Yanshanian–Himalayan geodynamic transformation of the northwestern Junggar Basin, southwestern Central Asian Orogenic Belt (CAOB), and its significance for petroleum accumulation

The northwestern Junggar Basin in the southwestern Central Asian Orogenic Belt is a typical petroliferous basin.The widely distributed reservoirs in Jurassic–Cretaceous strata indicate that the region records Yanshanian–Himalayan tectonic activity,which affected the accumulation and distribution of petroleum.The mechanism of this effect,however,has not been fully explored.To fill the knowledge gap,we studied the structural geology and geochemistry of the well-exposed Wuerhe bitumen deposit.Our results indi-cate that deformation and hydrocarbon accumulation in the northwestern Junggar Basin during the Yanshanian–Himalayan geodynamic transformation involved two main stages.During the Yanshanian orogeny,a high-angle extensional fault system formed in Jurassic–Cretaceous strata at intermediate to shallow depths owing to dextral shear deformation in the orogenic belt.This fault system connected at depth with the Permian–Triassic oil–gas system,resulting in oil ascending to form fault-controlled reservoirs(e.g.,a veined bitumen deposit).During the Himalayan orogeny,this fault system was deacti-vated owing to sinistral shear caused by far-field stress related to uplift of the Tibetan Plateau.This and the reservoir densification caused by cementation formed favorable hydrocarbon preservation and accu-mulation conditions.Therefore,the secondary oil reservoirs that formed during the Yanshanian–Himalayan tectonic transformation and the primary oil reservoirs that formed during Hercynian–Indosinian orogenies form a total and complex petroleum system comprising conventional and uncon-ventional petroleum reservoirs.This might be a common feature of oil–gas accumulation in the Central Asian Orogenic Belt and highlights the potential for petroleum exploration at intermediate–shal-low depths.

Co-evolution of paleo-environment and bio-precursors in a Permian alkaline lake,Mahu mega-oil province,Junggar Basin:Implications for oil sources

The co-evolution of paleo-environment and bio-precursors in alkaline lakes is of significance to understand the extreme environment system and associated enigmatic hydrocarbon potential.Here we used organic petrology and biomarker geochemistry to investigate the bio-precursors in a Permian alkaline paleo-lake in the Mahu mega-oil province within about a hundred miles of the Mahu Sag,Junggar Basin,China,and its effect on oil generation and accumulation.In general,the bioprecursors in the alkaline lacustrine source rocks of the Fengcheng Formation were mainly bacteria and algae,with a low abundance of higher plants.Therefore,these source rocks were mainly prone to oil generation.Two distinctive hydrocarbongenerating bio-precursors—Dunaliella-like algae and cyanobacteria—were identified.In addition to fossil evidence for these bio-precursors,the former results in a high C_(28)/C_(29)sterane ratio andβ-carotane abundance,and the latter results in the formation of medium-chain monomethyl alkanes in terms of biomarkers.The nature of the bio-precursors varied with the sedimentary paleo-environment,and was controlled by the salinity and stratification of the lake.Dunaliella-type source rocks were deposited in the central area of the alkaline lake,and cyanobacteria-type source rocks were formed around the lake margins.The crude oils in different parts of the Mahu mega-oil province within about a hundred miles have different sources.The bio-precursors in the saline lacustrine source rocks were jointly controlled by the age and water salinity of the source rocks.The physiological synthesis of carotenoids and sterols by haloalkaliphilic green algae may have affected the evolution of ancient green algae.