| Yin'E Basin is rich in the mineral resource,and is currently a hotspot for oil and gas exploration.In recent years,several important oil and gas breakthroughs have been obtained in the Mesozoic of Yin'E Basin.Shale reservoir with wide distribution,large sediment thickness and great hydrocarbon show has been discovered in Lower Cretaceous in Hari sag,north of Yin'e Basin.The unique sedimentary structure and mineral composition of Lower Cretaceous shale indicates a special genetic mechanism.Based on previous studies,many rock ore identification techniques,such as optical microscopy,cathodoluminescence,scanning electron microscopy,electron probe,X-ray diffraction analysis,and geochemistring testing methods major,such as trace and rare earth elements,carbon and oxygen isotopes,fluid inclusion analysis,were used to study the characteistics of petrology,mineralogy and geochemistry in this paper,which was used to verify the genesis of hydrothermal deposition,and analyze the source and properties of hydrothermal fluid.The distribution of hydrothermal deposition in the Early Cretaceous was quantitatively analyzed using the indexes of hydrothermal fluid.Combined with the distribution of the volcanic rock and deep fault,the characteristics and controlling factors of hydrothermal exhalation were discussed.Combined with the distribution of the oil and gas in the study area,the effects of hydrothermal deposition on shale oil and gas generation and accumulation were analyzed.Based on the above research,the results and understanding yield as follow:1.Under the regional extension of the Early Cretaceous,a number of deep faults that communicated with the basement and multi-period magma eruptions were developed in the study area with an unusually high geothermal background,which is the basis of a lacustrine hydrothermal sedimentary rock mixed with deep hydrothermal fluid and lake water.2.The study of petrologyand mineralogy indicated that the hydrothermal-sedimentary rocks in Hari Sag develop low-temperature hydrothermal minerals,such as iron dolomite,calcite,analcite,sodium zeolite,barite,siderite and pyrite;and is characterized with sediment structure,such as breccia structures,plaque structures,network vein structures,strip structures and syngenetic deformations,and mineral structure,such as mud-crystal structures,hydrothermal internal debris,and agglomerate structures.3.The study of geochemistry indicated that all the hydrothermal-sedimentary rocks in Hari Sag is characterized with the enrichment of Fe,Mn,Ba,high REE content,strong LREE/HREE differentiation,and negative Eu anomaly,and located in the hydrothermal sedimentation in the disciplinary plate of sediment,which indicates that the study area was affected by hydrothermal activity for a long time during the Early Cretaceous.4.The hydrothermal fluid is characterized by medium and low temperature,medium salinity and low density,with the density rang from 0.93 g/cm~3 to 1.06 g/cm~3,the temperature ranging from 121.8°C to 193.0°C,and the salinity ranging from 5.4 wt%NaCl to 18 wt%NaCl.The equilibrium temperature of mixed is between 44.84 and 103.90°C,which indicated the hydrothermal exhalative deposition is of"white chimney"type of low temperature.5.The hydrothermal sedimentary rocks in the study area are strongly heterogeneous in time and space,and are closely related to the distribution of volcanic rocks and fault.Compared with the Yingen period,the thickness of the hydrothermal sedimentary rocks in the Bayingebi and Suhongtu periods is larger,and the intensity of hydrothermal exhalation is greater.6.The hydrothermal fluids in Early Cretaceous can be divided into periodic transient hydrothermal fluids and continuous steady-state hydrothermal fluids.The two hydrothermal fluids showed significant differences in temperature and pressure conditions,driving force,flow channels,exhalative patterns,sedimentary structures,and associated minerals.The two hydrothermal fluid states tend to coexist and transform to each other under specific conditions.7.The relatively low temperature hydrothermal fluid in the study area promotes the enrichment of organic matter in source rocks,while the relatively high temperature hydrothermal fluid may destroy the preservation of organic matter in source rocks.In addition,the high geothermal field in the hydrothermal deposition area plays an important role in accelerating the hydrocarbon generation.Hydrothermal activity plays both constructive and destructive in the development of the reservoir.Considering the influence of hydrothermal deposition on oil and gas,the fracture developped zone and the high mature source rock distribution area should be the main targets for oil and gas exploration in the study area.. |
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