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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
6
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)
9
Machar Chalk Facies - Reworked
Homogeneous Reworking Debris Flow
10
• 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
11
• 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
12
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
13
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
14
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
18
• 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
19
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
20
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.
21
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
22
• 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
23
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
Interventions – addition of chalk perfs
26
• 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…
27
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
30
Biostratigraphic correlations
Tor cycles easily correlatable off-structure
31
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
Ek
ofi
sk
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