Using seismic and well data to determine processes of folding in the Pomeranian segment of the Caledonian Foredeep Basin, Poland

Abstract

Recognition of fractures and faults related to different types of folding is important for understanding how fluids flow in folds or which parts of folds represent a better productive sector of reservoirs due to, for example, changes in the intensity and distribution of fractures.In this paper, we suggest that seismic data alone may be insufficient to determine the style of buckle folding. The differences in mechanical properties of the rock masses favour the development of two main types of internal deformation within the buckle folds, that originated as a result of layer-parallel shortening: flexural slip/flow folding and neutral-surface folding. However, it is difficult to tell from seismic data alone which type of deformation had occurred.This paper presents the advantages of analyzing structures in core combined with results of amplitude variation with azimuth (AVAZ) analysis applied on the 3D seismic survey, when compared to the use of seismic data alone. To illustrate this, we use examples of seismic-scale folds, interpreted as buckle folds formed within the Pomeranian segment of the Caledonian Foredeep Basin.The analysis of structures related to the amplification of one fold known as the Opalino Anticline leads us to conclude, that this approach allowed us to recognize that folding within the mostly homogenous, isotropic Ordovician and Silurian mudstone formations is associated with bed-parallel and bed-perpendicular veins, reverse faults, thrusts, stylolites and normal faults, which formed at different depths within the fold. We consider the structures to have developed as a result of buckling of the mudstone formations and demonstrate that the process facilitated neutral-surface folding, which caused outer-arc extension in the higher parts of the Opalino Anticline and the development of an inner-arc contraction zone in the lower parts of the anticline. We show that this approach helps to predict the distribution of structures related with folding, which in turn aids in estimating the reservoir quality.