The Permian Zechstein Formation as a Potential Hybrid Unconventional Reservoir: A Sequence Stratigraphic and Sedimentological Evaluation of Organic-Rich Carbonates and Mudrocks from Shelf to Basin, Northern Germany
The Late Permian Zechstein Formation in north-eastern Germany has been a prolific conventional oil-producing formation primarily from shelf and slope carbonates that rimmed a basin that extended from eastern England through the Netherlands and Germany to Poland. Main reservoirs to date are grainstones rimming islands created by pre-existing paleohighs, from platform-rimming shoals, and slope carbonates. Lagoonal evaporites formed the seal for these carbonate and underlying sandstone reservoirs. The objective of this investigation is to evaluate potential unconventional reservoirs in lagoonal, slope, basinal, and organic-rich, fine-grained and/or tight lithologies of this formation. Therefore, a comprehensive, basin-wide study was conducted that included sedimentology, sequence stratigraphy, and geochemistry. The Zechstein Formation is divided into seven 3rd-order sequences that are dominated by regressive and transgressive cycles of shelf origin. Sequence stratigraphic correlations from shelf to basin are crucial in establishing a framework that will allow correlation of potential productive facies in fine-grained, organic-rich basinal siliceous and calcareous mudstones or interfingering tight carbonates and siltstones ranging from the lagoon to slope.
Most organic-rich shales worldwide are associated with 2nd- to 3rd-order eustatic transgressions. The basal Zechstein cycles, Z1 and Z2, contain organic-rich siliceous and calcareous mudstones and carbonates that form the transgressive deposits in the basin. Tight dolomitic layers, marlstones, and organic-rich limestones interfingered with organic-rich siliceous and calcareous mudrocks are candidates for forming a hybrid unconventional reservoir. A comprehensive database composed of core, wire-line logs, and outcrop is used to analyze the organic-rich mudstones and carbonates. Maturities range from over-mature (gas) in the basin to oil-generation on the slope with variable TOC contents. This sequence stratigraphic and sedimentologic evaluation of the transgressive facies in the Z1 and Z2 intervals in conjunction with chemostratigraphy will evaluate the potential for shale gas/oil and hybrid unconventional plays where interfingering organic-rich mudstones and carbonates might present new exploration possibilities and serve as analog to other hybrid plays such as the Bakken or Pearsall Formations in the USA.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California