importance of shelf edge delta for hydrocarbon exploration

Important reservoirs if associated with faults

Deltas are likely the most important agent to deliver sediment across the shelf & to deepwater beyond. The shelf edge is the staging area for this delivery

Regime of the shelf-edge delta is important

Somme et al., 2009

SHELF-EDGE DELTAS ARE REGULAR DELTAS BUT OCCUPY SHELF-EDGE SITES

Fluvial Dominated Wave Dominated Tide Dominated

River-dominated deltas: most efficient to deliver sediments basinwards. Wave-dominated deltas: lowest rate of sediment transfer basinwards. The dispersal currents are shore parallel Tidal-dominated deltas: sediment dispersal currents are shore normal

Courtesy C.Olariu

SPITSBERGEN

Porebski & Steel 2003

Log signatures of deltas on the shelf

River & coastal plain reservoirs

Deltas & other shoreline reservoirs

Shelf-Edge Delta

Slope mudstones and slope channels

Basin-floor fan reservoirs

RESERVOIRS

They tend to be thick (100m +) because of increased water depth at the shelf edge

They are often turbidite fronted if delta is strongly river-driven

Alternating beds of soft-deformed and undeformed mouth-bar deposits occur in rollover area

Porebski & Steel 2003

Washakie Basin, S Wyoming

landward basinward 80 km

TURBIDITE-FRONTED SHELF EDGE DELTAS

Distributary channels

Delta-front turbidites 60m

WASHAKIE BASIN/FOX HILLS: RIVER-DOMINATED SHELF-EDGE DELTAS

Downcutting distributary channels

15m

SPITSBERGEN

Shelf-edge incised channel

Thin-bedded delta-front strata

Mellere et al 2003

Ocean waves tend to be large at the shelf break, causing deltas there to be wave dominated. This hinders the deltas from delivering sand to deepwater

Orinoco shelf, Trinidad Steel et al 2007

WAVE-DOMINATED SAND BELTS AT THE SHELF EDGE

Orinoco shelf margin

The conventional model is that falling sea level drives them across the shelf edge, producing deepwater sand accumulation at lowstand

In the last 4-5 years we have been given many examples where cross-shelf transits of the deepwater delivery system happen with rising sea level, ie highstand fans. This requires a strong fluvial drive on the deltas

VARIABLES AFFECTING SHELF TRANSIT TIME (TRAVEL TIME BY DELTA TO SHELF EDGE) •Shelf width & gradient

•Coastal-plain gradient

•Sediment flux

•Sea level behaviour

Muto & steel 2001

WHEN SL IS RISING AT HOLOCENE RATES, SOME DELTAS ARE NOT ABLE TO REACH

THEIR SHELF EDGE

THERE IS A CATEGORY OF DELTA THAT NEEDS SL FALL TO REACH THE SHELF EDGE

Shelf-transit time…………

THE OTHER CATEGORY COULD BE TERMED ‘SUPPLY DRIVEN’ AND CAN DO IT WITH RISING SL

SPITSBERGEN

WASHAKIE BASIN, WY

Shelf edge trajectory shows alternating flat and rising segments (My)

Sydow et al 2003

Saller et al., 2007

Courtesy Josh Dixon, UT Austin

It can work with falling sea level, as in the conventional lowstand model

It can work with rising sea level if supply is sufficient (though much sediment is stored also on shelf

In either case it may not work if delta regime is not strong fluvial drive