Transcript LITHOSTRATIGRAPHY AND THE CRETACEOUS OF THE NORTH …

LITHOSTRATIGRAPHY AND THE
CRETACEOUS OF THE NORTH SEA
A Brief Outline of Concepts
It is not Easy!
Stephen Crittenden
Independent Geological Consultant
TALK STRUCTURE
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Define Stratigraphy
Onshore and Offshore
Define lithostratigraphy
Cretaceous Subsurface lithostratigraphy schemes
Criteria and Method
Forward and Onwards
FIRST, WHAT IS STRATIGRAPHY?
o Stratum = Latin
o Graphia = Greek
As trained geologists we are all familiar with stratigraphy.
Defined as ”the description of all rock bodies forming the earth’s crust and their
organisation into distinctive, useful, mappable units based on their inherent
properties or attributes in order to establish their distribution and relationship in
space and their succession in time, and to interpret geological history”.
Stratigraphy
Conventional Purist Stratigraphy Concept
Stratigraphy comprises:
• Lithostratigraphy
• Biostratigraphy
• Chronostratigraphy
• Sequence Stratigraphy
• Seismic Sequence Stratigraphy
• Magnetostratigraphy
• Other stratigraphies based on other properties of rock
bodies. For example ’Flow Units’, Chemical
Composition, Heavy Minerals
STRATIGRAPHICAL UNITS
Table of units
Stratigraphy Models
• As working petroleum geologists we deal extensively with sub-surface data rather than outcrop
data.
• For the North Sea explorer we deal exclusively with offshore material
• We must still retain a classical academic approach to stratigraphy – first principles, to underpin
our Pragmatic, Working Stratigraphy.
• We have to work with a pragmatic philosophy.
• Often the ’stratigraphies’ erected by the industrial petroleum geologist are neither strictly
defined nor procedurely correct. They are an immediate adequate means to an end but
inevitably will cause problems in the future.
• Stratigraphy is an important part of the geologists tool-box used by explorationists to find
hydrocarbons. Do it Right.
• But, not all geologists are stratigraphers!
OFFSHORE STRATIGRAPHY
The key to the offshore is the onshore
OFFSHORE STRATIGRAPHY
• Stratigraphical schemes for the offshore are constructed / erected with reference to
the vast database of onshore information.
• This can be illustrated by reference to the lithostratigraphy of the Lower
Cretaceous Albian Stage.
Lowermost part of the Rodby Formation & Upper part of Sola Formation equivalent
onshore UK
The Red Chalk Formation and Rødby Formation onshore equivalent = the Hunstanton Formation
The Albian Lithostratigraphy Offshore
WHAT IS LITHOSTRATIGRAPHY?
The part of stratigraphy that describes and names rocks based on lithology and stratigraphical
relationships and the use of that data to organise rock bodies into lithostratigraphical units.
• Lithostratigraphy is only part of the overall picture seen by the petroleum
geologist.
• Lithostratigraphy is an important building block for model generation in the
search for oil and gas.
• The aim of the petroleum geologist is the generation of an overall, allencompassing stratigraphy model which aids in the search for oil and gas.
Lithostratigraphy is a part of that model.
Cause and Affect Philosophy
Common Causes in the environment of deposition
affect
Faunas & Floras
Lithology
Which if all other factors are equal , in turn control
Drilling data incl.
ROP, Torque, Gas
LWD response
data
Fossil Assemblage
data
And are interpreted to derive
Lithostratigraphy
Which all together produce
INTEGRATED ’EVENT STRATIGRAPHY’
Biostratigraphy
Lithostratigraphy: a part of Stratigraphy
- a tool in the geologist’s tool-box
THE PRESENT IS THE KEY TO THE PAST
An Ideal Solution
An instant later the
time travelling
stratigrapher, with his
thermometer, is
obliterated leaving the
warm blooded / cold
blooded dinosaur debate
unresolved
Formal Lithostratigraphy Classification
An established Conventional Unit heierarchy. Units are recognised
by observable physical features.
Group – comprises two or more contiguous or associated
formations. Associated groups may be part of a
Supergroup.
Formation – the primary unit of lithostratigraphy
recognised solely on lithology. It has to be mappable.
Member – a named lithological subdivision of a formation
that may extend into other formations.
Bed – a named distinctive layer in a member or formation. A
key bed or a marker bed.
Unit Boundaries do not define time lines.
Fossil content may be a diagnostic lithological component.
ESTABLISHING LITHOSTRATIGRAPHICAL UNITS
Procedure
• Type localities / Stratotypes – clear and precise. With auxillary reference sections.
• Boundaries – positions of lithological change. In the subsurface define the
boundary at the top occurrence of the particular rock type.
Cretaceous Subsurface Lithostratigraphy Schemes
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A Progression from the simple to the complex.
Use both onshore surface and subsurface data with offshore subsurface data.
As a basin is explored the ’schemes’ erected become more and more detailed.
Schemes developed initially are parochial as each Oil Company and each country
involved in the basin erect their own ’secretive models’.
• Later Co-operation results in better understanding, data sharing and in better
models.
Lower Cretaceous Lithostratigraphy Models Compared– North Sea
Stratigraphical Synthesis – Lower Cretaceous
The Albian Stage – Rodby Formation onshore & Offshore UK
Upper Cretaceous Lithostratigraphy Models Compared – North Sea
Subsurface lithostratigraphy units and schemes
In practise identified and / or defined by the petroleum
geologist using both:
• Lithology
and
• LWD / Wireline log shapes
Pragmatic Lithostratigraphy
The Petroleum Geologist utilises all data at the wellsite for identification of
lithostratigraphic units; formal and informal.
Example Conventional Lithostratigraphy
1. Palaeocene section over the crest
of the structure is incomplete from
the base upward.
2. Formations pinch out around and onto the
flanks and are absent on the crest. This accounts
for the lack of the Vaale and Ekofisk formations
over the crest of the structure.
Paleocene section on flanks
of structure is more complete
and usually includes Vaale
and Ekofisk formations.
Hordaland Group
3. This also accounts for the patchy distribution
of the Cenodiscus Claystone Member
over the structure.
Pinch-out (onlap / offlap) of the
structure by the lower part of the
Palaeocene section has a major
impact on casing pick. The amount
of section to be drilled through the
Lista Formation is going to be
variable.
Formations
Rogaland Group
Informal Member status,
usually at wellsite can only
be reliably identified by
palaeo
Ekofisk Formation is part
of the Shetland Group
LWD Log ’Lithostratigraphy’ – Idealised Events
Crestal location
GR response trend is rather flat.
Clyst: lt gy – gy, lt blu grn gy,
frm, blky – sub fiss, silty,
sli calc, tr mica, glc.
GR response background trend change;
trend slowly increases in value downhole.
Stringers of Dol, reddish brn, brn gy, off wh,
hd, xln.
Clyst more varicoloured – bluish, prplsh with depth.
Reworked tuffs.
’False Balder Formation’ peaks
= reworked horizon (s).
Stringers of Chalky Lst, wh – lt brn,
frm – hd, brit, sucrosic. Associated
with increase in gas values.
Upper Red Claystone Marker.
Top Balder Formation
2613mMD, -2367m
Gas peak from Balder = higher resistivity.
Fork peak.
Top Sele Formation
2625mMD, -2378m
GR values higher
than Balder.
Top Lista Formation
2634mMD, -2386m
GR ’Bow’: peak,
cutback, peak.
GR peak characteristic of some crestal wells.
Top Tor Formation
2653mMD, -2403m
Tuff – tuffaceous clyst: lt – med gy,
speckled wh – gy, blky, sft – friable,
sli calc, qtz incl. Clst: varicoloured grn, lt grn.
Clst: dk - lt brn – earthy brn, lt-dk grn interbeds,
sft, slty, pyr.
Clst: varicoloured, lt grn, dk bluish grn, dk grn, gy,
lt gy, sft- frm, blky, smooth, waxy appearance,
pyr, glc/chl, siderite. Lst intbeds, gy- wh.
Clst, red brn, chocolate brn, = Lower Red Marker.
Subsurface Lithostratigraphy Models
 Good cuttings quality for Lithology description.
• Accurate lag time.
• Caught on depth.
• Hole in gauge and well cleaned.
• No caving.
• Close interval: 3m intervals.
 Spot cuttings samples when necessary.
 Good Core Data is an ideal.
 Good Drilling Data – ROP, WOB, Torque.
 GR & Resistivity response from LWD.
• Good data quality.
Lithological Description from cuttings
GEOLOGICAL DESCRIPTION
Description
Undiff E. Eocene
interval
Interval
(mMD)
2946 – 3009
Upper Red Claystone
3012 – 3030
CLAYSTONE (70 – 80%) dk brown dusky red,sft – frm, blocky, non – sli calc. (Upper Red
Claystone Marker)
CLAYSTONE (30 – 20%) varicoloured dk grnsh gy and med bluish gy and Lt bluish gy - violet, firm –
mod hd, platy, non calc, very micropyr i p.
LIMESTONE (tr – 10%) gy – lt gy, lt brnsh gy, orng gy, yllsh brn, firm – hd, blocky, argil.
3036 – 3066
TUFF (20 - 30%) pa-mod bluish gy – steel gy, sft-frm, cmby, cigarette ashy texture – gran tex, calc,
cmn spkld w crmy wh & dk gy incls, cmn prly dev, wk microlam (Balder Formation Tuff)
CLAYSTONE (80 –70%) varicoloured dk grnsh gy and med bluish gy and Lt bluish gy - violet, firm –
mod hd, platy, non calc, very micropyr i p.
Balder Formation Tuffs
CLAYSTONE (90 - 100%) varicoloured dk grnsh gy and med bluish gy, w<10% dusky brn, blksh red
– purple, violet, firm – mod hd, platy, non calc, very micropyr i p.,
LIMESTONE (tr – 10%) gy – lt gy, lt brnsh gy, orng gy, yllsh brn, firm – hd, blocky, argil.
Sele Formation
3072 – 3153
CLAYSTONE (100%) olv blk – brn blk, vfrm, blky, n-calc, micropy, tr vf carb flks, unifrm, earthy tex,
tr py nods (Sele Formation)
Lista Formation
3159 – 3171
CLAYSTONE (100%) mod-dk gnsh gy, med bluish gy, trc red brown, vfrm, brit, blky, splty, n-calc,
smth, unifrm tex, tr blk ?carb incls, tr crmy wh min incls, poss sl tuffaceous, pyr. (Lista Fmn)
3177 – 3183
Lower Red Claystone
3189 – 3222
Chalk
3228 - 3237
CLAYSTONE (100%) grnsh gy, grysh blue, firm - mod hd, blky, splty, non calc, smooth text, tr diss
pyr., tr red brown claystone
CLAYSTONE (20 –90%) reddish brn, gysh red, trc grn mottling, firm to mod hd, non to v sli calc,
smooth chocolate texture. (Lower Red Claystone Marker)
CLAYSTONE (80 – 10%) mod-dk gnsh gy, med bluish gy, vfrm, brit, blky, splty, n-calc, smth, unifrm
tex
LIMESTONE (trc – 10%) gy – lt gy, lt brnsh gy, orng gy, yllsh brn, firm – hd, blocky, argil.
CHALK (90%) yllsh gy, v pa orng gy, pa crm, sft to firm, crumbly, amorphous – micrxsln.
CLAYSTONE (10%) a mixture of liths as above.
Pragmatic approach for correct identification of the
lithological unit. Use all available data.
It is best to be prepared!
Cretaceous Lithostratigraphy
Monotonous Stratigraphy ?
”after all, it’s all white and Chalk is Chalk isn’t it?”
”anyway the Lower Cretaceous is all claystone”
Points of View – It is useful to step back and gain a
broader perspective
It’s the Plenus
Marl G0 Bed
It’s the
Herring
Formation
Actually, it’s the
Chalk Group
(and it’s a
mammoth not a
herring!)
Chalk Lithostratigraphy
Variety of schemes for various regions of the North Sea
• Chalk interfingers with the Shetland claystones. The previously illustrated slide
(22) shows the complexity and detail of the lithostratigraphy.
• Each lithostratigraphical unit has to be illustrated by a type well section.
• For Norlex: we have to use the existing defined and published type well sections.
• Norlex can illustrate and designate reference wells.
• Chalk is not monotonous: there are subtle and mappable differences.
Chalk – Lagerdorf Saturn Quarry, Germany
The white stuff we drill through.
Some layers and fractures / faults visible.
Faulted white stuff – Lagerdorf Saturn Quarry
Slightly more interesting – well defined lithological layers and clear faulting.
Reservoir Stratigraphy
 Geological Model for a Chalk reservoir is
complex; it is not a simple layered cream-cake.
 Faults and Fractures, both small scale and large
scale are present but not all are possible to model
from seismic.
Field Reservoir Stratigraphy
Perhaps there is more to Chalk than meets the eye!
Detailed Stratigraphy – flow units, ’lithostratigraphy’ and biostratigraphy
THE KEY TO SUCCESS IS TEAM WORK
Lithostratigraphical studies of the Cretaceous
Is it a success story?
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Effective people
Effective data and QC of data
Effective acquistion and interpretation of data
Effective modelling
Effective software
Effective communication
Effective documentation
Objective: A holistic understanding of all the subsurface data and how it all slots
together to achieve a lithostratigraphic model.
Forward Vision
Clear vision, rationale and plan for the process of building an effective
Lithostratigraphic Model.
Review Model sessions.
Feedback, consolidate and discuss.
Re-define vision, strategy (ies) and methods.
• Identify any changes necessary
• Identify any impediment to change
• Identify any other resources needed: skills, tools, people.
• Time frame required for change
Back up picture
LT.
PAL
MICROPALAEONTOLOG
Y
ZONES
(modified after King 1989 &
NANNOFOSSIL
MARKER
EVENTS
"EARLY"
VÅLE
FORAMINIFERID
+ RADIOLARIAN
MARKER
EVENTS
Calc. benthics
MT4
THANETIAN
Planktic inf lux, incl.
G.pseudobulloides
NNTp6 - 8
MT3
Common N.perf ectus
61.00
NNTp5
EARLY
PALEOCENE
62.82
NANNOPLANKTON
ZONES
(modified after Varol,1998 &
Burnett et al., 1998)
FORMATIONS
(after JCR Study, 1999)
TIME SCALE
(after G radstein et
al., 1995)
59.00
AGE
"LATE"
"MIDDLE"
DANIAN
G.daubjergensis/E.af f .trivialis
Inf lux P. dimorphosus
NNTp4
EKOFISK
b
Common P. tenuiculus
H. edw ardsii
NNTp3
iii
UC20
UC19
M AASTRICHTIAN
e
Common B.draco
FCS23
P.elegans
a/b
Z. compactus / C. obscurus
R. levis
d
c
UC18
T. orionatus
b
UC17
EARLY
UC16
71.30
iii
ii
i
v
B. parca constricta
FCS22
B.miliaris
A.bettenstaedti
Common unkeeled planktics,
incl. Rugoglobigerina spp.
Common A. velascoensis
a
R. anthophorus
G. af f . arca / T.capitosa
B. parca parca / H. bugensis
E. eximius
b
O. campanensis
iv
LATE
UC15
iii
H. trabeculatus
i
MAGNE
iv
iii
EARLY
UC14
S. granulata incondita
FCS21
B. geminicatillus
a
ii
CAMPANIAN
c
S. primitivum / G. obliquum
ii
i
Abundant
P.elegans/R.contusa
d
C. daniae
N. f requens
P.archaeocompressa (not
seen in JCR study)
MT1
Non rew orked Cretaceous
N. f requens & C.daniae
ii
i
iii
LATE
P.compressa
a
C.primus
B. hughesii
NNTp1
65.00
TOR
MT2
NNTp2
"EARLY"
C. biarcus / L. grillii
S. bif erula
B. parca constricta
S. exsculpta gracilis
B. enormis / C. crassus
ii
b
R. levis
FCS20
83.50
85.80
LATE CRETACEOUS
i
S. pommerana
B. parca parca
LATE
UC13
iii
THUD
SANTONIAN
Common C. obscurus
a
S. exsculpta exsculpta,
Cenosphaera spp.
FCS19
C. crassus
ii
i
MIDDLE
UC12
EARLY
UC11
iii
Common L. cayeuxii
Q. eptabrachium
ii
Common H. trabeculatus
i
LATE
A. cymbif ormis
Common S. granulata
polonica
FCS18
S. granulata granulata
b
T. virginica
Common M.marginata,
O.vacaensis, D.constricta
L. grillii
ii
FCS17
UC10
Q. intermedium / H. turonicus
CONIACIAN
i
MIDDLE
-EARLY
a
M. staurophora
S. granulata levis
ii
NARVE
89.00
b
UC9
Inf lux H. turonicus
FCS16
LATE
i
a
Q.eptabrachium / L.septenarius
TURONIAN
M IDDLE
UC8
UC7
EARLY
93.50
ii
K. magnif icus
i
E. eximius
Q. gartneri
c
FCS15
a
UC6
BLODØKS
UC4 & 5
LATE
H. chiastia
C. biarcus
b
R.cushmani
UC3
c
R.cushmani
L. ciryi inf lata
L. acutus
HIDRA
UC2
G. segmentatum
FCS13
b
EARLY
UC1
98.90
Inf lux Cenosphaera spp. +
D.multicostata
S. granulata humilis
P.stephani,P.gibba,D.aumalensi
L.globosa, W.
FCS14
M IDDLE
CENOMANIAN
Abdnt M.marginata,inc.P.cf .
stephani RRI+S. granulata
kelleri
a
C. kennedyi
Modified from Network Stratigraphic Services Ltd.
S. antiqua