Biomarker indicators of bacterial activity in the Upper Permian (Zechstein) carbonate microbialites and facies from the Southern and Northern Permian Basins of Europe

One of the most important hydrocarbon-bearing palaeobasins in Europe is the evaporite-carbonate Southern (SPB) and Northern (NPB) Permian Basin and especially their most petroliferous second carbonate unit (Z2C, also referred as the Main Dolomite, Roker Fm. or Staβfurt Karbonat, ca. 254 Ma) of the second Zechstein (Lopingian) cycle. Although extensively studied, there is still a lack of sufficient data on the extensive microbial facies in the Z2C which are the reservoir rocks, and probably the source rocks too for the hydrocarbons. Existing geochemical proxies such as scarce biomarkers, major and minor elements, sulphur and strontium isotopes do not provide sufficient information about the type of organic matter and sedimentary conditions in the Z2C sea. In order to circumvent this problem a biomarker-based approach was used on 100 samples from 15 boreholes and outcrops to characterize the biomarker distribution in the various Z2C depositional zones containing significant amounts of microbialite (stromatolites and thrombolites) from the northern and western and southern margin of the SPB, namely lagoonal, ooid shoal, slope, toe-of-slope apron and basinal facies. Sedimentologically, the lagoonal facies are characterized by dolomitized microbial boundstone and packstone (interfingering with evaporites (anhydrite), especially in NW Poland). Ooid shoal facies are built of dolomitized ooid grainstone; slope facies are largely represented by dolomitized or dedolomitized biolaminitic mudstone and turbidite packstones, microbial boundstone, floatstone and rudstone, and toe-of-slope apron facies consist of dolomitized interbedded units of oolitic packstone, carbonate conglomerates and debrites with microbialite fragments. Basinal facies are represented by very thin laminated clayey calcareous mudstones. The most diagnostic and useful signatures of Z2C organic matter in slope, toe-of-slope apron and lagoonal facies are the occurrence of 1) 2,3,6-aryl isoprenoids (C15 to C31), isorenieratane and β-isorenieratane derived from brown-pigmented green sulphur bacteria, 2) rare chlorobactane characteristic of green-pigmented green sulphur bacteria, 3) C31 and C32 2αβ-methylhopanes (only found in dedolomitized outcropping Z2C slope facies), essentially characteristic of cyanobacteria, and 4) a low sterane/hopane ratio (<0.2) suggesting microbial or microbially reworked organic matter. In addition, the occurrence of aryl isoprenoids (likely degradation products of isorenieratane), isorenieratane, β-isorenieratane and chlorobactane indicates that part of the photic zone, limited to the toe-of-slope apron, slope and lagoonal depositional zones, had become euxinic. Basinal facies are in general significantly depleted or lacking steranes, hopanes and polycyclic aromatic hydrocarbons, and largely contain terrestrial-type organic matter. Ooid shoal facies contain hopanes, low concentrations of steranes, a lack of aromatic carotenoids and benzohopanes. The thermal maturity, assessed from Ts/(Ts+Tm), C30 moretane/hopane, 20S/(20S+20R) and ββ/(ββ+αα) steranes, shows a mature character of the organic matter with respect to oil generation. The marginal parts of the basin and hypersaline lagoons during Z2C deposition were dominated by the activity of green sulphur bacteria which possibly coexisted with sulphate reducing bacteria, as suggested by the occurrence of very low amounts of native sulphur and microcrystals of pyrite. It also seems that cyanobacteria played an important role in carbon cycling and burial diagenesis, as suggested by the presence of specific biomarkers in the dedolomitized outcropping slope facies. Euxinic conditions may additionally have promoted microbial productivity and preservation, with only a low contribution of terrestrial and algal (phytoplankton) organic matter.