revised reservoir model for the paleocene mounds of the ... e… · revised reservoir model for the...

Revised reservoir model for the Paleocene mounds of the Utsira High, North Sea, Norway

John Wild (1) & Nowell Briedis (2)

(1) Mobil North Sea LLC

(2) Esso Exploration & Production Norway A/S

(ExxonMobil Subsidiary Companies)

DEVEX 12-13 May 2009

Summary

• Up until 2005, ExxonMobil’s geological model for the mounded Paleogene oil fields of the northern Utsira High consisted of sandy debrite and turbidite fan reservoirs of three distinct ages, linked by post-depositional sand injections.

• However, it was well recognised that some of the data did not fit this model.

• Recently improved seismic imaging does not show the expected geometries for the stratigraphic relationships. Stratigraphically-climbing injected sills are now recognised to be of much greater significance.

• In all the simple mounds, disruptions are imaged in the underlying Chalk that are interpreted to be large fluid escape features. Stratigraphically anomalous sands have been encountered by wells drilled through these features.

• We now re-interpret the Balder mounds to be extensive fluidised injection complexes. The mound morphology may be entirely secondary in origin, caused by fluid escape from the pre-Chalk section underlying the Utsira High.

• A new simulation model based on these ideas is more closely matching production history at Balder than previous models.

Location and early Tertiary setting

2000

1700

1800

1900

Depthm

Top Paleocene (Base Balder Fm.) Structure

A

Grane

Balder

Ringhorne

10km

EAST SHETLAND PLATFORM VIKING GRABEN UTSIRA HIGH

BALDER

PALEOCENESAND

SHALE

EOCENE

PALEOCENE

CRET

UJR

JR

TRNW SE

A A'

2000m

6000m

ca 90km

A’

Norway

Previous geological model: Compensational stacking of reservoirs

2000

1700

1800

1900

Depthm10km

A

A’

Base BalderStructure

• Five major Paleocene-Early Eocene sands were identified

• Heimdal and Hermod sand mounds were interpreted to be sandy debrites (shed from the East Shetland platform)

• Younger Balder sands were thought to be mostly turbidites

• All reservoirs were thought to be compensationally stacked, in a relatively unconfined deep-water setting.

• Much of the mounded relief was thought to be due to post-depositional sand re-mobilisation

Balder

Ringhorne

Grane

A A’Sand Re-mobilisation

Conceptual Line of Section

Unusual characteristics of the sand

• Sands are not found at consistent stratigraphic levels

• The sands are clean, fine to medium grained and consistently high porosity (~33%)

• Massive sands in the Heimdal, Hermod and Balder are visually indistinguishable and mineralogically identical

• Angular shale intraclasts are seen in core• No distal or off-axis facies encountered

(in over 150 wells)

GAS SANDOIL SANDWET SANDCLAYSTONETUFFCHALK

25/11-A4BH 25/11-A5H 25/11-5

600m 100m

Hei

mda

lH

erm

odB

alde

rTy

PE

TM

Th5

Th2

Yp2

25/11-A4BH 25/11-A5H 25/11-5

100m

Section A – Balder M5

Section B

Section A

Location

Balder

Ringhorne

Unusual characteristics of the shale

• Erosion of shale is minimal or absent

• For 25 years the uniform shale (thickness) model has been used to predict net sand prior to drilling

• Local variation in Paleocene isopach is due to variable sand thickness

25/11-2025/11-16 25/11-1 25/11-4 25/11-3 25/11-21S 25/11-2225/11-1525/11-18T2

Chalk

GAS SANDOIL SANDWET SANDCLAYSTONETUFFCHALK

Ty

Heimdal

Hermod

Complete Section

Shale Only

Balder

200m

100m

0m

Datum

>200 m

0 m

Net sand estimated from isopach

Beneath the mounds: intriguing anomalies in the Chalk

2500

1700

2100

Depthm

10km

• Detached rafts of chalk are associated with all simple mounds• Allocthonous glide blocks derived from the east?• But - matching underlying depressions in the Chalk surface

1,000 m ~5x Vertical ExWest East

Chalk Structure

Line of section

Chalk Rafts

Disrupted Chalk

Reflector

TWT

Seismic line with hole and raft

Chalk Rafts

Chalk

Base Balder

Map

Mound structure Attribute showing rafts

Balder “raft” penetration

500m

TWT

North South~2x Vert Exag

N

1

Chalk Raft

In-situ Chalk

25/11-C-13

Location

• The raft is elevated about 10m • Matching depression in the underlying Chalk• High quality massive sand beneath raft

Ringhorne raft, Ty and Heimdal sand relationship

• Sand beneath the raft is in seismic continuity with Heimdal sands drilled 250m away

• Well 25/8-C3 shows 20m repeat shale section over chalk

• The raft is elevated by ~35m • Size: ~ 1,100 x 600 x 15m• Mass: ~ 16 Million tonnes

25/8-C3 25/8-15S

Rep

eat

sect

ionRepeat with Lithlog

25/08-15S (pilot)

25/08-C3 (producer)

1,000 m NorthSouth ~2x Vert Exag

TWT

Location

Cretaceous Chalk clasts in Paleocene sands

• Chalk clasts have been penetrated in core

• Larger 25+ cm clast is in massive well-sorted fine-medium grained sand

• Smaller 5 cm clast is contained within angular intraclastic breccia, typical of thinner injection sands in area

• Many more wells show probable chalk clasts on wireline logs

Location25

/8-1

0S:

1737

m50

mm

25/8

-10S

T2:

1778

m10

0mm

Page 10

Chalk “rips”

1,000 m EastWest ~2x Vert Exag

Xl 1810

• Progression seen from:+ Short deflections in chalk (>100m) with dipping reflector

above, to a+ Rip – a normal to reverse fault rarely exceeding 200m in

length, sometimes with a small partly-detached raft

Location

Relationship of Heimdal, Hermod and Balder sands

1000 m ~2x Vertical ExWest East

• Rip in Chalk under mound• Reflectors cross-cut Base Balder, joining M3 to M4

M4 (Eocene)

M1 M2

M3

Balder

Relationship between Heimdal, Hermod and Balder sands

1,000m ~2x Vertical ExWest East

• Reflectors cross-cut biostratigraphy on mound margins• Higher continuity of sand is consistent with well

performance

M4 (Eocene)

M1 M2

M3

Balder

Distribution of Balder Fm. (Eocene) sand

70m

40m

20m

0m

• Massive (~40m, 100% N/G) high quality (Ø~33%) Balder sand is unusual for a distal turbidite• No source or feeder channels have ever been found• No contemporaneous sands have been drilled in main basin to west• East Shetland Platform undergoing transgression during Balder period - unlikely sediment source• We now interpret the majority of the Eocene Balder sand to be intrudites and extrudites

Sum of negamplitudes calibrated

with well data

Base Balder Structure

TWT

Attribute-based Balder Sand isopachBase Balder Structure

2km

M2M1

M3

1 2

Summary of Seismic-Stratigraphic relationships

1000 m ~2x Vertical ExWest East

• Most or all of the Hermod and Balder sands are interpreted to be sourced directly from the Heimdal mounds, rather than deposited compensationallyaround them

• All the mounds are underlain by disruptions in the Chalk

• We interpret there to be a genetic relationship between the two

Possible control on location of the Paleocene mounds

5,000m

Mesozoic

PaleozoicZechst

ein

Paleocene

Basement

A A’

EastWest ~2x Vert Exag

Balder M3

• The mounds overlie the flank of the Utsira High, a prominent tectonic high lying between the Viking Graben and Stord Basin

• The chalk rafts are generally confined to an area underlain by a subcrop of thick Paleozoic clasticsbelow a Lower Triassic angular unconformity

Base Triassic Structure

< Tr u/c

Tr u/c >

TWT

Summary of the new model

Heimdal

Hermod

• Our observations are consistent with most of the sand in the Paleocene mounds and the overlying adjacent Eocene being continuous bodies injected across stratigraphy

– This model can explain the:+ Minimal erosion of shale+ Lack of stratigraphic consistency+ Absence of transitional facies+ Lack of vertical variations in porosity+ Common, field-wide OWC and GOC+ Excellent reservoir communication on a

production time-scale

• The mounds are related to Chalk rips and rafts, which are thought to be fluid escape structures

• The location of mounds appears to be controlled by deeper structure

• The original distribution and geometry of depositional Paleocene sand is obscured by the extensive remobilisation

• The volume of injected pre-Paleocene sand remains uncertain

Balder M3

25/8-1 25/8-C24 25/8-3 25/8-10ST2

One sand body - injected across stratigraphy25/8-1 25/8-C24 25/8-3 25/8-10ST2 25/8-4

ChalkTy

Heimdal

Hermod

Balder

Potential importance for field production

• Modified mound geometries in a new field simulation is improving history match to production• Our model helps to explain:

– The high degree of communication between reservoirs of differing ‘ages’– Observed common oil and gas contacts over large areas– The shared aquifer over entire region

• ...And we hope will lead to the optimal placement of future wells

Revised reservoir model for the Paleocene mounds of the Utsira High, North Sea, NorwaySummaryLocation and early Tertiary settingPrevious geological model: Compensational stacking of reservoirsUnusual characteristics of the sandUnusual characteristics of the shaleBeneath the mounds: intriguing anomalies in the ChalkBalder “raft” penetrationRinghorne raft, Ty and Heimdal sand relationshipCretaceous Chalk clasts in Paleocene sandsChalk “rips”Relationship of Heimdal, Hermod and Balder sandsRelationship between Heimdal, Hermod and Balder sandsDistribution of Balder Fm. (Eocene) sandSummary of Seismic-Stratigraphic relationshipsPossible control on location of the Paleocene moundsSummary of the new modelPotential importance for field production

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