Gas reservoirs

Abstract ref. 2011:
     The gas saturated Zechstain Limestone Reefs within western and south western direction from Poznan have been recently discovered. These are Papro W reef
nearby Nowy Tomy l Town and Koscian - Bonikowo Kokorzyn reefs located nearby Koscian Town.  Koscian and Papro reefs and Bonikowo Kokorzyn reefs
were discovered with 2D and 3D seismic respectively.  The reefs were formed in the inner part of Zechstein See about 60-80 km north from the off shore carbonate
platform. The genesis of the reefs is strongly connected with a local Carboniferous paleo highs called Wolsztyn High (Fig 1). The paleo-highs are formed with
carboniferous sandstone (Koscian area) or Autunian volcanic rocks (Paproc_W area) and  they were covered by shallow see. In this shallows there were mainly
bryozoan reefs and oolitic-oncoidic carbonates deposited. Additionally, mostly at the top of the reef the stromatolite occurs. Porous reef body, characterised by
thickness of up to  90 m is surrounded by deep basin limestone facie which is deposited as thin (up to 5m thickness) tight mudstone. 
     Below this the most important parameters of the reefs are presented: 

     Porosity very good up to 35 % 
     Permeability variable 0.1 3000 mD

Paproc-W Reef, area 4 sqkm, top depth 2450m TVDSS, thickness 50-80 m -The first discovered by 2D seismic Zechstein Limestone Reef in Poland.  All its
thickness is gas saturated. The accumulated gas contains 40 % of hydrocarbons. 

          Koscian Reef ,  area 20 sqkm, top depth 2100m TVDSS, thickness 60 m. Koscian Reef is the largest of all discovered reefs body (Fig.3). 
          There is gas-water contact at the depth of 2208m.The resent discoveries allowed to located new Wielichowo and Ruchocice Reefs at NE from
          Koscian Reef. 
          Bonikowo Reef, area 9 sqkm, top depth 2260m TVDSS, thickness 90 m. 

     In 1999 several new reefs was discovered i.e. Rensko-Kokorzyn-Bronsko and it seems to be the largest discovered reef, however  shape of the reef was
interpreted from 3D seismic and wait for the wells drilling.  The reefs saturated  gas contains 81 % of hydrocarbons (80.5 % of methane) and 18 % of nitrogen. 
     It is very important to notice that the efficient reef prospecting is possible thanks to 3D seismic survey.  Both 2D and 3D seismic was shot by GEOFIZYKA
Toru.  Especially it is worth emphasising that final success is a result of a co-work of many people; geophysicists, geologists  and engineers. Presently, thanks to
excellent co-operation between POGC - Oil and Gas Zielona Góra and GEOFIZYKA Toru S.A. the exploration of the Zechstein Limestone reefs has become the
priority task for the companies operating in the western Poland