palaeozoic to the Tertiary. During the Early
Aptian, the area may have accumulated shallow shelf sediments (probably both carbonates
and siliciclastics) derived both from within the basin and from elsewhere. The basinal
depocentres and terrace margins adjacent to the high are prospective for Sola Formation
sandstones. In addition, there may be down-faulted areas on the high itself where Lower
Cretaceous sediments are preserved (similar to the conditions which occur in the Unst
Basin).
In the Forties field (blocks 21/10 and 22/6a), the oldest rocks penetrated are Middle
Jurassic volcanics overlain by a thin interval assigned to the Kimmeridge Clay Formation
and younger sediments. Where present, the overlying Lower Cretaceous comprises a . .
thin or condensed interval ofmudstones and sandstones containing volcanic rock fragments. ~
In Well 21110-1, Upper Cretaceous chalks lie unconformably upon Jurassic strata, and
a dark grey-brown sandstone is present at the base of the chalk interval above the
Kimmeridge Clay Formation. This sandstone may constitute an exploration target. A
similar "base- Chalk" transgressive sandstone/conglomerate ofEarly to "mid"> Cretaceous
age is present in the West of Shetland Exploration area (authors' observation), in the
Magnus area of the Viking Graben, and in the western part of the Halibut Horst. In Wells
22/11-1 and 22/11-2, a thin Upper Cretaceous chalk interval lies unconformably on
Triassic sedimentary rocks.
In the south-central part of the Forties-Montrose High (blocks 22/17 and 22/18),
Lower Cretaceous deposits comprise a thin sequence of Aptian to Albian chalky marls
which lie unconformably upon the Triassic. This thin section represents the extent of the
Early Aptian and Albian transgressions (Wells 22/17-T2 and 22/18- 1).
THE DISTRIBUTION OF THE SOLA FORMATION CLAYSTONES AND
SANDSTONES
The thickest Lower Cretaceous intervals are not located on Jurassic and pre-Jurassic
high blocks (which to date have been the main target for oil exploration). but in the
structurally- deeper, hanging-wall and back-basin areas, where little or no drilling for •
Lower Cretaceous targets has taken place. In some areas, deposition of material eroded
during the Late Aptian regression from the crests of active fault blocks formed hanging-
wall breccias and conglomerates.
Reworking of sediments from the shelfal areas adjacent to the basin-bounding faults has
resulted in what we interpret to be gulley- or canyon-fed fan systems. Good examples are the
sands in the Magnus Trough (authors' observation) and in the Agat field area (Norway
Quadrant 35) (authors' observations, and pers. comm., Brit Sauar). Other well-developed
examples are located in the West of Shetland exploration area in wells adjacent to the Rona
Ridge and the Shetland Platform. Deposition of coarse clastics with reservoir potential
took place in down-faulted basin margins, and. more importantly, in transfer-fault zones.
During the Early Cretaceous as in the Late Jurassic, salt movement and dissolution in the
Central Graben complicated the processes of erosion and redeposition. The principle
examples of Aptian sandstones are located in the Moray Firth, as discussed below.
The Moray Firth
Within the Moray Firth, the Banff Basin and Buchan Graben form a "corridor" that
extends from quadrants 13, 14, 19 and 20 in the west through block 2112 to the northern

13. 199S. Crittenden et aL.
liR.. Hr.v 1.'l'1·:RSf:( -ru'.·.' lt H..'
tI
/,,/ ..
// . '
/
,,",""",IO'-'t1
?
it" ,I ,,.W
Fig. 7. Relative lowstand depositional model for the Fladen Ground Spur during
deposition of the Late Aptian Sola Formation claystones and sandstones.
part of quadrant 22 in the east. The corridor extends to the Britannia field area in the
southern Witch Ground Graben and the Graben Intersection Area - quadrants 15/29 and
30. 16/26, 27 and 28, and 22/1 and 2 (see Fig s 2 and 5).
•
Reservoir sandstones of Aptian age are informally referred to on released composite
well logs as the Aasgard Sands and Captain Sands in the west on the western flanks of
the Halibut Horst, and as the Glenn Sands south of the Renee Ridge. In the east, south
of the Fladen Ground Spur, they are referred to as either the Bosun Sands or the Kopervik
Sands. This profusion of informal names is confusing. Johnson and Lou (1993) applied
.. a different, but nevertheless consistent and formalised, lithostratigraphical terminology
to these sandstones, and renamed the Sola Formation as the Carrack Formation.
Sediments of Late Aptian age were deposited along the tlanks of the Fladen Ground
Spur in the Witch Ground Graben, and in the Graben Intersection Area to the south. Thick
Sola Formation sandstones are present at the Britannia fieldjust to the south of the Fladen
Ground Spur in the south Witch Ground Graben and Graben Intersection Area (Fig. 7).
Over 275m (gross) of sandstones are present in Well 16/27-2, and these are almost
certainly sourced from the Fladen Ground Spur. Th e sandstones in Well 16/26-3 are
much reduced in thickness to only 5.5m gross, but the thickness rapidly increases westwards
into the southern part of quadrant 15 in the South Witch Ground Graben, and 112.5m
(gross) are recorded in Well 15/29a-2. Well 16/28-6 encountered Late Aptian gas­
bearing sandstones overlying a salt diapir. In Well 16/28-4, located on an intra-basinal
high, a very thin oil- bearing Aptian sandstone interval lies unconformably upon the
Kimmeridge Clay Formation. This marks the feather-edge of the Late Aptian sandstones
which pinch-out to the east onto the Maureen Shelf (Andrew Ridge).

14. r-­
200 The distribution oiAptian sandstones, Central/ Northern North Sea: part 2
These sandstones were derived from the tectonically-active SW margin of the graben
(the Renee Ridge), and were redeposited by gravity-driven processes. More importantly,
material was also shed from the tectonically-passive NE margin (Beach, 1984), i.e. from
the Fladen Ground Spur, and from the Maureen Shelf and Jaeren High on the eastern
flank of the Viking Graben during the Late Aptian regression. Sandstones entered
depocentres such as the Witch Ground Graben via (?fault-controlled) gulleys. This
occurred at the same time as the deposition ofcondensed Sola Formation claystones (the
Late Aptian-Early Albian S I, S2 and S3 units) on shelfal areas, and the deposition of
thick claystone intervals in parts of the depocentres which did not receive coarse clastic
material. We suggest that some of the sandstones in the depocentres were subsequently
reworked by submarine currents flowing parallel to the axi s of the graben and were
redeposited as contourites.
A lowstand depositional model for the Sola Formation sandstones in the Graben
Intersection Area is illustrated in Fig . 7; reference should also be made to Fig. 5 of
Crittenden et al. (1997). The area immediately to the NE and north of the Forties High
in the NE Central Graben is also prospective for the development of Aptian sandstones. ...
For example, the Sola Formation is 46-m thick in Well 22/8a-2. However, the basal S I ~
unit is only 3.3-m thick, and overlies the Early Aptian V6 unit of the Valhall Formation,
as indicated by an influx of tiny red/orange-stained Praehedbergella sp. assigned to the
KL6 Biozone. The thin S I interval may either represent condensation at the feather-edge
of the depositional area to the north and NW, or represent truncation associated with
erosional scour. In the nearby Well 22/3a-1, the Sola Formation claystones are 11- m
thick and overlie 75m (gross) of Sola Formation sandstones. The underlying Valhall
Formation is truncated, and the V6 and V5 units have not been identified. Similarly, in
Well 2212-2, 56m ofsandstones and claystones were drilled, while the underlying Valhall
Formation is truncated and the V6 and V5 units are missing. No sandstones are present
in the nearby Well 22/1-2a, but 32m of Sola Formation claystones, including green­
stained Praehedbergella sp. characteristic of the KL5B Biozone within the S I unit,
overlie the V6 unit of the Valhall Formation. The V6 unit in this well yielded a redl
orange-stained Praehedbergella sp. fauna, which is characteri stic of the KL6 Biozone.
Th e underlying V5 unit yielded brown-stained Praehedbergella sp. (H.D9) characteristic
of the KL7 Biozone.
NW into the Witch Ground Graben in quadrant 14, in the region of the Tartan and
Highlander fields, Sola Formation sediments are much thicker but no Aptian sandstones
have been recorded in wells for which data has been released. However, on the flanks
of the Fladen Ground Spur, Aptian sandstones may be expected in the form of mass-flow
deposits and their erosive-based feeder channels, forming good stratigraphic traps as the •
sandstones pinch-out up-dip. Indeed, the "Upper Valhall" sandstone (UVA ofCasey and
Romani, 1992) in the Saltire field, documented in Wells 15/17-23 , -21 and -22 (Casey
and Romani, ibid.; Casey etal. , 1993 ) agree with this model. However, their interpretation
was based upon incompletely-cored material from the 15/17-23 well.
The present authors disagree with Casey et £11.'s interpretation ofthe lithostratigraphy and
age ofthe sandstones in Well 15/17- 21. In that well, we interpret the UVB sandstones to have
been deposited in an erosive channel cut into the Valhall Formation claystones which has
removed the Fischschiefer (the V5 unit). We suggest that the Fischschiefer noted in the well
by Casey etal. has been wrongly identified, and is a claystone drape or minor flooding surface
within the Aptian sandstone package. This is consistent with the data, considering the
incomplete biostratigraphical coverage indicated by Casey et al. (ibid.) on their figures.
We suggest, therefore that the Piper Shelf, and the depositional area adjacent to it in the
Witch Ground Graben, is prospective for Late Aptian sandstone deposits.
The Halibut Horst and Halibut Shelfformed a major source area for sediments deposited
in the Banff Basin and adjoining Buchan Graben (Fig. 5). Within the axis of this basin, in
the region of the Ettrick field and to the south and west of it (i.e. in quadrant 20), Sola Formation

15. - - - - - - - -
201S. Crittenden et al.
a:!B Islands. - by granlIe bosses
~ sand prone lans ... sediment supply
' •• "'. deposKionalenvelope 01base01faunscarp,lan-slope aproncomplex
.~ .. ~ ...­
, - __ ~(CaledonianSouth Halibut Granite)
Fig. 8. Schematic "blrd''s-eye view" of the 14/26 - 13130 area during the Late Aptian sea­
level lowstand. Clastic sediments are being reworked from shallow-water areas on the
Halibut Horst into adjacent, basinal depocentres,
sediments are over 122-m thick. Most of the studied wells penetrated a great thickness
of Sola Formation (135m in WeIl20/7a-/) ; in some wells (e.g. 20/1-/), the S I unit was
•
thin. indicating condensation or erosion associated with nearby sandstone deposition.
Other wells had thin sandstone stringers (e.g. 20/3-1, -Za. -3); and yet others adjacent to
the northern margin of the basin recorded thick sandstones (e.g. 14/26-1. 13/30- I and 13/
30-3). A schematic depositional model for the 14/26 and 13/30 area is illustrated in Fig.
8. Wells in the basin centre are claystone rich, and Aptian sandstones are not recorded
on any of the released wells we have examined.
The Aptian sandstones in the Banff-Buchan Graben "corridor" to the south of the
Halibut Horst form part ofan east-west orientated, submarine fan - slope-apron complex,
consisting of small sandy debris-flow/turbidite sand lobes with interchannel muds. This
complex was supplied with poorly-consolidated sediments from sources lying to the
north. From the available data, we propose that these sandstone lobes did not extend far
into the deeper parts of the basin to the south, where only thick shale sequences (Sola
Formation) were laid down. However, Aptian sandstones are probably present along the
entire southern margin of the Halibut Horst, as already explained.
DEPOSITIONAL MODELS
Because the predictability ofthe Aptian sandstones has important economic implications,
the sandstones recovered by cores in the wells studied need to be carefully examined in

16. r
202 The distribution ofAptian sandstones. Central/ Northern North Sea: part 2
order to determine transport and depositional mechanisms. A variety of gravity- driven
processes (slides, slumps, debris flows, turbidity currents) can transport sediment from
the shelf edge and upper slope into deeper waters. A consideration of the models which
have been proposed for these gravity-driven processes is beyond the scope of this paper,
as is the differentiation of high-density turbidites and sandy debris flows which has
recently received much discussion (Shanmugam et al., 1994, 1995; Shanmugam, 1996).
We have studied numerous cores from Aptian claystones and sandstones, and have
identified a variety of sedimentological facies. These facies have allowed us to interpret
the probable mode ofdeposition and to suggest depositional models. Our regional studies
of Lower Cretaceous sedimentology (Fig. 9) and palynofacies indicate two major
depositional models for the Aptian sandstones of the Central and Northern North Sea:
(i ) We interpret some of the Aptian sandstones, particularly those in areas of more
rapid subsidence, to have entered the basin from a single, major point source. Thus, sand­
rich "borderland"- type submarine fans (c.f. Link and Nilsen, 1980), dominated by
channelised mid-fan facies associations were formed on slope aprons. This is exemplified
by Well 21/3a-4 (Fig. 5) (Crittenden et al., 1997). _
(ii) We interpret other sandstones to have been deposited in inner fan/slope areas. ­
These were supplied by a number of moderately-sized channels incised into interchannel
deposits (c..f. Chan and Dott, 1983), rather than a single point source. Locally, small fan
lobes were formed in the mid-fan area, at the termination of both inner-fan and mid-fan
channels. Examples are the SW t1ank of the Halibut Horst (13/30 and 14/26 area: see Figs
5 and 8).
Although evidence is absent from the cored sequences examined, these deposits can
be expected to grade rapidly into basin-plain sediments. Submarine fan - slope-apron
complexes comprise thick, sand-rich, laterally-continuous prisms. and consist of a range
of interbedded gravity deposits (sandy debris flows, slumps and slides, and turbidites).
Alternative depositional models for the sandstones in the "corridor area", such as a
single, large radial fan, or an east- west oriented turbidite system, have been considered.
However, the facies associations observed comprise well-developed channel/interchannel
units and thick, channelised "mid-fan" deposits. Associated outer-fan deposits are absent,
as are overall upwards-coarsening sequences wh ich are typical of radial submarine-fan
progradation (Walker, 1978; Stow, 1986). A turbidite system prograding from the west
along the "corridor" is considered to be unlikely. despite the occurrence of more distal
"mid-fan" type facies in wells in the east. Coarser-grained and more feldspathic sandstones
in Well 21/3a-4 in the east do not represent distal equivalents of the finer-grained. less
feldspathic sandstones observed in the west.
We therefore favour a fan - slope-apron complex sourced from a northerly direction •
for the Late Aptian sandstones in the "corridor" area (Figs. 5 and 8).The same depositional
model is proposed for the sandstones in the Graben Intersection Area. As in the Graben
Intersection Area, the basin-margins were ramped rather than composed of high-angle
fault scarps, and activity on the faults triggered slumping of sediment. The absence of
hanging-wall conglomerates and breccias indicates there was no major fault activity on
the margins of the "corridor". However, major lineaments such as the Wick Fault to the
north (see fig. 2 of Crittenden et al. , 1997) were expressed as fault scarps with which
hanging-wall conglomerates and breccias have been found to be associated. The southern
part of the Buchan Graben may be sand-prone as the result of sands derived from the
Peterhead Ridge to the south.
The Central Graben
We consider the Central Graben to be prospective for Late Aptian lowstand sandstones,
and although the area is not discussed in detail. a number of wells are now briefly
described.
The absence of the S I unit from the base of the Sola Formation in some wells suggests
that non-deposition, condensation and/or erosion has taken place. This is consistent with

17. 203S. Crittenden et al.
Facies Grain Sizel Bed Boundaries Structuresl Interpretation
Sorting Thickness Textures
1
CONGLOMERATE
Pebble 1·2m
Erosional bases!
sharpor
gradational tops
Matrix supported.
non-strati/jed.
disorgainsed
Deposited from highdensity
debris flows
2
BRECCIA
Pebble 1.5m +
Erosivebases!
sharptops
Clast supported,crsorqarused
Depositsof submarinetaun
scarp avalanches
3
UNGRADED
SANDSTONES
Poorly to well
sorted, nne
- grained
Several metres
Rare slightly
erosive bases,
sharptops
(3A) Structureiess
(3B) Bioturbated
(3C) Cross laminated/
convoluted
Deposits of high density
turbiditycurrents
4
GRADED
SANDSTONES
Very fineto
coarse. poorto
very well sorted
0.1 - 1.75m.
but
exceptionally
over 10m
Slightly erosive
basesI sharp
lOPS
Discont. planar am.,
dewateringI convoluted
structures. minorcross
laminae and bioturbation.
overallsequence
(4A) U. fining
(4B) U. coarse
(4C) No seq .
(40) Biolurb.
Depositedfromhigh - low
density turbidity currents
5
ARGILLACEOUS /
MICACEOUS
SANDSTONES
Very fine, well
sorted
15 - 50Cm
Frequent
convolutebases.
sharp tops
Low - angle lam.. rare cross
laminae I bioturbation
Deposited from low density
turbidity currents
6
THIN BEDDED
SANDSTONES
Siltstones; mainly
coarse silt.
Siltstones;
dew ems
Sandstone:
slightlyerosive
Siltstones;crsconnnuous
planar I lentic. lam.. rarely
bioturb.
Sandstonedepositedfrom
low densityturbidity currents
WITH
INTERBEDDED
SILTSTONES
Sandstones;fineI
very fine. very
well sorted
Sandstones
0.1 - o.am
bases I sharp
tops
Sandstones:bioturb.. small
rips., discont.parallel laminae
Siltstonesby combination of
suspension and turbidite
processes
7 Mud/siltstones; Mud/siltstones: Mudlsillslonesparafl. lam.. Mud/siltstones depositedby
MUDSTONES I clay to coarse silt. up to several Sandstone; lentic. lam., bioturb. combination 01 hemipelagicJ
SILTSTONES metres slightlyerosive turbo processes
INTERBEDDED Sandstone; very bases I sharp Sandstone; drscont, planar
WITH THIN fine grained,very Sandstone; < tops lam., conv. lam., rare ripsI Sandstone from low density
SANDSTONES weil sorted a.1m bioturbation turbidity currents
8
MUDSTONES I
SILTSTONES
Clay to coarse
silt, scattered
sand grainsin
places
Several metres
to low tens ot
metres
Typically sharp
Discontinuous and planar
laminae, bioturbatedin places
Muds/siltstones deposited by
a combinationof hemipelagic
I turbiditic processes
9 Very finely
GRADED
CARBONATES
crystalline
micritic, breccia
0.3 - a.6m Slightlyerosive Massive or bioturbated
Depositsof gravellydebris
flowsand turbidity currents
WITH BRECCIAS clasts<Scm
I 10
I GRADED
CARBONATES
Very finely
crystalline
Variable 0.1 ­
15m
Convolute I
slightly erosive
BioturbatedI slump planes
Deposited from turbidity
currents
11
UNGRADED
CARBONATES
Very finely
crystalline
0.15 - a.8m Sharp I convolute Stromatoporoids I brecciation
Platformmargincarbonate
build -up
12 Mud; micrite Mud; several
CARBONATE
MUDSTONES
WITH THIN Carbonate; very
10's cms
Carbonate;
Carbonate: sharp
/ convolute
Bioturbated/ slump planes
Muds depositedfrom
suspension, Carbonates
possibly penplattcrrn oozes
CARBONATE BEDS finecrystalline <O.2m
Fig. 9. Sedimentological facies identified from cored sequences of the Lower Cretaceous
strata of the UKCS.

18. 204 The distribution ofAptian sands tones, Central/ Northern North Sea : part 2
HIGHSTAND SYSTEMS TRACT - V61V5 UNITS
CAITHNESS HALIBUT HALIBUT HORST SOUTH HALIBUT
RIDGE PLATFORM BASIN
Shelfalareaof accumulation of sands, Very low rates of
. . - silts, carbonates & claystones; - ...~......__- - - sedimentation in
deposition concentrated in updip platform-slope-basin area
N portion of platform s
inactive fault
at Top Valtlall
Relative hi h sea-Ievel- VfiiV5 units
<c->:
-­-­...;:::--­
-=::­
--
Top Valhall. topography of the sea IIcor is very subdued. a time of tectonic quiesce nce and sloping margin s rather than fault scarps.
The faull controlled basins had become infilled as a consequence of the rising sea-level CUlminating in a maximum TST/HST in the Early Aptian.
Fig. 10. Schematic section across the Halibut Horst. Halibut Platform and Banff Sub-basin
during the Early Aptian transgression and accompanying deposition of the V6 unit.
the proposed Late Aptian sea-level fall, which elsewhere resulted in the deposition of
gravity-flow deposits as discussed above.
Aptian sandstones may be present in basinal areas in the west (e.g. 21/15) and east.
Well 23126a-2z to the east of the Forties High recorded thin sand stringers within the Sola
Formation. To the west of the Forties High, the Sola Formation is 68-m thick in Well 211
7-1, and overlies the V6/V5 units which are 16-m thick. In the Kittiwake field , the Sola
Formation in Well 21118-6 is 117-m thick and overlies the V6/V5 unit. In wells drilled
on the West Central Shelf adjacent to the Central Graben, Lower Cretaceous strata are
either very thin or absent. For example, in Well 2815-1 on the West Central Shelf, no
Lower Cretaceous sediments were recorded ; while in Well 2815a-2 , the Sola Formation
is thin, and overlies the V6 unit. However, Wells 29/23 -1. 29123h-2. 29/24-/ and 29/25-1
on the shelfal area flanking the graben recorded relatively thin Cromer Knoll Group
sediments, with a thin Sola Formation overlying the V6 unit. In wells on the flanks and •
uplifted shoulders of the Central Graben, Lower Cretaceous sediments are either absent
due to non- deposition or eros ion, or are very thin (rarely thick). Likewise, in wells in
the Gannet field area on the flanks of the Central Graben, Lower Cretaceous sediments
are either absent or are restricted to a thin veneer. However, in Well 2911b-l , the Sol a
Formation is 39-m thick and contains abundant reworked palynomorphs, although we
interpret it to be incomplete. The sedime nts may be a clay-rich mass-flow or slump
deposits associated with sandstones elsewhere.
The V6 and V5 units in the 280-m thick Valhall Formation are present in Well 2911h-1.
Well 2916a-J penetrated only 12.2m ofSola Formation above a truncated and incomplete
Valhall Formation. Wells 291/4h-1. 14h -2 and 14b-3 on the flanks of the Central Graben
encountered in the first case the Chalk Group lying unconformably upon the Kimmeridge
Clay Formation; and in the second and third cases, a thin Cromer Knoll Group section
(less than 15-m thick) sandwiched between the Chalk Group and the Kimmeridge Clay
Formation. In Well 29119-1a , Upper Cretaceous chalks overlie Triassic claystones, and
in Well 29/20-1, Upp er Cretaceous chalks overlie the Upper Jurassic Fulmar Sands.
In Well 29118-1, however, although the LowerCretaceous interval is relatively thin, a thin
V6 unit representing the Early Aptian transgression is present. Well 29119-2is only a few miles

19. 205S. Crittenden et al.
LOWSTAND SYSTEMS TRACT - SOLA FORMATION CLAYSTONES AND SANDSTONES
CAITHNESS HALIBUT HALIBUT HORST SOUTH
RIDGE PLATFORM HALIBUT
BASIN
Incision and erosion Incision and erosion of horst
on shell with sediment with sediment deposited in
E:S:SJ Valhall Fm. Vl-V4 IlI?1i!l Late Aptian Sandstones
_V6N5 units ~ Sola Claystones (Carrack Fm)
deposited in basin
1::::::::::::1 Valhall Fm Sandstones
basins to north and south
,•••••• Reworked V6N5 units
Lowstand sea-level s
Fig. 11. Schematic section across the Halibut Horst, Halibut Platform and Banff Sub­
basin during the Late Aptian regression, and accompanying deposition of the Sola
Formation claystones and sandstones.
to the NW of 29/19-1a, but here the Lower Cretaceous interval is relatively thick. The
Sola Formation includes a thin Late Aptian sandstone interval (7 -01thick), which truncates
the top of the Valhall Formation as the V6 and V5 units are missing.
In Well 29/4a-1a, a thick Lower Cretaceous interval is present including a thin Aptian
sandstone (12-01 thick); in Well 29/5a-1 the Sola Formation is 66-01 thick with a very
thin sandstone in the S I unit above the V6 and V5 units. Well 29/10-2 penetrated 118m
of Sola Formation, and the S I unit is sandy. Well 29/5a-3 nearby has a condensed and
incomplete Sola Formation above a thick V6 unit. In Well 29/8b-1, there is a sandstone
beneath the Redby Formation lying unconformably on the Triassic. In Well 29/8b-2, the
Sola Formation includes sandstone above the V6 unit. In Well 29/9a-1, the Sola Formation
is 94.5-01 thick and very sandy in the S I unit.
East of the Argyll area in the Central Graben, the Lower Cretaceous is quite thick in
Norwegian waters, as exemplified by Well 2/1 1-1. Well (UK) 31/27-1, on the western
flanks of the graben, penetrated a relatively thin interval of Lower Cretaceous sediments
lying on Triassic sediments. Chalk of Early Santonian to ?Cenomanian age lies
unconformably upon Late to Middle Albian, Cromer Knoll Group, Redby Formation
claystones and sandstones. These in turn lie above a thin succession of Albian - Aptian
sandstones, which in turn lie above a sequence of calcareous and sandy claystones
assigned to the Valhall Formation (Early Aptian and older).
Viking Graben
Little information is available on the prospectivity of the Lower Cretaceous interval
in the Viking Graben. However, hanging-wall and lowstand sandstones of Late Aptian
age present a good exploration target here. Sandstones ofAptian-to-Albian and Cenomanian
ages are present in the Agat field (Gulbrandsen, 1987; Shanmugam et al., 1994) on the
eastern flank of the graben in Norwegian waters.
In the South and Central Viking Graben, Boote and Gustav (1987) and Bisewski
(1990) interpreted a Late Aptian sandy fan to be present in blocks 16/12,16/13 and 16/
17, adjacent to the boundary-fault complex, and suggested that it was sourced from the

20. 206 The distribution ofAptian sandstones. Central/ Northern North Sea: part 2
~."posilion
oll~1iM
Fig. 12. Schematic "bird's eye view" of the East Shetland Basin during the Late Aptian.
Clastic sediments are being reworked from shallow-water areas on the East Shetland
Platform into the adjacent basin.
Fladen Ground Spur. For example. Well 16117-Ra penetrated a moderate thickness ofoil­
•bearing sandstones - a secondary reservoir at the Tiffany field (Kerlogue et al., 1994);
Well 16113a-2z penetrated 57111 of Sola Formation complete with sandstone stringers
(representing the feather-edge ofa fan system). In addition, regressive sandstone horizons
are present in the Late Albian to Early Cenomanian interval of Well 16113a- 2;;., similar
to the Agat field.
In Well 16117-/3 (Tiffany field), the thin Sola Formation lies above a 26.5-m thick
sandstone (known as the Shirley Sand on the composite log), which in turn lies above
the V6 unit. In Well 16123-1 within the graben, the Sola Formation is only 3.3-m thick
without sands, and overlies a thin Valhall Formation capped by a very thin V6/V5 unit.
In the North Brae area, the Lower Cretaceous interval is thin over the Jurassic structure
(wells 16I7a-/7b and 7a-19), but can be seen on seismic profiles to thicken in block 16/2
adjacent to the Fladen Ground Spur boundary-fault complex. The area is therefore
prospective, in that Lower Cretaceous sandstones derived from the Fladen Ground Spur
during the Late Aptian lowstand may be present. Well 9124b-2 penetrated a sandstone
within the Sola Formation, as does Well 16112a-4 further south.
_ _ _ _ _ _ _ _ _J

21. 207S. Crittenden et al.
EAST/NORTH
SHETLAND PLATFORM
N
t
retaceous
Fig. 13. The distribution of Aptian sandstones (shaded) in the Magnus Trough.
•
East Shetland Basin
This intracratonic basin (located on Fig. I) developed in an extensional regime resulting
from Mesozoic rifting and plate separation between Greenland and Norway. Both regional
and local tectonic intluences, together with eustatic(?) sea-level fluctuations, controlled
pattems ofsedimentation during the Early Cretaceous in this area.Extension and subsidence
in the Middle and Late Jurassic resulted in the alignment of horsts and half-grabens in
the East Shetland Basin, and the formation of numerous fault-blocks which were rotated
and tilted to the west. The eastern margin of the basin comprises a ridge of tilted fault
blocks. which runs in a broad curve north from Alwyn SE (block 3/15), through Alwyn
. North (block 3/9) and block 3/4, to the Brent and Statfjord fields. A spur branches-off
north of block 3/9 to the Gullfaks field . Approximately parallel to this trend and situated
about 25 km to the west, a second ridge of tilted fault blocks runs through theAlwyn South
(block 3/14). Ninian and Hutton, Dunlin and Murchison fields.
The majority of wells in the East Shetland Basin have been drilled on the crests of tilted
Triassic and Jurassic fault blocks. so that stratigraphic information for the Lower
Cretaceous is very sparse. In most wells, a basal limestone and calcareous claystone
interval (Mime Formation equivalent of the Norwegian sector) is present. On highs, this
is equivalent to most of the Valhall Formation; in basinal locations (which have been
penetrated by relatively few wells). it is only representative of the basal part of the
Valhall Formation (V I unit), the remainder being a relatively thick siliciclastic interval.
The onset of regional thermal subsidence in the latest Ryazanian to Early Aptian is
marked by a condensed interval in the East Shetland Basin, where the Val hall Formation
is carbonate-rich and encompasses many hiatuses and unconformities. A major regression
b

22. 208 The distribution ofAptian sandstones, Central/ Northern North Sea: part 2
occurred before deposition of the transgressive Late Aptian to Albian sediments, which
on lap the locally-eroded Jurassic-earliest Cretaceous topography.
The inherited, Late Jurassic north-south structural grain and renewed tectonic activity
had profound effects on patterns oferosion, sediment sourcing and sedimentation during
the Early Cretaceous. If sands were shed from platform areas and fault blocks during
relative sea-level lowstands and times of tectonic uplift, they may be expected to occur
in the adjacent lows. For example, sands occur within some sub-basins in the East
Shetland Basin; the low adjacent to the East Shetland Platform boundary- fault system;
and in the North Viking Graben itself. We suggest that if sandstones are present, they will
have been deposited by gravity-flow processes. Coarser clastics (conglomerates and
breccias) will be associated with footwall erosion.
A "birds eye" view of the East Shetland Basin during the Middle Aptian is presented
in Fig. 12. and shows our speculative interpretation of the palaeotopography at this time.
We postulate that during the Late Aptian, sediments were transported across and around
the East Shetland Basin area, and were deposited into the Viking Graben. Some of these
sediments are coarse siliciclastics. Sediments may also have been transported into the . .
Viking Graben from the east, across the shelf in the Norwegian sector. 'W{
The Magnus Trough (Fig. 13)
Tectonic activity, associated with the fault zones bounding the Magnus Trough and
related to proto-North Atlantic rifting, resulted in erosion of the NW flank of the trough,
and also of the upthrown fault block forming the SE margin (just NW of the Magnus
field). A considerable thickness of Lower Cretaceous sediments has been deposited in
the southerly-dipping half graben. These sediments are interpreted as mass-flow and
turbidite deposits, laid down in a fringe of coalescing fans. These were supplied from the
south (from the fault scarp), from the East Shetland Platform to the SW, and from the
Margareta Spur to the north and NW.
Wells on the NW flank of the trough (210/4-1 and 5-/) did not encounter Lower
Cretaceous sediments; in the former well, Upper Cretaceous mudstones of the Shetland
Group lie unconformably on Permo-Triassic redbeds; and in the latter well. they lie on
a thin Kimmeridge Clay Formation. Wells in the trough, however, penetrated Lower
Cretaceous sandstones; in Well 210/13-1, the thin Lower Cretaceous interval includes
water-wet sandstones of Barremian?-to-Aptian age. The Lower Cretaceous sediments
lie, according to the OTI composite log, unconformably upon Permo- Triassic redbeds.
In contrast, Well 210/15a-4 penetrated thick Lower Cretaceous sediments, including
Albian-?Bm-remian sandstones (in part therefore equivalent to the Sola Formation sandstone
member). and Hauterivian to Valanginian sandstones (Scapa Sand Member equivalent). . .
We interpret the upper sandstone interval probably to be equivalent to the sandstone
interval in 210/13-1. The base of this sandstone is here interpreted to rest on the mid­
Aptian unconformity, the ?Barremian age being interpreted from reworked fossils.There
is no well information for block 211/6 in the southern part of the embayment. although
it is prospective for Lower Cretaceous sandstones.
The thick sedimentary pile deposited during the Early Cretaceous in the Magnus
Trough is interpreted to be a base-of-slope fan which developed on the downthrown side
of the End of the World Fault Zone (Fig. 13). This is very similar to the LowerCretaceous
sequences recorded in (i) the Faeroe Trough (authors' observations); (ii) the Haltenbank
area off Mid-Norway (Hastings, 1987; Shanmugam, 1994; authors' observations)
(iii) the Moere Basin; (iv) and theAgat area in the Norwegian sector of the North Viking
Graben (Gulbrandsen. 1987; authors' observations). We have studied these areas extensively
in terms of their Lower Cretaceous prospectivity, but the results are outside the scope of
this paper. However, to place this study in a more regional perspective, Fig. 14 illustrates
our proposed stratigraphical model for the Lower and Middle Cretaceous in the West of

23. 209S. Crittenden et al.
WESTSHETLAHO BASINAONARtOGEFAE1K)E·8HE11.AHD
TROUGH
Fig. 14. Summary chart showing the Lower to Middle Cretaceous tectono-stratigraphy of
the West of Shetland - Faeroe Trough areas.
Shetland exploration area. Here, extensive lowstand sandstones are associated with
major lineaments including transform faults (e.g. the Judd Fault) which were active
during the Cretaceous opening of the NE Atlantic Ocean.
CONCLUSIONS
From this study of the Lower Cretaceous interval in the Central and Northern North
Sea, we conclude that deposition of the Sola Formation sandstones resulted from a
regional Late Aptian regression. These sandstones were deposited by mass-flow
processes, and represent a lowstand systems tract. In complete sections, they lie above
the highstand systems tract of the Early Aptian V6 unit of the Valhall Formation.
Recognition of these lowstand deposits, which are of reservoir potential, requires
careful mapping of the top of the Valhall Formation and of the areal extent of the Sola
Formation. It is particularly important to identify areas where the Valhall Formation has
been truncated. On seismic profiles, erosional incisions in the top ofthe Valhall Formation
and mounding of the overlying Sola Formation may indicate the presence of sand-rich
sediments.
The Late Aptian relative sea-level fall was associated with thermal subsidence, together
• with the opening of the proto-North Atlantic which may have caused reactivation of
basin-margin faults. These movements, together with the relative fall in sea-level, account
for the erosion and transport ofclastic material across shelfal areas, and for the reworking
of unconsolidated and unstable shelf/slope sediments by gravity-driven processes into
basin depocentres. This process was discussed by Shanmugam et al. (1994). In addition
to unconsolidated Aptian sediments, older Cretaceous and Jurassic sediments may also
have been reworked by slumping from adjacent highs. This is illustrated by Well 64071
5-1 off Mid-Norway, where a slump ofVolgian Kimmeridge Clay Formation equivalent
sediments (Spekk Formation) is found above Early Aptian sediments and below Late
Aptian sediments.
The occurrence of Late Aptian lowstand fans (composed of coarse-grained sands and
conglomerates) within hanging-wall half-grabens is restricted to major faults which were
active throughout the Early Cretaceous. For example, wells on the Halibut Shelf, located
in the hanging wall adjacent to the Wick Fault, penetrate very coarse sandstones and

24. r
210 The distribution ofAptian sandstones. Central/ Northern North Sea: part 2
conglomerates, as do wells in the hanging wall of the faulted NW margin of the Magnus
Nose. It is interesting to note that wells on the faulted margins of the Faeroe Trough and
the West Shetland Basin also penetrated very coarse sandstones and conglomerates
within the Aptian interval. Coarse sandstones are also recorded in the Agat area, which
is adjacent to the basin-margin fault complex.
We suggest that this methodology (i.e. integrating a consistently-interpreted well
data-base and a sequence- stratigraphic model with seismic sections) could contribute
to the search for other lowstand sandstones ofCretaceous age in the NE Atlantic margin.
Our studies of the NW European Continental Shelf indicate that lowstand sandstones
are present within a number of stratigraphic intervals: Valanginian and Hauterivian
(Scapa Sandstone equivalent); Aptian, Albian and Cenomanian (which, in Norway, are
collectively assigned to the Agat Formation); Cenomanian and Turonian (intra-Lange
Formation in Norway); latest Turonian to Coniacian (Lysing Formation in Norway); and
Santonian and Campanian (Nise Formation ofNorway). All these sandstones are associated
with major sea-level fluctuations. All are of reservoir potential, as evidenced by the oil­
and gasfields already discovered and by the numerous shows of these ages encountered
in exploration wells. Deep-water drilling in the NE Atlantic margin has confirmed the
significance ofthick Cretaceous (particularly Upper Cretaceous) strata as a target interval
which is within the reach of the drill.
ACKNOWLEDGEMENTS
The authors acknowledge discussions with numerous oil-company geologists engaged
in the search for hydrocarbons in the North Sea. In particular, S.c. acknowledges discussions
with Professor Malcolm B. Hart and Dr Ian Tunbridge (Plymouth University); Pedro
Barbeito, Wolfgang Witt, Kristos Kapellos and Richard Ribis (allformerly ofShe II UK);
Tony Dore (Statoil UK); and Steve Pinnock (Texaco UK) . He also thanks Brit E. Sauar,
Finn Livbjerg and John Gjelberg tNorsk Hydro, Norway) for extensive and enthusiastic
discussion on Cretaceous exploration in NW Europe. Finally, the authors acknowledge
Professor K. W. Glennie (Aberdeen University & Editorial Board) for constructive and
critical Journal reviews and for useful suggestions which have improved the paper.
Additional drafting was by Graffixx Consultancy (Henley-on-Thumes).
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