--> Complex Structural, Lithological and Subtle Gas Traps, Upper Rotliegend, Polish Permian Basin

AAPG ACE 2018

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Complex Structural, Lithological and Subtle Gas Traps, Upper Rotliegend, Polish Permian Basin

Abstract

Current discoveries of gas traps in the Polish Rotliegend basin show their diversity and complex structure. Most of the traps are found in eolian sandstones, less often in fluvial sandstones. Most often traps are sealed with Zechstein evaporites: salt and anhydrite. Less frequent are lithological seals represented by playa claystones or impermeable clayey sandstones (sand sheet beds). A separate category is diagenetic or capillary seal. This kind of internal seal is formed as thick units of impermeable, tight or porous sandstones.

The search for gas deposits is focused on the seismic deduction of subtle traps, both just below the Zechstein seal and within the Rotliegend sediments.

Sandstones, in most cases, have good reservoir properties. Improvements to these properties might result from the appearance of secondary porosity resulting from the dissolution of the feldspar lithic grains. Deterioration of these properties could result from diagenetic processes: the development of clay minerals (mostly illite and chlorite) and the carbonate and sulphate cementation, especially close to the Zechstein deposits. The patchy cementation areas are a challenge for finding the most favorable reservoir conditions.

Many gas fields have a complex structure. They are divided into compartments, separated by tight faults. In some cases, faults are associated with a dense net of impermeable deformation bands. This results in different gas-water levels in individual segments. The challenge is also to search for hidden faults that are not visible on the sub-Zechstein structural surface. Such faults can be impermeable barriers and thus formed subtle traps for gas accumulations. In some cases, multistorey or hybrid traps have been distinguished.

The tight gas was found in structural traps. The deep tight gas interpreted as bound with Basin Centered Gas System is still considered as probable. In some drills, overpressure was measured in the Rotliegend deposits. Locally it is interpreted as related to BCGS.

Gas production is carried out using conventional methods and by means of more complex methods such as fracturing or perforation in horizontal sections. The last method is particularly effective in extensive, low amplitude gas traps. Progress in three-dimensional seismic technology and interpretation methods yielded measurable results, especially for subtle traps, but further searching for the next subtle traps in Polish Rotliegend basin is necessary.