| Eyelid-eyeball-shaped limestone is widely distributed in the Middle Permian of Yangtze area.However,there are divergent opinions on its depositional environment and genesis,which has greatly restricted its exploration and development.Based on field outcrops,cores and thin sections,combined with SEM,XRD,geochemistry,physical property data,and NMR,this paper deals with eyelid-eyeball-shaped limestone in the 1st Member of Maokou Formation from central Sichuan Basin,systematically names eyelid-eyeball-shaped limestone and described the petrologic characteristics,then genesis of limestone is discussed from the perspective of analyzing genesis of clay minerals,finally the microscopic characteristics of eyelid-eyeball-shaped limestone reservoir are elaborated.The main minerals of eyelid-eyeball-shaped limestone include carbonate minerals,quartz and clay minerals,which cannot be named accurately by traditional carbonate naming methods.Therefore,this article uses the petrology classification of carbonate minerals(Car)-clay minerals(Clay)-siliceous minerals(Qz)creatively.After naming,it is found that eyeball-shaped limestone includes carbonate rocks and argilliferous carbonate rocks,and eyelid-shaped limestone includes transition rocks and carbonate rocks with a clay minerals or/and quartz content of more than 10%.Through outcrops and thin sections observation,it is found that the eyelid-eyeball-shaped limestone of the 1st Member of Maokou Formation in central Sichuan Basin is rich in clay minerals.Clay minerals mainly include sepiolite,talc,montmorillonite and illite mixed layer are showed by XRD results.Comprehensive analysis suggests that sepiolite was formed in the early diagenetic stage,and Mg and Si sources were from high-Mg calcite and silica-rich diagenetic fluid.During the burial stage,part of sepiolite began to transform into talc or montmorillonite.Otherwise,part of the montmorillonite and illite mixed layer came from terrigenous input which was from the same source area.The limestone in the study area is rich in bioclasts,such as foraminifera,brachiopoda,ostracoda,bivalve,bryozoan,and dasycladales.Combined with geochemical analysis,it is found that eyelid-eyeball-shaped limestone was deposited in shallow water with a strong hydrodynamic condition.The sedimentary environment was epicontinental sea with normal salinity.And eyelidshaped limestone was deposited in a relatively oxygen-rich and higher paleo-productive environment than eyeball-shaped limestone.In addition,the content differences and correlations of terrigenous elements indicate that although eyelids-shaped and eyeball-shaped limestones both contain terrestrial inputs,there is higher content in eyelid-shaped limestone,mainly including montmorillonite(illite mixed layer)and siliclistics,which are derived from mafic igneous rocks(peridotite)and felsic volcanic rocks(granodiorite),and the provenance region tectonic environment is mainly related to continental island arc area.Otherwise,the δEu negative anomaly and the content characteristics of Al,Fe and Mn of limestone samples excludes the possibility of hydrothermal origin of eyelid-eyeball-shaped limestone.In summary,due to the differences in terrestrial input,organic matter content,and paleo-productivity between eyelids-shaped and eyeball-shaped limestones,it is inferred that Milankovitch cycle caused original sedimentary differences firstly,and then differential compaction formed eyelids-eyeball limestone during diagenesis stage.On the whole,eyelid-eyeball-shaped limestone of 1st Member of Maokou Formation in central Sichuan Basin is a low-porosity and low-permeability reservoir,but eyelid-shaped limestone has better reservoir quality than eyelid-shaped limestone.Therefore,eyelid-shaped limestone in a and c submembers of 1st Member of Maokou Formation has a good reservoir quality.And reservoir space is mainly composed of organic pores and shrink fractures,which are both nano-scale pores with pore diameters mainly between 100 and 500 nm. |
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