The relationship between sedimentation rate and total organic carbon content in ancient marine sediments

Abstract

Sedimentation rate could become a new exploration tool for evaluating the source rock potential of sedimentary basins in frontier regions. Petroleum source rocks are defined on the basis of total organic carbon by weight percent. An analysis of Deep Sea Drilling Project (DSDP) cores indicates that there exists quantitative relationships between sedimentation rate and total organic carbon content in fine grained ancient marine sediments of Jurassic, Cretaceous and Cenezoic age. These relationships are independent of geographic setting, geologic age, and differential compaction, but are highly dependent upon lithology. For any given sedimentation rate, the total organic carbon content increases from calcareous to siliceous to black shale sediments. For each of these lithologies, the total organic carbon content increases with sedimentation rate due to reduced aerobic microbial degradation at higher burial rates. Above a critical sedimentation rate, the total organic carbon content may decrease with increasing sedimentation rate due to a clastic dilution effect. Aerobic microbial degradation, however, continues to be less efficient at higher burial rates. Therefore, even though the total organic carbon content may decrease, the quality of the organic matter preserved and the oil generation and oil migration potential of the sediment may continue to increase with increasing sedimentation rate. Similar relationships have also been established between total organic carbon and grain accumulation rate, and total organic carbon accumulation rate and grain accumulation rate. These relationships support both reduced aerobic microbial degradation and the clastic dilution effect. In the latter case, the lithologic control is less pronounced, and the relationship can be used to determine total organic carbon content even when the lithology is not known. The results of this study have important implications for petroleum exploration in frontier regions. Sedimentation rate and grain accumulation rate could be determined from seismic isopach and velocity data. When the lithology is not known, such as prior to exploration drilling, grain accumulation rates could be used to estimate the total organic carbon content, and the oil generation and oil migration potential of a sedimentary basin. Once the lithology is known, the source rock potential of the basin can be more accurately predicted. Future work should be directed toward testing the application of sedimentation rate and grain accumulation rate in the petroleum exploration of frontier regions.