Origin And Evolution Of The Lower Cambrian Black Shale DOM In The Eastern Upper Yangtze Region And Its Implication For Petroleum Formation

The organic-rich black shale of the Lower Cambrian Niutitang Formation is one of the main hydrocarbon source rocks in the Upper Yangtze region.Although the biogenetic derivation of this ancient Marine source rock is relatively simple and dominated by bacteria and algae,its maceral features are quite complex.Due to the complex thermal evolution and tectonic history of source rocks,most of the organic matter in source rocks is in the highly to over-mature stage,and the chemical composition and structure are gradually converging,which leads to the difficulty in identifying macerals based on chemical and optical properties.However,this evolution and "convergence" process must be accompanied by hydrocarbon generation and expulsion of organic matter in source rocks.Therefore,tracing the origin and evolution of dispersed organic matter(DOM)in ancient source rocks is not only of academic value in theory to increase new knowledge of the origin and evolution of sedimentary organic matter in ancient sediments,but also of great significance in practice to determine the origin and accumulation history of large Sinian-Cambrian gas fields in Sichuan Basin.A total of 247 samples(solid bitumen and shale)were collected from the Lower Cambrian Niutitang shale and Sinian-Cambrian carbonate in the Upper Yangtze region.Organic petrology,element and stable isotope geochemistry were closely combined to carry out the research.By using optical microscope and SEM(EDS),the composition and genetic characteristics of organic macerals in the Niutitang shale were analyzed in detail at the micro and sub-micro scales,so as to reverse and trace the formation process of organic macerals.Moreover,thermal evolution simulation experiments were carried out on the low maturity Middle Cambrian Alum shale,which has a similar maceral composition with the Niutitang shale,and the evolution of various macerals was forwarded and reproduced by comparative study combined with experimental simulation.In addition,the optical properties,trace elements and stable carbon isotopic composition of secondary solid bitumen in source rocks and reservoir solid bitumen are compared in detail.The following main understandings have been obtained:Contrastive studies and thermal evolution simulation experiments reveal the complex macerals of DOM in Niutitang Formation,and confirm the "Previous and present life" of the main macerals.The classification positions of protobitumen and secondary solid bitumen within source rocks were determined,and a new organic macerals classification system of ancient Marine source rocks was established.It is suggested that the DOM in Niutitang shale is composed of protobitumen,solid bitumen,zooclast,sapropel group and inertinite,and the characteristics and identification marks of each component are described respectively.Protobitumen and secondary solid bitumen within source are the characteristic components of the over mature old Marine source rocks including Niutitang Formation.The appearance of protobitumen is coupled with the increase of oxygen content in the palaeo-environment,which may be the relatively hydrogen-poor organic matter formed by the low aquatic organisms such as planktonic algae or bacteria under the action of intermittent oxygenation at the deposition interface.The secondary solid bitumen is the conversion product of bitumen that remains in source rock after primary migration.The simulation results show that the optical evolution of protobitumen and secondary solid bitumen in source is the same as vitrinite,which changes regularly with the increase of heating temperature and time,and the reflectivity increases gradually.It is confirmed that the protobitumen and secondary solid bitumen are indicators of the thermal evolution of organic matter,and the reflectivity can be used to indicate the maturity of organic matter in the pre-Devonian source rocks.Enrichment of in-source solid bitumen was found in some cracks within the calcareous concretion in overmature(Rbitumen > 3.5%)black shale of the Lower Cambrian Niutitang Formation from Songtao,Guizhou,China.As a result,the material basis for the comparison between in-source solid bitumen and reservoir solid bitumen has been laid.The in-source solid bitumen displays a relatively homogeneous optical structure with very weak anisotropy,which is visibly different from the migrated bitumen in the Ediacaran-Cambrian carbonate reservoir.This migrated bitumen in the reservoir displays strong anisotropy and optical structure similar to coke.Considering a strata like the lower Paleozoic in South China,which has no vitrinite and is highly over mature,the in-source solid bitumen is likely to be the most suitable organic component for measuring the reflectance and subsequently determining the thermal maturity of organic matter.Interestingly,the in-source solid bitumen displays some inheritance of its parent material.Not only are the stable carbon isotope ratios very similar to that of kerogen in the black shales(about-34‰ to-33‰),but also the distribution pattern of the trace elements and REE is consistent with that of the Niutitang black shales.The distribution patterns of the trace elements and REE in the migrated bitumen are basically consistent with those of in-source solid bitumen,however,their stable carbon isotope composition is slightly heavier(-33.6 ‰ to-31.2 ‰)than that in the source rocks.In other words,as a key link in the chain the in-source solid bitumen could connect the migrated bitumen in the reservoir with its source rock.This provides direct support for tracing oil and gas sources in the area by using the element geochemical characteristics of migrated solid bitumen in reservoir.It provides a new geochemical method for the analysis of the origin and accumulation history of natural gas in high to over mature areas.