Analysing Overpressure Distributions within Palaeogene Strata of the Central North Sea Jenna Robertson Department of Earth Science, University of Durham
Analysing Overpressure
Distributions within Palaeogene
Strata of the Central North Sea
Jenna Robertson
Department of Earth Science, University of Durham
Aims
� Use available pressure data from wells to map regional
variations in overpressure
� Determine directions of fluid flow and possible
connectivity between Palaeogene reservoirs
� Locate areas with the potential for hydrodynamic
trapping
Study Area59°
58°
57°
56°
55°
0° 5°
0° 5°
59°
58°
57°
56°
55°
OUTER MORAY
FIRTH
CENTRAL
GRABEN
Total of 331 wells
� UK (308)
� Norway (19)
� Denmark (4)
Data Availability
UK 15/29a- 9
v
v
vv
vvv v vv
Methodology
Forties Sandstone Member
Balmoral Sandstone Unit
Andrew Sandstone Unit
Constant Hydrostatic
gradient of 0.445 psi/ft
16 psi Overpressure
UK 15/29a- 9
v
v
vv
vvv v vv
Methodology
Forties Sandstone Member
Balmoral Sandstone Unit
Andrew Sandstone Unit
GWC
Hydrocarbon down to points
� Reservoirs containing hydrocarbons only
� Provide estimate of the maximum
overpressure in aquifer below
32 psi Overpressure
19 psi
Methodology
� Total 482 overpressure
points
� Each value assigned to
the corresponding
reservoir unit according
to the lithostratigraphic
nomenclature of Knox
and Holloway (1992)
Some overpressure values were
collected from within lower
permeability shale units
MO
NT
RO
SE
MO
RA
YS
TR
ON
SA
YN
OR
DLA
ND
Palaeocene
Eocene
Oligocene
Miocene
Pliocene
Quaternary
LARK FM
HORDA FM
BALDER FM
SELE FM
LISTA
MAUREEN FM
NORDLAND GP
FMv v v v v
Mey
Forties
Tay
Caran
Brodie
MO
US
A F
M
DO
RN
OC
H F
M
Sele
sands
Reservoir Distribution� Reservoir distribution was based on evaluation of wireline log data
from 664 Central North Sea wells – calibrated where available with
seismic data and biostratigraphic data
1°
1° 2°0°
57°57°
0° 1°
1° 2°0°
57°57°
0°
Tay Sand Isochron (TWT)
25 225122
� Additionally sources such as the Ternan Ltd Reports, Millennium
Atlas and various other published literature were also used
Reservoir Distribution
Mey Sandstone
� Laterally draining NW
towards the Moray Firth
� Max values of
overpressure ~ 2500 psi
recorded in Quad 30/8
� Steep pressure ramp
with implications for
hydrodynamics – Flyndre
field
Overpressure (psi)
-100
0
500
1000
1500
2000
2500
111 overpressure
points
Forties Sandstone
145 overpressure
points
� Max value of overpressure
recorded is ~1300 psi in
UK 30/8
� Magnitude of
overpressure not as great
as the Mey Sandstone
� Main flow direction SE
to NW, with drainage
also occurring towards
the western margin
Overpressure (psi)
-100
0
500
1000
1500
2000
2500
Forties Sandstone
145 overpressure
points
� Max value of overpressure
recorded is ~1300 psi in
UK 30/8
� Main flow direction SE
to NW, with drainage
also occurring towards
the western margin
(Dennis et al. 2000)
� Magnitude of
overpressure not as great
as the Mey Sandstone
Upper Sele Sands
Overpressure (psi)
0
50
100
150
200
250
300
33 overpressure
points
� Maximum of 336 psi
� Minimum of 29 psi
Tay Sandstone
52 overpressure
points
Overpressure (psi)
0
50
100
150
200
250
300
UK 21/29a- 843 psi OP
in Tay
� Maximum of 72 psi
� Minimum of 10 psi
TAY
~ 3250 ft (TVSS)
Residuals
POSITIVE RESIDUAL
� Overpressure B > Overpressure A
� +ve overpressure difference
� Pressure Transition Zone
� Red colour on maps
� Residual values of overpressure are the difference in
overpressure between reservoir units
� Residuals were calculated between each adjacent reservoir
unit to help identify areas of vertical connectivity
� Calculated using the deeper reservoir
overpressure minus the shallower
reservoir overpressure B – A = +ve
Residuals
UK 21/29a- 8NEGATIVE RESIDUAL
ZERO RESIDUAL
� Overpressure A > Overpressure B
� -ve overpressure difference
� Pressure Reversal
� Blue colour on maps
� Overpressure A = Overpressure B
� Zero overpressure difference (+/- 10psi)
� Reservoir units have the same overpressure,
they are in pressure connection
� Neutral colour on maps
B – A =
zero
B – A = -
ve
Mey-Forties Residual
Residual (psi)
+1900
- 400
0
Forties-Upper Sele Residual
Residuals range
from -40 to +130 psi
Upper Sele-Tay Residual
Residuals range
from -20 to +20
Conclusions� The Mey Sandstone Member and Forties Sandstone Members are
laterally draining SE – NW with additional drainage in the Forties
to the western margin
� Although only weakly overpressured the Upper Sele and Tay
sandstone units also appear to be laterally draining towards the
western margin
� Residual values indicate that there is overall good connectivity
between sands towards the western margin (Quad 21), which
may be a result of faulting or injectites
� Further work on structure is needed to support interpretations
Any Questions?
Special thanks to sponsors Maersk Oil Aberdeen and GeoPressure
Technology Ltd, Ternan Ltd for supplying Palaeogene play maps
and IHS for supplying pressure data