Research On Sedimentary Facies And Sequence Stratigraphy In The Upper Palaeozoic In The Northwestward Of Ordos Basin

Ordos Basin is known occurring on the stable North China Craton, and there are relatively faint tectonic events in the inner of the Basin, it is therefore important to make sedimentary facies and sequence stratigraphy research in order to clarify the distribution of reservoirs and explore oil and gas abundance areas. Based on the detailed research on the sedimentary rocks of northwest Ordos basin in their textures, structures, vertical sequences and logging response, six kinds of sedimentary facies can be recognized in this work, i.e., alluvial fan, stream, fan delta, braided delta, delta, lake, barrier beach and carbonate platform, respectively, and more detailed classification of sedimentary subfacies and microfacies for each sedimentary facies are also made in the work.Based on the variation of light/heavy mineral components of sedimentary rocks, the typomorphic characteristics of various debris, the cathodoluminescence color of quartzes, the direction of paleocurrent and the extension of sedimentary facies, two source direction of the studied area in Carboniferous and Permian can be recognized, one is in Bayanwusu, Wulan'ge'er and Hetaoyinshan area north of Hangjinqi in the north margin of Ordos, another is Alashan area in the northwest margin of Ordos.Based on the direction of sources and the ratios of sandstone to formation of sedimentary rocks, together with the sedimentary structures and textures, paleocurrent direction and the characteristics of paleontology, etc., the planimetric maps of sedimentary facies of each stages in the studied area are compiled in order to recognized the extension of dominant facies and the evlotionary history of sedimentary environment, hi the stage of Benxi and Taiyuan, lagoon-tidal flat sediments are widespread in the area, and the deposits of estuary, lagoon and small delta in the northwest margin of Ordos. The deposits in Shanxi Stage are, from north to south, alluvial fans, braided stream, meandering stream, delta plain, delta front and littoral sediments. During the early stage of middle Permian, the studied area is dominant, from north to south, of alluvial fans, braided stream, braided delta plain, braided delta front and littoral deposits, while the extension of sedimentary facies belts in the middle and the late stage of middle Permian is as the same as that of Shanxi Stage.The area was wholly uplifted with Caledonian orogeny in the early Palaeozoic, and underwent thereafter denudation. Most of areas are not deposited until Benxi Stage, except for Helan aulacogen, where the deposit began in Jingyuan Stage. In the late Carboniferous, rift bay deposit occurred in Helanshan, while widespread epicontinental sea deposit developed in the area east of paleo-mid-ridge. In the Taiyuan Stage of the early Permian, North China sea is connected with Qilian Sea to form uniform epicontinental sea deposit. The seawater thereafter withdrawed southward away from the area, and form abundant stream, lake and delta deposit in Shanxi Stage and the early Shihezi Stage and stream, lake and bog deposit in the lateShihezi Stage and Shiqianfeng Stage.Sequence stratigraphy for the upper Palaeozic strata of the area is also discussed on this paper. The sedimentary formation can be summarized as rift bay filling sequence, epicontinental sea filling sequence, offshore debris lake basin sequence and inner continental lake basin sequence. A framework of sequence stratigraphy for the upper Palaeozic strata is established in the area. The Helan aulacogen of Upper Carboniferous in the west margin of Ordos can be divided into 12 sequences, and most of the upper Palaeozoic strata in the area can be divided into 15 sequences. The characteristics of each stage in the sequence evolution are also summarized in the paper.