Download - 06 Facies Analysis
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Facies analysis
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Facies analysis
The meaning of the term facies
(1) Observational facies
(2) Interpretive facies
Walthers Law
The importance of vertical profiles
Depositional processes from wireline logs
The principle of the FACIES MODEL
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Different types of Facies
Lithofacies:Characterized by sedimentary attributes, such as
grain-size, sedimentary structures, bedding
Biofacies:Characterized by fossil content (body
fossils or trace fossils)
Seismic facies:Characterized by seismic reflection amplitude and
continui ty. (Interpreted in terms of large-scale lithologic
characteristics)
Facies: The observable attributes o f a sedimentary rock body that
reflect the depositional processes or environments that formed it.
Ichnofacies:
The assemblage of trace fossils
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The different meanings of Facies
as exemplified by l ithofacies
Descriptive Interpretive
Stromatoporoid Reef limestone
limestone
Crossbedded Fluvial
gravel-conglomerate conglomerate
Cairns Fm., a major Devonian
reservoir in Alberta
Sadleroch it Fm., Yukon,
Similar to major reservoir at Prudhoe Bay
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Facies criteria
All facies:
- bedding- sedimentary structures- trace and body fossils- types of vertical profile (cycle)- facies contacts (erosional, gradational)
- colour (with care - this may be a diagenetic product)- map distribution (isopach patterns)
PLUS:
Siliciclastic facies:
- grain size and texture (sorting)- vertical grain-size trends
Carbonate facies:- mud/allochem ratio
- allochem type
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Examples of facies descriptions(from Cant and Walker, 1976; describing a fluvial succession
Well defined trough crossbedded facies (B): This facies is composedof well-defined sets of trough crossbedding..., with trough depthsaveraging 15 to 20 cm (range 10 to 45 cm). The troughs areregularly stacked on top of each other, but in some individualoccurrences of the facies, trough depths decrease upward... Thesets are composed of well-sorted medium sand... A few of the
coarser sets have granules and pebbles concentrated at their bases.
Rippled sandstone and mudstone Facies (F): This facies includescross-laminated sandstones..., and alternating cross-laminatedsandstones and mudstones. [An example of the latter] is 1.5 m thickand consists of three coarsening-upward sequences, which grade
from basal mudstones into trough cross-laminated fine sandstoneand finally into granule sandstone. The sandstones capping eachcoarsening-upward sequence have sharp, bioturbated tops.
See application of these standard facies in next slide
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Examples of standardized facies classifications
Battery Point Sandstone (braided fluvial),
Cant and Walker, 1978
Standard limestone microfacies scheme
Wilson , 1975
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Advantages and disadvantages
of a standardized facies scheme
Advantages:-simplifies and standardizes observations-permits rapid logging of field sections
and drill core
Disadvantages
-discourages observations of the rareor unusual-may lead to inappropriate simplifications
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Facies variations on larger scales clastic example
If an outcrop is large enough, larger-scale deposit ional features may be mapped
(outcrop of Cretaceous fluvial channels and bars, Book Cliffs, Utah)
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Facies variation on a large scale
a carbonate example
Transect across a reef margin, Permian, new Mexico
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The hierarchy
of depositional
units
Stratigraphy is concerned
mainly with features at the
scale of d iagram (A) and larger.
Facies analysis is concerned
primarily with sedimentary
features at the scale of
diagrams (B) to (E).
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Walthers Law
and the importance of the vertical profile
Environments and processes
found side-by-side in nature
are represented in the same order
in vertical profile
(The fluvial point-bar model)
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The importance of the vertical profile
1. Vertical profiles are the main
form of data obtained from
drilling into the subsurface.
2. They may readily be documentedfrom wireline logs
3. Their analysis can make use
of Walther s Law to interpret
depositional processes andenvironments.
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Depositional controls
Autogenic processes:- those arising from natural redistribution ofenergy and sediment within a depositionalsystem.- e.g., channel migration and switching, beachregression, delta and slope progradation.
Allogenic processes:- those imposed on a depositional system fromoutside the basin.- e.g., sea-level change, tectonic movement,climate change
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The use of log shapes to interpret
autogenic depositional processes
NOTE: variable vertical scales.
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Facies analysis from log shapes
1. Funnel-shaped log
Prograding deltaic Deposits, Tertiary, Ellesmere Is.
Bayfill shale-sand succession, Carb., Kentucky
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Facies analysis f rom log
shapes
2. Bell-shaped log
Point bar,
Carboniferous,
Alabama
Fining-upward
Fluvial cycle,
Tertiary,
Ellesmere Is.
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Facies analysis from log shapes
3. other shapes
Distal turbidite,
Cretaceous,
South pf France
Fluvial channel.
Permian,New Mexico
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Examples of
deltaic processes
illustrated by
wireline log shapes
(from Tertiary of Texas Gulf
Coast)
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Mapping ribbon sand bodies
in the Mannville Group, Alberta.
Circle indicates location of log at left.
Arrow above points to scoured base
of channel
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Facies associations and
Facies models
Facies Association:-characteristic repeated association orassemblage of a limited suite oflithofacies and/or biofacies-may be characterized by typical cyclic pattern orvertical profile
Facies Model:-an interpretive device erected to explaina facies assemblage, e.g., a block-diagrampaleogeographic/depositional model
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From modern environment to facies model
Milk River, Alberta
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From outcrop architecture to facies model
Point bar,
Carboniferous,
Alabama
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From vertical profile to facies model
Standardized
suite of
lithofacies
for use in
logging sections
Stratigraphic section
consisting of a uniform
facies assemblage
Facies model for sandy braided river.
Letters correspond to locations where
the standardized lithofacies
are forming
SS
B
C
FG
William River,
Saskatchewan
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Walkers classic
facies-model diagram
Distilling away the
variability to extract
the pure essence
(or common features)
of an environment
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Depositional architecture is not necessarily
indicated by li thofacies assemblage
These outcrops of glacial
outwash (near Banff)were deposited in
gravel-bed rivers,
but the facies characteristics
do not indicate channel
size or organization.
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The attempt to define
a typical
submarine fan
Subdivision into
-feeder canyon
-upper fan,
-middle fan,
-lower fan,
-fan lobes-basin plain
Classification of major facies types
Distribution of facies types within the fan
Major depositional processes
(diagram from Shanmugam and Moiola)
T f b i f
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Types of submarine fan
Can we extract a single,
typical fan to serve
as the basis for a fan facies model ?
Bengal fan
shown against the
California coast
in order to indi cate
its magnitude
Mississippi
fan Amazon fan
Bengal fan
Navy
Fan,
California
10 km 100 km
1000 km
100 km
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Final thoughts
Facies schemes and facies models- serve to systematize and simplify observation andclassification- BUT can also serve to suppress innovation
Facies models are not available for all existingsedimentary environments (e.g., very little study of riversin rain-forest environments).
Some ancient environments are not fully replicated byconditions anywhere on Earth today
- e.g., large Paleozoic carbonate platforms- huge shelf seas of the Late Cretaceous- Precambrian vegetation-free tropics- large evaporite basins