The Study On Diagenesis Of Tuff-type And Its Influence On Pores In Pingdiquan Formation,Permian,Northeast Zhunggar Basin

Pingdiquan Formation of Permian in the northeast of the Zhunger Basin in Xinjiang Province formed in a rift-fault depression of salt lake of reduction-half oxidation condition.Affected by the volcanic activities and the alkaline magma hydrothermal fluid activities in sedimentary process,a combination of special rocks that was closely related to volcanic eruption and hydrothermal fluid activities formed.Because of the amount of alkaline tuff material and volcanic pouring out and splashing activities,the local temperature field,pressure field and chemical field of hydrocanbon source rocks changed and heat action,adding hydrocarbon catalytic accelerated the maturity and evolution of organic matter of lacustrine facies.So the research area,Pingdiquan Formation,have the characteristics of early maturity,early migration,early reservoir forming and source.According to comprehensive consideration of the regional tectonic background and sedimentary environment,and the observation of the large number of thin sections,combining with the analysis methods of scanning electron microscopy,electron microprobe,X-ray diffraction,cathode luminescence,this paper thinks that the rock types mainly include tuff,sedimentary tuff,tufaceous dolomicrite,tufaceous mudstone which are influenced by magma hydrothermal fluid activities,and micrite,limestone,terrestrial clastic rocks.The primary diagenesis of tuff is based on the alkaline diagenetic setting,and it not only contains alkaline diagenesis(quartz cornosion,carbonate precipitation,analcite,alkaline clay mineral,authigenic albite),but also includes acidic diagenesis(the corrosion of feldspar,analcite,carbonate mineral),presenting the feature of acid-base alternation and showing up a special diagenetic sequence and lithogenous phase(analcite cement,analcite and feldspar corrosion,carbonate mineral cement lithogenous phase).The tuff diagenesis is in a stage of A diagenesis,according to the rock fabric,vitrinite reflectance(Ro),maximum pyrolysis peak temperature.The effect of alkaline tuff material,magma hydrothermal fluid activities and the tuffaceous rocks diagenesis on the pore types and evolution is analyzed.The early diagenesis had the characteristic of alkaline diagenesis,then calcite and analcite cement formed which choked part of the early primary porosity and played a role of resistance to compaction,remaining partial left intergranular pore that makes sure the pore last not so fast.The middle early diagenesis was influenced by the sustainably eruptional and depositonal tuff with huge heat and catalytic hydrocarbon.The organic matter formed low-mature oil and the oil permeated into rocks and occupied the pore which lead to the reservoir rocks owning the resistance to compaction,and major primary porosity could be remained.At the same time,generated CO2 and organic acid partly corrode the early analcite and calcite cement which increased the content of the secondary pore.However,since organic acid and carbonic acid got consumed while corroding carbonate minerals and other minerals,pore medium of the reservoir gradually became acid from alkaline in the late early diagenesis,turning into alkaline diagenetic environment.In the middle diagenesis period-A,with the burial depth rising,rocks' temperature and pressure changed and organic matter in rocks achieved the oil generating threshold.Then a number of hydrocarbon began to form and organic acid was released that dissolved the alkaline minerals in rocks.The secondary pores were generated and the environment became acid.In the research area the special alkaline and acidic fluid alternate diagennesis may have an important effect on the pores and meanwhile provide new approach and thought for oil-gas exploration.