The Filling Structure Of Lacustrine Basin And Prediction On Tight Reservoirs In The Yanchang Formation,Ordos Basin

In recent years, more and more informations on sequence stratigraphy and sedimentology about Yanchang Formation have been revealed, owing to the higher quality of seismic data on Loess Plateau. And the oblique progradation of lake basin filling process was proved to be the cause of density current sand body of Chang 7. In this reseach, we discussed the filling patterns of lacustrine basins and its’ effect on tight reservoir of Chang7-Chang6. And we further established log facies patterns on the different combination of tight reservoirs. The techniques we took throughout the study were log response, seismic forward modeling, seismic inversion and seismic attribute analysis. And the main conclusions are as follows:1、Lake basin in the Yanchang Formation was not filled flatly but with Chang7-Chang6、 Chang7-Chang4+5 or Chang7-Chang3 in different direction and gradient, which is deltaic and gravity flow progradation. Tight sand rock in Chang7 is formed in the environment of delta-gravity flow deposition from southwest slope.2、By establishing the criterias for oilbearing grades, we could determine the reservoir quality, oil-bearing, lithology and electric property of compact sandstone. And we could further analyze the distribution, electrofacies and reservoir- cap assemblages of tight oil.3、The forward modeling of sandstone revealed that sand thickness is proportional to the amplitude, which meant that where there present a weaker amplitude, there may have developed a sandstone.4、We could discriminate tight sand from mudstone by post-stack seismic GR inversion, and we could also estimate a sandstone to be an oil-bearing one if its’ density is less than 2.56g/cm3 through DEN inversion. In a word, it will be better to predict tight reservoirs by the combination of GR inversion and DEN inversion.