Identification of Miocene gas deposits from seismic data in the southeastern part of the Carpathian Foredeep

Abstract

Seismic interpretations are normally made to help identify and locate structural and stratigraphic traps for oil. We focus on problems in interpreting seismic sections in sandy-shaly Miocene deposits which occur in the eastern part of the Polish Carpathian Foredeep. There, the structural picture yielded by the seismic section is not in good agreement with the known structure and a correct interpretation of the seismic wave field, based on seismic modelling, is needed to ensure proper location of exploratory and production wells. We show that the correct choice of petrophysical parameters in these deposits allows interpretation of the seismic image in terms of a multi-horizon gas body. A decrease in velocity, characteristic of gas-saturated beds, was not observed in velocity obtained from sonic measurements. Therefore, several versions of a seismogeological model were constructed based on the results of integrated log interpretation. A model using seismic wave velocity obtained from acoustic wave velocity and a quality factor Q, as a measure of attenuation of elastic waves, was of particular significance. In addition to the petrophysical parameters, the strata geometry necessary to construct a seismogeological model, was determined. Combining the interpreted geometry and information regarding depths of lithostratigraphic units an anticlinal structure was deduced in the gas-rich zone. A comparison of synthetic seismograms calculated using only sonic velocity and seismic velocity corrected for attenuation, with the recorded seismic traces, shows that the best agreement was obtained for a model which included the attenuation. Differences observed between the synthetic and field sections were a basis for determining local direct hydrocarbon indicators, which were then used to identify hydrocarbon deposits in the recorded seismic section.