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Northern German Unconventional Reservoirs in Upper Permian (Ca2) Microbial Slope and Basin Dolomitized Mudstones: Assessment of Oil to Source-Rock Correlations Within a Sequence Stratigraphic Framework


Upper Permian microbial carbonate source rocks were evaluated using sequence stratigraphic, sedimentologic and geochemical techniques to evaluate potential unconventional reservoirs in slope, basinal, and organic-rich, fine-grained and/or tight lithologies of the northern German Zechstein Z2 cycle. Carbonate microbial mudstones of the Southern Permian Basin have not been evaluated for its shale-gas/oil potential and incorporated into a sequence stratigraphic framework. Therefore, a comprehensive, basin-wide study was conducted that included sedimentology, sequence stratigraphy, and organic geochemistry. The Zechstein Formation is divided into seven 3rd-order sequences that are dominated by regressive and transgressive cycles. Sequence stratigraphic correlations from shelf to basin are crucial in establishing a framework that will allow correlation of potential productive facies in fine-grained, microbial dolomitized mudstones from slope and basin. The basal two Zechstein cycles, Z1 and Z2, contain organic-rich calcareous and dolomitized mudstones that form the transgressive third-order deposits in the basin. Tight dolomitic, microbial layers of carbonates interfingering with organic-rich mudstones are potential candidates for forming a hybrid unconventional or shale-oil reservoir. The basinal and slope succession of Z2 Zechstein sequence is dominated by dolomitized mudstones that is more organic-rich, laminated and fissile in the basin and more massive with microbial layers on the slope. Most slope and basinal mudstones exhibit TOC contents up to 0.8% with maturities ranging from 0.9% Ro to >2% Ro at burial depths from 2000 m to greater than 4000 m. Kerogen type was analyzed to be of Type II amorphous organic matter with increasing liptinites in less mature samples but hydrogen index of the mudstones is up to 200 mg HC/gTOC suggesting mixed type II/III kerogen. Biomarkers indicate in-situ source of microbial slope carbonates. Therefore, oil to source-rock correlation of microbial slope mudstones including isotopes and biomarkers corroborate a potential resource play in the Zechstein Z2 slope setting.