Stratigraphy and Correlation

Geological interpretation between wells – subsurface reservoir

framework

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What is correlation?

• Identification or demonstration of the linkage or equivalence of two or more geologic phenomena in different areas: – for example: this correlation implies that

between these two points this bedding plane is continuous.

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Learning objectives1. Identify correlation markers2. Correlate lithological units between wells using lithology and

wireline log information3. Understand what correlation means and how to use the

available data (seismic, logs, biostratigraphic or chronostratigraphic) to constrain a realistic correlation

4. Understand how interpretation of depositional environment affects correlation of rock units

5. Show how different models (such as sequence stratigraphy) or stratigraphic information can affect a correlation

6. Describe the pitfalls in correlation

• Correlation is the step before mapping - Exercises give useful experience

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Importance of correlation

• You need to correctly correlate lithofacies in the subsurface in order to identify flow units and to map the distribution, thickness and continuity of reservoir and seal facies

• Correlation is an interpretation of the available data – and therefore the interpretation may change as additional data becomes available

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What are we trying to do?• The answer to that question determines the

method used and the graphical result that is obtained…

• Usually you are trying to identify intervals or units in one well that are related to, or connected to units in another well– Could be the same age, the same lithology, same

chemical characteristics…• Use computer correlation techniques – or light

tables for visual comparison– Artform because it is purely subjective interpretation– Geological experience and judgement are essential…

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Stratigraphy – an essential concept for correlation

“The study of rocks and their distribution in space and time with the object of reconstructing Earth history” (Lafitte et al. 1972)

• Correlation or grouping of rocks by age, lithology, etc… for some purpose

• Different purposes require different KINDS of stratigraphy and therefore correlation

• So we have lithostratigraphy, chronostratigraphy, biostratigraphy, seismic stratigraphy, magnetostratigraphy, chemical stratigraphy, sequence stratigraphy….

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Lithostratigraphic correlation (all rock units are correlated between wells)…

Implies that the correlated units are continuous across the intervening space

Biostratigraphic correlation (all biostratigraphic markers (numbers) are correlated between wells)…

Implies that the correlated points are the same age in each well – the implication is that the bottom sand in W1 is older than the bottom sand in W2 – perhaps these do not connect between the wells?

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What data do we have?

• Log data – usually needs preparation– Need to use comparable data-sets (similar

logging and vertical scales)– Logs that have depth scales of measured

depth (MD), along hole depth (AHD) or below rotary table (RT) depths need to be converted to true vertical depth (TVD), and normalised to a depth datum (usually sea level or subsea SS), giving TVDSS.

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• Deviated wells –measured depth gives thicker units than true vertical thickness, meaning that correlations drawn using MD are distorted:

Why normalise scales?

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Ideal Composite Logs• Lots of data:

– Gamma Ray (GR): measures natural radioactivity, providing a lithology proxy (clay versus sands)

– Resistivity: measures resistance of the rock to an electric current, and shows up the type and amount of pore fluid (hydrocarbons, rock and fresh water have high resistivity while salt water has low resistivity). Since connected units often have the same pore water chemistry, resistivity will have similar profiles across these units, making it a good correlation tool

– Sonic: identifies seismic markers (for correlation with seismic lines) and hard or soft lithologies

– Density: detects density changes (lithology) and porous zones– Caliper: measures the borehole diameter. Increase in borehole diameter

indicates washed out zones, and therefore areas where the other log data will be unreliable (and also possibly areas of damage due to faulting or fracturing or soft lithologies)

– Dipmeter: identifies the dip of the rock units crossed, including faulted contacts. ‘Rock units’ in this context includes beds and features such as cross-bedding within beds (so that current direction can sometimes be obtained from dipmeterinformation). Changes in dip can indicate deformation, faulted contacts or stratigraphic changes.

– Mudlog: description of rock chippings, oil and gas shows

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Non-ideal Composite Logs

• Reduced data set or problems with conversions:– Reduced suite of geophysics (usually GR, resistivity

and sonic)– Modified by an approximate conversion factor from

MD to TVD– Measured depth logs (avoid unless they are honestly

vertical wells).

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How do we start?

• Need to identify equivalence between wells. We want to be correlating the same lithological units together, so that our correlations do mean connection of lithological units between wells...– Chronostratigraphy– Biostratigraphy– Magnetostratigraphy– Lithostratigraphy– Seismic stratigraphy or correlation using seismic data

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How do we start?

• Chronostratigraphy – identifying the age of units. We can use absolute radiometric dating (slow, costly), or isotope dating methods (ditto) or magnetostratigraphy (ditto), or biostratigraphy (much cheaper and relatively fast).

• Chronostratigraphy gives us tie-points in each well that we can use to identify the same formations...

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Chronostratigraphy

• http://activetectonics.coas.oregonstate.edu/nsaf_turbs.htm15

This is a geochronologic time scale (pure time). The terms period, epoch and age are used here, rather than system, series and stage, used in chronostratigraphy…

The rocks belonging to the Devonian System were deposited during the Devonian Period

Chronostratigraphic time scale

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How do we start?

• Biostratigraphy – using fossils to define rock units, either using extinction/evolution (disappearance/appearance) events or abundances of fossils, which can be correlated to a specific point in time. – IF appearance and disappearance are related to the evolution

and extinction, and not to facies changes (i.e. some species are restricted to x environment, while others can be deposited in almost any environment)

– IF evolution or extinction occurs at the same time everywhere (no significant barriers between areas)

– Therefore – pick the right kind of species to work with...

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Biostratigraphy

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BiostratigraphyMesozoic North Sea

example of using ammonite zones to subdivide and identify lithostratigraphic formations in the Cretaceous

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During the history of the earth, the orientation of the magnetic field has switched, and the strength of other parameters has also changed (such as the difference between the magnetic and geographical north pole, eddies in the field).

The switch from ‘normal’ to ‘reversed’ orientation is a global event, and if preserved in the rock record, will provide a global chronologically identical horizon worldwide.

The orientation of the field in the past is preserved in rocks by the orientation of magnetic minerals:

Igneous and metamorphic rocks - crystals of magnetite and other minerals, that preserve the orientation of the magnetic field at the time of crystallization.

Sedimentary rocks - tiny grains of detrital magnetic minerals, if undisturbed.

Magnetostratigraphy

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Magnetostratigraphy

• The rate of deposition will affect the thickness of the observed magnetostratigraphic zone… but the pattern will remain constant, so long as zones are not removed by erosion…

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http://nhm2.uio.no/norges/litho/rogaland.php

How do we start?

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• Lithostratigraphy – identifying units of the same lithology and relative stratigraphic position. These are formations or several similar formations may be part of a group. Formations may be subdivided into members, but these are of very restricted extent (formations must be mappable).

• Formation tops in wells, are often easily identified and picked from wireline logs, because they represent a large change in lithology.

Lithostratigraphy

• An Early Cretaceous lithostratigraphic and biostratigraphic framework for the Britannia Field reservoir (Late Barremian–Late Aptian), UK North SeaPetroleum Geoscience, December 2000, v. 6:345-367,doi:10.1144/petgeo.6.4.345

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Top Hidra Formation – note strong log shift

Erosion of units by unconformity (pinching out)

Marker bed used for correlation (or biostratigraphic marker)

Correlation

• Stratigraphic relationships are easier to display if you align the logs so that correlated rocks or units more-or-less line up. This is called hanging, and usually means that the sections are aligned so that a specific horizon or boundary is horizontal. For example…

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Stratigraphic Correlation Panel (Bajenovskaya marker)

Structural Correlation Panel TVDSS

Field B, 200225

Hanging

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Hanging

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Various geochemical properties of rocks can also be used in correlation. For example, specific beds may have unique geochemical signatures created by the conditions of deposition – which can then be used to identify these layers and use them as marker beds.

Common types of chemical stratigraphy include oxygen isotope stratigraphy (used to track sea-level changes) and strontium isotope stratigraphy (used as a form of dating marine rocks). Other isotopic ratios that can be used include C-isotopes (related to cycles of productivity), and S-isotopes.

Other types of chemical stratigraphy include major element chemistry (looking at the total composition of the rock, measuring Si, Al, K, Ti, Fe, Mn, Mg, Ca, Na, P and S) and trace element chemistry (for example measuring Sr, Pb, As, Ag, Au). Fluctuations in the amount of these elements can provide information about the provenance (source) of sediments, about the weathering processes occurring (and therefore climate information), about the depositional environment and about the diagenetic processes. More importantly for stratigraphy – fluctuations can be basin-wide, and therefore useful in correlation between outcrops or cores.

Chemical Stratigraphy

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Seismic Stratigraphy

• Correlation of seismic packages– Bounded by truncations of reflection events

• Advantages– Continuous interpretation in inter-well areas– Direct hydrocarbon indicators (DHIs)

• Disadvantages– Limited resolution, multiples, uncertainty as to

what seismic truncations really mean, must be ‘tied’ to well information for lithology and age

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Seismic Stratigraphy• Tying our wells to the seismic

– Using logs or VSPs– Can use to check likely position of

the same stratigraphic horizon in the next well, check...

• Seismic character– Helps identify lithology types– Chaotic, parrallel, clinoforms...

– Faults indicated by discontinuities30

Pinchout

Guidelines1. Use the log patterns to correlate, but beware of differences caused by fluid effects

(such as the presence of oil or water) on the resistivity logs2. Always correlate from the base upwards – this is how the rocks were deposited3. Always correlate from the large scale changes to the small scale – worry about the

formation tops and markers first, before correlating minor changes and beds4. Check for missing and repeated sections5. Always correlate both the top and bottom of a bed or formation6. Units that pinch out between wells are indicated by merging correlation lines (> or <)7. Never correlate the top or bottom of a well – these are artificial boundaries created

during drilling8. Check for mudstone (shale) colour changes in the mudlogs – these indicate changing

mudstone formations9. Keep an eye on the caliper log – indicates a loss of quality in the other logs but also

shows the location of less compacted or damaged layers10. The dipmeter log is also important – sudden changes may indicate the presence of

unconformities or faults11. Natural gamma signature is a good lithological indicator, many formations and

markers have distinctive signatures12. Volcaniclastics such as ash or tephra layers (eruption deposits) are excellent

marker beds, as they will be the same age everywhere, making them a chronostratigraphic marker, and they will often have a characteristic log signature

13. Non-geological features such as scale changes, casing shoes and sonic log cycle skips can sometimes mislead the unwary 31

Structural Discontinuities

• Normal Faults: Section missing

Drilled succession

True succession

Example from Tearpock and Bischke 1991

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Structural Discontinuities

• Reverse Faults: Repeated section

Drilled succession

True succession

Example from Tearpock and Bischke 1991

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Structural Discontinuities

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Using Wireline LogsA reminder of the relevant rules:

• Use the log patterns to correlate, but beware of differences caused by fluid effects (such as the presence of oil or water) on the resistivity logs

• Check for mudstone (shale) colour changes in the mudlogs – these indicate changing mudstone formations

• Keep an eye on the caliper log – indicates a loss of quality in the other logs but also shows the location of less compacted or damaged layers

• The dipmeter log is also important – sudden changes may indicate the presence of unconformities or faults

• Natural gamma signature is a good lithological indicator, many formations and markers have distinctive signatures

• Non-geological features such as scale changes, casing shoes and sonic log cycle skips can sometimes mislead the unwary

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Correlation of Wireline Logs• Correlate patterns: absolute values depend on variables other than

the lithology (such as instrumentation, well-bore fluid and so on), so are not reliable.

• http://www.ags.gov.ab.ca/publications/wcsb_atlas/A_CH21/CH_21.html

Note use of abrupt shifts in gamma to correlate units (coarsening-up)…36

Correlation of Wireline Logs

• This example of correlation of electrical logs from Kansas was made to demonstrate the continuity and uniform thickness of the units of this age, since they have subsequently suffered various tectonic disturbances.

• Data has been hung from a local datum – the top of the Stone Corral dolomite bed.

• Note the use of both log shifts and different character of the log patterns to define units (variable or non-variable, trends, shapes, internal patterns etc).

• http://www.kgs.ku.edu/Publications/Bulletins/162/06 pres.html

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Remember about non-geological changes…

• Scale shifts, casing shoes, fluid contacts and so on can all cause changes in wireline log patterns that resemble lithological changes:

base of pipe

tool not moving

base of first log run

top of second log run

first reading inside casing

casing shoe

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Exercise: Correlating Wireline Logs

Scale change – note no resistivity measurements: therefore this is probably inside casing or pipe.

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Pg 32

Exercise: Correlating Wireline Logs

Noisy resistivity suggests formation damage

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Correlation

• Displaying correlated logs:– Correlation panels or cross sections?

Horizontal log separation does not imply distance the logs are separated in reality.

Commonly used for purely stratigraphic correlation, but can be used to display structural relationships.

Horizontal log separation is proportional to the true distance the logs are separated in reality.

Used either to display stratigraphic or structuralrelationships.

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Cross sectionsShows relationships of rock units, and structure as it is…

Or interpretation of series of events.

Correlation

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Structural Cross Section

• Structural cross sections examples• Gulf of Mexico, from Tearpock and Bischke, 1991

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Fence diagrams

In these cases, lithological correlation, but other sorts of correlation could be used here.

Correlation

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Block diagrams – either 3-D image of current geology/geography, or interpretations of facies relationships, geological evolution etc.

Correlation

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Correlation

• Correlation is an interpretation of available data:– Interpretation is affected by our preconceptions– Interpretation must be geologically realistic – so we

must have an idea of the what the geology will be like before we start:

A ModelDepositional environments and sequence stratigraphy

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Geological Realism

Any correlation needs to be able to be explained by a reasonableseries of geological events. - Correlated layers will be consistent between wells (correlations lines ~ parallel). Some thickening and thinning may be expected. - One whole well is displaced a consistent amount relative to the adjacent well. - Divergent or inconsistent correlations are an indication that the units do not in fact correlate (channels, pinching out, isolated sand bodies).

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Large-scale studies of basins revealed that characteristically shaped packages of sediments were deposited during cycles of sea-level rise and fall. These were termed sequences and the concept of subdividing stratigraphy in a region in terms of packages of sea-level and therefore time related deposition has since been used globally and especially in the oil industry…

Useful for determining the location and likely extent, shape, and continuity of surfaces (along which fluids are likely to flow) and bodies of porous sandstone (reservoirs), or other lithologies of interest.

A Model

Sequence Stratigraphy

Relative vs. Eustatic sea-level changes….

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“A sequence is a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative conformities”

The sequence is subdivided into SYSTEMS TRACTS and PARASEQUENCES

Sequence Stratigraphy

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Sequence Stratigraphy• Packages of sediment – bounded by surfaces

(unconformities and correlative conformities)• Can be any scale• Model of deposition• Hierarchy of stratal elements – sequences,

systems tracts, bounding surfaces and parasequences

• Correlation of packages – not lithologies… packets may contain vastly different lithologies in different parts of the basin, but may belong to the same cycle of sea-level rise and fall

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Sequence Stratigraphy

Movie

High sea-level:

•Flooding surfaces

•Reservoir units close to shore/basin margin

•Shales deposited in basin centre

Low sea-level:

•Exposure of shelf, incision, erosion, unconformities

•Deposition of reservoir units in basin centre

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Sequence Stratigraphy

Applying Deltaic and Shallow Marine Outcrop Analogs to the Subsurface (Janok P. Bhattacharya) Search and Discovery Article #40192 (2006) Posted May 2, 2006 http://www.searchanddiscovery.net/documents/2006/06023janok/index.htm

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Sequence Stratigraphy

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Sequence Stratigraphy Exercise• On the following diagram, identify the major surfaces, an systems

tracts, and also label the parasequences.

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Correlation and Environmental Interpretation

• Accurate correlations depends on a little knowledge of the environment of deposition of the sediments. – In different environments, the geometry and

extent of sediment bodies is different.• For example in marine settings sand bodies may

extend several kilometres, but in fluvial settings they would rarely be wider than a few hundred metres as each body represents a channel fill.

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Stratigraphic Cross Section

Shallow MarineCook Fm., L. Jur.

Fluvial-deltaicNess Fm., M.Jur.

From Livbjerg and Mjos, in Collinson, 1989

From Ryseth, in Collinson, 1989

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Channels in correlations…• Channels are erosive features, deeper in the centre, thin to the

edge, elongate in the downstream direction, shorter in cross-section. • They are generally filled with coarse material (sand etc), although

abandoned channels can be filled with mud (oxbow lakes). • The fill is always flat topped (law of horizontality)…

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If tops are the same depth (relative to a marker), the channel may be connected between wells. If not, the channels are different (although overlap may lead to connection…)

Exercise 4: Environments make a difference Pg 29

These panels of well logs show alternating sandstones (dots) and mudstones (grey). Relative to the green marker bed at the top of the wells, are the sandstones at roughly the same depth? If so then they are likely to be laterally continuous and therefore correlatable. If not…

If you think the sandstones correlate, then connect the top and bottom of each bed…

If not, then perhaps they are channel sands, so draw them in as ‘boat shaped’ bodies.

Although there is roughly the same volume of sand in each panel, how does your correlation affect the reservoir connectivity and volume?

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Exercise 4

1. a marine environment (think extensive sheets of sandstones) 61

Exercise 4

2. a fluvial environment (think localised channel-shaped sandstones) 62

Stratigraphy and Reservoir Performance

• Stratigraphy – and the depositional environment – give you the architecture of the reservoir…

• Locations of traps, likely flow paths and parameters…

• This information can give you estimates of the reservoir performance and the recovery likely from that reservoir.

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Flow Units (after Ebanks 1987)

• Characterized by the same poro-perm properties

• Not necessarily uniform• Recognizable on logs• Correlatable between

wells• Include pay and non-pay• Include fluids within• May be connected

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• Mixture of high and low permeability sedimentary layers and bodies:

• variable overlap of high permeability bodies, variable difference of permeability vertically and horizontally. • Sometimes called ‘jigsaw’ architecture.

Reservoir ArchitectureGeologically realistic model from Weber and van Geuns (1990)

• Layered reservoirs:• Low permeability contrasts between vertically stacked layers which are laterally extensive• Sometimes called ‘layercake’ architecture.

• Isolated high permeability bodies within low permeability ‘background’:

• variable overlap of high permeability bodies, variable difference between sandbodies and background sediment.• Sometimes called ‘labyrinth’ architecture.

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Example: Aeolian Environments

• Sheet-like

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Example: Fluvial Environments

• Channels – either isolated or larger stacked channel bodies. In sheets of floodplain muds, with wedges of crevasse splay deposits, and sheets of silt-sand deposited during flood events.

• This example from a deltaic system, so underlain by more sheetlikeshallow marine sediments.

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Example: Shallow Marine Environments

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• Sheet-likeFulmar Formation – shallow marine

Piper Formation – wave dominated delta front

Example: Deltaic Environments

• Pods, lenses, channels (in delta plain)

• Sheets and lenses (delta front)

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Ness = delta top fluvial. The whole unit correlates, but the individual sand bodies do not.

Etive = shallow marine

Rannoch = delta front

Example: TurbiditeEnvironments

• Sheets and lenses (fans versus channelized; distal versus proximal)

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Brae = proximal; sheets and channels, conglomerates and sandstones

Forties = distal; sheetlikesandstones

Architectural MatrixVE

RTI

CA

L H

ETER

OG

ENEI

TY

HORIZONTAL HETEROGENEITYLOW MODERATE HIGH

HIG

HM

EDIU

MLO

W

Wave-dominated (proximal) delta

Sand-rich strand plain

Barrier island

Wave-modified (distal) delta

Eolian

Submarine fan (Turbidite)

Distributary mouth bar

Proximal delta front

Tidal deposits

Mud-rich strand plain

Meandering fluvial (single point bar)

River dominated delta

(single package)

Back Barrier (single package)

Shelf bars

Alluvial fan

Fan Delta

Distal delta front

Wave modified delta (proximal)

Braided river

Tide dominated delta

Meandering fluvial Braid plain

River dominated delta (stacked packages)

Meandering fluvial (Stacked pt. bars)

Back barrier

(stacked packages)

Submarine fan (stacked packages)

(from Tyler and Finlay, 1991)

Layered architecture(Layercake)

Mixed architecture(Jigsaw)

Isolated architecture(Labyrinth)

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Oil Recovery and Strategy

• from Tyler and Finley (1991)72

Layercake

Labyrinth

Jigsaw

Layercake

Jigsaw

806040200

Shallow marine (Piper Sd)Shallow Marine (Fulmar Sd)

Lacustrine (Lewis)Turbidite (Katrine)

Aeolian (Auk)Submarine fan (Andrew)

Fluvial (Crawford)Shallow marine (Piper)

Submarine fan (Forties)Turbidite (Magnus)Debris flow (Brae)

NORTH SEA RESERVOIRS

Primary Recovery

Water Injection

Recovery and Geology

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Compartmentalisation

• Sedimentary structures or stratal architecture (facies models)– Turbidites (Forties), fluvial reservoirs (Brent Ness

Formation), deltas etc…• Faulting

– Seismically resolvable faults (Gullfaks)– Sub-seismic faulting (Thistle)

• Combination - faulting and architecture (NW Hutton)• Some fields have no compartments

Is the reservoir subdivided into discrete flow unit areas or compartments? Could be caused by:

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Learning objectives1. Identify correlation markers2. Correlate lithological units between wells using lithology and

wireline log information3. Understand what correlation means and how to use the

available data (seismic, logs, biostratigraphic or chronostratigraphic) to constrain a realistic correlation

4. Understand how interpretation of depositional environment affects correlation of rock units

5. Show how different models (such as sequence stratigraphy) or stratigraphic information can affect a correlation

6. Describe the pitfalls in correlation

• Correlation is the step before mapping - Exercises give useful experience

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Exam Question – worked answer

• The figure shows wireline logs and interpreted lithology from 5 appraisal wells in an onshore oil field. The interval shown on the logs is approximately 110 m thick and the logs have been ‘hung’ on a well-established stratigraphic datum. Twelve biostratigraphic samples were taken from different depths in the 5 wells (shown alongside logs). Samples 3, 7, 10 and 12 indicate a marine environment, whilst all the others indicate non-marine deposition.

• Using the lithology and shape of the wireline logs for guidance, draw a correlation panel for the wells, using your understanding of the depositional system represented (11 marks). Remember that the aim of this type of stratigraphic correlation is to indicate the lateral extent of potential reservoir sandbodies.

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marinemarine

marine

marine

non-marinenon-marine

non-marine

non-marine

non-marine

non-marinenon-marine

non-marine

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79

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http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch24/ch_24.html

Deltaic mouth-bar, Delta top

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Exam Question – b, c and d

• B) Give reasons for your correlation (5 marks)

• C) Describe the broad environment of deposition (4 marks).

• D) How would the reservoir behaviour of the sandbodies in the upper half of the panel differ from those in the lower half (below 65 m) (5 marks)?

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Exam Question – marking schedule

• Neatness (1)• Correlation of both bed surfaces (1)• Coals correlate across the panel (except the eroded one (2)• Lenticular shape of lower sandstone (2) and boat shapes of upper

sandstones (2)• Sandstone in well three erodes coal (1)• Sharp based ?channel sand in well three (1)• Interfingering of lower sandstone with mudstone in well two (1)• Marine sandbody – likely to be bar and channel, lenticular in shape,

correlates across several wells. (2)• Channel sandstones likely to be lenticular in cross-section, but could be

elongate down-dip/flow direction. (3)• Environment of depositions: Lower half = delta front mouth bar with

associated distributary channel; upper half = fluvial or delta plain with coals and channel sandstones (4).

• Reservoir behaviour: Good lateral sweep and pressure support in lower sandstone (2) , channel sandstone may act as preferred conduit for flow (being better sorted (1)). Upper sandstones are likely to be isolated, some may connect laterally or vertically (2). 83

Correlation Exercise

December 2013 Exam QuestionWith Fault!

Question Text• You have been given 5 wells with interpreted lithology and

selected wireline log information (Attachment B2). Fossil samples from the locations 1, 2, 5, 7, 8, 10 and 11 contained a palynomorph assemblage consistent with temperate non-marine environments, while samples 3 and 9 contained a fossil assemblage consistent with shallow marine conditions, and samples 4 and 6 contained a microfossil assemblage indicating a deep marine environment. An angular unconformity has been identified using image logs and biostratigraphic information, and is indicated in the logs using a wiggly line. A clear disruption assumed to be due to a fault has been identified in Well 3 (indicated on the attachment), assume that there are no other faults disrupting the stratigraphy.

Question Text

• On the attachment, correlate the various lithologies present. Is this correlation a cross-section or a correlation panel and why?

• It is a correlation panel, because the wells are equally spaced, whereas in reality they are probably unevenly spaced. 2 marks

Nonmarine

Shallow marine

Deep marine

First transferring the information from the text…

Start with the bottom section: The unconformity is horizontal – correlate that.

Everything below the unconformity is at different heights, while everything above is at the same height (relative to our marker). Why is that?

We are told the unconformity is angular, that means the beds below are orientatedifferently to the beds above…

Look at the beds – are there some that consistently change position between wells? Correlate those… (remember that all lines between adjacent wells should be consistent)

We now see that we have beds dipping to the left, but they have been disrupted by the fault we know is in well 3. What kind of fault? Missing section or repeated section?

Look at the coal – it is thinner and the overlying mudstone is missing. Therefore this is a normal fault, which means the right side being downthrown is the hanging wall. Now draw it!

Now for the sand bodies below the coal – are any at the same height relative to the coalin adjacent wells? These might be connected… if not, they are isolated channels.

Remember that interpretation of channels as being isolated or connected is your Interpretation, so either solution is ‘correct’. The only way to tell would be a pressure test…

In the upper section, everything agrees in terms of depth relative to our marker, so we can correlate across the panel.

The sand-mud relationship in well 3 is ‘interfingering’ – caused by shifting location of the shoreline and therefore the facies being deposited. This is shown by pinching out in both directions. 9 marks

Question Text• Explain your choice of correlation, justifying your

decisions and specifically mentioning your interpretation of the changing depositional environments.

• Depositional environment in lower part indicated as temperate non-marine (palynology). Although sand bodies correlate between a few wells in the lowest section, they vary in thickness, and do not correlate across all 5 wells. They display fining upwards trends in gamma. They are probably therefore fluvial channels interbedded with floodplain mudstone (1).

• The coal indicates changing depositional environment, and is superseded by thick mudstone which may indicate lacustrine environments, which is superseded by interbedded sandstone and mudstone possibly indicating a return to braided fluvial depositional environments (see very variable, uncorrelatable gamma logs), all offset by a normal fault. (2)

• The unconformity correlates across the panel. (1)• Shallow marine environments above the unconformity should and do correlate

(blocky wireline signature), with interfingering into the deeper marine environment to the east. A general transgression indicated by limestone and then deep marine turbidite layers (fining upwards) follows (3) 7 marks

Question Text• What processes must have acted on the lower

sedimentary succession in order to create the observed geometrical arrangement of strata? What type of fault is present?

• Fault marks a missing part of the succession in well 3, so is a normal fault. (1)

• Processes in order - deposition, burial, compaction, lithification, tilting, faulting, uplift, erosion. (3)

4 marks

Question Text

• Identify a potential trap on your correlation. Is this a stratigraphic or a structural trap?

• Stratigraphic traps of sands etc pinching out against muds, (either the channels in the lower half or the sand bodies in the upper half assuming an appropriate structural tilt) and also against the unconformity in the east where it is overlain by muds. Pinchouts require appropriate tilting of beds (not given). 3 marks

Correlation from the beginningHere is an example for you to correlate:– Three wells, A-B 6 km apart, B-C 21 km apart. – Biostratigraphic zonation provided for each

well. – Try lining the wells up by depth – it is difficult to

match the curves. – Line the wells up by one or other of the

biostratigraphic zones – much easier to see the similarities.

– Correlate lithologic ‘units’. – Make a correlation panel, and a cross-section. – Try taking your stratigraphic correlation and

creating a structural arrangement – put them back into depth order.

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Answer• The real example comes from one

formation – Kimmeridge Clay Formation.

• There are two limestone bands, but the rest of the variation is due to alternating carbonate, mud and organic rich layers within the shale.

• However it could have been alternating sandy and shaleyunits...

• Your subdivision is your interpretation based on the available information.

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