Machar Chalk Characterisation

Zoë Sayer, Hannah Beattie, Mary Ward and Andy Ronald BP ETAP Reservoir Management Team May, 2015

Machar Field

• Discovered in 1976, on production since 1998

• High-relief structure, draped over a salt diapir

• ~500 MMBO STOIIP

− 70% in Chalk, ~30% in Palaeocene turbidites

− Minor celestite caprock reservoir

• Subsea development tied back to ETAP CPF, 32 km to NW over

Marnock

• Produces from both Palaeocene sandstone and fractured Chalk,

with water injection via three wells into the Chalk

• One of several producing diapir fields in the ETAP area

2

MACHAR

Machar Oil Column 1330m (TVD)

OWC ~2500m

Top Chalk Depth Structure – viewed from the south

“Ben” Machar

• Oil column is ~1200 m

• Relief from contact to crest is approx.

the same as that of Ben Nevis!

Machar Diapir height cf surrounding top chalk

~2.5 km

Ben Nevis height -1344m (TVDSS)

Structure - Seismic

4

• Steep dips, salt and gas chimney

adversely affect the seismic

• Three surfaces mappable:

− Top Balder, Top Chalk, Base

Chalk

• Internal reservoir surfaces and

structure not visible

• Reservoirs drape salt diapir

• Significantly thinner Chalk than off

structure

− <300 m c.f. ~1000 m

Reservoir Stratigraphy

5

• Chalk

− Cretaceous Tor (main) and Hod

− Danian Ekofisk

• Palaeocene

− Maureen (main), Lista and

Forties

• Celestite

− Diagenetic alteration of Zechstein

anhydrite

• Sealed by Sele Fm muds passing

up into Eocene Balder Formation

Structure – Faults and Fractures

• Salt movement results in abundant faults

and fracturing

− Radial – regional influence?

− Concentric – diapir?

• Fracture “highway” around shoulder area

leads to injection water bypass and

unswept downdip areas

− Proven by tracer data

• Smaller fractures essential for chalk

production

− Imbibition of water into matrix releases

oil for production via fractures

− Not conventional injection sweep

• Multiple OOWCs

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Main seismic faults mapped at top Ekofisk

Fracture “highway”

OWC regions

2450 m

2405 m

2500 m

2463 m

Chalk Characterisation

• Study carried out in 2013 to apply Chalk geology to static and dynamic models

for infill screening and blowdown simulation

7

• ~500 m of core logged at 1:100 scale for

facies interpretation

• External petrographic study

• Image log electrofacies interpretation

− no well had both core and image logs!

• Wireline log facies interpretation

• Biostrat used to subdivide Tor and Ekofisk

• Rock typing attempted

• Petrophysical properties derived for each

facies by formation

• Model populated using depositional facies

maps Machar cored wells

and image logs

Machar core data coverage

Chalk Deposition

8

• Deep, basinal carbonates

• Mostly coccolith plates with minor

planktonic and benthonic foraminifera and

rare echinoids/bivalves

• Pelagic rain of coccolith “pellets”

• Pelagic clay where argillaceous

• Laminated/clearly bedded

• Deposition on slope causes reworking

− Debris flows, slumps, slides Leads to facies interpretation….

?

Facies

• 7 facies identified in core

• 7 corresponding electrofacies identified in image logs

• Not all evident in logs though facies “Types” e.g. pelagic, reworked,

interbedded could be recognised (sim. Brasher et al., 1996)

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Machar Chalk Facies - Reworked

Homogeneous Reworking Debris Flow

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• Grain re-organisation

− Massive

• No internal structure

− Burrowed bed tops

• Very low, blocky GR

− Cycles correspond to

individual reworking

events in core

• Mottled/uniform FMI

• Same but with clasts

• Fines-upwards

− Grades into

Homogenous

Reworking

− Burrowed bed tops

• Same GR

− Can’t distinguish from

logs alone

• “Speckled” FMI

Machar Chalk Facies – in situ

Pelagic/Argillaceous Pelagic

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• Deposition from pelagic settling

• Finely laminated, burrowed, stylolitised

• Higher, serrated GR

• Clear lamination in FMI with constant dips

• Pelagic

− clean chalk, white-buff, lower GR

• Argillaceous pelagic

− contains more clay, local quartz, grey, higher GR

Machar Chalk Facies - Other

Slump Dense Zone

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Calciturbidite

Slump

fold

Burrowed

dense

zone

Not identifiable on logs

alone:

Image logs have

chaotic dips…

Chalk types

• Not all facies evident in logs

alone

• Can be grouped into 3 chalk

types

− Type 1 – laminated, serrated

GR – pelagic

− Type 2 – intermediate, includes

interbedded and slumped

− Type 3 – massive, blocky GR –

reworked/debris flow

• Dense zones not correlatable

between wells, so not included

in RDEs

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Rock Properties Framework - Why do Facies Matter?

• Best reservoir quality in reworked chalks

• Grains reorganised during early resedimentation

enhancing porosity

• Rock-typing attempted but unsuccessful to date

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Homogenous reworking 27% Ø, 1.4 mD K

Pelagic 19% Ø, 0.15 mD K

Argillaceous pelagic 5% Ø, 0.01 mD K

Debris flow 20% Ø, 0.31 mD K

crestal well,

Ekofisk

Rock Typing

• Little petrophysical

difference between

different chalk rock types

• Chalk rock types 1 – 5

share the same space on

density neutron xplot

covering a wide porosity

range

• Coloured by rock type

and include 1,2,3,4,5

FA5a 1 Pelagic

FA5a2 2 Argillaceous Pelagic

FA5b 3 Debris Flow

FA5c 4 Homogenous Reworking

FA5d 5 Slump

FA5e 6 Dense Zone

FA5f 7 Calciturbidite

FA5g 8 Injected sands

FA5h 9 Slumped Debris Flow

Rock Typing

• Histogram of GR

coloured by chalk

rock type

• All chalk rock types

cover a range of

GR from ~5 gapi –

30 gapi

Rock Typing

• When 23/26A-13 is used in

blind tests it predicts type 5

(slump) when it should be 4

(homogenous reworking)

• Even if we lump 4 and 5

together we are still only

accurate 43%

• i.e. no better than random

Facies Maps and Depositional Development

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• Facies maps

− Based on log chalk types

− Layering based on nannostratigraphy

• Maps revealed information about potential structural evolution of the diapir

through changes in distribution of pelagic, reworked and slumped chalk

including

− Location and timing of major slumps

− Areas of localised slumping

• Timings correspond to regional structural understanding, but more work is

required for further clarification

− e.g. Seismic onlap mapping

Lower Tor

• Mostly pelagic chalk (Hod also mainly

pelagic).

• Deposition over low relief, relatively

stable structure

• Minor reworking associated with slopes

• Overall poorer reservoir quality

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Middle Tor

• Major period of reworking

• Continued increase in in relief led to more

widespread slumping

• Main episodes during Middle-Upper Tor,

Late Maureen and Post Sele

• During Middle Tor:

− crest of structure moved upwards,

− first development of local highs and

shoulder area

• Field dominated by reworked chalk, with

minor pelagic preserved on present flanks

• Best reservoir quality

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Ponding in rim syncline?

Upper Tor

• Crestal highs expand with continued

growth leading to larger areas absent

zones.

• No net deposition on the crest, all

sediment reworked down onto shoulders

and lower flanks.

• Shoulders pond thick reworked chalk.

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Unconsolidated T1-4

chalk reworked into

basin

Semi-lithified T5-

10 slumped

downslope

• Flanks comprise

thinner interbedded

pelagic and reworked

chalk.

• Slumps develop in S

and W (illustrated)

Lower Ekofisk

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• Unlike the Tor, the Ekofisk has regional

depositional trend evident in cyclicity

− Correlatable on- and off-diapir

• Indicates a period of structural

quiescence

• Earlier slump scars now sites of

deposition

• Deposition occurred post-KT extinction

− Different, smaller coccoliths =>

different reservoir quality

− Higher porosity but lower perms

− Sand and clay influx (tsunami-ite??)

− Proto-Maureen sand influx?

Upper Ekofisk

• Regional depositional trend continues

• Clastic influx ceased

• Reworking increases towards the end of

Ekofisk deposition, and the youngest

sediments are all reworked

• Oversteepening or renewed movement?

• No slumping – earlier scars still

depositional sites

− pond overlying Maureen turbidites

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Static modelling

• RDEs used to populate RMS model

− Unfaulted

− Faults put in dynamic as TMZs

• Petrophysical modelling based on

Chalk Type averages

• Zonation based on nannostrat

− 2 zones in Ekofisk

− 3 zones in Tor

− 1 zone in Hod

• Chalk previously had uniform

values

• Characterisation study enables

more geology to be put into the

model

• Successfully history-matched in

base case and downside

− Used for blowdown simulation 24

Down-dip section –

internal reservoir

surfaces isochored

Transmissibility

zones and faults

Static Modelling - Facies and Properties

Lower Ekofisk

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Middle Tor

NTG nPORO

Interventions – addition of chalk perfs

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• Three reworked chalk

intervals to be perforated,

picked from a slumped

section. Facies identified

using image logs and logs

Conclusions

• Chalk facies can be identified from

standard data

• RDEs can be mapped and used to

control model inputs

• Rock typing was problematic

− Possible with more work?

• Petrophysical modelling populated

in model by facies

• Modelled chalk geology has been

successfully history mapped and

used for forward modelling for

blowdown

• Chalk character used to inform

intervention decisions

• Not all chalk fields are the same…

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Backup

28

Structural Framework

29 Modified from Foster et al., 1993; Glennie et al., 1998; Starmer,

1995

• Central N Sea, post-rift

succession

• Eastern Trough

• Cluster of fields following

movement of Zechstein Salt

− Diapirs – Machar, Mungo

− Salt withdrawal – Marnock

• Main phases of salt movement

coincident with regional tectonic

events

• Pre-, syn- and post-depositional

salt movement

Comparison to other N Sea Chalk

• Machar follows regional

diagenetic trends

− No overpressure

− Late charge

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Biostratigraphic correlations

Tor cycles easily correlatable off-structure

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Cyclicity breaks down on-structure –

can push correlation through within

nanno framework but less convincing

Regional Ekofisk character –

argillaceous base with sands, passing

up into clean chalk

Same character seen on Machar,

though condensed

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