Abstract:Finegrained sediments extensively occur in the southwestern Ordos Basin. Based on a volume of drilling and logging data, redox condition and geochemical features of core, this study discusses the distribution rule and major controlling factors of the finegrained system. The results show that the 7th Member of the Yangchang Formation in the southwestern Odors basin deposited in a warm and humid condition and is extensively distributed in semideep or deep lacustrines, with organic finegrained shale well developed. Organic matterrich shale formed on the bottom of lake basin and was affected by sedimentary modes of sand body. During the deposition of the Chang 73 Member, lake water invaded rapidly, increasing depth and size of the lake. The high productivity resulted in the layering of water body (salinity) and formed extensive anoxic environment, which are conductive to development and preservation of organic matterrich shale. In silent deepflat lake, insufficient clastic supply and high organic carbon gave rise to type Ⅰ and type Ⅱ kerogens, while in slope semideep lake close to the delta front, sediments are affected by terrestrial clastics and dominated by silty mudstone, with type Ⅱ kerogen and development of type Ⅲ kerogen.