Sedimentary Facies and Petroleum Source Potential, Upper Permian Lucagou Formation, Junggar Basin, Northwestern China

A. R. Carroll, S. A. Graham, M. S. Hendrix, Jinchi Chu, C. L. McKnight, Yunhai Liang, Xuchang Xiao

Very thick lacustrine sedimentary sequences were deposited in the Junggar basin of northwestern China during rapid tectonic subsidence in the Late Permian. Two measured sections from the southern Junggar document up to 700 m of finely laminated oil shale deposited in an anaerobic to dysaerobic lake, probably during periods of year-round stratification. Lake-margin oolitic and pisolitic carbonates and fine-grained rippled and mud-cracked sandstones are interbedded in places with the oil shales and may record brief shoaling intervals. Permian-Triassic fluvial sandstones and conglomerates were deposited over the Lucagou as clastic sediment supply began to exceed subsidence rates.

Rock-Eval pyrolysis of over 100 Lucagou Formation oil-shale outcrop samples indicates a maximum TOC content of 34%, with maximum hydrogen indices in excess of 900. Kerogen is predominantly type 1. These values equal or surpass the best Green River Formation oil shales, placing the Lucagou among the richest petroleum source rocks in the world. Generated oil can be seen seeping from cracks in outcrops in the southern Junggar; vitrinite reflectance values from these outcrops are about 0.6%, indicating that they have just entered the oil window. Preliminary time-temperature modeling suggests that although the Lucagou may be overmature in the deeper parts of the basin, it is within the oil window near the Karamay field in northwestern Junggar. The Lucagou Formation could underlie as much a one-third of the Junggar basin, suggesting a substantial volume of oil may remain undiscovered.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.