Effect of the basin-fill features on oil shale formation in Paleogene,Huadian Basin

Abstract：On the basis of sequence straigraphy,three third-order sequences was divided in western basin,and oil shale occured in transgressive system tract and high stand system tract.Considering the characters of tectonic evolution,in the initial subsiding stage and the atrophic stage,Huadian Basin was filled with shallow water,and mainly develope alluvial fan,persistent fan-delta and shallow lake deposits,and the depth and scale of semi-deep and deep lacustrine dark mudstone were small.In the maximum subsidence stage,Huadian Basin was deep and stable.The intermittent fan-delt was deposited in the northeast part of this basin.The depth and scale of semi-deep and deep lacustrine dark mudstone reached to its maximum amount.Dozens of high-quality oil shale layers was deposited in this stage.Based on the distribution features of strata thickness,sedimentary facies,thickness of and oil yeild of oil shale,oil shale with the most high-quality and the maximum thickness was not deposited in the maximum depth water without sandbody,but was deposited in the deep water where was near the basin margin and without fan-delta.The interbedded of semi-deep lake and fan-delta front sediments brought nutrients into water,which caused high productivity of lake.During the deposition of semi-deep lake,the reduction deep water is the best place to preserve and get organic enrichment,then forme high-quality oil shale.