Study On The Characteristics And Formation Mechanism Of Upper Sinian Dengying Formation Chert In East Chongqing

In this thesis, we have aimed the Sinian Dengying formation chert in East Chongqing, and studied the texture types, group, evolution characteristics under polarizing microscopy and scanning electron microscopy. Petrological characteristics have showed that chert is an origin of replacement of dolomite. Geochemical characteristics of minor elements, and REE have been detailed studied, and revealed that formation of chert is related to hydrothermal fluids and seawater, controlling the sedimentary facies and diagenesis process. Formation mechanism and time of chert have also been discussed, so is the role of chert in petroleum geology. The following conclusions have been listed:Chert is mainly distributed in 3rd member of Dengying formation, and always interbeds with dolomite. The lithological boundary between dolomite and chert is very clear, and with significant differences. In thin section scale, aphanitic quartz, chalcedony, microcrystalline quartz and mega quarz, whose are characterized by varied crystal form and size, have been showed different texture element features. The texture elements features includes cryptocrystalline , microcrystalline, mosaic (include laminated,fenestrae, broken or curved laminated texture ) and so on. And in the scanning electron microscope scale, residual dolomite crystals have been found in quartz. Quartz have still hold the form of dolomite and some dolomite crystals are very whole because of partly silicification.Main elements in chert have showed that content of SiO₂,TFe₂O₃,CaO,MgO and LOI is high, but the others is low. According to the diagrams of Fe/Ti,(Fe+Mn)/Ti,Al/(Al+Fe+Mn, there is clear that chert originates from hydrothermal fluids.Elements of Ba and As show significant depletion in chert, but richment of Cr. This is very different from the chert deposited directly from hydrothermal fluid. The content of REE is low, with richment of LREE and depletion of HREE. Most chert sample have showed negative anomalies and not clear anomalies ofδCe, even positive anomalies ofδCe. The value ofδEu is obvious negative anomalies, which has different from traditional hydrothermal cherts (with weak positive anomaly). The above distribution characteristics of main, minor and REE have been argely controlled by the original depositional environment and the activity grade of hydrothermal fluid.The values ofδ³⁰Si have varied from -0.6‰ 0.8‰, belong to the range of chert origin from hydrothermal fluid. The values ofδ³⁰Si of quartz in caves have ranged from -0.4‰ 0.5‰. The similar composition between chert and quartz mineral have showed that their have the same fluid source, but precipitation in different ways: replacement of dolomite and filling /deposited in pores water. The value ofδ¹⁸O in chert have ranged from 15‰to 23.5‰. Calculating formation temperature of chert have ranged from 60℃to 140℃according to the values ofδ¹⁸O in chert. Petrographic and geochemical characteristics of chert have revealed that its origin has been controlled by the sedimentary -diagenesis process and seawater-hydrothermal fluids. Comparison to the traditional upper Sinian Liuchapo formation chert, the characteristics of chert around Sichuan basin show obvious origin of replacement of dolomite. And the formation time of chert has ranged from late Sinian to early Cambrian(536.3±5.5Ma). The hydrothermal fluid has been related to the early Xingkai taphrogenesis thermal activity. And the main source of fluids rich in silica is deep thermal fluid through the deep faults on platform margin slope belts and permeable sedimentary strata.Chert has very important effect on the oil&gas exploration of Sinian Dengying formation. Large amount of hydrothermal fluids have caused a local hypoxic environment, where is much helpful to the burial and preservation of organic matter.Meanwhile, the hydrothermal fluids rich in silica have improved the reservoir rocks by dissolution, Moderate silicification and strong neomorphism of silica. The reservoir rocks can be an effective for nature gas accumulation. However, quartz infilling the primary pores or cavies has destroyed the storage spaces, and caused the reservoir rocks more dense. On the other hand, dense cherts have a significant sealing effective on the downward movement of hydrocarbon fluids.