The partitioning of oxidized and reduced species of exogenic carbon and sulfur, as calculated from secular Phanerozoic trends in ^dgr³⁴S and ^dgr¹³C, suggests a strong coupling between major reservoir transfers and global changes in sea level due to geotectonic mechanisms. The stoichiometry of the major reservoir transfers can be approximated by two tectonic-geochemical end-member scenarios.

Scenario I--high ridge volume, high spreading rates, high global sea level:

Me²⁺CO₃ + SiO₂ + 8CaSO₄ + 2Fe₂O₃ + 15CH₂O ^rarr Me²⁺SiO₃ + 4FeS₂ + 8CaCO₃ + 15H₂O + 8CO₂.

Scenario II--low ridge volume, low spreading rates, low global sea level:

Me²⁺SiO₃ + 4FeS₂ + 8CaCO₃ + 15H₂O + 8CO₂ ^rarr Me²+CO₃ + SiO₂ + 8CaSO₄ + 2Fe₂O₃ + 15CH₂O.

Scenario I tends to be a time of globally widespread carbonates, elevated carbon dioxide, warmer temperatures (greenhouse effect), extensive iron sulfides, light ^dgr¹³C and heavy ^dgr³⁴S. Conversely, scenario II represents a time of globally widespread evaporites, red beds, reduced carbon, carbon dioxide consumption, more frequent glaciation, heavy ^dgr¹³C and light ^dgr³⁴S.

These secular trends which track the first-order sea-level curve have important bearing on global hydrocarbon-reservoir and source-rock strategies. Owing to elevated carbon dioxide in scenario I, the widespread carbonates on the flooded shelves would tend to be composed of allochems and marine cements of metastable aragonite and/or Mg calcites greater than 8 mole % Mg. Such compositions are vulnerable to becoming excellent secondary porosity reservoirs. The carbon dioxide concentrations might also enhance the "anoxic" preservation of source-rock organic matter in areas where slow depositional rates would normally lead to oxidation of reduced carbon before burial. Scenario II, on the other hand, would yield less favorable conditions for carbonate reservoir development. This would be a esult of both a decrease in areal extent (lower sea level, increased clastic input) and a general decrease in potential secondary porosity development owing to the lower carbon dioxide levels which lead to a dominance of more stable non-aragonite Mg calcitic (less than 8 mole % Mg) allochems and marine cements. However, the source-rock potential at this time would tend to be generally favorable owing to the greater global storage of reduced carbon. Furthermore, scenario II would also represent a time of widespread areas for the potential application of a variety of evaporite and red-bed play concepts.

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