High resolution sequence analysis and sand body prediction of Chang 9 oil set in Hongjingzi area, Ordos Basin

Based on core observation, casting thin sections, petrological features, sedimentary structures, paleontology marks, and mineralogical and geochemical analysis, combined with well log interpretation results of Chang 9 oil set of high resolution sequence stratigraphy and the plane distribution of sand body in Hongjingzi area, the authors have found that, in the period of Chang 9 oil set, this area developed a typical shallow water delta depositional system in which the sand body showed blanket distribution characteristics. High resolution sequence stratigraphy studies show that Chang 9 oil set can be divided into two lake transgressive-retrogressive cycles, i.e., the upper Chang 91 and the lower Chang 92. The base level cycle structure analysis revealed that Chang 9oil set in this area can be identified as being composed of a large regional lake transgressive sequence and two secondary lake transgressive-retrogressive cycles, and can be divided into a long-term cycle sequence, two middle-term cycle sequences corresponding to the Chang 91 and Chang 92 oil set respectively, and four base-level rising and falling phases corresponding to small layers, including lake transgressive-retrogressive depositional systems tracts. In addition, the authors set up the isochronous stratigraphic framework of Chang 9 oil set, and carried out the tracking correlation and prediction of small layers sand bodies.