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Previous HitAlainTop Y. Huc1, P. Bertrand2, D. A. V. Stow3
(1) IFP, Rueil Malmaison Cedex, France
(2) Université de Bordeaux, France
(3) University of Southampton, United Kingdom

Abstract: Depositional processes of source rocks in deep offshore settings: Quaternary analogs

According to the conventional view, deep-water settings are not favorable for source rock formation because organic material is subjected to intensive degradation during its transit through the water column. However, mass transport (via turbidity currents and debris flows for example) of organic sediments previously deposited in shallow water provides one means for the accumulation of terrestrial and marine derived organic rich sediments in deep offshore situations. Moreover, highly biological productive areas, such as active upwelling zones appear to deliver sufficient quantities of organic charge to outbalance the degradative capacity of the system leading to the formation of organic rich sediments even in deep and not strictly anoxic conditions. This has important implications for a better understanding of the distribution of source rocks in deep offshore petroleum systems.

Two examples of Quaternary deposits are presented to illustrate the sedimentology of organic matter in deep water environments: Sediments deposited in the Makassar Strait, off the Mahakam Delta (Kalimantan, Indonesia) at water depths down to 2229m. These sediments exhibit high organic content (with individualized intervals exceeding 5% TOC), which can be rationalized in terms of glacioeustatic changes.

The second example is related to the Benguela current offshore Namibia, which is controlling one of the present day most prolific upwelling systems in terms of primary organic productivity. Recent piston cores sampled at depths 1028m, 2910m and 3606m are respectively 7.7% TOC, 3.81% TOC and 1.77% TOC. These high TOC provide clear evidence for the possibility of the accumulation of organic rich pelagic/hemipelagic deposits in deep oceanic settings.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana