The nature and origin of petroleum in the Chaiwopu Sub-basin (Junggar Basin), NW China

The Chaiwopu Sub-basin is a minor, ex-tension of the Junggar Basin, NW Chin a, and covers an area of about 2,500 sq. km. It is bounded to the east and north by the Bogda Shan and to the south by the Tian Shan ("Shan " meaning "mountains " in Chinese). Four wells have been drilled in the sub-basin; co ndensate and gas have been produced in noncommercial quantities at one of t he wells (Well C), but the other three wells were dry. In this paper, I inv estigate the nature and origin of the petroleum at Well C. Three of the four wells in the Chaiwopu Sub-basin penetrated the Upper Perm ian Lucaogou Formation. Previous studies in the Junggar Basin have establis hed that laminated lacustrine mudstones assigned to this formation comprise a very, thick high quality source rock. However; the analysis of cores fro m wells in the sub-basin shows that the Lucaogou Formation is composed here of shallow lacustrine, fluvial and alluvial deposits which have very low p etroleum generation potential. Overlying sediments (Upper Permian, Triassic and younger strata) likewise have little source potential. Around 1, 000 m of Upper Permian laminated oil shales crop out at Dalongkou and Tianchi on the northern side of the Bogda Shan. On the southern side o f the Bogda Shan, however; only, 30 m of Upper Permian oil shales occur at Guodikong. Shades and oil seeps from these locations were analysed using st andard organic-geochemical techniques. The physical properties of the petroleum present at Well C, and its carbon isotope and biomarker characteristics, suggest that it has migrated over lo ng distances from its source rock, although an alternative explanation for its origin is not precluded. Burial history modelling indicates that hydroc arbon generation and migration may have occurred before the uplift of the B ogda Shan in the Late Jurassic - Early Cretaceous, the orogenic episode whi ch resulted in the differentiation of the Chaiwopu Sub-basin from the Jungg ar Basin.