鄂尔多斯盆地三叠系延长组长8—长7油层组高分辨率层序地层格架及其地质意义

鄂尔多斯盆地三叠系长8—长7油层组位于延长组中下部,代表湖盆最大扩张期,主要属于湖成三角洲相。运用高分辨率层序地层学原理,详细分析了鄂尔多斯盆地三叠系延长组长8—长7油层组地层沉积的基准面旋回特征,识别出1个长期旋回、3个中期旋回(MSC3、MSC4、MSC5)和15~25个短期旋回,分析得出长8油层组为三角洲环境,到长7油层组则演变为半深—深湖环境。3个中期旋回当中,MSC3、MSC4是以上升半旋回为主的不对称旋回,并在MSC4上升半旋回顶部形成盆地最大湖泛面,MSC5以下降半旋回为主的不对称旋回,发育浊积扇。对不同井基准面旋回的分析得出:储集层段多发育于短期基准面上升半旋回的水下分流河道、河口坝和浊流沉积砂体中,且砂体与前三角洲泥构成的烃源岩相匹配,形成岩性油气藏。运用高分辨率层序地层学原理建立地层格架对指导鄂尔多斯盆地延长组油气勘探具有重要指导意义。

The High-resolution Sequence Stratigraphy Framework and Its Geological Significance of Triassic Yanchang Formation Ch8-Ch7, Ordos Basin

Triassic Yanchang Formation Ch8-Ch7 of Ordos Basin was situated in the middle and lower parts of Yanchang Formation,which represented maximum extension of lake basin and was chiefly ascribed to lacustrine delta facies.The character of basal level cycle of Triassic Yanchang Formation Ch8-Ch7 in Ordos Basin is analyzed by the theory of high-resolution sequence stratigraphy and is totally subdivided into 1 long-term base level cycle,3 middle-term cycles and 15 to 25 short-term cycles.Based on the above,the environment of Ch8 is delta and that of Ch7 evolved from semi-deep lake to deep lake.MSC3 and MSC4 of 3 middle-term cycles are unsymmetry cycles dominated by ascending half cycles.During the period of the top of MSC4 ascending half(cycle),maximum-flooding surface was formed.MSC5 is an unsymmetry cycle dominated by a drooping half(cycle).At the same time turbidite fan developed.According to the different wells' base-level cycles,it indicated that the reservoir developed in the sub-distributary channels,the debouchure bars and the turbidities in the half ascending short-term cycles,and it is crossed with the prodelta mud,so lithologic reservoir is easily formed.Establishment of isochronous stratigraphic framework based on the theory of high-resolution sequence stratigraphy should further direct significantly the exploration activity of Yanchang Formation in Ordos Basin.