Występowanie gazu ziemnego zamkniętego w piaskowcach czerwonego spągowca Polski

The paper presents the first unconventional tight gas field discovered in tight low permeability (K > 0.1 mD) eolian sandstones in the Polish Upper Rotliegend Basin. The tight gas field has been found in area east of Poznań (Siekierki zone). In that area, reservoir eolian sandstones lost almost all permeability and partly porosity due to compaction and illitization processes. The origin of the gas trap was analyzed in many variants, from which one was selected as the most probable. It is assumed that main source for illite crystallization were Zechstein hypersaline brines rich in Ca, Na, K and SO4, which infiltrated the Rotliegend sandstones on tectonically uplifted block. The gas generation and migration were taking place at the same time as processes responsible for decrease in sandstone permeability have been ceasing. As a result, tight gas field from the Siekierki zone can be defined as an unconventional gas field occurring in conventional structural trap. It is expected that tight gas fields could also originate in deeper parts of the Polish Rotliegend Basin but under conditions of BCGS (Basin-Centered Gas System). Gas fields originating under this system are without classic seal and its capacity can be significantly bigger than those of conventional gas fields. The type of organic matter occurring in mature Carboniferous rocks was determining composition of gas formed at gas generation phase. It also affected the chemical content of infiltrating fluids and, in this way, had significant influence on the course of diagenetic processes. In the fault zones, diagenetic cements formed impermeable barriers which separate conventional and unconventional gas fields, as well as barriers separating individual parts of these fields. Such compartmentalization was surely important for origin of the tight gas reservoirs in the Polish Upper Rotliegend Basin.