鄂尔多斯盆地奥陶系层序地层格架

通过大量露头、钻井与地震层序地层学综合分析,建立了全盆地奥陶系层序地层格架。研究提出了“碳酸盐岩层序地层划分与对比五要素”分析方法。应用该分析方法在奥陶系识别出3个二级层序界面、6个三级层序界面,将奥陶系划分为2个二级层序和8个三级层序。盆地不同构造环境形成不同的层序地层格架:在盆地西部窄大陆边缘北部奥陶系发育层序 Osq3-层序Osq7五套地层,持续时间从早奥陶世弗洛阶到晚奥陶世桑比阶末,南部发育层序Osq1-层序Osq7七套地层,持续时间从早奥陶世特马道克阶到晚奥陶世桑比阶末,总体上西部地层西厚东薄,南北向条带状展布,向伊盟隆起-庆阳古隆起上超覆尖灭;在盆地南部宽大陆边缘奥陶系发育盆地所发现的8个层序,持续时间从早奥陶世弗洛阶到晚奥陶世凯特阶早期,地层南厚北薄,向庆阳古隆起上超覆尖灭;在盆地中东部台内洼陷奥陶系仅发育层序Osq3-层序Osq5 TST,以盆地东部洼陷东侧最厚向伊盟隆起-庆阳古隆起上超覆尖灭;盆地北部伊盟古隆起、西南部庆阳古隆起主体一直处于隆起剥蚀状态,二者的鞍部仅发育Osq4 TST层序,表明盆地西部的祁连海槽与盆地东部的华北海在中奥陶世晚期有过短暂连通。

Sequence Stratigraphy of the Ordovician System in the Ordos Basin

Through comprehensive analysis of  various outcrops, drilling and seismic sequence stratigraphy, Ordovician sequence stratigraphic framework in Ordos Basin has been established. In view of poor quality of Ordovician seismic data in the basin, a set of suitable sequence stratigraphic analysis method is very necessary. So,we meticulously researched Ordovician sequence stratigraphy of Ordos Basin in the eleventh five-year plan, and put forword "the five elements analysis method of carbonate strata sequence stratigraphic classification and correlation". Using the analysis method, 3 secondary sequence interface and 6 tertiary sequence interface have been identified in the Ordovician, and the Ordovician can be divided into 2 secondary sequences and 8 tertiary sequences. OMsq1 including Osq1 (equivalent to Yeli Group), Osq2 (equivalent to Liangjiashan Group), Osq3 (LST: equivalent to Majiagou 1 Formation;TST: equivalent to Majiagou 2 Formation;HST: equivalent to Majiagou 3 Formation), Osq4 (TST: equivalent to Majiagou 4 Formation;HST: equivalent to Majiagou 5 Formation) and Osq5 (equivalent to Majiagou 6 Formation /Kelimoli Group);OMsq2 including Osq6 (equivalent to lower Pingliang Group/Wulalike Group + Lower Lashizhong Group), Osq7 (equivalent to Upper Pingliang Group/Upper Lashizhong Group + Gongwusu Group) and Osq8 (equivalent to Baiguoshan Group). Different basin tectonic environment formed different sequence stratigraphic framework. Ordovician of the northern portion of western narrow continental margin in the basin can  be divided into Osq3-Osq7 of sequence stratigraphy  of  which geologic time is from the Floian of Early Ordovician to the Sandbian of Late Ordovician. Ordovician of the southern portion of western narrow continental margin in the basin can  be divided into Osq1-Osq7 of sequence stratigraphy of which geologic time is from the Tremadocian of Early Ordovician to the Sandbian of Late Ordovician, The sequence stratigraphy's thickness is 0~1 600 m which exhibit a bolt in shape extend in a north-south ward direction on the norrow edge which is sloped gently toward west and overlapping and pinching out to Yimen-Qingyang Ancient Uplift . In the southern wide continental margin of the basin, we have discoved eight sequence stratigraphy in Ordovician  of  which geologic time is from the Floian of Early Ordovician to the Early Katian of Late Ordovician. The sequence stratigraphy's thickness is 0~1 800 m which exhibit a bolt in shape extend in a west-east ward direction on the wide edge which is sloped gently toward south and overlapping and pinching out to Qingyang Ancient Uplift. In the eastern platform inner sag of the middle-eastern Ordos Basin, we only divided Ordovician into three sequence stratigraphies (Osq 3-Osq5 TST) which thickness is 800m in the east of the depression and overlapping and pinching out to Qingyang Ancient Uplift. Northem Yimeng Ancient Uplift and Qingyang Ancient Uplift have been uplifted in eroding state, which developed the only Osq4 TST ,and show that the Qilian Sea in western Ordos Basin and the North China Sea in Eastern Ordos Basin have been fleetingly connected in the late of Middle Ordovician.