The Genetic Mechanism And Geological Significance Of Eyeball Limestone In Maokou Formation In Eastern Sichuan

This paper utilizes outcrops cores and thin sections observations,X-ray diffraction(XRD),electron probe,carbon and oxygen isotope and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)analyses to study macro-and micro-characteristics and geochemical features of eyeball-shaped limestone within Middle Permian Maokou Formation at eastern Sichuan Basin,clarifying its distribution feature and environmental significance,and further discussing the genetic mechanism,which provides an important material to further understand paleoclimate and paleoceanography significance of the development of eyeball-shaped limestone.Through field and thin section identification,we find that a large number of brachiopods that with high levels biotaldiversity,larger individual,striking decoration of shell,and dasycladalean algaes that living in shallow water develop in eyeball and eyelid of eyeball-shaped limestone within Middle Permian Maokou Formation,eastern Sichuan Basin.Combined with regional geological background in the study area,the eyeball-shaped limestone deposited in shallow-water carbonate platform,which coincide well with statistic results that distribution of the eyeball-shaped limestone in South China.Meanwhile,the part of eyelid is short of mollusks fossils which are made up of aragonite.Geochemical research shows that the eyeball and eyelid of eyeball-shaped limestone have similar characteristics of rare elements content with high Sr and low Mn,ratio of Al2O3 and TiO2 and REE distribution pattern that reflect the feature of the original seawater composition,which suggest that the eyeball and eyelid of eyeball-shaped limestone within Middle Permian Maokou Formation at eastern Sichuan Basin formed in the same sedimentary environment with no difference in their original sediment.Nowadays,the difference between eyeball and eyelid may be formed by the differential diagenesis.And the dissolution of mollusks which are made up of aragonite is the key to formation of eyeball-shaped limestone by marine burial differential diagenesis during the penecontemporaneous period.Afterward,differential compaction enhanced the difference between eyeball and eyelid result from marine burial differential diagenesis during the penecontemporaneous period,and formed the eyeball-shaped limestone with different occurrences.Researches on reservoir characteristics of eyeball-shaped limestone show that this type of composition may incorporate into an unconventional porous reservoir featured by low-porosity and low-permeability.In general,this eyeball-shaped limestone,extensively found in the whole Sichuan basin,can be served as a relatively tight limestone reservoir for future hydrocarbon exploration,and the exploration potential cannot be ignored.Besides,the content of calcite,clay and quartz,the value of oxygen isotope and species and content of palaeobios will be changed via marine burial differential diagenesis during the penecontemporaneous period.Thus,we should attach more importance to the diagenetic reconstruction signals when studying the cyclostratigraphy.