A Review of Research Progress on Re-Os Isotopic System of Carbon-enriched Geological Samples

Re and Os are relatively enriched in organic-enriched sedimentary rocks under an anoxic environment due to their organophilic property, and the seawater Os isotopic composition is recorded during the deposition of sedimentary rocks. Under certain geological conditions, the Re and Os in organic-enriched sedimentary rocks will transfer to oil-gas reservoir samples through the process of hydrocarbon generation. The enrichment mechanism of Re and Os in these carbon-enriched geological samples, such as organic-enriched sedimentary rocks, low-metamorphic sedimentary rocks, lacustrine sediments, coal, and oil-gas reservoir samples is the basis of wide geological applications. Re-Os isotopic system of organic-enriched sedimentary rocks has been applied to directly date deposition ages or stratigraphic boundary ages or oil-gas reservoirs, has been used to provide the occurrence time and explore the mechanism of major geological events, reconstructing paleo-environment, and deducing the evolution of petroleum, which has resulted in many achievements.The accumulation mechanism of Re and Os in organic-enriched sedimentary rocks plays an important role in understanding the geological significance of Re-Os isotopic data obtained from organic-enriched sedimentary rocks. In anoxic seawater, Re and Os are reduced to lower valence states of Re(Ⅳ) and Os(Ⅲ), respectively, and then Re and Os enter into organic-rich sedimentary rocks by combining with organic matter. Re and Os are enriched and fractionated during this process, enabling the Re-Os isotope system of organic-enriched sedimentary rocks to directly determine stratigraphic boundaries and sedimentary ages. The Re-Os isotope system has successfully provided accurate stratigraphic boundary ages of Devonian—Carboniferous, Cambrian—Ordovician, Ordovician—Silurian, Permian—Triassic, middle-upper Triassic, and Jurassic—Cretaceous. Furthermore, the Re-Os isotopic system has unique advantages for the strata lacking biological fossils or volcanic interlayers, which have provided the sedimentary age of the Niutitang Formation black shale in Guizhou province and Doushantuo Formation in South China. During the process of hydrogenous source Os into organic-enriched sedimentary rocks, the relative flux of Os from continental weathering, hydrothermal and cosmic dust was recorded, which was reflected as the initial Os isotopic ratio of sedimentary rocks. The coupling relationship between the variation of Os isotopic ratios, atmospheric oxygen level, crustal weathering degree, glaciation events, meteorite impact events, and Large Igneous Provinces can provide the occurrence time and explore the mechanism of major geological events in the history of the earth. The paleoseawater environment and paleoclimate during sedimentation can be further reduced in multiple dimensions by means of the coordinated variation between Re-Os isotopic data, organic matter accumulation, enrichment degree of redox-sensitive elements, and other geochemical proxies.The behavior of Re and Os in the oil-gas system has always been the focus and difficulty in the research of oil-gas reservoir chronology, which is an important basis for understanding the geological significance and wide application of oil-gas reservoir-related samples. The chemical behavior of Re-Os varies in different stages of hydrocarbon formation and evolution. It has been found that the Re-Os isotope system of source rocks will not be destroyed during the process of hydrocarbon generation, and Re/Os fractionation and redistribution of Re and Os will occur during the process of hydrocarbon expulsion and migration. During the process of deasphaltizing, biodegradation, thermal alteration, thermal cracking and oil-water reaction, the asphaltene in crude oil will change correspondingly, and Re and Os in crude will migrate and fractionate accordingly, which will affect the closure of Re-Os isotope system, and even reset the Re-Os isotope system in crude. The different chemical behaviors of Re and Os in the process of hydrocarbon formation and evolution determine the geological significance of Re-Os isochrones obtained from oil-gas samples.Despite the Re-Os isotope system obtaining many achievements and advances in the application of carbon-enriched geological samples, there are still many crucial problems that need to be solved, which restrict the wide application of the Re-Os isotope system. Through statistical analysis of the Re-Os data (including Re and Os contents, ages and initial Os uncertainty) of carbon-enriched geological samples from different sedimentary ages, it is found that the Re and Os contents of organic-enriched sedimentary rocks are at the level of 10^-9-10^-12g/g, and the error of the Re-Os isochron age and initial Os isotopic ratio of many samples exceeds 10%, especially the larger error of initial Os data. In the process of Re-Os isotope analysis, the accuracy of initial Os isotopic ratio data will not only be affected by sample content, procedural blank of chemical treatment method, error propagation and amplification during calculation, but also by geological factors in the process of sampling. The accuracy of the Re-Os data of carbon-enriched geological samples restricts the accurate identification of the paleoenvironment, the occurrence time of major geological events, the true source of seawater Os, and the degree of geological process by using the initial Os isotopic ratio and the Re-Os isochron age of organic-enriched sedimentary rocks. Moreover, the error of Re-Os isotopic data in the oil-gas system is larger than that that in the organic rich sedimentary rocks. Studies on Re-Os chemical analysis methods and sampling methods for different types of oil-gas samples are still insufficient, and the fractionation mechanism of Re-Os in the process of hydrocarbon formation and evolution is still unclear. This hinders the further understanding of the chemical behavior of Re and Os, and the geological significance of Re-Os isochron ages during oil-gas evolution. In addition, the lacustrine sediments and coal have more complex material sources, geological processes, and the impact of terrigenous detrital during the formation process that is difficult to obtain effective sedimentary ages by Re-Os isotope dating at present.On the basis of existing Re-Os isotope analysis techniques, improving the Re-Os data quality as much as possible is the key to studying the Re-Os enrichment mechanism of carbon-enriched geological samples, especially exploring the chemical behavior of Re and Os and geological significance of the Re-Os isochron age of the oil-gas system. There are four aspects that can improve the accuracy of Re-Os isotopic data of carbon-enriched geological samples.(1) Sampling methods: In addition to collecting fresh and unweathered organic-carbon geological samples, the deposition rate can be a reference as a sampling interval. This can guarantee the samples have relatively uniform initial Os isotopic value and a wide range of the Re-Os isotopic ratios and can improve the accuracy of the Re-Os isochron and initial Os values. Through different pretreatment, such as mixing for a long time, or extracting with organic reagents improvements to the homogeneity of oil-gas samples and the quality of Re-Os data can be made.(2) Dissolution methods: Except for the traditional H₂SO₄-CrO₃ and reverse aqua regia method, researchers established two dissolution methods which were HNO₃-H₂O₂ and H₂SO₄-Na₂CrO₄ to solve the problem of high Re blank of the traditional H₂SO₄-CrO₃ method and improve the accuracy of Re-Os data. As the principle of choosing a suitable dissolution method, the Re and Os content of the samples especially the Os content should be considered first and the method with lower procedural blank should be selected.(3) Enrichment methods: Previous studies presumed that the ¹⁸⁵Re/¹⁸⁷Re data obtained by acetone-NaOH extraction method and anion resin exchange method were consistent. However, recent studies have confirmed that different enrichment methods can produce Re fractionation. The acetone-NaOH extraction method can obtain more consistent results than anion resin exchange method during Re enrichment. The acetone-NaOH extraction method is recommended for samples with Re content less than 1ng/g and for oil-gas samples.(4) Selection of reference materials: Matrix matched reference materials are very important in monitoring the process, data quality and method accuracy of Re-Os isotope analysis. At present, the main reference materials used are shale and oil shale standard samples (SBC-1, SCo-2 and SGR-1b) of the United States Geological Survey (USGS) and crude standard samples (NIST RM8505) of National Institute of Standards and Technology (NIST). There are also calcareous organic-enriched shales from the USGS and sedimentary rock samples from the Japan Geological Survey. According to the summarized Re-Os data obtained by different dissolution methods, SBC-1, SGR-1b and SCo-2 have relatively uniform Re/Os content and isotope ratio on the whole, which can be used for Re-Os data monitoring of shale and oil shale, respectively. The Re-Os data of calcareous organic-enriched shale standard samples have been reported recently and show good uniformity which have great potential in monitoring Re-Os data. NISTRM8505 has relatively uniform Re-Os isotope ratio and may be more suitable for isotope ratio monitoring.In conclusion, the Re-Os isotope system of carbon-enriched geological samples provides much key information for the determination of sedimentary age, paleomarine environment evolution, occurrence time of major geological events, and oil-gas evolution, which is significant for understanding Earth's history. It is important to improve the accuracy of Re-Os isotope data of carbon-enriched geological samples by different methods to better understand chemical behavior of Re and Os and the geological significance of Re-Os isochron age. Enriching and improving Re-Os analysis methods, databases and reference materials of carbon-enriched geological samples will provide important support for the study of Re-Os enrichment mechanism of carbon-enriched geological samples and its wide application