Sequence Stratigraphic Characteristics Of Ordovician Majiagou Formation In Middle-eastern Area Of Ordos Basin

Under the guidance of new principles and methods of Sequence stratigraphy, modern Sedimentology, Reservoir Geology, Carbonate diagenesis, based on the observation of cores, conventional thin section and casting thin section identification, X diffraction, cathodo luminescence, electeonic probe spectrum and carbon and oxygen isotope analysis etc., together with physical properties, established the sequence stratigraphic framework, systematic studied characteristics of sedimentary facies, diagenesis and the diagenetic environment of Majiagou formation in Middle-eastern area, Erdos Basin under a sequence stratigraphic framework.The most rock are dolomites, associated with calcilutite and micrite in Majiagou formation of Middle-eastern area in Erdos Basin. Types of Sedimentary Facies are mainly composed of wide platforms, evaporative carbonate platform, which mainly belong to evaporative carbonate platform; Surfacie is mainly tidal fiat.Transgressive system tract and late highstand system tract deposition, is characterized by particle deposition and upward shallowing sequence, which belongs to best kind of the primary pore reservoir; Ma54-1member belonging to the highstand system tract of the sequence4, Ma52-1member belongs to the deposition of late high system tract, highstand system tract, is characterized by penecontemporaneous slightly salty seawater mud powder dolomite, combined with nearby the top of Ordovician weathering crust; Pleokarst had been brought about in the meteoric water of earths surface, when the crust rose, having the very strong leaching dissolution, which can produce a large number of pores, should be advantageous to develop high porosity, connectivity good natural gas reservoir.On the comprehensive analysis of a great quantity wells data and combines with the key surface, lithological combination, electrical characteristic and the micro facies, we have identified7parasequences in Ma5Member, Ordos basin. The seven parasequences are all meter-scale cyclic sequences, with the thickness between5m and13m. There is a very good corresponding relationship between parasequences and GR; the bottom depositional environment is mainly inter-tidal zone and sub-tidal zone; the upper depositional environment is mainly supra-tidal zone; the top sedimentary facies is gypsum-dolomite tidal facies, which reflects that the top has suffered from the durable exposure. Based on the micro-examination of thin section analysis, core observation, cathodoluminescence and the data of carbonates geochemistry, we have discussed the control of subsequence on digenesis.Though statistical analysis the data of lithology, petrophysical parameters and porosity, permeability, combined with actual output, with porosity of2%and permeability of0.02×10-3μm2as the lower limit of reservoir, the carbonate reservoir of studying area is divided into class Ⅰ, class Ⅱ, classⅢ, classⅣ, porosity of above4%as the class I favorable reservoir, porosity of3%～4%as the class Ⅱ favorable reservoir, porosity of2%～3%as the class Ⅲ favorable reservoir.