The Study On The Depositional System And Sequence-based Lithofacies-paleogeography Of Xiashihezi Formation In Northwest Of Ordos Basin

Ordos Basin is a large multicycle craton superimposed basin, in the middle tectonic strip of sedimentary basin of china, an important basin with strong potential for natural Gas exploration, and one of the backbone resource bases for " West to East Gas Pipeline Project" .In this thesis, the article apply the basic theories and methods of Sedimentology, High-resolution sequence stratigraphy and Petroleum Geology to research the sedimentary system , high-resolution sequence stratigraphy and lithofacies paleogeography characteristic of Xiashihezi Formation north-west of the Ordos Basin, and offer important basic data to further gas exploration in the Xiashihezi Formation of the Ordos Basin.Based on sedimentary pattern and Characteristic, Xiashihezi formation was indentified as the Continental sedimentary system including alluvial depositional systems , lacustrine-delta depositional system and lacustrine depositional system. Obviously, two river-delta systems developed in the east and west respectively. According to regional tectonic setting and evolution of sedimentary of Basin, Combining with the dominated factors of the fluvial types, hinterland topography (slope), depth of water body .etc. It is concluded that the delta in this area belongs to braided delta(member 8 of xiashihezi formation) and meandering delta(member 5,6,7 of xiashihezi formation) , noting that the deltas have characteristics of river-shallow lake delta. Reservoir sand-bodies are distributary channel deposition and underwater distributary channel deposition, the micro facies such as bayou sand bars and front matting shaped sand-bodie and far sand bars are not well developed.This thesis reseached the high-resolution sequence stratigraphy of the Xishihezi formation in the northwest of Ordos Basin, summarized the sound markers of stratigraphic division and correlation, and built the high resolution sequence stratigraphic framework of Xishihezi formation. Consequently, one Super Long-term Sequence Cycle(SLSC) was indentified , and then , subdivided into five Long-term. Sequence Cycle(LSC) and 10 Middle-term Sequence Cycle(MSC) respectively. Furthermore, it went to study the characteristics and genesis of sequence boundary, systems tracts and sequences.On basis of the above research, using of the sequence-based lithofacies paleogeography mapping methods, drawing the maps of sequence-based lithofacies paleogeography within the long-term and middle-term sequence cycle, disclosing the character and evolvement of the lithofacies paleogeography and sand body spreading regularity of Xiashihezi Formation, show it's paleogeography composed by alluvial depositional systems, lacustrine-delta depositional system and lacustrine depositional system.