Thin-bedded strata and tuning effect as causes of seismic data anomalies in the top part of the Cenomanian sandstone in the Grobla–Rajsko–Rylowa area (Carpathian foreland, Poland)

Abstract

A high-amplitude anomaly in seismic signal, i.e. a bright-spot, which is the main direct hydrocarbon indicator, may also occur due to interference between the reflected signals from the top and base of a thin bed (the tuning effect). In such settings the main problem is to distinguish amplitude anomalies associated with lithological changes or changes in gas saturation from anomalies originating from the tuning effect. We provide a method of interpreting the composite seismic signal produced by the interference between reflections. Such  reflections occur along the top of the Cenomanian sandstone in the area of the Grobla–Rajsko–Rylowa 3-D seismic dataset (southern Poland). The non-standard method presented here is based on interpretation of seismic data which was rotated by 270°. The results of seismic modelling were used to develop criteria for interpretation of such data. These criteria are based on the difference between information carried by the peak-to-trough amplitude and that by the total amplitude of rotated seismic data. The difference was used to define the transfer function and to remove the high-amplitude anomalies caused by the constructive interference between the reflected signals from the top and bottom of the thinning stratum. The final outcome of the study was a map showing the seismic amplitude response at the top Cenomanian reflector unaffected by the tuning effect. The map shows the extent of the area where high-amplitude anomalies do not correspond with changes in thickness and petrophysical parameters within the top part of the Cenomanian. These anomalies are caused by a lateral increase in thickness of the Turonian and/or an increase in the velocity within the bottom part of the Senonian (Coniacian) strata. The map can be used to determine precisely the boundaries of the Rajsko and Rylowa gas reservoirs as well as to locate the potential extension of the reservoir zone to the east of the Rylowa 6 borehole.