Abstract: In early Oligocene, the depositional environment of Qaidam Basin had its own features. The basin was in a semiarid climatic zone and surrounded by mountains. On the northwest it was bounded by the Aerjing mountain with a steeper slope; on the northweast by the Qilian mountain with a gentle slope; on the south by The Kunlun mountain, where the basin slope was between the two above. within the basin, Sub-basinal fault and trough with different depth developed on the base- ment of gentle-slope depression Basically, the basin was a closed drainage basin, in which the lacustrine waterbody had a relatively high salinity and the lake boundary changed greatly, the expanding and shrinking of the water body was obvious. During the period of the lowest water level; The lake covered an area in accordance with that of the deep part of the subbasinal fault, while during the period of lake water expanding, water could submerge not only the other part of the subbasinal fault but also the gentle slope region, especially in the gentle-slope depression water body would cover a much larger area with slightly rising of water level.So in the area between maximal and minimal extents of the lake water, an expanding lake basin, lacking waterbody at intervals, was finally formed. Studying the strata formed in such environments ( mainly the lower part of Lower Gan Caigou Formation of Oligocene but its upper part involved occasionally in the marginal area ) , we find that the sedimentation of this kind of lake basins has the following characteristics: In steep slope area, the gradient is high, the peak period of water input and expansion is in accordance with that of sediment input, the progradation of both shorewards lacustrine deposits and lakewards terrigenous sediments is vertically great, sediments is thickening more quickly than those in other areas at the same time, so alternate-progradation is very active. Diluvial-lacustrine depositional system is well developed. Clastic rock bodies can be divided into two kinds of assemblages: diluvial fan-lacustrine beach and diluvial fan-sudaquatic diluvial fan. The latter may be easily mistaken for the Gilbert delta, the distinction is that the topset of subaquatic fan is lacustrine deposits and the alternate-progradation structures are well developed. In sub-steep slops, affeced by overfast velocity and aggradation of flow as well as intermittently expanding and shrinking of the lake area, hidden-bedding sandstones and conglomerates which are mainly deposited in the expanding lake area are well-developed. The framework of sedimentary system mainly consists of conglomerate and sandstone bodies of diluvial fans, mainstream channel of river and unconventional deltaic facies. According to the change of lake water body, unconventional delta can be devided into three types, i. e. drying up, expanding and interactive. In the extensive gentle slope area, meander stream channels develop very well in the fluvial deltaic system. Since streams are far away from their sources, well-sorted thick sandstone bodies can be formed. Among the above mentioned sandstone and conglomeratic bodies, those of steep slopes and sub-steep slopes are near the source area of oil and gas generation so they are the preferable targets for oil and gas exploration. Sandstone bodies of unconventional delta are easily sealed and often filled with pore fluids with abnormal high pressure, which is favourable for preserving primary pores and forming secondary pores. Even burried in great depth, they still have enough spaces for preserving oil and gas, So they are more important for petroleum exploration.