Challenges in the subsurface interpretation of a fold and thrust belt and how to get the best insights: examples from the Romanian Carpathians

Fold and thrust belts are a notoriously challenging environment when it comes to providing structural models for the subsurface, and the Romanian Carpathians are no exception. Hosting the largest onshore oil fields in Romania, this is a highly mature hydrocarbon area, with most of the fields producing since the late nineteenth century.

The structural style of this fold and thrust belt is influenced by a number of parameters which includes multiple detachments (including salt), combined with multiphase tectonic event and reactivation of basement structures which adds to the complexity and variability of the structural style.

As a consequence, the reservoirs, especially the Oligocene - lower Miocene (sub-salt), thought very prolific are structurally complex, heterogeneous, and compartmentalized. It is a constant struggle for geologists to create structural maps of these reservoirs due to complex deformation, and insufficient or ambiguous geological/geophysical data. Some of the most significant issues are related to scattering dip data and the overall difficulties in correlating well logs. In some cases, even the logs of the side-track well do not correlate with the initial log.

Single-method approaches (e.g. only seismic interpretation) often lead to uncertainties or contrasting models regarding the structural style. In this study, we adopted a multiscale/multimethod approach such as forward-modeled regional cross-sections that rely on seismic reflection and well data, UAV-based digital outcrop models, fieldwork, microstructure analysis, and scaled analogue modelling.

This holistic approach enhanced our understanding of fold and thrust belts and provided better constraints on the subsurface structural style in the Romanian Carpathians, as well as explaining complexities that have hitherto been ignored. These new models can reduce subsurface uncertainties regarding structural style, and unlock the full potential of the area which will significantly enhance future exploration programs.