sequence stratigraphy - the basics

7/30/2019 Sequence Stratigraphy - The Basics

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Sequence Stratigraphy - The Basics


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Vertical and Lateral relationship of Rocks

STRATIGRAPHY

Lithostratigraphic method uses time-transgressive lithofacies boundaries

Biostratigraphic methods use boundaries, which are not physical surfaces.

Allostratigraphicunit"a mappable stratiform body of sedimentary rock

that is defined and identified on the basis of its bounding discontinuities"

(NACSN,1983).

Sequence stratigraphy defines sequence boundaries based on a

chronostratigraphic framework of cyclic, genetically related strata.

Sequence stratigraphy is another form of stratigraphic analysis

where the bui lding block is theSEQUENCE


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Lithostratigraphydefined based on observable characteristics of rock

Fundamental rock unit is a formation

mappable, lithologically distinct body of rock having recognizableboundries (contacts) with other formationsformations subdivided into members or grouped into groups

Correlation and Time


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LITHOSTRATIGRAPHY

versus

ALLOSTRATIGRAPHY

Lithostratigraphy is facies-driven and doesn't honour BiostratigraphyOn the contrary Allostratigraphy and Biostratigraphy should co-operate


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BIOSTRATIGRAPHYBiofacies

Law of Faunal SuccessionBiostratigraphic events- defined by the presence of a taxon in its timecontext; as derived from its position in a rock sequence.

FAD

LAD

Common or peak occurrence

Events are the result of biological evolution of life on Earth

USES Correlation

Paleo-ecology & Paleo-geographical studies

Bathymetry Age

Method Biozones

Correlation

Quantitative Biostratigraphy PMI


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Example of definition of biostratigraphic events.


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BIOSTRATIGRAPHYFOSSIL ABUNDUNCE MINIMAS ARE OFTEN

ASSOCIATED WITH SEQUENCE BOUNDARIES

MICROFOSSIL ABUNDUNCE PEAKS OFTEN

INDICATE CONDENSED SECTIONS

VARIATIONS IN BOTH ABUNDUNCE

PATTERNS AND SPECIFIC FOSSIL CONTENT

CAN BE USED TO CHARACTERIZE

DEPOSITIONAL SYSTEM TRACTS


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BIOSTRATIGRAPHYBATHY BENTHIC

FORAMSPLANKTICFORAMS

CALCARNANNOS

DINOCYST CLIMATE KEROGEN REWORK ACCUMRATE

SH/SSTSyst.Tract

TST

SB

HST

Mfs

TST

TS

LST

SB

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BIOSTRATIGRAPHYBiostratigraphy can support sequence analysis in the

following ways:

Development of a constrained time framework.Analysis of biofacies.

Understanding temporal and spatial relationships

within and between systems tracts.

Assisting development of play fairway concepts.

Predictive modelling of relationships between

depositional systems.


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SEQUENCE

State of being Sequent or following Order of succession A series of things following an order Unbroken series A set of things that belong next each other on some

principle of order

A relatively conformable succession of genetically related strata

bounded at its top and base byunconformitiesand theircorrelative conformities (Vail et al., 1977)

A succession of genetically linked deposition systems (systems

tracts) and is interpreted to be deposited between eustatic-fall

inflection points (Posamentier et al., 1988).


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Sequence Stratigraphy

A New Paradigm?

Most concepts in sequence stratigraphy not new

Relies on understanding of how sea level change effects deposition of sediments.

Suess 1885: plotted extent of marine transgression and estimated water depth on

basis of lithology and fossils.

Modern sequence stratigraphy: origins 1940: Sloss

OK What is it?

Study of genetically related sediments which are bounded by surfaces of erosion or non-

deposition

Sequence represents a group of sediments reflecting large scale global sea level change

KEY:unconformity based stratigraphy - trying to correlate the gaps in time: trying to

correlate unconformites that formed at the same time.

Sequence stratigraphy: LOADS of terminology - will try to keep it to a minimum


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SEQUENCE STRATIGRAPHY(Definition, Factors and Controls)

Sequence stratigraphy - a new correlation tool that integrates out-

crop, well-log and seismic data to identify and define the genetic

character of different types of physical surfaces and stratigraphic intervalswithin the rock record.

The building block is the SEQUENCE Within sequence stratigraphic framework, the distribution ofdepositional environments and the lithofacies tracts are defined.

It bears both chrono-stratigraphic and genetic stratigraphic meanings. Sequence stratigraphic units are scale independent both spatially andtemporally.

1st order to 6th order sequences are defined.


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More effective in predicting sandstone continuity and trenddirections of reservoirs, superior to shale tops

Improved methods for predicting reservoir, source and sealfacies away from wells

Better at locating sands: Basinal; shoreface; incised valleys;HS and LS regressive sands

Prediction of diagenesis, porosity and permeability

Definition of new stratigraphic play types

Improved ability to define and locate subtle stratigraphic traps

Reevaluate producing fields to increase reserves More integrated stratigraphic framework for risking new plays


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CYCLE CHART

THE DURATION AND EPIDOCITY OF

GEOLOGICAL EVENTS AND

STRATIGRAPHIC CYCLICITY SPANDIFFERENT ORDERS OF CYCLICITY


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Nature of the Cycle Chart

A working model based on best available data in 1987;

An averaged global curve;

Events on the chart included only if found in three ormore non-contiguous basins;

Firmly tied to European Stage Stratotypes;

Biostratigraphic resolution improves upwards - Further

improvements to be expected as new data accumulates.

Usefulness of Cycle Charts

Age Estimation

First approximation in frontier basins

First order global correlations

Depositional Trends

Duration and magnitude of unconformities

Duration and magnitude of Condensed Sections

Extent of lateral migration of Facies

Duration of exposure in Carbonates


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Cycle Duration (m.y.)1st order 200-500 A global supercontinent cycle generated

by assemblages of supercontinents in

space and time (Wilsons cycle of sea

opening and closing), Rifting-seafloor

spreading and drifting of continents andfinally collision and reassembling

2nd order 10-100 Eustatic cycles induced by volume

change at MOR and tectonic subsidence

of basement

3rd order 1-10 Regional to local cycle of basement

movement induced by regional plate

kinematics and intra-plate stress regime

4th order 0.2-0.5 Global cycles generated by orbital forcing,

& glacioeustasy, productivity cycle etc.5th order 0.01-0.2


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Sequence stratigraphy

Sequencestratigraphy is the study of rock relationshipswithin time-equivalent depositional successions bounded by

surfaces of erosion or nondeposition.

An interruption in sedimentation (discontinuity) is commonly

accompanied by a period of erosion that can cause a significant

gap in the rock record (unconformity).

In sequence stratigraphy, unconformities define the ends of

depositional sequences and the beginnings of new sequences.

Sequence stratigraphy can be used as a lithologicalpredictor

and as a tool for unraveling basin-fill history.

High resolution sequence stratigraphy is useful inpetroleum

reservoir correlation and modelling.


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Transgression


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Forced Regression


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AccommodationThe potential space available for sediment to fill

Accommodation is a function of changes in relative sea level (Jervey, 1988,

It is also a function of rates of sedimentation; e.g.

if the sea level rises and there is a zero or low sediment flux, thentransgression results.

if sea level rises and there is a low rate of sediment flux, then

retrogradation of the coastal parasequence results.

if sea level rises and the rate of sediment flux matches the sea level rise,

then aggradation of the coastal parasequence results.

if sea level rises and the rate of sediment flux exceeds the sea level rise,

then progradation of the coastal parasequence results


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Position of of sea surface relative to a fixed datum near the sea floor: takes into

account two components: eustasy and vertical movement of the sea floor (tectonism


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SEQUENCE AND DEPOSITIONAL SEQUENCE

Sequence - a relatively conformable successionof

genetical ly related strata bounded by unconformities.

Original definition has been broadened to include

" .....and their correlativeconformities

Thus, a depositional sequence (DS) is defined as a conformable

succession of geneticall y related strata bounded above and

below by unconformities or their correlative conformities. Laterally includes two portions:the landward unconformable portion and the

correlative basin-ward conformable portion.

Sequences composed ofdepositionalsystems tracts


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(After Loutit et al., 1987)

Wheller Diagram:


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Parasequence

Parasequence-a relatively conformable succession of genetically

related bed or bed set bounded by flooding surface or their correlative

surfaces.

Parasequence set

a succession of genetically related parasequenceforming a distinctive stacking pattern bounded by major flooding

surface and their correlative surfaces.

Parasequence Characters

oGenetically related package exhibiting shoaling succession of vertical

facies

oBoundaries (flooding surfaces) represent surfaces above which there hasbeen rapid sea level rise

oParasequence boundaries approximate time lines

oCan be used to create high-resolution seq. strat. framework

oGrain-size increase upwards

oBed & bed-set thicken upwards

oTypically 5-30 m thick

oBest expressed in shallow marine settings


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System TractL inkage of contemporaneous depositional systems, defined

objectively by stratal geometr ies at bounding sur faces,

position within sequences and internal parasequence

patterns.

Dip direction - fluvial, delta, and shelf and slope/basin systems

Strike section - delta, barrier-bar/lagoon and strand plain, and tidal-flat/estuary systems

Each is associated with a specif ic segment of the eustatic curve

1.Eustatic lowstand - lowstand wedge

2.Eustatic rise - transgressiveEustatic highstand -highstand system tracts


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Tertiary eustatic changes of sea level after Vail and Hardenbol

(1979).

Metres above or below sea level are tentative.

Surface Features of the Earth


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Surface Features of the Earth

Oceans cover 71 % of Earth's surface -- average depth 3.7

km. Land covers remaining surface with average of 0.8 km

above sea level

Ocean BasinsContinental Shelf , Slope, and r ise

Abyssal Plains

Oceanic ridges

Oceanic


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Lowstand and highstandHIGHSTAND

LOWSTAND

SEA LEVEL

SEA LEVEL

SHELF/SLOPE BREAK

SHELF/SLOPE BREAK

MB NB ON MN IN


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S B d i (SB)


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Sequence Boundaries (SB)Fundamental stratal unit

Relatively conformable, genetically related succession of

parasequences bounded by unconformities or correlative

conformities

SB in response to relative fall in base-level

TYPE 1- subaerial exposure, concurrent erosion, stream

rejuvenation; basinward shift in facies, downward shift

coastal onlap, onlap of overlying strataTYPE 2 - subaerial exposure, no erosion, downward shift

in coastal onlap landward of shoreline break

Criteria for RecognitionRegional truncationOnlap

Abrupt basinward facies shift (regional)

Abnormal subaerial exposure

Major biostratigraphic breaks


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Van Wagoner et al. (1990)

Van Wagoner et al.,1990

Type 1 Sequence

sea-level fall below the shelf-slope break


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Type 2 Sequence

sea-level does not fall below the shelf-slope break

Van Wagoner et al, 1990


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Lowstand systems tract(reprod uced from Van Wagoner et al.)


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Lowstand System Tract (LST)

Relative lowstand of sea level

Above Type 1 boundaries

Progradation of shorel ines on mid- and outer shelf

(Lowstand Prograding Complex)

Basin-f loor fansdevelop when sea level falls beneath

shelf -slope break

F luvial incisement discharges sediment onto slope

Downslope accumulations due to gravity flow(turbidites)

Laterally progresses into deeper shale facies

E l Ph L St d S t T t


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Early Phase LowStand System Tract

a. Falling stage of relative sea level induced by eustasy falling rapidly

and/or tectonic uplift outpacing the rate of change in sea level position

b. Fluvial incision with formation unconformity or sequence boundary and

the focus of sediment input at the shoreline

c. Forced regressions induced by the lack of accommodation produces

stacking patterns of downward stepping prograding clinoforms

d. Slope instability caused by the rapid deposition of sediment from the

fluvial systems

e. Basin floor fans formed from sediment transported from the shelf margin

when this fails under the weight of the rapid sediment accumulation

f. Shelf margin and slope fans form when rates of sedimentation slows and

slope instability is reduced

g. Onlap of sediments onto the prograding clinoforms below the shelf break


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TRANSGRESSIVE SYSTEMS TRACTRetrogradational parasequence set Finning and thinning upward units with shale near top:

Beach and shore face sands near base:

Pelagic shales in the basin:

Retrogradational parasequence are time transgressive. Sequence Boundary: Erosion during low-stand ; Incised valley with low-stand

Sands: retrogradational units onlap on to the lower boundary updip and downlap in

basinward direction. The top of the system tract is the downlap surface

Maximum Flooding Surface

Lowest resistivityhighest Gamma due to abundance of organic richne

Clay/Shale with abundant planktonics;

May show apparent truncation below boundary.


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Condensed Section

Thin marine stratigraphic units consisting ofpelagic to hemipelgic

sediments characterised by very low sedimentation rates.

Arealy most extensive at the time maximum regional transgression

of the shoreline.

Associated commonly with marine hiatus and often occur either as

thin but continuous zones of burrowed, slightly lithified beds(omission surfaces) or marine hard-grounds.

Characterized by abundant planktonic and benthic microfossil

assemblages, authigenic minerals (Glauconites, Phosporites, and

Siderites), Organic Matter, and Bentonites.

Possess concentrations ofplatinum elements like iridium.


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Transgressive sys tems tract (reprodu ced from

Van Wagoner et al.).


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HIGHSTAND SYSTEMS TRACT

Below Type 1 or 2 boundaries

Form during:

Late part of sea-level riseEarly Highstand Systems

Tract (EHST)

Stillstand

Early part of sea-level fallLate Highstand Systems

Tract (LHST)

Aggradational to progradational with fluvial sediments

in latter part of systemClassic regressive deposition on shelf

Strata downlap onto Maximum Flooding Surface

Terminated by unconformity by next sea-level fall


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Highstand systems tract(reprod uced from Van Wagon er et al.) .


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Terms: Top Boundary: Truncation, Toplap, Concordance

Bottom Boundary: Onlap, Downlap, Concordance.


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High-resolution seismic profile showing conformable U. Paleocene-

Eocene deposits overlain by S-SE prograding Oligocene-M. Miocene

deposits, onlapped by aggradationel M. Miocene - L. Pleistocene

depoits.


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Santa Cruz Terrace Deposits Downlapping onto unconformity


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Genetic Stratigraphic Sequences

Galloway (1989), described genetic stratigraphic sequence as apackage of sediments recording a significant episode of basin-margin

outbuilding and basin margin flooding.

Product of a depositional episode.

Depositional episode records one complete relative sea-level f luctuationfr om maximum f looding to maximum f looding

RST followed by a TST together form a genetic stratigraphic sequence,

bounded by maximum flooding surfaces(MFS).

Xue and Galloway (1993) used three terms: Progradational Systems tract (PST)

Lowstand Prograding Complex (LPC)

Retrogradational Systems Tract (RST)

FACTORS CONTROLLING STRATIGRAPHIC


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SEQUENCESDynamic interplay of three principal factors: eustatic

changes, sediment supply and basin subsidence rate.

EUSTACY

Eustatic Sea-level change (E) - glacial, tectonic & geoidal

Glacial eustatic changes result from the changing volume of continental

ice caps and have potential change rates of 10 to 100 m/1000 years (Pitman,

1978)

Tectono-eustatic changes results from changes in ocean volume by large-

scale lithospheric plate interactions like variation of sea-floor spreading rate,volume loss in subduction etc.

Geoidal eustacy is due to change in the ambient geoid (sea-level surface).

The geoid is the equipotential surface of the combined rotational and

gravitational potential fields and corresponds to mean sea level.


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Subsidence (T)-Product of tectonics (crustal extension, cooling, tectonic loading) or

sedimentary loading.

Total subsidence changes along the dip direction of the basin margin.

(1) Rate of subsidence increases towards sea (Intracratonic or passive margins

as well as along some leading edges of active margins)

(2) Rate of subsidence increases towards land (Foreland basins on the marginadjacent to orogenic belt)

(3) Rate of subsidence is constant along a dip profile (basins where the localtectonic activity is dormant and where the crust has cooled sufficiently

so that little differential subsidence occur).

Sediment Supply (S)

Overall terrigenous clastic supply is a complex response to source-to-basin

relief and climate of the source area.

Relief, in turn, is controlled by tectonics of the source terrain and immediate

transport pathways to the basin.


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Interplay of the different Factors

Accommodation-Eustacy and subsidence together generate the space i.e.the accommodation for the sediment.

Jervey (1988) combined the effect of both these factors with a broader term

ofrelative sea-level (RSL) changes and defined accommodationas the spacemade avai lable for potenti al sediment accumulation.(which) is a function of

both sea-level f luctuation and subsidence.

Change of accommodation is independent of sediment influx. Rather the

thickness of potential sediment fill is a function of accommodation.

(1) dS/dt > dA/dt - systems tract progrades.

(2) dS/dt < dA/dt - systems tract retrogrades

(3) dS/dt = dA/dt - systems tract aggrades.


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MAJOR VARIABLES

eustacy

climateSediment supply

subsidence

These three together control the water-depth changes


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W = R- S (1)

Or, W= E+T-S . ...(2)

Differentiating,

dW/dt = dR/dt-dS/dt .(3)

Or, dW/dt = dE/dt +dT/dt dS/dt ..(4)

i) A positive rate of change of water-depth (i.e . dW/dt>0 or dR/dt>dS/dt,

deepening), results upward-deepening facies (UDF), transgression.

ii) A negative rate of change of water-depth (i.e. dW/dt


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If magnitude of one of the variables is always much larger than the

others, then the effect of the smaller variables will be effectively

suppressed.

The higher-frequency variable will be the driving force behind the

high-frequency changes in the stratigraphic record.

Eustasy is the higher-frequency variable (Posamentier and James, 1993).

Tectonic subsidence/uplift may be the driving force in many foreland

basins.

Sequence and their systems tracts are controlled by the interplay of

- the rate of change in accommodation

- the rate of sediment supply


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sequence stratigraphy - the basics

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