--> Abstract: Tectonic Controls on Upper Permian Lacustrine Oil Shales in the Junggar Basin, NW China, by A. R. Carroll, S. C. Brassell, and S. A. Graham; #91004 (1991)

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Tectonic Controls on Upper Permian Lacustrine Oil Shales in the Junggar Basin, NW China

CARROLL, A. R., S. C. BRASSELL, and S. A. GRAHAM, Stanford University, Stanford, CA

Collision of the Tarim craton with the southern margin of Asia during the Late Carboniferous-Early Permian resulted in uplift of an ancestral Tian Shan range and geographic isolation of the previously marine Junggar basin. Dramatic shifts from marine to nonmarine sedimentation took place in both the southern Junggar and northern Tarim basins during the Permian. Paleocurrent analyses indicate that by the Late Permian, coarse-grained sediments in both basins were being supplied predominantly from the area of the Tian Shan. In addition, sandstone petrographic trends in both basins record the unroofing of granitic plutons in the Tian Shan, suggesting that the uplift of this range was on the order of at least several kilometers.

During the Late Permian, the southern Junggar received in excess of 5000 m of nonmarine sediments, including approximately 1000 m of laminated, highly organic-rich lacustrine mudstones (oil shales). These deposits commonly have TOCs of 20-30%, and Rock-Eval pyrolitic yields reaching 200 mg/g, ranking them among the most prolific petroleum source rocks in the world. Based on a comparison of the distribution of steranes and extended tricyclic terpanes, these Upper Permian oil shales appear to be the primary source of oils in the giant Karamay field in the northwestern Junggar basin. Biomarker distributions in bitumen extracts from the southern Junggar oil shales suggest that these rocks were mostly deposited in a fresh- to brackish-water lake. Biomarkers in oils from Karamay field are d minated by beta-carotane, however, suggesting a hypersaline depositional environment. This apparent paradox can be resolved by considering possible geographic salinity gradients within the 300-km-long Late Permian Junggar lake. It is proposed that the Late Permian Tian Shan induced orographic precipitation on its northern flanks, providing an influx of fresh water into the southern margin of the lake. In contrast, in the northwestern Junggar and other areas more distant from the range, evaporation exceeded freshwater influx. Ancestral uplift of the Tian Shan thus produced a complex tectono-hydrologic partitioning of the Late Permian Junggar basin, which exerted a strong influence on the character of petroleum source rocks deposited within the basin.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)