Overpressure G

eneration and Vertical Transfer in the Sarqala-1 W

ildcat: Kurdistan R

egion of Iraq1*

11

William

Matthew

s BS

c, K

e Lovan BS

c, G

eorge Pinckney B

Sc P

hD1

* W

esternZagros Resources Lim

ited, Calgary, A

lberta, Canada; C

orresponding author

Abstract

Overpressure is a significant drilling hazard in m

uch of the Zagros Fold Belt. R

apid sedimentation throughout the P

lio-P

leistocene combined w

ith tectonic compression during form

ation of the fold belt led to the development of m

oderate to severe overpressure beneath the regional M

iocene Fars evaporite seal. Mud w

eights in excess of 17PP

G have been

required to successfully drill discoveries where overpressure has been encountered. The S

arqala-1 wildcat encountered

severe overpressure in younger stratigraphy than ever before seen in N. Iraq. O

verpressure was unexpectedly

encountered in the clastic alluvial Upper Fars (Injana) form

ation. Pre-drill basin m

odeling indicated a very high probability of encountering norm

al pore pressures. Wireline log data indicates the shales in the Fars clastics are norm

ally to slightly overpressured suggesting overpressure w

as vertically transferred through hydro-fracturing of the Fars evaporite seal.

Discussion

Two hydrocarbon-bearing, overpressured sands w

ere encountered in the Upper Fars form

ation. The uppermost sand is

moderately overpressured (~1450kg/m

3) and is nearly in pressure equilibrium w

ith the surrounding mudstones

(~1400kg/m3). The deeper sand is highly overpressured (2020kg/m

3) and pore pressure greatly exceeds that of the surrounding m

udstones (~1300kg/m3). Light, high-G

OR

oil recovered after penetrating these units is geochemically

similar to oils encountered deeper in the w

ell beneath the Lower Fars evaporite seal. The sim

ilarity of the oils in the sands to the deeper, sub Fars reservoirs and the pressure disequilibrium

with the encasing m

udstones suggest that overpressure in these sands is the result of vertical transfer of highly pressured fluids from

a deeper stratigraphic level. P

ore pressures at Sarqala-1 approach the fracture pressure beneath the Low

er Fars evaporite seal and may have, in the

past, exceeded it’s mechanical seal capacity. If so, hydraulically induced fractures could have allow

ed upward m

igration through the seal and into the overlying clastic section charging perm

eable beds with hydrocarbons. S

ubsequent to overpressuring the pore pressure in the shallow

sand re-equilibrated to the surrounding mudstones, likely through lateral

connection to surface at the Shakal Thrust. The deeper sand did not equilibrate due to either lim

ited permeability or a lack

of connection to surface. This analysis suggests that LWD

data would facilitate real-tim

e pore pressure tracking and may

alleviate wellbore stability issues caused by insufficient m

ud weight. N

o pre-drill prediction would detect anom

alous, vertically transferred, pore pressures due to its lim

ited effect on the surrounding mudstones.

Acknow

ledgements

The authors would like to thank M

ark Becker and D

avid Reeve for their contribution of engineering data and discussion.

Naw

zad Aziz provided valuable assistance in the field aspects of this project. W

e would like to thank W

esternZagros m

anagement, Talism

an Energy and the K

urdistan Regional G

overnment, M

inistry of Natural R

esources for supporting this w

ork.

3G

radient (kg/m)

Depth (mMD)

Pore P

ressure Gradient vs. D

epthS

arqala-1 Wildcat

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Upper BakhtiariPliocene

Lower BakhtiariMessinian - Pliocene

Lower FarsLanghian

Upper FarsSerravallian

Lower Fars

Saliferous B

eds

0

500

1000

1500

2000

2500

3000

350010001200

14001600

18002000

22002400

2600

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Shakal Thrust

tmQ

uickbird im

age of the Lower Fars to U

pper Bakhtiari section exposed at the S

hakal thrust.Yellow lines represent surface

trace of sandstones in the Upper Fars and Low

er Bakhtiari. S

urface cut is approximately a strike section w

ith flow to the S

W.

Channels have w

idth to thickness aspect ratios of 100:1 to 500:1.

3m thick sandstone of the U

pper Fars Fm. S

harp, irregular base. Fines upw

ard from M

g to Fg sand. Underlying silty-sandy shales

show early stages of pedogenesis

A

Lateral pinchout of a 2m thick sandstone w

ith trough cross bedding and sharp base. Thinning of the sandstone is abrupt. Lateral tim

e equivalence of the siltstones is evident.

B

Repetitive sequence of sandstones, sandy shales and siltstones.

C

Time (ms)

Time (ms)

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Resistivity

Resistivity (Ώ

m)

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Sonic Interval

Transit Time

ITT (µs/ft)

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VS

P IntervalVelocity

Velocity (m/s)

Depth (mMD)

Overpressure

BAC

Up . B akht.

Low.F ars

4m

Sh

Ss

FgM

gC

gC

ong.

Ripple Lam

inated

Trough Cross B

edding

Tabular Cross B

edding

Massive

Planar and R

ipple Laminated

Typical Upper Fars S

andstone

0

500

1000

1500

2000

2500

3000

3500

4000

4500

500010001200

14001600

18002000

22002400

2600

Depth (mMD)

3G

radient (kg/m)

BakhtiariUpper FarsLowerFars

Pre-FarsCarbonates

Pre-D

rill Pore P

ressure Prediction

Lower BakhtiariUpper Fars

Lower Fars

Saliferous Beds

Transition Beds

Jeribe

Shiranish

Qam

chuqa

Gotnia

SQ

-1

View to N

W

View to N

W

View to S

E

Shakal Thrust

A’

A

Sarqala-1

A

A’

K44

2km

N

Covered

Covered

Covered

Pore pressures calculated using the E

aton method. S

onic and resistivity derived pore pressures show

good agreement. P

ore pressure derived from

VS

P interval velocity lower than log based analyses due to averaging of

non-mudstone lithologies in the interval average increasing the velocity.

Overall character betw

een VS

P and log based methodologies very sim

ilar. Fracture gradient calculated using a M

atthews and K

elly approach with K

calibrated using loss events.

1967m K

ickG

ood agreement betw

een log derivedm

udstone pore pressure and measured

pore pressure in the sand

2307m K

ickP

ore pressure measured in the sand

considerably higher than adjacentshale interval

Loss Event

Major loss event caused by

high density drilling fluid

Iraq

Kurdistan Region

Regional P

ore Pressure D

atabaseLow

er Fars Isopach and Geology of Iraq

Pressure D

ata P

etroleum G

eological Services A

nalysis Ltd., Quantock G

eological Services, S

akerG

eological Services and U

niversity of Reading, 2006. P

etroleum G

eology and Exploration P

otential of Iraq. Multi-client report.

Lower Fars Isopach

Dunnington, H

.V., 1958. Generation, M

igration, Accum

ulation and Dissipation

of Oil in N

orthern Iraq, in Weeks, L.G

., (Ed), 1958 Habitat of O

il, 1194-1251

Geology

Jassim, S

.A., H

agopian, D.H

. and Al-Hashim

i, H.A

.J., 1990. Geological M

ap of Iraq, State Establishm

ent of Geological S

urvey and Mineral Investigation, B

aghdad

2125m

2125m

1875m

1625m 1375m

1125m875m

625m

5km

Lower B akhtiari

Upper F ars

The overall character of the Upper Fars is of a low

net to gross section of sands and m

udstones. Sands fine upw

ard abruptly w

ith tabular and trough cross bedding at the base and ripple lam

ination at the top. The basal contact is sharp and

erosive w

ith som

e decim

eter scale

irregularity. O

ccasionally dessication

cracks in

the underlying

mudstone are filled w

ith the overlying sands. Aspect ratios

are very high (100-500:1) and sands pass laterally into thin fine

grained sandstone

and silt

sequences. Flow

directions are consistent to the S

W but vary from

W to S

. The m

udstones are red to purple and show incipient

pedogenic structures but no well developed soil or root

horizons. Bedding is poor to absent.

The environment of deposition is likely an aggradational

alluvial plain dominated by braided river deposits and

associated floodplain muds. R

ainfall was seasonal w

ith flash floods dom

inating the depositional history. Rivers

were relatively unconfined due to a lack of vegetation to

stabilize the floodplain. The environment is consistent w

ith the present clim

ate of Northern Iraq.

The alluvial clastic section is underexplored in Northern

Iraq due to a lack of trap at most of the previously drilled

structures. Initial studies have shown the section to have

some

potential to

develop reservoir

quality. N

earby G

ilibat-1 tested the lowerm

ost section of the Upper Fars

and reported a flow rate of 2000bbl/day from

these sands suggesting perm

eability may be adequate.

Data Type

Brow

n: mud w

eightB

lack: DS

T measured

“Best Fit” N

ormal C

ompaction

Lines in Brow

n