Hydrocarbon-forming Organism Characteristics And Their Petroleum Geological Significance Of High-quality Source Rocks In The Lower Paleozoic In Southern China

Studies have shown that the Lower Cambrian Niutitang Formation(or Qiongzhusi Formation and Jiumenchong Formation at the same level)and the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation black marine source rock in southern China have good hydrocarbon generation potential.However,the source rocks of lower Paleozoic are generally characterized by high-over maturity.It is difficult to accurately evaluate the quality of source rocks by conventional geochemical parameters.In particular,the study of the characteristics of organic protoplasts(hydrocarbon-forming organisms),which are essential for the hydrocarbon generation and reservoir evaluation of source rocks,is very weak.In this paper,systematic petrology,paleontology and geochemistry studies of the Lower Cambrian shale in the Huangping depression of the southwestern Sichuan and the Upper Ordovician-Lower Silurian black shale in the Sichuan Basin are carried out.The results show that the Lower Paleozoic hydrocarbon-generating organisms are various and abundant,but there are differences in different regions and times.Among them,the collophanite deposits in the bottom of the Lower Cambrian Qiongzhusi Formation in the Well Jin-1 in southwestern Sichuan contain cryptospores,cyanobacteria,tubular organisms and Polymorphic fossils.The Jiumenchong Formation of the Well HY-1 is dominated by bacteriodecomposed,which preserves Leiosphaeridia,lophominuscula and tubular organisms.The graptolite is an important animal organic matter type in the Wufeng Formation-Longmaxi Formation.A large number of radiolarian fossils are also preserved at the bottom of the Wufeng Formation-Longmaxi Formation in the Well LY-I of Eastern Sichuan.Energy spectrum and micro-FTIR analysis showed that the composition of the graptolite may be mainly collagen.Its gas potential is low,however,the pores are formed between the multi-layer fibers of the graptolite,The inner tube of graptolith is hollow and the graptolith exists in the stratum.These void structures are conducive to the storage and migration of shale gas,and the suspected aromatics have high components and strong hydrocarbon generation capacity.Under the scanning electron microscope,the surface of the Acritarch is loose,rich in organic pores and missing pores,providing space for the storage and migration of shale gas.The electron probe shows that the biological shell preserved in the bottom of the Well Jin-1 is fluorocarbon apatite,which is presumed to have a metasomatism during the burial process.There are two significant positive drifts and two significant negative drifts in the organic carbon isotope of the Lower Cambrian Qiongzhusi Formation in the Well Jin-1.The section I of Well Jin-1 is affected by the extinction of the Ediacara and shows a large negative drift.It is located in the Lower Cambrian stage Fortunian.The positive drift of the II section corresponds to the prosperity of the small shell animals,while the III stage organic carbon The negative anomaly of the isotope may represent the extinction of the small shell fauna.The predecessors found two iconic trilobite bands in the top VI of the Well Jin-1,indicating that the chronostratigraphy is located in the stage3,while the organic carbon isotope The positive drift also indicates the prosperity of the Chengjiang fauna.The paleontological identification and trace element analysis combined with the tectonic background showed that the bottom of the Lower Cambrian Qiongzhusi Formation in the Well Jin-1 developed in the basement fault depression formed by the basement fault depression.The forced transgression caused the water body to deepen and form an occlusion and hypoxia.The environment and the simultaneous rupture also bring about hot water activities.The cyanobacteria and suspected sources have a lot of burgeoning.After the accumulation of dead,a large amount of phosphorus is released,and the bottom is preserved as a hydrocarbon-rich bio-phosphorus ore.After that,a slow sea-returning water body changes.The upper carbonaceous shale layer and the silty shale layer are deposited shallowly.