Source and accumulation process of Jurassic biodegraded oil in the Eastern Junggar Basin,NW China

Biodegradation usually obscures or even radically alters the original characteristics of oil biomarkers.The mixing of oil from multiple sources makes each source difficult to trace.Identifying the source of biodegraded oil from multiple sources has always been a hard nut to crack.Rising to this challenge,in this study-we carried out a comprehensive investigation of biodegradation impacts,oil-source correlation,and oil charging history to trace the source and reveal the mixing process of biodegraded oil in the Toutunhe Formation(J_(2)t)in the eastern Junggar Basin,NW China.The oil of this area was biodegraded to different extent,consequently,many commonly used biomarker parameters(e.g.Pr/Ph,Pr/n C_(17))became less powerful for oil-source correlation.To address this problem,the resistance of many biomarkers to biodegradation was analyzed,and those of high bio resistance were selected to generate a more reliable oil-source correlation.The results revealed that biodegraded oil was a mixture of oil sourced from Lucaogou Formation(P_(2)l)and Xiaoquangou Formation(T_(2-3)xq).Core sample observation,microscopic fluorescent analysis and fluid inclusion analysis were combined to analyze comprehensively oil charging history.The analysis of accumulation process exhibited that the existing oil in J_(2)t was a mixture originated from the P_(2)l and T_(2-3)xq source rocks in two separate charging stages when it underwent a complicated process of charging,biodegradation,recharging and mixing.

The coupling control of biological precursors and environmental factors onβ-carotane enrichment in alkaline lacustrine source rocks:A case study from the Fengcheng formation in the western Junggar Basin,NW China

The organic-rich mudstones and dolostones of the Permian Fengcheng Formation(Fm.)are typically alkaline lacustrine source rocks,which are typified by impressively abundantβ-carotane.Abundantβ-carotane has been well acknowledged as an effective indicator of biological sources or depositional environments.However,the specific biological sources ofβ-carotane and the coupling control of biological sources and environmental factors on the enrichment ofβ-carotane in the Fengcheng Fm.remains obscure.Based on a comprehensive investigation of the bulk,molecular geochemistry,and organic petrology of sedimentary rocks and the biochemistry of phytoplankton in modern alkaline lakes,we proposed a new understanding of the biological precursors ofβ-carotane and elucidated the enrichment mechanism ofβ-carotane in the Fengcheng Fm.The results show that the biological precursors crucially control the enrichment ofβ-carotane in the Fengcheng Fm.The haloalkaliphilic cyanobacteria are the primary biological sources ofβ-carotane,which is suggested by a good positive correlation between the2-methylhopane index,7-+8-methyl heptadecanes/C_(max),C_(29%),andβ-carotane/C_(max)in sedimentary rocks and the predominance of cyanobacteria with abundantβ-carotene in modern alkaline lakes.The enrichment ofβ-carotane requires the reducing condition,and the paleoredox state that affects the enrichment ofβ-carotane appears to have a threshold.The paleoclimate conditions do not considerably impact the enrichment ofβ-carotane,but they have some influence on the water's paleosalinity by affecting evaporation and precipitation.While it does not directly affect the enrichment ofβ-carotane in the Fengcheng Fm.,paleosalinity does have an impact on the cyanobacterial precursor supply and the preservation conditions.