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Controls on Lower Carboniferous (Dinantian) prospectivity in the
Mid North Sea High region
Matthew Booth1, John Underhill1, Rachel Jamieson1, Rachel Brackenridge1.
(1) Heriot-Watt University, Edinburgh, UK.
The discovery of, and subsequent production from, the Breagh gas field in Quadrant 42 of the
Southern North Sea (Figure 1) challenges long-held views concerning the limited potential
prospectivity of the Mid North Sea High region. The location of the Breagh field approximately 30
km NW of the major axis of Carboniferous gas fields in the Southern North Sea (SNS) marks it as an
outlier. Furthermore, the gas is located in Lower Carboniferous reservoirs as opposed to the Upper
Carboniferous or Permian reservoirs typically associated with SNS gas fields. However, the
occurrence of the field attests to a working petroleum system having been active in the area,
something that lends encouragement to further exploration efforts in the basin.
We have integrated seismic interpretation of two proprietary 3D pre-stack time migrated (PSTM)
volumes (one of which covers the Breagh field), a regional grid of 2D data acquired as part of the Oil
and Gas Authority (OGA) Frontier Basins research program, well log, core description and field-data
from coastal exposures in Northumberland. This multi-method approach has been used to identify and
interpret the structural and depositional history of the Breagh region and to help determine the
primary controls on prospectivity.
The Breagh field was discovered in 1997 by well 42/13-2 which was drilled by Mobil to a total depth
of 2672.8 m (TVDSS), after passing through approximately 400 m of Lower Carboniferous sediments
and a gas column that is over 120 m thick. The well is the deepest in the study area and reached total
depth within the Scremerston Formation, part of the Farne Group (Figure 2). The seismic data
indicates that the Carboniferous sediments continue below well penetration depth and are probably
underlain by sediments of Devonian age; including the Kyle Limestone which forms a prominent
marker horizon in some areas. The Lower Carboniferous (Dinantian) sediments of the Farne Group
(including the Scremerston and Yoredale Formations) accumulated in fluvio-deltaic to shallow-
marine depositional environments that were variably influenced by eustatic sea level change,
extensional tectonics and delta abandonment processes (Maynard & Dunay 1999; Collinson 2005).
The sandstone reservoirs of the Breagh field were originally thought to be part of the Scremerston
Formation. However, proprietary biostratigraphic analysis and lithological limestone correlation have
shown that the sandstone reservoirs are in fact part of the stratigraphically younger Yoredale
Formation. The sediments were sourced from northerly quartz-rich Caledonian terranes and
transported southwestwards initially depositing into localised fault controlled depocentres that were
juxtaposed with buoyant granite-cored highs. Later the faults ceased moving leading to regional
subsidence and stratigraphic linkage of the basins (Johnson 1984; Fraser & Gawthorpe 1990). The
extensional faults affecting the Carboniferous and older strata are defined by relatively planar
geometries (Figure 3 & 4) and are typically orientated NW-SE; these orientations are likely controlled
by underlying Caledonian lineaments (Leeder 1982; Fraser & Gawthorpe 2003). The faults rarely
affect strata above the Base Permian Unconformity (BPU). It is likely that sedimentation continued
throughout the remainder of the Carboniferous (e.g. Namurian and Westphalian) in a similar manner
to adjacent offshore and onshore areas. However, uplift, deformation and sub-aerial exposure
attributed to the Variscan orogeny eroded most of these later Carboniferous sediments from the
Breagh area leaving the Dinantian Yoredale Formation sub-cropping against Upper Permian strata.
The Variscan orogeny caused inversion on a regional scale with extensive and well documented
evidence present onshore and offshore Britain (Corfield et al. 1996; Glennie & Underhill 1998).
Figure 1 – Map showing the offshore geology at sea bed to the northeast of Flamborough Head together with the structures
recognised at sea bed all redrafted from BGS data. The Breagh gas field and exploration wells in the study region together
with selected additional gas fields are also shown. Inset map shows quads and outline of UK.
Figure 2 – Chronostratigraphic and lithostratigraphic summary diagram for the Breagh area. Typical facies and prominent
seismic horizons are also indicated.
Figure 3 – Geo-seismic section orientated west-east located to the south of the Breagh field showing some of the important features discussed in the text. 3D seismic available to the south
of Breagh is of much higher quality thanks in part to the absence of chalk in that area (Figure 1). The seismic is displayed in two way travel time with the time in milliseconds indicated on
Figure 4 – Geo-seismic section orientated WNW-ESE through the Breagh field. The 3D seismic data over the Breagh field is of
limited quality due to the presence of variably thick Cretaceous chalks and Zechstein evaporites making interpretation and
subsequent depth conversion difficult, but all the more important. The section shows a syn-sedimentary graben structure infilled
with Mesozoic sediments and including thin Bacton Group deposits. Note the salt mobilisation within the Zechstein Group and
the postulated salt wing identified from well data but not easily distinguished on seismic. The seismic is displayed in two way
travel time with the time in milliseconds indicated on the left.
Further south, in a line roughly following the boundary between the two seismic datasets (Figure 1),
Permian Rotliegend Silverpit Formation sediments can be observed thickening ~southwards whereas
over the Breagh area no Rotliegend material has been encountered and Zechstein evaporites directly
overlie the angular unconformity (Figures 3 & 4). A complete Zechstein sequence (i.e. Z1-Z4) is
present in some wells and typically comprises halite, polyhalite and anhydrite although prominent
Plattendolomit rafts are easily distinguished in the seismic data. Lower Triassic sediments
conformably overlie the Permian and are represented mainly by shales and later sandstones of the
Bacton Group which display a uniform thickness across much of the study area and records deposition
in effectively a ‘pre-rift’ phase of sedimentation. Shales of the Haisborough Group were deposited
during the Upper Triassic with thin shale and interbedded anhydrite belonging to the Rot Halite
Formation at the base. The conformable Lower Jurassic sediments comprise shales of the Lias Group
with Middle Jurassic mixed sandstone and shale deposits belonging to the West Sole Group. The
Jurassic sediments vary in thickness in the wells and often display prominent wedge-shaped
geometries thickening into fault zones indicative of syn-sedimentary extension (Figure 3) and are
characterised by slow velocities in the seismic data.
The extension is characterised by basin-scale listric faults that terminate within lithologically weak
rocks, mainly the Zechstein evaporites but also the Triassic Rot Halite. The extension probably began
in the Middle to Upper Triassic with faults oriented NNW-SSE and NE-SW that display a curvilinear
shape in map view. This Mesozoic extensional episode is linked with extensive halokinesis of the
Zechstein evaporites resulting in thin to absent Zechstein deposits within the graben areas and
occasional grounding of the overlying Triassic sediments directly against the Carboniferous rocks.
Furthermore, the halokinesis and listric extenstion led to the formation of turtle-back structures, salt
diapirs and salt wings in some inter-graben areas, the full appreciation of which governs depth
conversion and accurate mapping at the reservoir level. Examples of these features are shown in the
geo-seismic sections in figures 3 and 4. These graben structures and thick wedge-shaped Mesozoic
sedimentary deposits and the associated halokinesis represent a ‘syn-rift’ phase of sedimentation.
Uplift and erosion in the Upper Jurassic to Lower Cretaceous has planed off most of the Middle and
Upper Jurassic sediments over Breagh. In the south of the study area in wells 42/18-1, 42/18-2 and
42/23-1 (see figure 1 for well locations) sediments belonging to the Upper Jurassic Humber Group are
encountered at seabed (Figure 3). It is likely that these sediments were also deposited over the Breagh
area but have subsequently been eroded. However, deformation was relatively minor such that the
unconformity is relatively shallow-angle in nature and often difficult to confidently interpret in the
seismic due to the lithological similarity of the deposits in the overlying Cretaceous Cromer Knoll and
underlying Jurassic Lias Groups. The Jurassic sediments are unconformably overlain by interbedded
sandstones and shales followed by clays belonging to the Cromer Knoll Group that may also have
been deposited during syn-sedimentary extension indicating reactivation of the Mesozoic listric fault
zones. The Cretaceous Chalk Group occurs gradationally above this and is present over the north of
the study area but subcrops to seabed where it is erosionally truncated and therefore not present in the
southern part of the study area. The chalks drape over the Mesozoic faults indicated a ‘post-rift’ phase
of deposition. No rocks younger than the Cretaceous chalks were encountered in the study area.
The results of this work show that the Breagh field lies within a partially fault-controlled 4 way dip
closed structure in fluvio-deltaic reservoirs of Lower Carboniferous (Dinantian) age at Base Permian
Unconformity (BPU) level. The structure formed in response to Palaeozoic folding to create a closure
of erosionally truncated (subcropping) and highly faulted (compartmentalised) reservoirs which are
then sealed by Upper Permian (Zechstein Group) evaporites. The absence of the Upper Carboniferous
(Coal Measures Group) across the area implies that gas charge comes from Lower Carboniferous
coals (and potentially also lacustrine, lagoonal and marine mudstones) within the Yoredale Formation
as well as those belonging to the stratigraphically older Scremerston Formation. Alternatively, the gas
may have migrated from the traditional Westphalian source area to the SE via fill and spill of
intermediary traps. Regional interpretations show that an Early Cenozoic (Paleogene) tilt was also
imparted and is attributed to the Atlantic opening and associated Icelandic plume emplacement. This
had an important effect since it led to the Breagh field lying on a westerly (re-)migration pathway.
Maturation may have begun prior to the Variscan uplift but probably climaxed during the Cretaceous
prior to this Paleogene regional tilting. Additionally, the absence of the Rotliegend Silverpit
Formation is likely a critical factor in the success of this field as the gas could accumulate in the high
reservoir quality fluvial sandstone deposits of the Yoredale Formation rather than the poor reservoir
quality claystone and siltstone deposits of the Silverpit Formation. Understanding the detailed
depositional history of the Carboniferous rocks together with the structural evolution of the entire
basin is essential for future exploration success around Breagh and further north on to the Mid North
Keywords: Carboniferous, Dinantian, tectonic, basin evolution, Breagh gas field.
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