| The Lower Paleozoic Marine argillaceous hydrocarbon source rocks,including the Lower Cambrian and the Lower Silurian dark gray to black fine-grained sedimentary rocks, distribute widely in South China and are rich in biological organic matter. These source rocks possess good shale gas exploration potential. Conventional organic geochemical parameters are difficult to accurately evaluate the maturation of these source rocks as their high thermal evolution degree, old age and complex tectonic history., Systematical organic petrology research and classification of macerals were utilized to the Lower Paleozoic Marine source rocks from the Upper Yangtze Region. The analyses of micropaleontology, kerogen carbon isotope, and Raman spectroscopy were combined to study the origins, types and differences of macerals.. The sedimentary environment and development pattern of Niutitang Formation were analyzed according to the biological organisms and element geochemical characteristics.Most Lower Cambrian samples are good hydrocarbon source rocks with the average TOC (total organic carbon) content (5.85%) and average equivalent vitrinite reflectance Ro (2.27%) which indicating the over mature state. The average TOC content of Lower Silurian source rocks is 2.39% indicating medium-to-good rank. Their degree of thermal evolution are mainly slightly lower than the Cambrian source rocks which are mainly in high mature stage and over mature stage, with the average equivalent vitrinite reflectance Ro of 1.88%.The results show that the macerals are grouped into atructural and amorphous macerals. The atructural maceral retained some biological structure or morphology, including plant algae, animal organic matters, sclerotinite, and vitrinite-like maceral. The main components of amorphous maceral are amorphous body, microsome and bituminite, mineral bituminous matrix and solid bitumen, etc. The various algae contain lamellar algae, structural algae 1 and structural algae 2, and the former two are from planktonic algae while the latter is benthic algae-forming. Animal organic matters also show abundant varieties, including sponges, foraminifera, hyolithes, graptolites, chitin shell, zooplankton, etc. The graptolites and foraminifera only exist in the Lower Silurian stratum.The biological origins of macerals have a significant characteristic of multiple attributes. In addition to a large number of Plankton type, including Acritarcha, planktonic algae, there are also many benthos such as large algae and sponge, even some lower degeree sporopollen and other animals in the Lower Silurian rocks. Depending on the kerogen carbon isotope, the organic matter types of lower Cambrian rocks are mainly type I, containing a small amount of Ⅱ1 type and the Lower Silurian organic matter type is given priority to type II, with a small part of type I. The diversity of biological origin results in the differences in organic matter division. Therefore, they should be the basis in the research of organic matter type of the Lower Paleozoic source rocks.The similarity of Raman spectroscopy between two kinds of parent material is that they both have the disordered structure peak D and graphite structure peak G in the range of 1000cm-1～2000cm-1, which demonstrates their organic properties and hydrocarbon generation. The differences mainly exist in the spectroscopy parameters, consisting of displacements and intensity ratio between peak D and G. Compared with benthos, planktons have smaller displacements and larger intensity ratio. The polymerization degrees of the macerals originated from benthos are greater than those of planktons, and the order degree of phytoplankton is greater than benthic algae. Raman spectrum method can support the organic petrologic study of high-over mature hydrocarbon source rocks of Lower Paleozoic.The distribution characteristics of microfossils, organic macerals and element geochemistry redox index in the west Hunan 4202 borehole section reflect that the redox conditions change significantly. During early sedimentary period of Niutitang Formation, The oxygen contents are poor so it is mainly reducing environment, but some short oxygenated events happened in this time. On the later depositional stage, the sea level gradually declined and water depth became shallower, leading to the increase of oxygen contents and considerable development of benthos. In addition, geochemical characteristics show the polymetallic is mainly hydrothermal origin. Synthesizing these results with geological background, the sedimentary environment of Niutitang Formation is attributed to be neritic shelf with oxygen contents and water depth changing among the whole period. The depositional model of Niutitang Formation is marine rich organic matter productivity-hydrothermal activity-upwelling-shelf changing shallower. |
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