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Conceptual Depositional Model of Organics in Marine Environment: Integrating Biology and Sedimentology

Huc, Alain-Yves *1; Granjeon, Didier 1; Chauveau, Benoit 1
(1) Geology, IFP Energies Nouvelles, Rueil Malmaison, France.

Predicting the occurrence and organic attributes of source rocks is a mandatory for modern 3D basin models and shale gas. A conceptual model liable to be translated into forward numerical approach can be proposed for marine source rocks. The first parameter is the organic flux exported from the euphotic zone. The second aspect is the transfer of the organic matter to the sea floor. According to the low density of organic matter a ballast effect is required. In this respect organo-mineral transport vectors are mainly fecal pellets and marine aggregates. During the residence time within the water column a part of the organics is degraded. The organic flux decreases with depth according to a power function which has been recognized in many areas of the world ocean. The preservation of the organics on the sea floor is favored by anoxia which is controlled by the oxygen demand and then by the flux of organics reaching the sea floor.

Fecal pellets and aggregates are rapidly disentangled due to the destruction of the chitineous peritrophic membrane packing the fecal pellets and of the polysaccharide exudates of the phytoplankton trapping the organic and mineral particles within aggregates.

This biological driven control is relayed by sedimentary processes since the released organics can recombine with fine grain minerals forming "floccules". The latters play a major protective role for the adsorbed organics and is the main means of transportation for marine organic matter within the bottom boundary layer. The floccules behave as fine grain sediments and their displacement proceeds by suspension, advective processes and traction transport according to bottom currents or water movement induced by storms. These floccules are mandatory to explain the avalanche process controlling organic deposition within observed small scale foresets. The organics follows gradient of water energy and have the tendency to settle down in bathymetric lows of epicontinental seas, carbonate platforms, continental shelf as well as on continental slopes. This water energy control explains the shift of the organic depositional zone in a sequence stratigraphy framework. Two last points need to be considered: a) the linear accumulation rate including a minimal value to allow the preservation of organics during the early burial and the dilution effect when the linear accumulation rate increases, b) the degradation of a part of the organics by the deep biosphere during diagenesis.

 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California