The Hoh Xil basin, with a total sediment pile of 6,055.2 m thick and an area of 101,000 km2, is the largest Tertiary hinterland basin in the Qinghai-Tibet plateau. The Tertiary strata include the Eocene river-fan-delta-lake Fenghuoshan Group, the early Oligocene riverlake Yaxicuo Group, and the early Miocene fandelta-lake Wudaoliang Group from lower to upper. The geological ages of the Fenghuoshan Group and the Yaxicuo Group are decided as 56-30 Ma ago by the magnetochronostratigraphical methods. Oil shale of 21 cm thick is found in the lake sediments of the Wudaoliang Group with the geographic position of N34° 58' 34" and E93° 00' 01", the Wudaoliang region.
The dark-brown oil shale, which developed in the lower part of the Wudaoliang Group, lies under the caprock of gray-yellow silty mudstone, and intercalates gray calcareous mudstone. The oil shale contains the organic content of 2.46%, the bitumen content of 663.7 ppm, and the hydrocarbon content of 338.1 ppm. The oil source rock of the Wudaoliang Group is 236.2 m thick, which belongs to the good class. On the other hand, the Fenghuoshan Group and the Yaxicuo Group contain the oil source rock of 16.9 m thick, which belongs to the middle class. The total reserve strata of the Hoh Xil basin are measured as about 2,800 m thick. Therefore, the discovery of the oil shale shows excellent petroliferous prospects in the Hoh Xil basin, northern Qinghai-Tibet, "The Roof of the World".
AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid