Cambro-Ordovician mudstones

upper Cambrian and Carboniferous gypsum rocks and the relatively tight lithologies have blocked the vertical migration of formation fluids in Tarim basin.In Paleozoic

Tarim basin experienced 6 tectonic movements

including Caledonian(early

late and last)and Hercynian(early

late and last).Except the last stage of Hercynian

all the other tectonic movements were mainly of elevation and subsidence movements and weak eompressional movements

thus they are characterized by limited destruction scopes and faults of small magnitude

resulting in strong vertical compartmentalization of formation fluids.The salinity of Paleozoic formation water in Tarim basin is of brine water level

which is the highest in the Ordovician

the next is in the Carboniferous

while that in the Silusian is the lowest.Preservation of the low salinity features in Silurian indicates that the Paleozoic formation fluids have been strongly compartmentalized;as a result

most hydrocarbons would mainly migrate laterally along the Paleozoic regional unconformities.Vertically

hydrocarbons are mainly distributed along the source beds

while laterally they are distributed both in depressions and uplifts along the migration pathways.