Qaidam Basin

ever developed with sulphate water

is typical in salts and petroleum coexisting continental lacustrine basins.Various kinds of salts

such as gypsum

halite

mirabilite

bloedite

epsomite

potash

bischofite

borate and celestite are found in Qaidam Basin

mainly distributed in Pliocene to Holocene series.Salt formation could be divided into two stages

i.e.the first

Pliocene to early and middle Pleistocene

and the second

late Pleistocene to Holocene.The former mainly occurred in the western part of the basin

sulphate waters of which were precipitated typically in continental environment;the later is proceeding in all the modern saline lakes of the Qaidam Basin

the western part of which contain sulphate waters

while those in the eastern part contain chlorate waters.It is assumed that the salt precipitaton was proceeded after vigorous uplift ofthe present region

under dry climate

and by ways of different crystallization process and salt separating sequence for different types of waters.Therefore

two salt-formation patterns have been suggested:the"tear drop"pattern and the "bull's eye"pattern.The former is important not only for the formation of salt

but also for oil and gas.The major oil source rocks are in Oligocene and Miocene in the western part of the basin

while main gas source rocks are in the middle-lower Pleistocene in its eastern part.They were formed in deep lacustrine lakes

where slightly saline to simi-saline sulphate waters under the more humid climate while the region suffering stable downwarping.According to spacial and temporal relationship between the formation of salt as well as oil and gas

three patterns can be suggested:1.Typically

oil and gas are formed before the salt precipitation;2.Oil(-gas)and salt are formed at the same time

but not at the same place;3.Salt and oil-gas are formed alternately.