Enrichment characteristics of oil and gas in Jungar Basin

Abstract

Jungar Basin is a large sedimentary basin with abundant oil and gas resources and the petroliperous characters in every structural element.Based on the crystalline basement of the Pre-Cambrian and the folded basement of the Pre-Carboniferous,the basin has undergone four history stages including foreland basin from the Late Carboniferous to the Middle Triassic period,moderate extension and depressive basin from the Late Triassic to the early phase of the Middle Jurassic period,trans-compressive basin of the late phase of the Middle Jurassic to the Cretaceous period,foreland basin of the Cenozoic Era.In the four tectonic evolutional stages,the superimposition of the different proto-types of sedimentary basins has resulted in the wedge-shaped geological structure being the thickest in the south and the thinnest in the north,which controls the accumulation pattern of oil and gas.The basin was vertically subdivided into four reservoir intervals by the thick mudstones in Baijiantan Formation of the Upper Triassic series,the thick mudstones of the Tugulu Group of the lower Cretaceous series and the extensive abnormal pressure sealing beds or the basin-scale compartment,including the Carboniferous to the Middle Triassic (C?T 2),the Upper Triassic to the Lower Jurassic (T 3?J 1s),the Middle Jurassic to the Lower Cretaceous (J 2?K 1) and the Upper Cretaceous to the Neogene System (K 2?N).The vertical migration of oil and gas along the faults and the interval vertical breakthrough of abnormal high-pressure fluid system has resulted in a typical hydrocarbon migration and accumulation model as well as resulted in hybrid pools sourced from the different source rocks or source kitchens of different ages.The distributions of oil and gas fields show that the four basement fracture belts trending in north-north-east direction have become the proper paths of oil and gas migration.Along these paths,there has occurred four favorable belts for oil and gas accumulation.The formation of oil and gas fields and the difference of hydrocarbon abundance are chiefly controlled by the coupling style between the basement faults and the sedimentary cover faults.The Jurassic System and the Cretaceous System are favorable for the development of oil and gas reservoirs with the features of distant source,gentle slopes and secondary characters,while the Permian System and the Triassic System are favorable for the reservoirs with the near source,steep slopes and primary characters.Theaccommodationpositionsoffaultandlithologicalbodysuchassandbody ,sandy-conglomerateandconglomeratebodiesarethemainsitesfor hydrocarbonentrapment .OilandgasintheJurassicSystemandCretaceousSystempredominantlyoccurredinthenorth-eastornorth-east- easttrans-compressivestructuralbelts .ThosebeltsmaybetheimportantplaysforthecomingpetroleumexplorationinJungarBasin .