--> ABSTRACT: Biomarker Analysis of Upper Permian Lacustrine Oil Shales, Junggar Basin, NW China, by Alan Caroll; #91003 (1990).

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ABSTRACT: Biomarker Analysis of Upper Permian Lacustrine Oil Shales, Junggar Basin, NW China

Alan Caroll

Upper Permian lacustrine oil shales containing up to 34% TOC (total organic carbon) underlie approximately 50,000 km2 of the Junggar basin in western China, and appear to be the principal source of oils in the giant Karamay field in the northwestern Junggar and in several recent discoveries in other areas of the basin. The siliceous oil shales were deposited in a sediment-starved foreland basin during a period of predominantly humid climate. Previous biomarker studies of crude oils from Karamay field have documented an abundance of ß-carotane (which in some cases dominates the aliphatic hydrocarbon distribution) and gammacerane, suggesting a source bed deposited under hypersaline conditions. However, relatively complete outcrop exposures of finely laminat d oil shales in the southern Junggar conspicuously lack evaporites, extensive dessication horizons, or other sedimentological evidence of playa lake environments. Indeed, the aliphatic hydrocarbon distribution in bitumen extracts from southern Junggar oil shales appear characteristic of freshwater to brakish water deposition of organic matter in an anoxic lake. Normal alkanes show a slight odd-over-even preference with relatively low levels of the C22 n-alkane, pristane/phytane ratios close to unity, low ß-carotane and gammacerane levels, and the absence of C34-C35 hopanes. This apparent difference in source bed depositional environments may be due to tectonic partitioning between separate depocenters of the Late Permian Junggar basin. Alternatively hypersaline oil shale facies may be limited to deeper basinal areas, whereas upslope southern Junggar sediments record highstands in lake level or influx of fresh water from the adjacent drainage areas.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990