Volcanics-Forming Environment And Its Hydrocanbon Significance Of The Lower Permian Volcanic Rock In Northwestern Margin,Junggar Basin

In modern volcanology,according to the source of energy that drives volcanic eruptions,the eruption styles are divided into three types: magmatic eruption,phreatic eruption and phreatomagmatic eruption.However,so far people have poor understood or systematic researched on the volcanic eruption processes,models,and their effects on volcanic rock features and reservoir significance in different environments in lacustrine petroliferous basins.This research intends to take the Lower Permian volcanic rocks in the Northwestern Margin of Junggar Basin,where the volcanic rock has a long petroleum exploration history,as an example.Based on data from seismic,drilling logging,core data,thin-slice observation,scanning electron microscopy(SEM),elemental geochemistry and electron probe microanalysis(EPMA),the rock types,petrological characteristics,geochemical characteristics and distribution characteristics of the Lower Permian volcanic rocks in the Junggar Basin are studied.Results as follwed:(1)There are 4 major categories,8 sub-categories,and 14 types of the Lower Permian volcanic rocks in the Northwestern Margin,Junggar Basin.Ash aggregates-bearing lava and welded breccia have an abundance of pores which have great potential of petroleum exploration in study area.(2)Based on literature research about subaqueous eruptions,this research established the discrimination marks for the volcanics-forming environment of volcanic rocks.Volcanic eruptions were widely involved water,which was named as hydrovolcanic eruptions.Normal sediments covered directly by volcanic rocks are the indicators of eruptive environments.In addition,microtextures,special structures,lithofacies or facies sequences,and geochemical index of volcanic rocks can also provide significant evidences.Clearly,those marks are not equally reliable for the judgement of eruptive environments,and most of the marks can be effective in limited application only.(3)Two kinds of eruption styles,phreatomagmatic eruption and magmatic eruption,are recognized.In detail,phreatomagmatic eruption is characterized by peperite,welded tuff and ash aggregates-bearing welded breccia.While,magmatic eruption is characterized by the cementing earlier ash aggregates,forming ash aggregates-bearing breccia lava and lava.The revolution of paleogeographic environment near vent controls the eruption styles and its transition,which then influence the characteristics of eruptive products.This reveals that volcanics-forming environment has significant influence on eruption styles,eruptive process and features of eruptive products.(4)The difference is distinctly obvious in porosity and permeability of different rock types.The deeper reason why the reservoir properties of volcanic rocks are controlled by rock types is that the physical properties of volcanic rocks are controlled by the development of ash aggregates.In the vertical movement of volcanic ash,the droplets wet or nucleate the particles,or gather the particles around them to form spherical cavities,or attach to the surface of the particles to form crescent-shaped cavities;in the lateral movement,the main accretion occurs,and the ash aggregates continually adheres to volcanic ash,with ever-increasing rims.Therefore,the volcanics-forming environment rich in water has a critical controlling effect on the development of pores in ash aggregates,which indirectly affects the reservoir properties of ash aggregates-bearing pyroclastic lava.(5)The volcanics-forming environment changes the eruption styles and the features of eruption products,and greatly affects the developmental types,degree and disciplines of primary reservoir space.Therefore,the influence of volcanics-forming environment on the quality of volcanic reservoir should be considered to improve the accuracy of volcanic reservoir prediction in volcanic reservoir assessment.