DEPOSITIONAL CHARACTERISTICS OF THE EARLY AND MIDDLE JURASSIC ON THE NORTHERN MARGIN OF THE QAIDAM BASIN AND THEIR IMPLICATIONS FOR PETROLEUM GEOLOGY

On the northern margin of the Qaidam Basin(NQB),a Jurassic fossil-rich clastic sequence is well developed.From bottom to top it composed of the Formations of Xiaomeigou,Dameigou,Caishiling and Hongshuigou.The Lower Jurassic includes the Xiaomeigou FM and the Lower Dameigou FM and the Middle Jurassic consists of the Upper and middle of Dameigou FM.During the Early-middle Jurassic,NQB was suffered from intensive and complicated tectonic movement.As the result,there formed many small fault basins,filled by the deposits of shallow lakes,river-flood plains,lake-fan deltas and braid deltas in Early Jurassic,and the deposits of lake alluvial plain,shallow-deep lake in the Middle Jurassic,as the environment changed into a reducing deep water.The Lower Jurassic consists of conglomerate,fine sandstone,siltstone,carbonaceous mudstone and shale,characterized by changing thickness;The Middle Jurassic mainly consists of fine-grained clastic deposits and thick black shale.Thick lacustrine mudstone occurs in the Lower Jurassic and a coal-bearing sequence in the Middle Jurassic in the NQB.Source rocks are rich in organic matters of typeⅡand III.The clastic deposits with high porosity could be a high quality reservoir,with mudstone and shale acting as seal rocks.The combination of source rocks,reservoirs and seals has provided excellent geological conditions for the accumulation of small scale hydrocarbon deposits,in particular gas deposits,in the Jurassic of the region.