The Contribution Of Graptolite Organic Matter To The Burial Organic Carbon And Its Geological Siginificance

The Upper Ordovician Wufeng and Lower Silurian Longmaxi Formations in the Middle-Upper Yangtze region are a set of graptolite-bearing black shale system.It is not only an important source rock strata,but also an important target strata for shale gas exploration.In the past,it was generally believed that the main contributor of organic carbon in source rocks were the primary producers in the ancient ecosystem.However,as an ancient animal,graptolite is a kind of consumer,and there are many organic fragments preserved.Therefore,the contribution of graptolite organic matter to the burial organic carbon in source rocks is worthy of further research.In this paper,the over-matured graptolite-bearing shales in the Upper Ordovician Wufeng and the Lower Silurian Longmaxi Formations in the Middle-Upper Yangtze region of China were selected for a comparative study with the low-mature Alum shale of northern Europe and Liten shale deposited in the Barrandian basin of the Czech Republic.The combination of organic petrology and geochemistry were employed to analyse the perspective of the burial characteristics of graptolite hard fossils as well as the evolution of optical properties of graptolite periderms.Meanwhile,elemental geochemistry and biomarker parameters were used to analyse the sedimentary environment of graptolite-bearing shale,and determine the source of organic matter,the sedimentary environment and the controlling factors of the enrichment mechanism.Finally,the contribution of graptolite organic matter to burial organic carbon was discussed by comparing the maceral quantification with the separation of graptolite organic matter.The research work has achieved the following main achievements:(1)The distribution characteristics of graptolite fossils has a good positive correlation with TOC in the Wufeng-Longmaxi Formations.In the upper strata of Longmaxi Formation,most of the graptolites are in the form of moulage with low TOC.However,in the Lower Wufeng Formation,the graptolites are mostly preserved in the form of organic hard bodies with high TOC.Under the Stereoscope,the graptolite looks steel gray with metallic lustre,dense distribution,complete form,thick visual effect and kind of stereoscopic sense.(2)The macerals of the graptolite-bearing shale with high maturity in the middle-upper Yangtze region are simple,and the main microstructures of the organic matter are divided into the non-granular graptolite periderms and the secondary solid bitumen.While the macerals of Alum shale in northern Europe with low maturity are rich and various,and the main microstructures that constitute the organic matter are graptolite periderms,lamalginites,solid bitumen and mineral-bituminous groundmasses.Among them,the Alum shale in Sweden mainly contains the non-granular graptolite periderms,while that in E-1 mainly contains the granular graptolite periderms.The maceral characteristics and composition of Alum shale and Liten shale with high maturity are similar to those of the Wufeng-Longmaxi Formations.Thus,The Alum shale in northern Europe and Liten shale in Czech republic with low maturiy are the"past life"of the over-matured shale in Wufeng-Longmaxi Formations.(3)The accumulation of sedimentary organic matter in the late Paleozoic Marine environment may be controlled by many factors,not only the high productivity but also reductive environment.The Yangtze region and Europe generally have a high level of paleoproductivity,but the redox of sedimentary environment is more likely to be the key factor,that is,the reduction environment of water stagnation is benificial to the preservation of graptolite fossils and the enrichment of organic matter.The Wufeng Formation is deposited in a deep water shelf environment with a relatively strong reduction and strong degree of water stagnation,and the preservation of graptolite organic hard bodies are usually good with a high content of TOC.While the shales in Longmaxi Formations are deposited in a shallow shelf environment with slightly poor reductibility,and the water turbulence leads to poor preservation of graptolite organic hard bodies and low content of TOC.(4)The results of maceral quantification showed that the graptolite organic matter in the shale of the Wufeng-Longmaxi Formations accounts for 0.74%-15.19%of the total dispersed organic matter in the rock,which has a positive correlation with the content of TOC,indicating that the graptolite organic matter has a significant contribution to the burial organic carbon.But in general,because the origin of secondary solid bitumen is related to the residue of hydrocarbon generation evolution of various hydrogen-rich components,it is somewhat similar to the ultimate maceral in source rocks.Therefore,it is the most important maceral and the main form of burial organic carbon in the over-matured shales in the Wufeng-Longmaxi Formations,whose content is directly positively correlated with TOC.The TOC content of the seperated organic matters of pure graptolite is 66.14%-72.00%,which is not affected by graptolite species and sedimentary areas.The graptolite organic matter has properties of kerogen type?₂,and has a certain hydrocarbon generation potential,which suggests that it would be a good parent material of gas generation.(5)The result of Scanning Electron Microscopy showed that organic pores were developed both in graptolite organic matter and secondary solid bitumen.Graptolite not only contains a certain amount of organic pores,but also many microfractures with surrounding inorganic minerals.There are a large number of organic pores in the secondary solid bitumen,which are spongy,but with no microfracture.Thus,the existence of graptite organic matter may have a great influence on the reserve of shale gas in the Wufeng-Longmaxi Formations.