Aspects of the petroleum geology of the Junggar Basin, Northwest China

Abstract The Junggar Basin is one of a family of productive interior basins in Northwest China. These are generally referred to as ‘flexural’ or ‘orogenic’ basins nucleated by Variscan orogenic episodes. Their uniqueness is enhanced by later rejuvenation and reactivation in the successive orogenic phases affecting Central Asia. The northwest margin of the Junggar Basin has been productive since the discovery of the Karamay-Wuerhe group of fields in the 1950s. The source rocks are generally considered to be Permian lacustrine sediments which provide mostly liquid hydrocarbons to basin margin alluvial fan reservoirs of Permian, Triassic and Jurassic age. The Karamay Thrust Belt, which forms the northwestern margin of the Junggar Basin, was initially formed during Variscan movements and re-activated during several subsequent structural episodes. According to KOC work there are many trap permutations associated with the Thrust Belt in ‘hanging-wall’ and ‘footwall’ situations. Recent studies have focused on the southwestern part of the Junggar Basin. Here marginal production from Himalayan periclinal structures adjacent to the Tien Shan, dates back to the beginning of the century. More recently Carboniferous ‘basement’ rocks on the southward extension of the Karamay Thrust Belt, the Hong Che Fault Belt, have proved productive. The stratigraphy shows the fairly continuous existence of lacustrine conditions in the mid basin since the Late Palaeozoic. Other studies have shown that ‘black mudstone’ conditions are prevalent in the Permian, Triassic, Jurassic and Early Tertiary and that oil-prone algal-rich intervals, indicating permanent or semi-permanent anoxia, are most likely in the Permian. The stratigraphic development also demonstrates the occurrence of suitable reservoir units in marginal alluvial fan-fluviatile clastics at most stratigraphic levels. The source-reservoir spatial relationships demonstrate favourable conditions for primary and secondary migration. The maturation history shows ‘deep’ Permian source rocks becoming mature for oil generation in the Triassic and for gas-generation from Cretaceous times. More gas-prone higher level source rocks generated hydrocarbons progressively from the Cretaceous through the Tertiary. The structural history shows migration routes controlled by regional tilting into the Tien Shan foredeep, and local migration and entrapment by re-activation of the Karamay Thrust Belt along the northwestern basin margin, and of intra-basinal late Variscan transpressional trends. Late Himalayan structuring along the southern margin and basin-wide tilt has effected a phase of re-migration and further funnelling of hydrocarbons to the productive zones of the northwest.