Sedimentary Geochemical Records And Organic Matter Accumulation Mechanisms In The Sinian-lower Cambrian Strata

The Sinian-early Cambrian is a critical transition period in Earth history,during which there were complicated interactions among the geochemical recycling of carbon element and trace elements,the sedimentary environment and biological development in paleo-oceans.The studies on trace element and carbon isotope distribution in the Sinian-lower Cambrian strata are of indicative significance to understanding the synergistic mechanisms among the marine environment,biological evolution and organic matter accumulation near the Sinian-Cambrian boundary.In this study,the Sinian-lower Cambrian strata from different sedimentary settings in South China and Tarim Basin were research objects.Based on the geochemical analyses of total organic carbon content(TOC),trace elements,carbonate carbon and oxygen isotopes(?¹³C_carb and?¹⁸O_carb),and kerogen carbon isotope(?¹³C_ker)from the stratigraphic sections,four subtopics were discussed:(1)the co-evolution of trace elements,redox conditions and the Ediacaran Biota during the Sinian-early Cambrian in South China;(2)the factors constraining the organic matter accumulation in the lower Cambrian organic-rich strata from the Keping and Kuluketage areas in the Tarim Basin;(3)the development conditions of lower Cambrian source rocks and the petroleum geochemical characteristics from well Luntan 1 in the Tarim Basin;and(4)the comparisons between the Tarim Basin and South China on sedimentary geochemical characteristics.The organic and carbonate carbon isotopes of the Sinian-lower Cambrian strata from the the slope Dongkanshang section to the basinal Fengtan section in South China showed a decreasing spatial trend resulting from the different dominant microbial taxa at different water depths and the redox stratification of the ocean.The redox-sensitive trace element ratios U/Th,V/(V+Ni),V/Cr,and Ni/Co indicated that the deepwater of the Sinian-early Cambrian ocean in South China was dominated by anoxic-euxinic conditions with interferences from several transient oxidation events that favored biological development.In addition,the relative concentrations of Co,Cu,Ni and Mo were generally higher in kerogens than in the bulk rocks,indicating a selective enrichment of these trace elements by the organisms in the Sinian-early Cambrian ocean.The increasing concentration of the bio-essential trace elements from kerogens was corresponding to the rise of the Ediacaran Biota during the middle of the Sinian period,reflecting a synergistic evolutionary relationship between those trace elements and organisms in the Sinian-early Cambrian ocean.Geochemical analyses of trace elements from the Yuertusi Formation(the Shiairike section in Keping area in the northwestern Tarim Basin)and Xishanbulake and Xidashan Formations(the Yaerdangshan section in Kuluketage area in the northeastern Tarim Basin)demonstrated an anoxic depositional environment and high productivity in both.The enhanced productivity was promoted by the enrichment of nutrient elements such as Ba,Cu,Mo,Ni,V,and Zn from the upwelling fluids and hydrothermal events,and their concentrations were as high as 10000 ppm,130 ppm,79 ppm,66 ppm,1935 ppm,and 244 ppm,respectively,which were much higher than the average content in the upper crust.The lighter kerogen carbon isotope values of the Yuertusi Formation(average-34.89‰)indicated a benthic algal dominated biotype in the Shiairike section,while the relatively higher kerogen carbon isotope values of the Xishanbulake and Xidashan Formations(average-32.65‰)indicated a mixed benthic and planktonic algal biotype in the Yaerdangshan section during the early Cambrian.Higher abundance of planktonic algae,more detrital input and faster sedimentation rate of the Xishanbulake Formation led to a lower organic carbon content but greater thickness of the source rocks than that of the Yuertusi Formation.The accumulation and preservation of the organic matter of the lower Cambrian strata from the well Luntan 1 in Tarim Basin were controlled by high productivity and reducing conditions,similar to that in the northwest of Tarim Basin.The light crude oils from the Cambrian reservoirs in well Luntan 1 displayed an overwhelming predominant of tricyclic terpanes compared with pentacyclic terpanes.It might be related to its high thermal evolution degree,far migration distance,and special parent sources.A comprehensive investigation of the geological background and the correlations of organic carbon isotope,biomarker distribution,and maturity between the crude light oil and the potential source rocks were carried out.The results showed that the light crude oil in well Luntan 1 might be originated from the underlying Yuertusi Formation source rocks.The sedimentary geochemical characteristics and development of the lower Cambrian source rocks in the Tarim and Yangtze Blocks are comparable.The negative anomaly of carbonate carbon isotope at the bottom of the lower Cambrian in the two blocks reflected an anoxic deposition environment during the transgression event.Influenced by the sea level rise and tectonic activities,a set of high-quality source rocks from the lower Cambrian were developed in the two blocks,providing a significant resources to the deep oil and gas resources.The lower Cambrian source rocks from the Tarim Basin and Yangtze region were enriched in various trace elements,which were indicative of a high paleoproductivity.Some of them also have a catalytic effect on the hydrocarbon generation process,especially the transition metal elements with catalytic properties,such as Ni,Fe,Mo and V.Overall,the transition metal element concentrations from the Lower Cambrian Niutitang Formation of the Yangtze region are higher than that from the Yuertusi Formation of the Tarim Basin.This could be a potential factor contributed the difference of the two regions in the degree of organic matter evolution in the lower Cambrian strata.