06 facies analysis

7/22/2019 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

06 facies analysis

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