LITHOSTRATIGRAPHY AND THE CRETACEOUS OF THE NORTH SEA

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LITHOSTRATIGRAPHY AND THE CRETACEOUS OF THE NORTH
SEA

• A Brief Outline of Concepts
• It is not Easy!

Stephen Crittenden Independent Geological
Consultant
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TALK STRUCTURE

• Define Stratigraphy
• Onshore and Offshore
• Define lithostratigraphy
• Cretaceous Subsurface lithostratigraphy schemes
• Criteria and Method
• Forward and Onwards

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FIRST, WHAT IS STRATIGRAPHY?

• Stratum Latin
• Graphia Greek
• As trained geologists we are all familiar with
  stratigraphy.
• Defined as the description of all rock bodies
  forming the earths 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.

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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

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STRATIGRAPHICAL UNITS

• Table of units

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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!

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OFFSHORE STRATIGRAPHY

• The key to the offshore is the onshore

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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.

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Lowermost part of the Rodby Formation Upper
part of Sola Formation equivalent onshore UK
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The Red Chalk Formation and Rødby Formation
onshore equivalent the Hunstanton Formation
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The Albian Lithostratigraphy Offshore
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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.

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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
Fossil Assemblage data
LWD response data
Drilling data incl. ROP, Torque, Gas
And are interpreted to derive
Lithostratigraphy
Biostratigraphy
Which all together produce
INTEGRATED EVENT STRATIGRAPHY
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Lithostratigraphy a part of Stratigraphy- a
tool in the geologists tool-box
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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
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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.

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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.

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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.

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Lower Cretaceous Lithostratigraphy Models
Compared North Sea
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Stratigraphical Synthesis Lower Cretaceous
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The Albian Stage Rodby Formation onshore
Offshore UK
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Upper Cretaceous Lithostratigraphy Models
Compared North Sea
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Subsurface lithostratigraphy units and schemes

• In practise identified and / or defined by the
  petroleum geologist using both
• Lithology
• and
• LWD / Wireline log shapes

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Pragmatic Lithostratigraphy

• The Petroleum Geologist utilises all data at the
  wellsite for identification of lithostratigraphic
  units formal and informal.

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Example Conventional Lithostratigraphy
Paleocene section on flanks of structure is more
complete and usually includes Vaale and Ekofisk
formations.
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.
Hordaland Group
3. This also accounts for the patchy distribution
of the Cenodiscus Claystone Member over the
structure.
Formations
Rogaland Group
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.
Informal Member status, usually at wellsite can
only be reliably identified by palaeo
Ekofisk Formation is part of the Shetland Group
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LWD Log Lithostratigraphy Idealised
EventsCrestal 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.
Tuff tuffaceous clyst lt med gy, speckled
wh gy, blky, sft friable, sli calc, qtz
incl. Clst varicoloured grn, lt grn.
Top Sele Formation 2625mMD, -2378m
GR values higher than Balder.
Clst dk - lt brn earthy brn, lt-dk grn
interbeds, sft, slty, pyr.
Top Lista Formation 2634mMD, -2386m
GR Bow peak, cutback, peak.
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 peak characteristic of some crestal wells.
Top Tor Formation 2653mMD, -2403m
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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.

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Lithological Description from cuttings
Undiff E. Eocene interval
Upper Red Claystone
Balder Formation Tuffs
Sele Formation
Lista Formation
Lower Red Claystone
Chalk
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Pragmatic approach for correct identification of
the lithological unit. Use all available data. It
is best to be prepared!
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Cretaceous Lithostratigraphy

• Monotonous Stratigraphy ?
• after all, its all white and Chalk is Chalk
  isnt it?
• anyway the Lower Cretaceous is all claystone

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Points of View It is useful to step back and
gain a broader perspective
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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.

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Chalk Lagerdorf Saturn Quarry, GermanyThe
white stuff we drill through.Some layers and
fractures / faults visible.
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Faulted white stuff Lagerdorf Saturn
QuarrySlightly more interesting well defined
lithological layers and clear faulting.
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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.

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Field Reservoir StratigraphyPerhaps there is
more to Chalk than meets the eye!

• Detailed Stratigraphy flow units,
  lithostratigraphy and biostratigraphy

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THE KEY TO SUCCESS IS TEAM WORK
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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.

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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

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Back up picture
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