Deepwater Depositional System in the Frame Work of Sequence Stratigraphy

Debabrata Biswas and N. C. Mandal
Exploration, ONGC, Kolkata, India

Explanation of deep water depositional system within conventional frame work has it’s own limitations that has posed challenges for hydrocarbon exploration in that set up. The objective of the present paper is to define the depositional system of deepwater province within the frame work of sequence stratigraphy. The data from outcrops, modern analogs, cores, well logs, seismics and biostratigraphy have been utilized for the present study. Tectonics, sediment supply and base level changes are the main controls of the deepwater system. Sequence boundaries are erosional in shelf/basin margin and may be conformable in the deep water areas. Conventional understanding of sequence boundaries and condensed sections that bound the deep water system is one of the limitations for explaining it within sequence stratigraphic framework. In any basin where erosion and base level changes are common phenomenon low stand system tract(LST) may be eroded and the erosional unconformity will be overlain by a thin transgressive system tract (TST) followed by thicker high stand system tracts (HST) overlain by the next sequence boundary on the shelf area. As the sequence boundary and the maximum flooding surface (MFS) that is the top of TST, follow towards basin, the thickness of TST increases and HST decreases towards the deep basin. This is more appropriate explanation of deepwater system in the frame work of sequence stratigraphy. Although paucity of coarser clastics in deepwater areas is one of the facts, it occurs in the form of basin floor fan, slope fan, prograding complexes and turbidities that are found mostly in LST. Coarser clastics come to this set up through channels, submarine canyons, incised vallies. Deepwater system of Mahanadi Basin, India is one of the examples for the present study.

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