准噶尔盆地南缘下侏罗统三工河组地球化学、沉积环境及源区特征分析

Geochemistry,Sedimentary Environment and Source Characteristics of the LowerJurassic Sangonghe Formation in the Southern Margin of Junggar Basin

准噶尔盆地南缘是重要的油气勘探地区,也是研究盆地构造属性、沉积环境演化的重要区域。南缘下侏罗统三工河组由上部主体浅青灰色-灰绿色长石岩屑砂岩、粉砂质泥岩、泥质粉砂岩和下部土黄色长石岩屑砂岩、含砾粗砂岩、砾岩组成,对上部样品进行岩石学、岩相学和地球化学研究显示:样品化学蚀变指数(CIA)为77.57,反映物源区经历中等风化作用;碎屑物质磨圆和分选较差,表明沉积物搬运距离短,成分成熟度低;Cu/Zn、V/Cr、V/(V+Ni)、Ni/Co及U/Th值指示古沉积环境为富氧的氧化环境-贫氧过渡环境;Sr/Cu、Al_(2)O_(3)/MgO值指示温暖湿润的气候特征;Li,Sr,Sr/Ba,Th/U值指示淡水环境;综合岩相学特征认为,其上部主体为浅湖亚相沉积,下部为辫状河三角洲平原亚相沉积。样品中含流纹岩岩屑,La/Th-Hf、La/Sc-Co/Th和Ni-TiO_(2)判别图指示三工河组源岩主要来自于上地壳长英质火山岩;出现少量安山岩岩屑和Th/Co、Th/Cr比值显示源岩有少量中-基性岩。通过样品与不同构造背景杂砂岩成分对比、Th-Co-Zr/10、La-Th-Sc、Th-Sc-Zr/10判别图显示物源区应为大陆岛弧构造背景。

The southern margin of the Junggar Basin is an important area for oil and gas exploration.It is also an important area for studying the tectonic attributes of the basin and the evolution of the sedimentary environment.The previous research on the sedimentary environment and the characteristics of the clastic source area of the Sangonghe Formation in the southern margin is partially inadequate,which is not conducive to further exploration and development.The Sangonghe Formation of the Lower Jurassic in the southern margin is composed of the upper main body of light blue-grey to grayish green feldspathic lithic sandstone,silty mudstone,muddy siltstone,and lower soil yellow feldspathic lithic sandstone,pebbly coarse sandstone,and conglomerate.Petrological,petrographic,and geochemical studies on the upper samples show that the chemical alteration index(CIA)of the sample is 77.57,indicating that the source area has undergone moderate weathering;The poor grinding and sorting of detrital materials indicate that the sediment transport distance is short and the compositional maturity is low;Cu/Zn,V/Cr,V/(V+Ni),Ni/Co,and U/Th values indicate that the paleosedimentary environment is an oxygen rich oxidation environment to a oxygen poor transition environment;Sr/Cu and Al_(2)O_(3)/MgO values indicate warm and humid climate characteristics;Li,Sr,Sr/Ba,Th/U values indicate freshwater environment;Based on the comprehensive petrographic characteristics,it is believed that the upper part is mainly composed of shallow lake subfacies sedimentation,while the lower part is composed of braided river delta plain subfacies sedimentation.The sample contains rhyolite debris,and the discrimination maps of La/Th-Hf,La/Sc Co/Th,and Ni TiO_(2)indicate that the source rocks of the Sangonghe Formation mainly come from felsic volcanic rocks in the upper crust;The presence of a small amount of andesite debris and Th/Co,Th/Cr ratios indicates a small amount of intermediate basic rocks in the source rock.By comparing the composition of samples with different structural backgrounds of sandstone,Th Co Zr/10,La Th Sc,Th Sc Zr/10 discrimination maps,it is shown that the source area should be a continental island arc tectonic background.