A Review on DeepSea Sedimentation Theory: Significances to Oilgas Exploration

Key words: 
• Bouma Sequence /
• sediment gravity flow /
• bottom traction current /
• turbidity current and turbidite fan /
• submarine fan /
• deepsea sequence stratigraphy /
• deepsea oilgas exploration

Abstract: It has been challenged for the sedimentological theories, such as “Bouma Sequence” and “turbidite fan”, in the direction of deepsea oilgas exploration. This is because, as Shanmugam et al. say, the “Bouma Sequence” is not a unique product of turbidity sedimentation, but results from cooperation of several sedimentation, i.e., sandy debris flow, turbidity current, internal tide and wave, contour current. Following the interpretation by Shanmugam et al., it is summarized as: sediments of the “Bouma Sequence” section A could be caused by sandy debris flow besides of turbidity current, those with crossbeddings within the sections BD be traction current; only those by turbidity current could be called turbidite; socalled “high density turbidity” is not real turbidity but sandy debris flow; turbidity current is Newton current and turbulent sediment gravity flow; there are no suspended layer and reverse grading within turbidite. It lacks linkage for deepsea traction current between theory and practice although advances have been made. And as an aspect of sedimentation, deepsea sequence stratigraphy is to some degree different from that in shallow sea：there are at least three types of sequence boundaries——slope erosional surface, downlap surface of lowstand systems tracts (LST), and submarine sediment hiatus surface; when it is dominated by a terrigenous depositional setting, LSTs would be composed of submarine fan sediments, transgressive systems tracts (TSTs) and early highstand systems tracts (HSTs) are probably of noncalcareous pelagic sediments, and few HSTs are found; when it becomes carbonate setting, LSTs are dominant of slump and debris sediments, TSTs and early HSTs could spread on submarine fan, and late HSTs could be consisted of calciturbidites. Support remarks and suggestions are proposed by review and experienced works of deepsea sedimentation: it would be better to invert comma “Bouma Sequence” to make a mark showing a note of notunique causal turbidite result; it is necessary to use the term “submarine fan” and/or "deepsea fan instead of “turbidite fan”; study of both theoretic deepsea traction currents (e.g. internal tide and wave, contourite) and practical field needs to promote in future. It is great potential and prosperous for oilgas exploration in deepsea sediment, especially for reservoir prediction of LST turbidite, sandy debris and bottom traction sandy bodies. However, to get a new burst in deepsea oilgas exploration, a key is that more precise and detailed study on theory of deepsea sedimentation and depositional procession would be engaged in future.