Cutting-edge reverse time migration imaging of the hydrocarbon deposits in the Outer Carpathian fold-thrust belt

The Western Outer Carpathian fold-thrust belt is an example of strongly tectonically disturbed rock formations characterized by steeply dipping, narrow folds. The results of seismic surveys conducted in that region were used to evaluate the efficiency of the Pre-Stack Depth Migration using Reverse Time Migration (PreSDM RTM) processing workflow to enhance and detail the seismic imaging of geological structures. The analysis focused on seismic imaging of three hydrocarbon accumulations discovered using surface geological mapping methods long before the seismic methods were implemented into the hydrocarbon exploration.

The key problem in the application of the RTM algorithm is the construction of a highly accurate seismic velocity field. In this study, two inversion methods were used for this purpose: gravity inversion and the Early Arrival Waveform Inversion, employing as initial velocity model one derived from well-log data and standard refraction tomography. The effectiveness of the PreSDM RTM was assessed using both isotropic and anisotropic algorithms, particularly employing the TTI anisotropy variant due to the geological structure.

The results obtained showed that RTM migration enables the imaging of challenging details of strongly tectonically disturbed structures in the Carpathian fold-thrust belt, such as steep and tight folds or small-displacement faults. The best seismic imaging was achieved using TTI RTM algorithm, whose additional advantage is the accurate horizontal positioning of fold culminations being potential hydrocarbon traps, which is crucial for the proper location of drilling sites. The RTM efficacy was confirmed by good imaging of shallow hydrocarbon accumulations, whose geological structure is recognized by drillings.