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An analysis of Deep Sea Drilling Project (DSDP) cores indicates quantitative relations between sedimentation rate and total organic carbon content in fine-grained ancient marine sediments of Jurassic, Cretaceous, and Cenozoic ages. These relations are largely independent of geographic setting, geologic age, and differential compaction, but are strongly affected by lithology.
For any given sedimentation rate, the total organic carbon content (TOC) by weight percent increases from calcareous sediments to calcareous-siliceous sediments to siliceous sediments to black shale. The total organic carbon content also varies with the sedimentation rate for each lithology. Starting at low rates of sedimentation, TOC first increases with sedimentation rate owing to a more rapid passage of the sediment through the near-surface zone of intense organic degradation. Above a critical sedimentation rate, TOC typically decreases with increasing sedimentation rate owing to clastic dilution of the organic input. Near-surface organic degradation, however, continues to be less efficient at higher burial rates. Therefore, the oil-generation potential of oil-prone organic constit ents should continue to increase with increasing sedimentation rate.
Similar relations were also established between total organic carbon accumulation rate and grain accumulation rate and support both reduced near-surface organic degradation and the clastic dilution effect. By using the TOC cutoff for a petroleum source rock, these relations can be used to identify the presence of petroleum source-rock formations in marine sedimentary basins.
The results of this study may have important implications for petroleum exploration in frontier regions. Sedimentation rate can be determined from seismic reflection records or seismic isopachs in basins where only limited well data are generally available. Seismic records can thus be used to identify potential petroleum source rocks and to estimate the relative oil generation potential of source rocks in various frontier sedimentary basins prior to and during the early phases of exploratory drilling.
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