Geo-seismic characteristics of thin reservoir intervals in West Siberia: interpretation problems and their solutions

The Cretaceous and Jurassic terrigenous section of the West Siberia basin is known from well data to consist of quasi-homogeneous thin layers with thickness ranging from 0.5 to 15 m. Lithologies vary from clays to sands, with many intermediate types of rock with different compaction and clay to sand ratios. Reflection coefficients are generally low (less than 0.1), and seismic multiples are not a severe problem. Only high velocity sparse thin (0.5–1.5 m) coal beds and dense carbonized sandstone add some slight complications to the seismic wave field. Tectonic activity in the region has been very low and most seismic horizons can be treated as quasi-horizontal. However, the porosity and permeability characteristics of the layers are very laterally variable. Even in a Cretaceous clinoform complex of Neocomian age and Jurassic faulted continental intervals, the lateral lithology shows more fickleness than the geometry of the layers. The situation is ideal for regional seismic stratigraphic interpretation, but not for the creation of models based on seismic data to improve field development or the identification of small structural/stratigraphic traps that at best are hardly visible on seismic sections. Many of the oil- and gas-saturated intervals of Cretaceous and Jurassic age are beyond seismic resolution, and most of the reflectors seen by interpreters are a result of the interference of numerous reflected waves. The main difficulty of seismic interpretation in the West Siberia region results from an inadequate regard for this thin layering of the geological strata. Exploration and production activity in the region has been increasingly targeted towards small (1–4 km2) structures with closures of 20–40 m or even less, and effective pay thicknesses of about 1.5–12 m. As operating oil companies are only partly replacing their oil production by putting new reserves on stream, increased priority will have to be given to innovative interpretation (N.A. Krylov 1998). In this situation it is very important to assess the resolution limits of existing geophysical methods and interpretation techniques. So what is the precise influence of some typical thin-layered sand reservoirs on seismic and sonic wave fields, what is the impact of various fluid saturations, and what could be done to improve the interpretation process in the biggest Russian oil province?