Optical Characteristics Of In-source Solid Bitumen In Different Slice Directions And Its Geological Significance

Vitrinite reflectance is considered the most reliable indicator of organic matter maturity.Vitrinite comes from higher plants,but there are no higher plants in the preDevonian strata.Therefore,determining its organic matter maturity has become a major challenge.As early as the 1980 s and 1990 s,domestic and foreign scholars used the reflectance of microscopic components such as solid bitumen,graptolite,chitinozoan,and scolecodont to convert equivalent vitrinite reflectance,and achieved many significant research results.Especially the in-source solid bitumen,as the most common microscopic component in the pre-Devonian sediments,is closely related to the formation of oil and gas,but its genesis is complex,causing significant difficulties for research.This paper attempts to study the optical characteristics of in-source solid bitumen with different slice directions in source rock,and then evaluate the maturity of organic matter in pre-Devonian marine source rock.In this study,a total of 148 samples were collected from the Doushantuo Formation,Niutitang Formation,and Wufeng-Longmaxi Formation in the Middle and Upper Yangtze region,as well as the Cambrian and Ordovician strata in the Tadong Uplift region and the Alum shale in Sweden.Optical microscopy and scanning electron microscopy were used to study the microscopic features,composition,and reflectance characteristics of the samples.Having gained the following understanding:(1)The macerals of pre-Devonian marine source rock can be divided into Sapropelic group,Vitrinite-like group,Zooclasts group,Solid bitumen group and Inertinite group.The in-source solid bitumen is the main maceral in the pre-Devonian high over mature marine source rock;(2)By comparing the reflectance of in-source solid bitumen in different slice directions,it was found that there is a good correlation between them,and the reflectance of in-source solid bitumen in random,vertical,and parallel bedding directions is close;There is a good correlation between the random reflectance of in-source solid bitumen and the random reflectance of graptolite,especially in the vertical bedding direction;(3)Due to the intense tectonic movement in the Tadong uplift area,the reflectance of in-source solid bitumen in the same layer in different areas varies greatly.The reflectance of in-source solid bitumen can be used to evaluate the maturity of organic matter in the Lower Paleozoic source rock in the Tadong area,and these strata have entered over mature stage.This paper describes the maceral composition of the pre Devonian marine Source rock,which is of great significance for the origin and evolution of organic matter and oil and gas in the ancient marine Source rock;The conversion formula between the reflectance of in-source solid bitumen and graptolite is constructed,which can guide the maturity evaluation of organic matter in pre Devonian marine Source rock;The evaluation of organic matter maturity of Lower Paleozoic Source rock in the Tadong uplift area by using the reflectance of solid bitumen in the source is helpful to the exploration in the Tadong uplift area.