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

Abstract: 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 9 oil 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.