Depositional Environment and Sedimentary System of Chang 10 Stage Yanchang Formation in the Ordos Basin

Triassic Yanchang Formation was main oil bearing formation in Ordos Basin,and Chang 10 oil layer was new layer for oil and gas exploration in basin.According to data of outcrop,core observation,grain size analysis,and palaeogeomorphologic reconstruction,etc.,the paper systematically investigated characteristics of sedimentary facies,depositional environment and sedimentary system in lower Yanchang 10 interval.The result indicated that ①Chang 10 oil layer mainly developed alluvial fan and fluvial,and fluvial was chief types of sedimentary facies;②During Chang 10 stage,fluvial sedimentary system was typically controlled by western and northern material source system,but was affected weakly by southern material source system;③In Chang 10 period,Ordos region developed two types of meandering and braided rivers,and braided river sedimentary system mainly distributed in middle-western basin,meandering river sedimentary system was chiefly in south of basin.④ During Chang 10 stage,the Ordos today was discharge zone,then collection area may be located in western Henan province-the East Qinling,and its ancient geographical environment may be similar to present Mississippi River;⑤ Channel sands of Chang 10 were matched with lacustrine oil shales of Chang 9,which may form the source rock on reservoir hydrocarbon accumulation assemblage in longitudinal direction.