GEOCHEMISTRY OF OILS FROM THE JUNGGAR BASIN, NORTHWEST CHINA

The Junggar basin of northwestern China is a structural basin containi ng a thick sequence of Paleozoic-Pleistocene rocks with estimated oil reserves of as much as 5 billion bbl. Analyses of 19 oil samples from nine producing fields and two oil-stained cores in the Junggar basin r evealed the presence of at least five genetic oil types. The geochemis try of the oils indicates source organic matter deposited in fresh to brackish lake and marine environments, including coaly organic matter sources. The volumetrically most important oil type discovered to date is produced from Late Carboniferous-Middle Triassic reservoirs in the giant Karamay field and nearby fields located along the northwestern margin of the Junggar basin. Oil produced from the Mahu field, located downdip in a depression east of the Karamay field, is from a differen t source than Karamay oils. Unique oil types are also produced from an upper Permian reservoir at Jimusar field in the southeastern part of the basin, and from Tertiary (Oligocene) rocks at Dushanzi field and L ower Jurassic rocks at Qigu field, both located along the southern mar gin of the basin. Previous studies have demonstrated the presence of U pper Permian source rocks, and the possibility of Mesozoic or Tertiary sources has been proposed, but not tested by geochemical analysis, al though analyses of some possible Jurassic coal source rocks have been reported. Our findings indicate that several effective source rocks ar e present in the basin, including local sources of Mesozoic or younger age for oil accumulations along the southern and southeastern margins of the basin. Future exploration or assessment of petroleum potential of the basin can be improved by considering the geological relationsh ips among oil types, possible oil source rocks, and reservoirs.