LITHOSTRATIGRAPHY AND THE CRETACEOUS OF THE NORTH SEA

A Brief Outline of Concepts

It is not Easy!

Stephen CrittendenIndependent Geological Consultant

TALK STRUCTURE

o Define Stratigraphyo Onshore and Offshoreo Define lithostratigraphyo Cretaceous Subsurface lithostratigraphy

schemeso Criteria and Methodo Forward and Onwards

FIRST, WHAT IS STRATIGRAPHY?

o Stratum = Latino 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”.

StratigraphyConventional 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 UNITSTable 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, all-encompassing stratigraphy model which aids in the search for oil and gas. Lithostratigraphy is a part of that model.

Cause and Affect PhilosophyCommon Causes in the environment of deposition

affect

Lithology Faunas & Floras

Which if all other factors are equal , in turn control

LWD response data

Fossil Assemblage data

And are interpreted to derive

Drilling data incl. ROP, Torque, Gas

Lithostratigraphy Biostratigraphy

Which all together produce

INTEGRATED ’EVENT STRATIGRAPHY’

Lithostratigraphy: a part of Stratigraphy

- a tool in the geologist’s tool-box

THE PRESENT IS THE KEY TO THE PASTAn 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 ClassificationAn 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 UNITSProcedure

• 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

• 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

Formations Rogaland Group

Hordaland Group

Ekofisk Formation is part of the Shetland Group

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 formationsover the crest of the structure.3. This also accounts for the patchy distribution of the Cenodiscus Claystone Memberover 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.

Paleocene section on flanks of structure is more complete and usually includes Vaale and Ekofisk formations.

Informal Member status, usually at wellsite can only

be reliably identified by palaeo

LWD Log ’Lithostratigraphy’ – Idealised EventsCrestal location

Clyst: lt gy – gy, lt blu grn gy, frm, blky – sub fiss, silty, sli calc, tr mica, glc.

Stringers of Dol, reddish brn, brn gy, off wh, hd, xln.

Stringers of Chalky Lst, wh – lt brn,frm – hd, brit, sucrosic. Associated with increase in gas values.

Top Balder Formation2613mMD, -2367m

’False Balder Formation’ peaks = reworked horizon (s).

GR response background trend change;trend slowly increases in value downhole.

Top Sele Formation2625mMD, -2378m

Top Lista Formation2634mMD, -2386m

Top Tor Formation2653mMD, -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.

GR response trend is rather flat.

Upper Red Claystone Marker.

Clyst more varicoloured – bluish, prplsh with depth.Reworked tuffs.

Gas peak from Balder = higher resistivity.

GR peak characteristic of some crestal wells.

Fork peak.

GR values higher than Balder.

GR ’Bow’: peak,cutback, peak.

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 Interval (mMD)

Description

2946 – 3009 3012 – 3030 3036 – 3066 3072 – 3153 3159 – 3171 3177 – 3183 3189 – 3222 3228 - 3237

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. 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. 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. CLAYSTONE (100%) olv blk – brn blk, vfrm, blky, n-calc, micropy, tr vf carb flks, unifrm, earthy tex, tr py nods (Sele Formation) 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) 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.

Undiff E. Eocene interval

Upper Red Claystone

Balder Formation Tuffs

Sele Formation

Lista Formation

Lower Red Claystone

Chalk

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 Herring

Formation

Actually, it’s the Chalk Group

(and it’s a mammoth not a

herring!)

It’s the Plenus Marl G0 Bed

Chalk LithostratigraphyVariety 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, GermanyThe white stuff we drill through.Some layers and fractures / faults visible.

Faulted white stuff – Lagerdorf Saturn QuarrySlightly 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 StratigraphyPerhaps 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 CretaceousIs it a success story? 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 VisionClear 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

Modified from Network Stratigraphic Services Ltd.

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MARKER EVENTS

Non rew orked Cretaceous N. f requens & C.daniae

C. daniae

N. f requens

S. primitivum / G. obliquum

Z. compactus / C. obscurus

R. levis

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B. parca constricta

R. anthophorus

B. parca parca / H. bugensis

E. eximius

O. campanensis

H. trabeculatus

B. geminicatillus

C. biarcus / L. grillii

S. biferulaB. parca constricta

B. enormis / C. crassus

R. levis

B. parca parca

A. cymbiformis

Common C. obscurus

Common L. cayeuxiiQ. eptabrachium

Common H. trabeculatus

T. virginica

L. grillii

Q. intermedium / H. turonicus

M. staurophora

Inf lux H. turonicus

Q.eptabrachium / L.septenarius

K. magnif icus

E. eximiusQ. gartneri

H. chiastia

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Abundant P.elegans/R.contusa

Common B.draco

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A.bettenstaedtiCommon unkeeled planktics, incl. Rugoglobigerina spp.

Common A. velascoensis

G. af f . arca / T.capitosa

S. granulata incondita

S. exsculpta gracilis

S. pommerana

S. exsculpta exsculpta,Cenosphaera spp.

Common S. granulata polonica

S. granulata granulata

Common M.marginata, O.vacaensis, D.constricta

S. granulata levis

S. granulata humilis

Inf lux Cenosphaera spp. + D.multicostata

L.globosa, W.

Abdnt M.marginata,inc.P.cf . stephani RRI+S. granulata kelleri

R.cushmani

R.cushmani

L. ciryi inf lata

S. antiqua

C. biarcus

L. acutus

G. segmentatum

C. kennedyi

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Inf lux P. dimorphosus

Common P. tenuiculusH. edw ardsii

C.primusB. hughesii MT1

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