Seismic works in Domanik-type formations exploration and prospecting

The authors discuss the physical principles of seismic methods application in identifying and mapping of the Domanik-type formations using the surface- and well-based measurements in the north-eastern shoulder of the Mukhanovo-Erokhovsky trough (southern branch of the Kamsky-Kinel’sky system of troughs). It is shown that the main parameter causing anomalies of elastic properties in the Upper Devonian carbonate-siliceous deposits is the volume of the high-carbon part of the rock, primarily kerogen. The kerogen content in Domanik-type formations can reach 25-30 %. A sharp increase in natural radioactivity measured in wells using gamma-ray logging is typical of the intervals having high kerogen content in Upper Devonian deposits. It is shown that gamma-ray anomalies correlate significantly with anomalies of elastic wave travel velocity and bulk density on diagrams of sonic and density logging. A question of kerogen amount versus mobile oil in the Domanik-type formations is beyond the capability of seismic exploration technique, and must be solved using other methods. Calculation of synthetic seismograms showed that anomalies of elastic properties associated with the high kerogen content create rather intense reflected waves comparable in amplitude to the reflections from Top and Bottom of the carbonate sequence, which are the regional marker horizons in the east of the Russian Platform. The main difficulty in identifying anomalies associated with the Domanik-type formations are the zones of argillaceous formations intercalated in the Upper Devonian sequences and observed in the shoulders of some paleo-arches; they create acoustic anomalies in the section, which are similar to anomalies from Domanik-type formations. Superposition of multiple reflections on useful signals in the Upper Devonian interval poses a main difficulty for seismic measurements. Suppression of these multiple reflections requires sufficiently powerful grouping in the acquisition stage and subsequent modern sophisticated data processing. The theoretical conclusions are supported by 2D CPD data processing carried out in parallel in three companies using the leading-edge Russian and western processing systems. It is shown that all the interpreters have identified the persistent, well traced signals reflected from intervals with higher kerogen content in the sequence under investigation. Inversion transformations allowed obtaining a picture of acoustic impedance distribution with resolution corresponding to the capabilities of the seismic technique