1. Laboratory of Structure �� Sedimentary Reservoir��Exploration �� Production Research Institute��SinoPEC��Beijing 100083��China; 2. Logging Branch of North China Bureau��SinoPEC��Zhengzhou 450006��China

The study of hydrocarbon source condition��formation mechanism of reservoirs and trap characteristics of the Ordovician weathering crust gas accumulation in southern Ordos Basin showed that the Huangling paleouplift separated the Qinling aulacogen and Huabei sea during Early Ordovician��and formed evaporate platform slope gypsum dolomite sediments in northern paleuplift�� The gypsum dolomites and gypsum-bearing dolomites formed various porous reservoirs by differences dissolution in the period of Caledonian orogeny in northern Huangling paleouplift�� which uplifted accompanying with the Ordos Basin�� The widespread dolomites were characterized by finegrained dolomite honeycomb-like��dolomite pinhole-like and dolomite breccia shown in drilling�� During the Hercynian- Indosinian��the paleouplift at the superior site by accepting the migration fluid came from Qinling aulacogen�� and more old reservoirs have been found on the north and south wings of the Huangling paleouplift�� The oil that stored in old reservoirs was transformed into gas by high palaeotemperature during the Late Jurassic-Early Cretaceous�� which provided adequate gas source to Fuxian slope�� Fuxian-Ganquan area became favorable for gas exploration as the dense rock band in Mizhi depression which provided regional trap condition for weathering crust natural gas gathering��