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