Prediction Research Of Ordovician Pre-salt Dolomite Reservoirs In Eastern Ordos Basin

The Ordovician pre-salt dolomite reservoir in the eastern Ordos Basin is an important natural gas exploration layer,with huge potential of exploration.In recent years,several high-yield industrial gas fields have been explored in Majiagou Formation pre-salt dolomite reservoir.However,the Ordovician pre-salt dolomite reservoir has the characteristics of fast lateral change,strong heterogeneity,thin layer in single,which restricts the oil-gas exploration development of this formation.Therefore,it is of great practical significance to overcome these problems and form a set of prediction techniques suitable for the Ordovician subsalt reservoirs in the basin.Taking the 4th member of Majiagou formation in zhao26 area as an example,based on fully investigating the regional geological background,guided by theories of multidisciplinary including seismic reservoir prediction,hydrocarbon accumulation theory,seismic sedimentology.Integrally using information of geology,log,seismic and so on,combined with paleogeomorphology restoration,variable window waveform clustering analysis and pre-stack inversion,this paper qualitatively and quantitatively predicts the distribution of pre-salt dolomite reservoir in eastern Ordos from three aspects of fine interpretation of structure,analysis of sedimentary microfacies and reservoir prediction.On the basis of the above research,The distribution law of dolomite reservoir is discussed and favorable exploration targets are put forward.Through this research,the following four main achievements and understandings have been achieved:First,the present tectonic morphology of the study area is relatively simple and inherited.In the north and southeast of the study area,two groups of large-scale NW-trending and NE-trending main faults are developed,and many small-scale faults and fractures are developed along the main faults.The paleogeomorphology analysis shows that the study area is characterized by“alternation of uplift and depression”.Secondly,there are four sedimentary microfacies in the study area,including grain beach,cloud flat,ash cloud flat and ash mud mound.Affected by tectonics and ancient grooves,the study area is divided into four regions,and each region has a ring-like transition from ash mud mound to cloud flat.Third,the photoelectric cross-section index Pe property can be used as a sensitive parameter to distinguish between reservoirs and non-reservoir areas,and the pre-stack inversion is used to predict the thickness of dolomite reservoirs.The prediction results show that AVO inversion can identify 3 m thin reservoirs,which is highly consistent with the logging interpretation.The time window of the 4^th member of Majiagou formation is extracted from the inversion results to calculate the thickness of dolomite reservoirs on plane map.The results show that the reservoirs with thickness greater than 20 m are mainly concentrated in the southern and northeastern regions,and the distribution characteristics are also highly consistent with the distribution law of sedimentary microfacies.Fourth,oil and gas accumulation in this area is mainly controlled by four factors:structure,deposition,fracture and dolomite distribution.Based on the above research results,four favorable exploration targets are optimized.Through the application research of techniques and methods of reservoir prediction in the research area,a technique of predicting Ordovician pre-salt dolomite reservoirs in the eastern Ordos basin is proposed,which provides a right way for the next phase of exploration and development and important reference value for researches in similar areas.