Anatomy of the organic carbon related to the Miocene syn-rift dysoxia of the Rudeis Formation based on foraminiferal indicators and palynofacies analysis in the Gulf of Suez, Egypt

摘要

The syn-rift deposits from the Gulf of Suez witness different episodes of anoxia and dysoxia conditions. In many areas, the sediments of Rudeis Formation that represent the deposition in the time of maximum rift subsidence are considered as the principal organically rich and oil prone sources, while in other areas they are not. These differences are attributed to relative positions to the location of the syn-rift dysoxia and amounts of the carbon flux during active paleoproductivity time. In the present study, an extensive analysis of the syn-rift dysoxia and paleoproductivity during the Miocene is conducted in Burdigalian Rudeis Formation in Ramadan-1 well to determine their influence on the accumulation of organic carbon. The analysis depends on the statistical investigation of the benthic foraminiferal assemblages. The foraminiferal assemblages are discriminated into two biofacies in R-mode (suboxic and dysoxic) and three biotopes in Q-mode (suboxic (S), dysoxic DA and DB sample groups). The sediments are divided into three stratigraphic sequences. Five oxygen minimum zones are determined at the extreme oxygen depletion intervals and they are defined by suboxic (S) and dysoxic (DA) sample groups in descendent order of oxygen depletion intensity and mostly coincide with the high sea levels. The distribution of the organic matter showed large enrichment with partial fluorescence amorphous organic matter indicating kerogen type II in the three biotopes as clarified from palynofacies analysis. The organic carbon enrichment from the studied samples ranged from fair to very good (0.51 and 2.38 wt%) and the relatively larger enrichments are found to be correlated with dysoxic sample group (DA) of intermediate oxygen depletion. The pyrolysis gas chromatography (PGC) specifies the algal rich Type I kerogen that deposited in oxygen depleted marine environment. The mathematical calculation of the paleoproductivity (oligotrophic to eutrophic range 106-352 gCM(-2) yr(-1)), carbon flux (13.77 and 36.19 gCm(-2) yr(-1)) indicates that higher levels of organic productivity are correlated with dysoxic sample group (DA). The larger organic carbon values are found in the Rudeis Formation when the dysoxic conditions from the deeper topographic lows extend to the higher tectonic position of Ramadan-1 well during the sea level rise that furthermore enhances the paleoproductivity.