Gas Chimney Record in Seismic and Surface Geochemical Data - Case Study From the Carpathian Foredeep (SE Poland)

Abstract

In a pair of similar structural traps developed above adjacent basement highs in the Miocene Carpathian Fordeep fill (SE Poland), one is charged with gas and the other turned to be dry. Seismic analysis has shown that the dry trap is surrounded by a gas cloud, which represents the first finding of such phenomenon in this basin. Surface geochemical survey has revealed elevated concentrations of light hydrocarbons, which are well above the background levels. The shape and composition of these anomalies indicate relatively fast microseepage of alkanes from gas and gas-condensate subsurface accumulations and are consistent with the presence of the gas chimney below. The gas derives mainly from an intra-Miocene kitchen (microbial methane), as well as probably from a deeper source located within basement rocks. A direct cause for such massive gas leakage along a concentrated path remains yet to be solved. However, this study demonstrates also that combination of seismic with surface geochemical data may significantly aid in a better selection of potential drilling targets for gas in the Carpathian Foredeep and similar objects elsewhere.