Abstract:  The geological structure of shale oil reservoir of the Qianjiang Formation in the Qianjiang sag of the Jianghan basin is thinlayer shale under the background of rhythmic layers. It is difficult to accurately describe the spatial distribution of the thin intersalt shale oil reservoir thickness by the conventional seismic exploration methods due to the limitation of seismic resolution. This study is based on the existing seismic data to explore the method of thinlayer seismic prediction suitable for shale thin layers in rhythmic structure. In this paper, the seismic propagation matrix method is used to calculate and analyze the seismic response characteristics of the target rhythmic layer in the study area and the dynamic characteristics of seismic reflection when the thickness and velocity of the target shale thin layer change in the nonuniform thin interbedded structure. It is found that the amplitude and phase of seismic reflection waveform change obviously with the velocity and thickness of the nonuniform shale thin layer. On this basis, combined with logging analysis, a rhythmic geological and geophysical model is constructed, with 99 combinations of shale thin layer velocity and thickness being considered in the model space. Through finding the best match between the actual seismic reflection waveform of the target layer and the seismic waveform in the constructed model space, a seismic prediction method of the thin shale reservoir thickness is established, and by which, the thickness of the thin shale reservoir in the study area is predicted: The thickness varies from 520 m, showing an increasing trend from southwest to northeast. There is a high consistency between the thickness of the predicted shale thin layer and that of the logging interpretation, which verifies the effectiveness of this seismic prediction method and provides important information for the identification and evaluation of shale oil reservoirs.

Key words: Qianjiang sag, intersalt shale oil, rhythmic formation, seismic response, thin layer thickness, seismic prediction