Abstract:
Laminated shale oil in the interval of Chang 7
3 of the Ordos Basin mainly occurs in the multi-lithology complex strata at the bottom of the source rock, with a formation thickness of 5–20 m. These strata are composed of shale, tuff, silty mudstone, and minor fine sandstone in a laminated stacking pattern, with reservoirs dominated by centimeter-scale tuff and silty mudstone. Due to the small thickness below the λ/4 resolution limit and the weak geophysical property contrasts between reservoirs and surrounding rocks, the seismic reflection signals of the reservoirs are overshadowed by the strong reflections from the shale, lacking independent responses. Thus it is challenging to accomplish reservoir prediction using conventional seismic inversion methods. To tackle this issue, we propose the concept of seismic geobody identifiability (SGI), defined as the quantifiable degree to which a geobody alters the waveform of its enclosing seismic reflection event. The SGI value depends on the geophysical property contrast between the geobody and surrounding rocks, geobody scale and structure, and seismic data quality. Using SGI as the quantitative criterion, we establish a seismic prediction technology system for unconventional reservoirs characterized by “hierarchical identification and classified implementation”. When SGI ≥ 1, mature techniques such as conventional inversion and amplitude attribute analysis are directly applied for rapid prediction. When SGI < 1, targeted identification and prediction techniques are developed based on geobody structural characteristics. For geological parameters that are difficult to predict directly using conventional methods, such as TOC and abnormal formation pressure, indirect prediction is performed by exploiting the inheritance and regularity of geobodies. This system surpasses the conventional framework of “seismic resolution” and formulates differentiated technical strategies centered on geobody properties. It extends reservoir prediction from elastic parameters to geological parameters for the multi-type shale oil accumulations in Chang 7
3, providing methodological support and a practical case for unconventional oil and gas exploration.