Influence Of Volcanic Ash On Source Rock Organic Matter Enrichment In The Lucaogou Formation,the Jimusar Depression

The Jimusar Depression locates in the Eastern Junggar Basin,and the Permain Lucaogou Formation is an important tight oil reservoir there.The results demonstrate that the Lucaogou Formation in the Jimusar Depression is a lacustrine mixed rock with characteristics of low clay content and high tuffaceous matter and carbonate.Volcanic ash further development,and rich in the Lower and upper“sweet spots”.Based on microscopic observation,biomarkers,element property and carbon and oxygen stable isotope of carbonates,the dissertation analyzes volcanic ash's influence on the organic matter accumulation,ultimately summarizes organic matter accumulation pattern.The Lucaogou Formation is a thick source rock widespread in the Jimusar,and its average TOC is 3.51%,belonging to good source rock,and the organic matter mainly belongs to?₁type,and its maturity of source rock is mature.Geochemical results demonstrate that the lower sweet spots and basal of the Lucaogou Formation own highest Sr content,highest U content,highest?-carotene content,lowest Pr/Ph,positive anomaly Ce,depositing in brackish-saline and anoxia environment;the lower section between two sweet spots and the upper sweet spot own relatively high Sr content,high carbonate oxygen isotope,high?-carotene content and high gammacerane content,relatively low Pr/Ph,depositing in brackish and semi-anoxia environment;the upper section between two sweet spots owns lowest Sr content and lowest?-carotene content,Pr/Ph over 2 some sections,depositing mostly in brackish and semi-anoxia environment,and oxidize fresh water sometimes.Widespread volcanic ash in the Lucaogou Formation remarkably raises paleoproductivity,organic matter enrichment and paleoproductivity in ash-containing interlayers are higher than others.Algal blooming is critical mechanism,tuffaceous rocks contain high organic content,and algal blooming is frequent in these rocks.olcanic ash enriched layers'paleoproductivity is in the same level of other layers,indicating too much volcanic ash cannot raise paleoproductivity linearly.In the lower“sweet spot”,a large amount of volcanic ash provides the material basis for sulfate reduction in anoxia environment,much of organic matter is degraded into pyrite.What's more,volcanic ash in the lower“sweet spot”causes deterioration of living environment,and Cyanophyta blooms in this section.Plenty of volcanic ash rises the paleosalinity of the upper“sweet spots”depositional period.The organic matter accumulation patterns of the Lucaogou Formation in the Jimusar Depression include“preservation pattern”and“productivity pattern”.During the upper“sweet spot”and the base sediment period,the sediment environment belongs saline reducing lake basin,and plenty of volcanic ash inputs,and the survival stress of lake basin organism increases,leading to the blooming of algae,meanwhile,the degradation of organic matter from sulfate reducting bacteria controls organic matter occumulation in this interlay.Interlayers above upper“sweet spot”belongs to“productivity pattern”.Inspite of paleosalinity and the amount of volcanic ash are fluctuant,the paleoenvironment in the period is semi-oxidation,paleoproductivity controls organic matter accumulation.