Seismic and geochemical anomalies related to vertical migration of gas in the Radlin gas field

Abstract

In the Radlin Rotliegend gas field (southwestern Polish Lowlands, Fore-Sudetic Region, Poznań Depression), modelling and reinterpretation of seismic data combined with surface gas surveys (free gas method) help identify “gas chimneys” and locate pathways of vertical migration. An almost impermeable Zechstein evaporite cover reduces the stream of vertical gas migration so that a dry surface geochemical zone occurs at the surface and no seismic disturbances are observed directly above the pay zone of the gas field. The surface gas anomalies form a halo-type pattern surrounding the gas field. The majority of surface hydrocarbon microseepages from the subsurface agrees with the seismically visualised geological structure. The belts of surface gas anomalies and the dry zones separating them run along the strike of Permian structures and faults. Vertical pathways of gas migration are recognisable as zones of seismic discontinuity and chaotic distribution of distinct reflections and diffracted waves. Seismic “gas chimney” effects coincide with surface hydrocarbon gas anomalies. Particularly, they appear above the upper terminations of faults disturbing the Zechstein deposits. The faults create the main a venues for secondary vertical migration of gas from the reservoir to the surface. “Gas chimneys” are observable in a shallow zone above the Muschelkalk (Tm) where higher gas saturation of more porous rocks produces a seismically traceable velocity effect