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DEEPWATER/SUBMARINE FANS


DISCUSSION SUMMARY : JUNE 24, 1999

Introduction
Definition:
- defined on the basis of processes rather than on bathymetrical numbers
- absence of tide and wave influences, without 'fluvial' related processes
- turbidite deposits can be found in few meters depth (small lakes/ponds) to more than 200 m.
- ichnofossil indication: Zoophycos and Nereites

Processes
- bottom traction currents (semi-)permanent contourites, lag deposits
- fluid escapes sedimentary structures as the indication of rapid depositional process during deposition.
- settling from suspension (semi-) permanent (hemi) pelagic muds, organic rich oozes as impact of slowing down the depositiona process (low-density turbidite), or usually called "turbidity tail".

Source rock of the deep water play
- insufficient knowledge of ancient insitu source rock. Current deepwater plays in have source rock whose potential depends very much on carbonaceous material supply landward.
- in Kutai basin, no kerogen components show insitu deep-water material; all are transported from terrestrial.
- oxygen minimum zone (OMZ) naturally formed in 1800-2000 m water depth.
- in order to produce high-potential source rock, it needs restricted conditions (e.g. Weser, 1977), pronounced upwelling, upper slope OMZ, anoxic waters in intraslope (Howell & Normark, 1982)
- modern example on average organic carbon value
- Hemipelagic muds in restricted basins: 2-2.5%; in open ocean: 0.5% (Nelson & Nielsen, 1984)
- Areas with higher sedimentatin rate: 1% (Wilde, 1978)
- Extreme case: 25% in Mediterranean Sea - low sea level & conditions of water circulation (Sammerhayes, 1977)

Back to basic geochemistry
- preservation of organic material is a key
- standard maturity window and quality classifications are some time mis-leading
- source rocs are not necessarily penetrated / identified (Western Indonesia - NW Borneo, Gulf of Mexico source rocks are not really understood but prolific)
- number of organic material might not concentrate in particular beds but should be enough to charge in many basins.
- in relation with 'confined' and 'unconfined' system, intra-deepwater source-rock preservation is better in 'unconfined' system because of the un-settlement-trend of deposition (avulsion) which allows reduction (for source rock preservation) condition in places, away from clastic fairways.

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