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The Jurassic strata are composed mainly of Lower- - Middle Jurassic Dameigou Formation and Upper Jurassic Caishiling Formation��exposed along the Gesigou to Heishuigou in western Altyn area as well as northwest- ern Qimantagh Mountain��showing NEE- trending distribution�� Based on the borehole and seismic data��the Jurassic strata are widely discovered under the Cenozoic strata in western Altyn area�� The Dameigou Formation is character- ized by alluvial fan facies- fluvial facies- coastal lake facies��with abundant dark mudstone and minor amounts of coals��whereas the Caishiling Formation is represented by the pluvial- alluvial fan facies sedimentary conditions�� with development of clastic rocks�� The sedimentary characteristics in western Altyn area are consistent with those in western and northern Qaidam Basin and the Tarim Basin��indicating that the Qaidam and Tarim were probably con- nected with each other at the time when Jurassic deposited��forming a huge Jurassic prototype basin�� At that time�� the Altyn Fault is not exist�� Subsequent to the Oligocene��the rapid uplift of the Altyn Mountain corresponding to the strike- slip activities of the Altyn Fault resulted separation of former Jurassic prototype basin into the present Qai- dam and Tarim basins�� On the other hand��the analyses of organic geochemistry on dark mudstone in the Lower- - Middle Jurassic Dameigou Formation are carried out in this study�� The presented results show that the mudstones are rich in organic matters��belonging to medium- good hydrocarbon source rocks�� The kerogen types of organic mat- ters are mainly type II 2 and type III��with the thermal evolution history in mature to high- mature stage��implying a good hydrocarbon generative potential�� In addition��the source rocks��reservoir beds and cap rocks match well��to- gether with the well developed traps��suggesting that the Jurassic in western Altyn area possesses relatively better petroleum geological conditions and oil exploration prospect��