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Tight Gas in the Rotliegend Reservoir in the SW Polish Basin

Kiersnowski, Hubert 1; Poprawa, Pawel 1; Kuberska, Marta 1
1 Polish Geological Institute, Warszawa, Poland.

The Upper Permian continental Rotliegend basin in Poland is a eastern part of large South Permian Basin (SPB), covering significant part of central and NW Europe. Rotliegend is a classic reservoir for conventional gas with very mature exploration, however recently in different parts of the SPB a significant tight gas potential was recognized, confirmed by discoveries in eastern British offshore and western Germany. Recent studies and results of Trzek 1 well drilling indicates also potential for thigh gas exploration in the SW part of the Polish Basin, referred to as Fore-Sudetic Monocline (FSM). Tight gas potential was studied here for the eastern FSM by integration sedimentologic and petrologic studies with burial and hydrocarbon generation modelling. The studied area is located east of conventional gas exploitation, in the zone were upper part of the Rotliegend complex is developed in eolian and fluvial facies with good primary porosity. Deep burial causes however partial porosity reduction, and significant permeability decrease. Recent depth of the reservoir burial in this region is 3600 m or more, however part of the Mesozoic section was eroded, therefore maximum burial was even higher. Porosity and reduction is related to mechanical compaction, calcite, quartz, dolomite, anhydrite and clay minerals cementation. Important agent causing reduction of permeability is fibrous illite growth, which according to K/Ar dating took place mainly during late Jurassic. Gas was generated from the Lower and/or Upper Carboniferous source rocks presumably during late Triassic to Jurassic time, therefore might predate main phase of permeability reduction, favorable for thigh gas. In the eastern part of FSM within the top part of the Rotliegend section there is at least 100 m thick interval, which during previous drillings gave no spontaneous flow of gas, however show no water saturation also. Porosity for this interval is in a range of a few % to 20 %, and permeability is a few mD to a few tens of mD. Test fracturing of 5 m thick interval in Trzek 1 well (2007/2008) resulted with 215.000 m3 gas/day, confirming tight gas concept. Existing data allows to extrapolate tight gas zone further SE for considerable part of the eastern FSM, where the target formation is at the recent burial depth of 3500-4000 m or more, previously regarded as not perspective. Considering experience from Jupiter-Ganimedes (UK North Sea) zone with no recent structures might be also perspective.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009