deltas reservoir system slides medco 2006

8/12/2019 Deltas Reservoir System SLIDES MEDCO 2006

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Deltaic successions form a significant

proportion of the Worlds siliciclastic

petroleum reservoirs

1 km10 km


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Review of delta sedimentology andstratigraphy

Coastal processes on deltas

Delta types and reservoirs

Effects of sea level change on deltas

Deltaic reservoirs- correlation & mapping


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Definition, Delta morphology Scale of deltas, size and relationship to

geological time

Deltaic cycles and key surfaces


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Herodotus 445 BC - Nile Delta

a triangular alluvial tract at the

mouth of a river enclosed ortraversed by its distributariesOxford dictionary

a discrete shorelineprotruberance formed when ariver enters the ocean or other

large body of water (Elliot,1986)

shoreline regression typically

associated with progradation


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Alluvial plainfluvial channel

Delta plain

distributaries Delta front

mouth bars Prodelta

shelf muds >10m

10m70m


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Deltas range in scale, from 100m wide to >100km,but are individual delta are typically 10-60m thick

, and


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If sediment supply is high, then most deltas canprograde to shelf edge (10-100s km) in a few

thousand years

Most modern deltas less than 6000yrs old(progradation since global sea level maxima

Most below biostratigraphic resolution, so

geological record comprises deltaic cycles

representing progradation bounded by flooding

surfaces


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Delta classification Fluvial influences and facies

Wave influences and facies Tide influences and facies

Effects of fluvial sediment supply vsaccommodation


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Mississippi

Volga

Fluvial

birdsfoot

lobateWave Tide


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MISSISSIPPI DELTA

Fluvial dominated, no tides,

some waves


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MAHAKAM DELTA

Mixed fluvial and tide

dominated, no waves

Meandering alluvial

plain (incised)

Head of passes

Active distributaries

Abandoned distributaries

Mouth bars Prodelta

10m

70m


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Niger

Baram

Beach ridges on strandplain

Lobate shoreface, tidal channels

Fluvial

Wave Tide


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NIGER DELTA

Wave dominated with tidal influence

Meander belt in

alluvial plain

Abandoned estuarinedistributaries

longshoredrift

Active distributaries


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Landward limit of coastal processes First major bifurcation (head of passes)

Change in gradient, almost flat surface

Decrease in marine or brackish infaunalbioturbation; but bioturbation does occur in

the alluvial plain Alluvial plain typically meandering,

whereas distributaries usually straight


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Distributary channels Mouth bars

Levees Crevasse splays and crevasse deltas

Prodelta muds


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Typically straight, but sinuous thalweg

Depths up to 40m, but mainly 5-20m Widths from 10s to 100s m, up to Kms

Width/thickness ratios 1:50- 1:100

Fluvial dominated channels are zone of sandbypass; lateral bars ->lateral accretion

Tide dominated more heterolithic

May be isolated and mudplugged; can beinterconnnected with mouth bars


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Miocene Mahakam


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Formed when flow velocity declines at the rivermouth and sediment falls from suspension andaccumulates as a bar (coarsening-upward) and can

be fluvial, tidal or wave dominated Fluvial bouyancy dominated flows produce lobate

mouth bars & subaqueous levees

Fluvial friction dominated flows producetriangular mouth bars

Tidal suspension/friction dominated flows produceelongate mouth bars

W/T ratios typically >1:1000, 5-15m thick and

>4km wide; wave dominated >>5km


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Mouth bar

Fluvial channel

Distributary channels

1 km

Crevasse

Splays

Neales Delta, Lake Eyre


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Levees formed in floods, both subaqueousand subaerial; vegetated; climbing ripples;

gentle primary dips

Crevasse Splays; channels and mini-deltas,

with both fining-up and coarsening motifs

Prodelta mainly muddy, dips seaward andonlaps onto shelf


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Floodbasin

Crevasse

Delta

Main

Channel

Distributary Channel

Floodbasin

CrevasseDelta

Distributary Channel


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Delta

successions


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Normal regression during rising or falling

sea level due to greater rate of sediment

supply (progradation) Normal transgression during rising or

falling sea level due to greater rate ofaccommodation than sediment supply

Forced regression (drop in sea level and

resulting incised valleys and lowstand deltas


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Flooding surfaces: transgressive surface and

maximum flooding surface these define

parasequences (or p..sets)

Erosion surfaces: local erosional scour at the base

of fluvial channels or at the fairweather wave base(all minor)

Unconformities & correlative conformable

surfaces that separate older sediment from

younger sediment (sequence boundaries)


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Basic building blocks of the stratigraphic

record produced by one cycle of infilling

accommodation

Represent conformable successions of

genetically related strata bounded by marine

or lacustrine flooding surfaces May either coarsen upward (in deltas,

crevasse splays etc.) or fine-upward

(fluvial/tidal channels)


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If the rate of sediment supply is greater

than the rate of accommodation (S>A)

then sediments will PROGRADE

(progradational stacking pattern,forestepping, or regression)

Eg. deltas!

PROGRADATIONAL PARASEQUENCES


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PROGRADATIONAL PARASEQUENCES

ALLUVIAL PLAIN

UPPER DELTA PLAIN

LOWER DELTA PLAIN

PRODELTA


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If the rate of sediment supply is less than

the rate of accommodation (S


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RETROGRADATIONAL PARASEQUENCES

ALLUVIAL PLAIN

UPPER DELTA PLAIN

LOWER DELTA PLAIN

PRODELTA


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Identify facies (cores, cuttings, fossils)

Identify stacking patterns (wireline logs)

Identify key surfaces (cores and logs)

Use analogues for width/thickness ratios of

distributary channels and mouth bars

Correlate using flooding events notsand

bodies

Interconnectedness of flow units

deltas reservoir system slides medco 2006

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