Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin

Journal Pre-proof Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin

Marta Sośnicka, Volker Lüders PII:

S0009-2541(19)30582-0

DOI:

https://doi.org/10.1016/j.chemgeo.2019.119453

Reference:

CHEMGE 119453

To appear in:

Chemical Geology

Received date:

29 September 2019

Revised date:

22 December 2019

Accepted date:

23 December 2019

Please cite this article as: M. Sośnicka and V. Lüders, Fluid inclusion evidence for lowtemperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin, Chemical Geology (2019), https://doi.org/10.1016/j.chemgeo.2019.119453

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© 2019 Published by Elsevier.

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Fluid inclusion evidence for low-temperature thermochemical sulfate reduction (TSR) of dry coal gas in Upper Permian carbonate reservoirs (Zechstein, Ca2) in the North German Basin Marta Sośnicka1,2*

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Volker Lüders1 GFZ German Research Centre for Geosciences, Telegrafenberg, D-14473 Potsdam, Germany

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Friedrich Schiller University Jena, Institute of Geosciences, Burgweg 11, D-07749 Jena,

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Germany

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*Corresponding author: [email protected]

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Abstract

Upper Permian Zechstein carbonate Ca2 gas reservoirs in the southern part of the

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Pompeckj Block in the North German Basin locally contain up to 36 vol.% hydrogen sulfide

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(H2S) produced by thermochemical sulfate reduction (TSR). TSR was triggered by migration of dry to extremely dry coal gas from Upper Carboniferous into the Zechstein carbonate reservoirs. Methane reacted with dissolved sulfate at temperatures of less than 150°C, as inferred from fluid inclusions hosted in fracture-filling minerals and cements in the carbonate reservoir rocks. Such low temperatures for methane-dominated TSR are unique and were not observed so far, as it was widely believed that alteration of super dry methane requires much higher temperatures. Here we present detailed compositional and carbon isotope data of reservoir gases as well as those of gases trapped in fluid inclusions hosted in cements and fracture-filling minerals in Zechstein Ca2

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Journal Pre-proof carbonate reservoir rocks. We constrained the P-T conditions of gas entrapment, hydrocarbon reactivity and the lower temperature limit for TSR. The results of this study decipher three major stages of gas migration in the Pompeckj Block. Stage I commenced in the Late Triassic during burial when Zechstein Ca2 reservoirs were charged with dry CH4-CO2±N2 gas sourced from mature Upper Carboniferous coals. Burial continued through the Jurassic and caused alteration of Ca2 reservoir gas by sulfate reduction

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reactions due to increasing temperatures. Entrapment of CH4-H2S-CO2-N2 gases in fluid

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inclusions, hosted in cements and fracture-filling minerals, occurred at temperatures between 100

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and 152°C and was related to Stage II uplift in the Early Lower Cretaceous. In the Late

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Cretaceous (Stage III) deep burial of the Pompeckj Block led to charge of the Zechstein Ca2 carbonate reservoirs with Upper Carboniferous-derived CH4-CO2±N2±C2+ coal gas and/or

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dilution of existing reservoir gas at temperatures of 144-167°C. Highly variable δ13CCH4 values

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from -18.7 to -8.7‰ and very negative δ13CCO2 values (-22.4 to -18.9‰) of H2S-rich fluid inclusion gases as well as negative δ13C values (-10.4 to -4.6‰) of host calcites reveal

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compelling evidence for participation of methane in TSR. Fluid inclusions imply that CH4-

dissolved Mg2+.

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dominated TSR proceeded at Tmin of 135°C in the presence of catalyzers such as H2S and

This study demonstrates that fluid inclusions serve as an excellent and accurate tool for tracing H2S concentrations in hydrocarbon gases through time and space, which is not possible using the present-day compositions of natural reservoir gases. It also contributes to the understanding of carbonate reservoir-hosted hydrocarbon-bearing fluid systems and processes that significantly control the quality of reservoir gases. Keywords: TSR, hydrocarbons, methane, hydrogen sulfide, fluid inclusions, stable isotopes

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1. Introduction Hydrogen sulfide (H2S) is a commonly occurring non-hydrocarbon gas in carbonatehosted gas reservoirs with proportions varying from trace amounts up to about 30% (e.g. Worden and Smalley, 1996; Machel, 2001; Cai et al., 2004, 2013; Zwahlen et al., 2019). H2S can be produced by thermal decomposition of sulfur-bearing organic matter in kerogens, thermal

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cracking of sulfur-containing crude oil, or by biotic or abiotic redox reactions between sulfate and

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hydrocarbons (e.g. Orr, 1977). There are several reactions leading to the formation of H2S by sulfate reduction (e.g. Ohmoto and Goldhaber, 1997; Krooss et al., 2008). In low-temperature

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sedimentary environments (T < 80°C) sulfate reduction by organic matter is the principle reaction

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to produce H2S (Machel et al., 1995; Machel, 2001). At temperatures above 80°C most sulfate-

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reducing bacteria cease metabolism and at temperatures ≥120°C thermochemical sulfate reduction (TSR) is the major process generating H2S in deep carbonate gas reservoirs (e.g.

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Worden and Smalley, 1996; Machel, 2001; Krooss et al., 2008).

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TSR has an important impact on the economic potential and environmental issues of gas

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reservoirs in deeply buried carbonate host rocks. The quality of gases in sour gas-bearing carbonate reservoirs is not only determined by concentrations of hydrogen sulfide but also carbon dioxide, which both are major products of TSR (e.g. Machel et al., 1995; Worden and Smalley, 1996; Hao et al., 2015). Over the past decades many experimental and geologic studies have been performed that improved the understanding of TSR (Goldstein and Aizenshtat, 1994; Machel et al., 1995; Worden and Smalley, 1996; Machel, 2001; Bildstein et al., 2001; Thom and Anderson, 2007; Anderson and Thom, 2008; Ma et al., 2008; Zhang et al., 2008; Cai et al., 2013; Liu et al., 2013; Truche et al., 2014; Jiang et al., 2015; Meshoulam et al., 2016; Barré et al., 2017; Kotarba et al., 2017; He et al., 2019; Zhao et al., 2019). Although much is understood about TSR, there is 3

Journal Pre-proof still some debate about reacting hydrocarbons and minimum reaction temperatures (Krouse et al., 1988; Worden and Smalley, 1996; Machel, 1998; Cai et al., 2004; Amrani et al., 2008; Yuan et al., 2013; Hao et al., 2015). During TSR hydrocarbons react with dissolved sulfate producing hydrogen sulfide, bisulfide, elemental sulfur, carbon dioxide, and water, following the simplified reaction after e.g. Worden and Smalley (1996) or Maystrenko et al. (2008):

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hydrocarbons + SO42- → H2S ± HS- ± S ± altered hydrocarbons ± solid bitumen + CO2 + H2O (1) TSR reactions are mainly controlled by temperature, availability of reactive sulfate as well

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as origin and type of reacting hydrocarbons (e.g. Hao et al., 2015). Methane-dominated TSR has

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only rarely been observed in some gas reservoirs (Hao et al., 2015). Only little attention has been

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paid to CH4-dominated TSR since methane is considered to be the least reactive hydrocarbon (Machel, 2001). However, some studies of deep (> 3km) gas reservoirs suggested that CH4 dry

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gases can be the dominant reducing agent during TSR at temperatures of about 160°C in the

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absence of higher hydrocarbons (e.g. Worden et al., 1995; Worden and Smalley, 1996; Hao et al.,

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2015). In contrast, even much higher temperatures of >200°C are considered for CH4-dominated TSR in the Mobile Bay gas reservoirs, where high molecular weight hydrocarbons are abundant (Mankiewicz et al., 2009).

Experimental studies of TSR reactions performed so far mostly utilized high temperatures above 300°C due to slow reaction rates in the absence of catalysts such as H2S or dissolved Mg2+ (He et al. 2019). Experimental studies of methane-dominated TSR by Yuan et al. (2013) and He et al. (2019) required onset temperatures of 200°C and 250°C, respectively. Such high onset temperatures of the experimental studies exceed by far the temperature conditions of natural high H2S accumulations in carbonate reservoirs. However, a recent study of fluid inclusions hosted in 4

Journal Pre-proof pore-filling calcite cements from the NE Sichuan Basin in China yielded minimum temperatures of ~161.5°C for methane-dominated TSR (Li et al., 2019). Thus fluid inclusions trapped in minerals and/or cements from natural carbonate-hosted H2S-bearing gas reservoirs may provide a means to unravel the timing and conditions of H2S formation in carbonate reservoirs during basin evolution. Following this idea we studied C-H-O-S-N fluid inclusions hosted in abundant fracture-

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fill mineralization and cements in Zechstein carbonate Ca2 reservoirs rock from wells that were

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sunk into the southern flank of the Pompeckj Block (PB) in the North German Basin (NGB) close

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to the border to the Lower Saxony Basin (LSB) (Fig. 1). The deep-seated Zechstein carbonate

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Ca2 reservoirs were charged with methane-rich dry gas sourced from highly mature Upper Carboniferous coals (Schoell, 1988). Previous geochemical studies provided evidence for high

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concentrations of H2S (5-35 mol%) in Zechstein carbonate Ca2 reservoirs in the NGB are derived

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by TSR (Mittag-Brendel, 2000; Biehl et al., 2016; Sośnicka and Lüders, 2019). Here we present, for the first time, a comprehensive study of fluid inclusions in order to

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decipher the onset and reacting conditions of TSR in Zechstein Ca2 carbonate rocks in the NGB

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and to constrain the P-T conditions during fluid evolution and migration in the reservoirs. Besides microthermometric measurements and laser-Raman spectroscopy we performed stable C-O isotope analysis on selected calcite chips containing gas inclusions of different compositions (i.e., H2S-rich or CH4-rich). Furthermore, we conducted bulk analyses of δ13CCH4 and δ13CCO2 isotopic compositions of fluid inclusion gases being released by crushing (Plessen and Lüders, 2012) of minerals hosted by Zechstein Ca2 reservoir rocks. Molecular and carbon isotopic compositions of fluid inclusion gases are compared with those of natural reservoir gases from Ca2 reservoirs located close to the southern margin of the PB (Fig. 1).

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2. Geological setting The NGB, situated in Central Europe, forms part of the complex intracontinental Southern Permian Basin (SPB), which was superimposed on the Variscan fold-and-thrust belt and the Westphalian foreland basin during Late Carboniferous-Early Permian times (e.g. Ziegler, 1990; Baldschuhn et al., 2001; Maystrenko et al., 2008). The study area is situated on the south-western marginal area of the NGB encompassing the southernmost part of the PB and northernmost

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flanks of the LSB (Fig. 1). These two tectonically different geological units resulted from the

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complex differentiation and deformation history of the SPB (Betz et al., 1987; Glennie, 1986;

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Brink et al., 1992; Petmecky et al., 1999; van Wees et al., 2000; Littke et al., 2008).

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The major source rocks for hydrocarbon gases in Upper Carboniferous tight gas reservoirs as well as in Rotliegend sandstone and Zechstein carbonate reservoirs in the NGB are numerous

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coal seams and coaly shales that occur within Westphalian (Upper Carboniferous) >3 km thick

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rock successions (Schoell, 1988; di Primio et al., 2008). The transition from Late Carboniferous to Permian in the NGB is marked by extensive magmatic and volcanic activity, which was

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followed by deposition of alternating sequences of continental siliciclastic Rotliegend red beds

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and lacustrine evaporites (Breitkreuz et al., 2008; Maystrenko et al., 2008). In the study area, however, most of the Rotliegend rock units are lacking with an exception of scarce volcanics (Breitkreuz et al., 2008). During Upper Permian times, the Paleozoic strata was covered by up to 1.5 km thick evaporitic sequences, consisting of stacked shales, including Kupferschiefer black shale, carbonates, sulfates, salt rock (halite) and Mg-K salts, which accumulated due to transgression of Zechstein Sea (Breitkreuz et al., 2008; Maystrenko et al., 2008). The Zechstein platform carbonates (Staßfurt carbonate Ca2), which belong to the second evaporitic cycle, are one of the most important gas reservoir rocks in the NGB. In the western part of the NGB shale units within Zechstein evaporites are organic matter-enriched (<0.5 up to 1% TOC) and also 6

Journal Pre-proof locally acts as a source rock for gas generation (Littke et al., 1996; di Primio et al., 2008). Zechstein salt beds (Na2 and Na3), covering platform carbonate facies, generally acted as sealing lithology for the Ca2 gas reservoirs (Petmecky et al., 1999; Karnin et al., 2006). Zechstein carbonate Ca2-hosted gas fields occur in the PB as well as in the LSB. However, the highest H2S concentrations of up to 36% are found in gas fields situated in the PB, whereas the H2S content in the LSB commonly does not exceed 10% (industrial data). The LSB

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and PB experienced significantly different burial histories and structural evolution (Fig. 2). The

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LSB developed to the south of the PB during rifting in the Early Jurassic to Early Cretaceous

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(e.g. Betz et al., 1987; Kockel, 2002; Littke et al., 2008). During the Triassic, rifting and

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subsidence rates triggered by extensional tectonics, were much more intense in the PB compared to the LSB (Stollhofen et al., 2008; Bruns et al., 2013). This period is also marked by an onset of

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halokinetic movements focused in areas where thick salt beds accumulated above the deep

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basinal carbonate facies. During the Kimmeridgian, subsidence rapidly intensified in the LSB (Fig. 2), which led to high maturities of Westphalian source rocks (Petmecky et al., 1999;

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Adriasola Muñoz et al., 2007; Bruns et al., 2013). Whilst the LSB was deeply buried in Early

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Lower Cretaceous times, the adjacent PB was strongly uplifted in response to the Kimmerian tectonic phase (Bruns et al., 2013) (Fig. 2). This is expressed by a large stratigraphic gap beneath Albian strata in the PB compared to the LSB (Betz et al., 1987; Petmecky et al., 1999). According to numerical basin modelling by Bruns et al. (2013), intense erosion during the Kimmerian inversion led to removal of maximum 3 km overburden in the study area located north of the PB/LSB margin and SW of Bremen (Fig. 1). In the study area maximum burial depths were reached in the late Jurassic prior to Early Cretaceous inversion. In contrast, the more northern part of the PB experienced deepest burial from the Late Cretaceous through present times (Fig. 2) (Schwarzer and Littke, 2007; Bruns et al., 2013). During Late Cretaceous inversion 7

Journal Pre-proof the Zechstein Ca2 fairway in the LSB was strongly uplifted whereas the PB remained tectonically unaffected (Kockel, 2002; Kley and Voigt, 2008). It is assumed that gas generation from Westphalian coal-bearing succession in the LSB and PB commenced in the Late Triassic (Karnin et al., 2006). In the LSB gas generation most likely ceased after the basin inversion. In contrast, some of the deeply buried Zechstein carbonate Ca2 reservoirs in the PB are still charged with CH4-rich dry coal gas (industrial data). Lateral

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migration of late sourced dry gas from the northern part of the study area during the Tertiary

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locally changed the gas composition in carbonate-hosted reservoirs in the southern part of the PB

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as well as in the LSB (Karnin et al., 2006).

3.1

Sample material

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3. Sample material and analytical methods

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Core samples (Fig. 3) for this study were collected from 13 wells drilled at depths

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between 3.8 and 4.2 km during gas exploration from the southernmost part of the PB and the PB/LSB border zone (Fig. 1). The sampling suite comprises Zechstein Ca2-carbonates as well as

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cements and differently composed fracture-fill mineralization (Fig. 3). Compositional and isotope data of reservoir gases were provided by the industry.

3.2

Raman spectroscopy The molecular compositions of fluid inclusions were measured at GFZ Potsdam using a

LabRAM HR Evolution (Horiba Jobin Yvon Technology) instrument and an excitation radiation of 532 nm (green laser). For internal calibration silicon (520 cm-1) was used. Raman spectra of gas phases in fluid inclusions were acquired in the spectral range between 1200 and 3000 cm-1, whereas the range of 200-1200 cm-1 was used for mineral phases. A 2x20s acquisition time was 8

Journal Pre-proof used for measurements of aqueous inclusions and gas inclusions. Lower laser energies and acquisition times were used for measurements of sulfur-dominated inclusions to avoid recrystallization. Acquired spectra were processed using LabSpec software (ver. 6.4.4.16). Compositions of gases were plotted in the quaternary diagrams using CSpace (Torres-Roldan et al., 2000).

Microthermometry

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3.3

Microthermometric measurements of fluid inclusions in doubly polished wafers were

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conducted at the GFZ using a FLUID INC. adapted USGS heating-freezing system mounted on a

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BX50 Olympus microscope with a 40x objective (N.A. = 0.55). The USGS stage was calibrated

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with synthetic Synflinc fluid inclusion standards. The accuracy is ±0.1°C for Tm(ice) and ±1.7°C

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for Th>350°C. Salinities of aqueous fluid inclusions were calculated from corresponding Tm(hydrohalite) and Tm(ice) values using HOKIEFLINCS_H2O-NACL spreadsheet (Bodnar,

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1993; Steele-MacInnis et al., 2012; Sterner et al., 1988). The properties of C-H-S-N-O and C-H-

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O±N fluids were evaluated using software package FLUIDS ver.2 (Bakker, 2009) and V-X diagrams for CH4-CO2 system from Thiery et al. (1994). The estimated molar volumes and gas

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compositions determined by Raman spectroscopy were used to calculate isochores for the C-H-SN-O and C-H-O±N fluid systems applying equations of state provided with the software package FLUIDS ver.2 (Bakker, 2009).

3.4

Cathodoluminescence microscopy Cathodoluminescence imaging has been performed at GFZ Potsdam using a hot-cathode

HC3-LM microscope with a coupled DP74 digital camera system attached to an Olympus microscope. The electron gun was operated under high vacuum (10-5 bar) at a voltage of 14 keV with a filament current of 0.2-0.3 mA. Thick sections were carbon coated for CL imaging. 9

Journal Pre-proof 3.5

Stable isotope analyses of calcites Calcite samples were analyzed for δ13C and δ18O isotopic compositions using a Finnigan

MAT253 mass spectrometer connected to an automated carbonate-reaction device (KIEL IV) at the GFZ. Samples were finely powdered prior to analyses in order to remove gas inclusions, which could contaminate the isotopic signatures from calcite hosts. Powdered samples of around 60-90 µg were automatically dissolved with 100% H3PO4 at 72°C and the isotopic compositions

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were measured on the released and cryogenic purified CO2. Replicate analysis of reference

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material (NBS19) reported relative to VPDB yielded standard errors of 0.06 ‰ for δ 13C and 0.06

Stable isotope analyses of fluid inclusion gases

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3.6

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‰ for δ18O, respectively.

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For stable isotope analyses of fluid inclusion gases a sample crusher connected via a GCcolumn to an elemental analyser (EA)-IRMS system was used. The analytical setup at the GFZ

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includes a sample crusher, GC-column, EA, a ConFlo III interface and a Thermo DeltaplusXL

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mass spectrometer (Plessen and Lüders, 2012). The crusher is coupled to the EA via a He carrier gas line from which the carrier gas, He (purity 5.0), passes through the crusher at 300 ml/min.

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The crusher consists of a special hardened steel chamber and piston, which is not only useful for soft minerals, such as calcite, fluorite and sulfates but also for quartz and other silicates. The crusher is equipped with a septum port for direct injection of gases for blank runs. Its volume of about 2 cm3 allows the crushing of up to 1g of sample material. The reproducibility for C-isotope ratios of fluid inclusion gases in individual samples was found to be better than 0.6‰ (Lüders and Plessen, 2015). After crushing of sample chips (previously characterized by microthermometry and laser-Raman spectroscopy) the He/gas mixture passes through a GCColumn and separates CH4 and CO2 from each other. The gas species enter the oxidation column 10

Journal Pre-proof of the EA, where CH4 gets oxidized to CO2 with simultaneous injection of O2 at 960°C. In order to obtain complete oxidation of CH4, a 25 ml oxygen loop was used for the He/O2 purge line. After passing the reduction column and water trap, the gas species, CO2 from CH4 oxidation and original CO2 from inclusions are separated again in a second molecular sieve and enter the IRMS via a Conflo III interface. The isotopic ratios of carbon dioxide were measured online using reference gas calibrated against NBS19 for CO2. Reproducibility of bulk analysis

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3.6.1

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Using a bulk technique requires critical assessment of the results as the measured values

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represent isotopic compositions of a gas mixture released from all generation of gas inclusions

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within each analyzed sample. Previous studies have shown that bulk stable carbon isotope

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compositions of CH4 and/or CO2 released by crushing from fluid inclusions fall into narrow ranges (± 0.3 to 0.5‰) for individual samples during repeated runs (up to 30 repeated analysis)

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although different generations and fractions of heterogeneous fluids are trapped in inclusions in the studied samples (Lüders et al., 2012; Lüders and Plessen, 2015; Prokofiev et al., 2019). This

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implies that the δ13CFIgas values of the analyzed fluid inclusion gases are dominated by the supply

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of CH4 and/or CO2 during a major fluid event. For this study, detailed and extensive fluid inclusion analysis showed that gas inclusions predominantly contain CH4-rich dry gas (>99%) besides non-hydrocarbon gases. Repeated analysis of fluid inclusion gases released by crushing of mineral grains from some individual samples show reproducibility better than 0.1 ‰ for CH4 and 0.5 ‰ for CO2 (Table 1). Therefore, the measured δ13CCH4 values are representative for the isotopic compositions of predominant primary and pseudo-secondary dry gas inclusions. Contamination by secondary gas inclusions is negligible. Moreover, similarity of ranges of δ13CCH4 values for reservoir and fluid inclusion gases also supports reliability of the bulk isotope analyses of fluid inclusion methane. 11

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4. Results 4.1

Petrography and mineralogy

Fluid inclusions are hosted in pore and vug-filling cements as well as fracture-fill mineralization (Figs. 3-5) in Zechstein-2-Carbonate. The Ca2 is composed of shallow water dominantly dolomitized oolitic grainstones and mudstones deposited in a slope environment. The oldest cements comprise dolomite I matrix and dolomite II in vein fillings and they

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both show magenta to red luminescence (Fig. 5A, C-F). Dolomite II cement was replaced by

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diagenetic, weakly luminescent calcite I that forms mosaics of interlocking brownish euhedral to

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subhedral crystals (Fig. 4A, 5B, 7A). Subsequent calcite II forms nodules showing brighter

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orange luminescence (Figs. 4A, 5B). Calcite III and IV are associated with parallel-bedding stylolites post-dating dolomite II cements (Fig. 5C). Calcite III shows homogeneous orange

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luminescence, whereas calcite IV exhibits patchy sectoral zoning (Fig. 5C).

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Fracture-filling cements precipitated on reverse faults (Fig. 3A), vertical fractures (Fig. 3C) or are associated with oblique to vertical tectonic stylolites (Fig. 3B). The fractures are

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usually from <1 cm to about 4 cm thick and their orientations are commonly vertical or at a steep

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angle to the bedding (Fig. 3A, B). They commonly contain fragments of older calcite generation and/or calcite V, which is often replaced by fluorite (Fig. 3C, 4D), and locally minor galena. Coarse calcite V and fluorite also cement vuggy pore spaces (Fig. 4D, 5D, F) and interstitial spaces between ooidic aggregates in grainstones (Fig. 5E). Calcite V cement shows orange luminescence (Fig. 5A, D-H), whereas in wider fractures it exhibits prominent sectoral zoning expressed in orange luminescence of different intensities (Fig. 5G). Pyrite-calcite fracture-fillings contain up to three generations of pyrite (Fig. 4E-F). The earliest porous pyrite I is overgrown by euhedral plain pyrite II (Fig. 4E-F), which in turn is overgrown by euhedral pyrite III. The latter exhibits prominent growth zones (Fig. 4F). Anhydrite is associated with stylolites oblique to the 12

Journal Pre-proof bedding (Fig. 3B) or forms prismatic crystals directly overgrowing calcite cement I (Fig. 4B) or post-dates calcite V in fracture-fillings. Fluorite shows blue luminescence (Fig. 5F-H) and sectoral zoning (Fig. 5H), which is characteristic for fracture filling fluorites in drill cores from the NGB (Nadoll et al., 2018, 2019). For summary, a generalized paragenetic scheme of the mineral sequences is illustrated in the Fig. 6.

Fluid inclusions

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4.2

Based on petrographic observations (Fig. 7) and Raman spectroscopy (Figs. 8-10) three

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major compositional fluid inclusion types were distinguished in Ca2-hosted cements and fracture

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filling minerals from selected wells in the PB: a) type 1a gas-rich C-O-H±N±S (with H2S<7

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mol%) and type 1b gas-rich C-O-H-S±N gas inclusions (V), b) type 2 aqueous two-phase and

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three phase (with halite daughter mineral) inclusions containing gas portions in the vapor phase (L+V±H±S), and c) less frequent type 3 S-rich inclusions ± HS-, H2S and/or CH4 (L±V). In the

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frame of this study 224 fluid inclusions were analyzed for gas compositions, which are

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summarized in Table 2 and shown in quaternary diagrams (Fig. 10b-d). Microthermometry was performed on 152 aqueous and gas inclusions (Tables 3, 4, Electronic Annex). Compositions of

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fluid inclusion gases are very consistent within individual fluid inclusion assemblages – FIAs (FIA according to Goldstein and Reynolds, 1994) (Table 2). 4.2.1

Calcite I-II cements

High methane contents of 92-93 mol% are typical for type 1a inclusions hosted in calcite I and II cements (Fig. 4A, 7A, 10d). Besides methane, the type 1a inclusions contain 6-7 mol% CO2 and <2 mol% N2. The gas inclusions form clusters with co-genetic aqueous two-phase (L+V) inclusions (Fig. 7A). Since the latter are not arranged along trails they are considered to be trapped contemporaneously with type 1a gas inclusions during calcite precipitation. Aqueous 13

Journal Pre-proof two-phase inclusions (<5-12 µm) are predominantly spherically or cylindrically shaped and show homogenization temperatures of Th(LV→L)=102-127°C (median: 110°C) (Table 4). Type 1a gas inclusions hosted in re-crystallized oncolites (calcite I) contain CH4 (87-94 mol%), CO2 (5-9 mol%), N2 (2-3 mol%), H2S (1-2 mol%). Solid bitumen, which was identified as two prominent broad bands at 1341 cm-1 and 1590 cm-1 (e.g. Fig. 8a) in the mid-frequency region of the Raman spectrum (e.g. Zhang et al., 2007), was detected in some of the gas inclusions hosted in calcite

Calcite V

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4.2.2

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cements I and II.

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Methane-dominated (49-73 mol%) type 1b gas-rich inclusions hosted in calcite V (Fig.

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7B-D) from late cements and fracture-fillings show variable contents of non-hydrocarbon gases:

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H2S (14-40 mol%), CO2 (6-16 mol%) and N2 (<4 mol%) (Figs. 8d, 10b-d). Type 1b gas inclusions are typically co-genetic with primary two-phase (L+V) aqueous inclusions, which

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frequently enclose insoluble dolomite solids (Fig. 7E). Vapor bubbles of the aqueous inclusions additionally contain CH4 (44-62 mol%), CO2 (33-51 mol%) and H2S (5-10 mol%) (Fig. 10c).

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Type 2 aqueous inclusions homogenize to the liquid phase at temperatures of Th(LV→V)=100-

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140°C (median: 119°C) (Fig. 11). Assuming gas saturation during fluid entrapment (Roedder and Bodnar, 1980) the homogenization temperatures of gas-bearing aqueous inclusions reflect the original trapping temperatures Tt. Low first melting temperatures (Tfm<-47°C) indicate that the aqueous inclusions contain NaCl-CaCl2-H2O brines (Table 4). Final melting occurred by disappearance of hydrohalite-Tm(hh) at temperatures from -12.4 to -8°C, albeit some of the inclusions showed metastable melting of hydrohalite (Davis et al., 1990) and therefore, they were excluded for salinity calculations. The Tm(hh) values indicate high corresponding salinities of 24.5-25.1 wt.% NaCl equiv. of H2S-bearing aqueous inclusions (Fig. 11). Zechstein Ca2-hosted

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Journal Pre-proof two-phase brine inclusions of similar salinities and melting behavior were observed in the NGB by Zwart and Touret (1994). Calcite V also locally hosts primary fluid inclusions that contain native sulfur and bisulfide (HS- or SH-), which show yellowish color and co-exist with H2S-rich gas as well as aqueous inclusions (Fig. 7B-C, 9). Prominent Raman peaks at 150 cm-1, 218 cm-1 and 473 cm-1 are typical for native sulfur and were interpreted based on the native sulfur reference from the

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RRUFF database (Fig. 9). Some of the S-HS- inclusions contain additional phases filled with

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CO2, CH4 as well as vapor and liquid H2S (Fig. 9) and thus provide compelling evidence of co-

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genetic origin of the S-HS- and CH4-H2S-CO2 gas inclusions. Type 2 aqueous two-phase

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inclusions co-existing with S-HS- and CH4-H2S-CO2-N2 gas inclusions in calcite (Fig. 7B-C) yielded homogenization temperatures of 146-149°C (median: 147°C) (Table 4).

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Raman spectroscopy proves the presence of type 1a H2S-free, CH4-CO2 gas inclusions

4.2.3

Anhydrite

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(84-89 mol% CH4, 11-16 mol% CO2) in calcite fracture-fillings in one well (Fig. 10b,d, Table 2).

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Primary type 1a gas inclusions in anhydrite are oriented parallel to the c-axis of prismatic

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crystals (Fig. 7F), which overgrow calcite I, and contain methane (Fig. 10d) and in some cases solid bitumen. Trails of secondary type 1a inclusions, which were entrapped perpendicularly to the longest crystal axis (Fig. 7G), contain CH4 (91-94 mol%), CO2 (5-6 mol%) and N2 (2-3 mol%) (Fig. 10d, Table 2). Both primary and secondary gas inclusions homogenize to the liquid phase in the temperature range between -76.1 to -74.4°C, whereas solid melting occurs between 91.5 and -90.1°C (Table 3). Type 2 aqueous inclusions co-existing with secondary gas inclusions in trails homogenize to the liquid phase at 125-135°C (median: 127°C). Gases trapped in type 1b fluid inclusions hosted in prismatic anhydrite from fracture-fillings contain 61-69 mol% CH4, 2129 mol% H2S, 6 mol% CO2 and 3-4 mol% N2 (Table 2, Fig. 10d). 15

Journal Pre-proof 4.2.4 4.2.4.1

Fluorite Type 1b inclusions

Fluorite from fracture fillings hosted by dolostones (e.g. Fig. 3A, C), contains large (up to 100 µm) primary type 1b inclusions (Fig. 7H-I) that occur in three-dimensional clusters. Discrete trails of gas inclusions are also present in some samples, however, they contain smaller inclusions but compositions are similar to primary gas inclusions as shown by Raman spectroscopy (Fig.

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10d). It is likely that these inclusion trails follow sectoral zoning (Fig. 5H) and are of primary

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origin, too. Primary gas inclusions contain 58-71 mol% CH4, 17-30 mol% H2S, 7-12 mol% CO2

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and <4 mol% N2 (Fig. 8c, 10d). The inclusions homogenize into the vapor phase in the temperature range between -40.4 and -28.6°C, whereas melting of solid phases occurs between -

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96.5 and -89.7°C (Table 3).

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Aqueous type 2 inclusions frequently co-exist with gas-rich inclusions indicating

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heterogeneous entrapment of fluid and gas. Vapor bubbles of aqueous type 2 inclusions contain 12-73 mol% CH4, 5-44 mol% CO2 and 4-44 mol% H2S (Fig. 10c). Co-genetic formation of

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aqueous and gas inclusions is also evidenced by the presence of thin aqueous rims surrounding

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some of the large gas-rich inclusions (Fig. 7I), gas compositions of vapor phases in type 2 inclusions (Fig. 10c) and/or dolomite solids accidently trapped within gas inclusions. Type 2 inclusions are most frequent in studied fluorite samples and may additionally contain a halite daughter crystal or enclose patches of black organic matter and insoluble dolomite solids (Fig. 7J). Primary type 2 inclusions yield homogenization temperatures of 124-152°C (median: 139°C) (Fig. 11). Pseudo-secondary inclusions that are arranged along discrete trails show slightly lower homogenization temperatures of 121-138°C (median: 129°C) (Fig. 11). Primary and pseudosecondary type 2 inclusions show similar salinities (24.2-24.8 wt.% NaCl equiv.) as calcitehosted type 2 inclusions. 16

Journal Pre-proof 4.2.4.2

Type 1a inclusions

Fluorite-hosted primary H2S-poor type 1a gas inclusions contain 78-97 mol% CH4, 3-20 mol% CO2, <7 mol% H2S and <10 mol% N2 (Table 2, Fig. 10b-d). In the northernmost studied well, fluorite-hosted type 1a primary gas inclusions co-exist with type 2 aqueous inclusions, which yielded Th values of 149-167°C (154°C) and salinities of 24.5-25.6 wt.% NaCl equiv. (Fig. 11). The gas inclusions from this well homogenize to the liquid phase at temperatures between -

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83 and -77.4°C, whereas melting of the solid phase occurs at temperatures between -101.5 and -

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95.5°C (Table 3).

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Secondary CH4-CO2±N2 gas inclusions (88-91 mol% CH4, 1-2 mol% N2, 9-11 mol% CO2, C2+<1%), that are arranged in a trail crosscutting a fluorite cement (Fig. 4C, 7K), show

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Tm(solid) between -79 and -76.5°C and homogenization to the liquid phase between -70.7 and -

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69.9°C (Table 3). Type 2 aqueous two-phase inclusions that occur in the same secondary trail

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show Th values of 144-151°C and salinities between 23.7 and 24 wt.% NaCl equiv. (Fig. 11). This secondary fluid inclusion trail post-date clustered primary H2S-rich inclusions in fluorite

Stable C isotopes of fluid inclusion gases

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4.3

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cement present within the same sample from the NE part of the study area.

The isotopic compositions of fluid inclusion gases measured in Zechstein Ca2-hosted fracture-filling calcites and fluorites are presented in Table 5 and Fig. 12. The δ13CCH4 values of H2S-rich inclusions range between -18.7 and -8.7‰, whereas the δ13CCH4 values of inclusions with low or undetectable concentrations of H2S are considerably more negative and range between -26.5 and -20‰ (Table 5). The δ13C values of carbon dioxide measured in H2S-rich fluid inclusion gases cover a wide range between -22.4 and -18.9‰, whereas the δ13CCH4 values typical for H2S-poor inclusions are less negative to positive and vary between -3.9 and 8.4‰ (Table 5). 17

Journal Pre-proof 4.4

Stable C-O isotopic composition of calcites

Fracture-filling calcites from nine wells were analyzed for stable carbon and oxygen isotopic compositions (Table 6, Fig. 13). Calcites show a wide range of carbon isotopic compositions ranging between -10.4 and 6.1‰ (Fig. 13). Calcites from four wells containing type 1b H2S-rich inclusions yielded negative δ13C values from -10.4 to -4.6‰. In contrast, calcites hosting type 1a H2S-free and/or H2S-poor inclusions always have positive δ13C values (2.7-

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6.1‰). Oxygen isotopic compositions of brown to beige colored calcites, which precipitated at 18

O

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the vein-wall rock boundaries, range between 26.7 and 28.8‰, and are more enriched in

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when compared with isotopic compositions of white and colorless calcites (δ18OSMOW=20.7-

4.5

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δ18OSMOW values of 18.8-19.9‰ (Fig. 13).

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24.6‰). TSR-related calcites hosting H2S-rich inclusions yielded the lowest and very consistent

Chemistry and isotopic compositions of reservoir gases

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Present-day natural gases from Ca2 reservoirs in the PB are classified as dry gas as

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indicated by high dryness coefficient (C1/ƩCn) of 98-100% (Fig. 14). Overall concentrations of C2+ hydrocarbons, including ethane, propane, isobutane, isopentane, heptane and octane, do not

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exceed 1.77 mol%. Methane content varies between 52.8 and 93.9 mol%, whereas H2S is the dominant non-hydrocarbon gas with concentrations of up to 35.6 mol% (Fig. 15). The highest gas souring index (GSI) values for reservoir gases reach up to 40.3 mol% (Fig. 14). CH4 contents in Zechstein gas fields decrease with increasing H2S concentrations towards the south, whereas CO2 concentrations (4-18 mol%) remain at relatively constant level (Fig. 15). Nitrogen is usually the least abundant gas in analyzed wells (median: 3.5 mol%, Fig. 15). Methane and ethane δ13C values in Zechstein Ca2 reservoir gases show ranges of -25.7 to -6.3‰ and -27.1 to -12.9‰, respectively. The δ13CCO2 values of carbon dioxide in reservoir gases vary from -20.1 to 4.5‰. 18

Journal Pre-proof Present-day reservoir temperatures recorded in the studied wells lie between 125 and 155°C (median=136°C).

5. Discussion 5.1

Origin of hydrocarbons

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Dry gases, which were charged into Zechstein carbonate-Ca2 gas reservoirs, are sourced from deeper, highly mature and over-mature Westphalian kerogen type III coals (Schoell, 1988)

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(Figs. 14, 16). Sulfur-rich compounds are scarcely produced during thermal decomposition of

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type III kerogen type (Tissot and Welte, 1984) and thus exclude H2S supply from Upper

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Carboniferous coals into Ca2 reservoirs of the NGB. This is in line with fluid inclusion studies

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that have never reported H2S-bearing fluid inclusions from fracture-fill mineralization hosted by pre-Zechstein strata in the PB or LSB (e.g. Reutel et al., 1995; Mumm and Wolfgramm, 2002;

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Lüders et al., 2012; Wüstefeld et al., 2017). Therefore, the major sources of reacting sulfate for

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TSR in the PB was dissolved anhydrite.

The maturity of organic matter from source rocks in the PB area at the time of gas

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generation falls into a range of ~1.5-2% Ro (Bruns et al., 2013). Using the vitrinite reflectance data for terrestrial-humic type III kerogens from the southern PB, the carbon isotopic signature of instantaneously sourced methane can by calculated after Schoell (1980): δ13C = 0.6logRo - 28‰ (2) and equation based on more recent data from Lüders et al. (2012): δ13C = 3.9107lnRo - 28.077‰ (3) For both equations, the calculated range of stable carbon isotopic compositions for instantaneously expelled CH4 from Upper Carboniferous coals (~1.5-2% Ro) vary between -26.5 19

Journal Pre-proof and -25.4‰. These values are used as reference for isotopic compositions of Upper Carboniferous coal-derived gases in the southern area of the PB in the following discussion.

5.2

Spatial variability of gas compositions

Data from present-day reservoirs and fluid inclusion gases generally show similar compositional trends (Fig. 10a-b). Fracture-fillings from the northernmost wells contain CH4-

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dominated gas inclusions that locally only show very low concentrations of H2S of up to 1-7 mol% (Fig. 10b). In contrast, H2S-rich gas inclusions are prevalent in fracture-fill mineralization

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from the southernmost wells. This trend is generally consistent with compositions of reservoir

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gases, which show increasing H2S content towards the south (Fig. 15).

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In the quaternary diagrams the molecular compositions of gas inclusions cluster more

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compared to compositions of the present-day reservoir gases (Fig. 10a-b). Nevertheless, the molecular compositions of reservoir gas, stored in individual reservoirs, fall within narrow ranges

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(Fig. 10a). Fluid inclusion gases commonly show very consistent molecular compositions within

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individual wells and FIA’s, however, in four wells two end-member compositions were measured as expressed by two different clusters (Fig. 10b). This data distribution suggests that bulk

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compositions of reservoir gases may represent pure sour gas end-member or a mixture of sour gas and H2S-free sweet gas. Distinguishing between these two cases is possible when using fluid inclusion data since they provide insights into a discrete sequence of gas migration events and reveal true gas compositions at the time of gas entrapment. Fluid inclusion analysis proves that sweet gas is contained in secondary fluid inclusions hosted in cements and fracture-fillings, whereas the sour gas is contained in primary inclusions in the same samples suggesting later migration of sweet gas into the reservoir.

20

Journal Pre-proof The highest H2S contents of up to 40 mol% were detected in gas inclusions hosted in fracture-filling calcites (Table 2). The highest H2S concentration in reservoir gases was measured to be 35.6 mol% (Fig. 15). Analyses of natural reservoir gases in some of the wells show much lower present-day contents of H2S compared to the H2S contents measured in fluid inclusion gases. This implies that H2S concentrations in the reservoir gas were locally higher in the geological record than today. In the central LSB, TSR-derived H2S was consumed by deposition

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of Ca2-hosted massive Zn-Pb sulfide mineralization (Sośnicka and Lüders, 2019). Massive

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sulfides are absent in the PB, however three generations of fracture-filling pyrite and later galena

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are indicative of H2S consumption by sulfide formation. Lower present-day H2S content could

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also have resulted from mixing with sweet gas that was sourced from Upper Carboniferous coals during deep burial from the Late Cretaceous onwards. Large-scale lateral migration of sweet gas

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sourced from mature coals from the PB towards south is supported by the presence of secondary

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H2S-poor and H2S-free CH4-rich gas inclusions in fracture-filling minerals from wells in the PB as well as at the northern margin of the LSB (Duschl et al., 2016).

Dry coal hydrocarbon gas reactivity during TSR

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5.3

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Methane in H2S-free and H2S-poor gas inclusions yielded δ13CCH4 values (-25.9 to 24.3‰), which are similar to the range of the calculated δ13CCH4 values for the unaltered dry gas sourced from Upper Carboniferous coals (Fig. 12c, d). In contrast, methane contained in H2S-rich fluid inclusion gases shows much less negative δ13CCH4 values (from -18.7 to -8.7‰) compared to the calculated reference isotopic compositions of -26.5 to -25.4‰). Carbon isotopic and molecular compositions of fluid inclusion gases as well as recent reservoir gases show similar trends. There is a strong positive correlation (R2=0.92) between the maximum H2S contents detected in gas inclusions and increasing δ13CCH4 values (Fig. 12c). A 21

Journal Pre-proof similar positive trend (R2=0.63) is observed for reservoir gases (Fig. 12a). The observed shift of the δ13CCH4 values in fluid inclusions as well as in reservoir gases strongly supports the idea that dry coal gas was the dominant reacting hydrocarbon during TSR in the study area. Reaction between dissolved sulfate and CH4 led to

13

C enrichment in the latter and thus to less negative

δ13C values in the residual CH4 gas pool. Negative linear correlation between decreasing CH4 content and increasing δ13CCH4 values of reservoir gas additionally confirms that alteration of

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isotopic signatures of CH4 is caused by consumption of CH4 by TSR and contemporaneous

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generation of CO2 (Fig. 12b).

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Very negative δ13CCO2 values in the range between -22.4 and -18.9‰ as well as negative

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correlation (R2=0.95) between δ13CCO2 values and δ13CCH4 for H2S-rich fluid inclusion gases is indicative for TSR origin (Fig. 12d). Partial incorporation of TSR-derived CO2 is also supported

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by negative carbon isotopic signatures of some calcites (-10.4 to -4.6‰), which host H2S-rich gas

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inclusions (Fig. 13). In contrast, non TSR-derived calcites show positive δ13C values similar to those of marine Permian carbonates (e.g. Magaritz et al., 1981; Veizer et al., 1999).

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The positive linear correlation (R2=0.56) between δ13CC2H6 and GSI values for the ethane

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gas in Ca2 reservoirs implies consumption of C2+ hydrocarbons during TSR reactions. The observed isotopic trend of more positive δ13CC2H6 values compared to δ13CCH4 values (Fig. 12a) is typical for advanced stages of TSR and extreme gas dryness (e.g. Liu et al., 2013). Following the entire consumption of ethane the δ13CCH4 values become less negative (Fig. 12a).

5.4

P-T conditions of gas migration

Based on fluid inclusion data three major stages of gas generation/migration in the southern PB can be distinguished: Stage 1) reservoir charge with methane-rich gas and alteration during subsequent burial, Stage II) migration of altered H2S-rich gas along fractures in the 22

Journal Pre-proof carbonate reservoir rocks in response to inversion tectonics and trapping as fluid inclusions in fracture filling minerals, and Stage III) post-inversion deep burial-related sweet gas migration. 5.4.1

Stage I

Stage I gas migration was associated with progressing calcitization and bedding-parallel stylolitization (calcite I-IV cements), which post-dated early dolomitization and dolomite dissolution (Figs. 6, 5C-F). In the PB these processes took place during intermediate burial

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starting in Early Triassic (Schoenherr et al., 2018; Humphrey et al., 2019). Migration of CH4-

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CO2±N2 gas is recorded by type 1a inclusions hosted in calcite I-II cements (Figs. 4A, 7A).

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Hydrocarbon gases were trapped in inclusions at temperatures of 102-127°C and pressures of

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350-400 bars (Fig. 17). These values correspond to maximum depths of about 3.3 km assuming

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slightly over-pressured conditions (Fig. 17b). The inferred depths and gas migration are consistent with the major hydrocarbon-producing event, which took place in Late Triassic times

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(Fig. 18). From that time on until the Early Lower Cretaceous the reservoir gases were altered by sulfate reduction reactions due to increasing temperature upon burial. Solid bitumen detected in

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some of the gas inclusions resulted rather from cracking of early stage hydrocarbons than TSR as

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the inclusions are H2S-poor or contain solely trace amounts of H2S. These sulfur compounds could have been produced by thermal cracking of sulfur-containing crude oil (e.g. Orr, 1977). Stage I dry gas, which was charged into the reservoirs during the Late Triassic, was altered by TSR from that time on until at least the beginning of the Stage II. Thermal decomposition of methane, reacting with dissolved sulfate in Ca2 reservoirs, led to the generation of H2S, which was most intense in the southern part of the studied area in the PB.

23

Journal Pre-proof 5.4.2

Stage II

Stage II marks the change from extensional to compressional regime in the Early Cretaceous resulted in the formation of tectonic stylolites, fractures and small reverse faults, which were filled with calcite, anhydrite and fluorite (Fig. 3). At least three H2S-flow events are recorded by three pyrite generations (Fig. 4E-F). Euhedral non-porous pyrite overgrowths covering the porous cores precipitate by interaction of hydrothermal Fe-rich fluids with TSR-

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derived H2S similarly as shown for sulfides from the Harz Mountains (e.g. Lüders and Ebneth,

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1993). Small proportions of altered gas were trapped as CH4-H2S-CO2-N2 gas inclusions in

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fracture-filling minerals. The latter precipitated from highly saline brines at continuously increasing temperatures (Fig. 11). The fracture mineralization commenced by calcite

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precipitation at T=100-149°C (median: 119°C) and continued during replacement of calcite by

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fluorite at temperatures of 124-152°C (median: 139°C) (Fig. 11). Low density type 1b CH4-H2S-

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CO2-N2 gas inclusions hosted in fracture-fillings yield low-pressure conditions of P=170-210 bars for fluid entrapment (Fig. 17). Such low-P conditions may be underestimated as accurate

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equations of state defining such complex gas mixtures do not exist. The estimated depths of ~1.7

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km under hydrostatic conditions suggest that migration of decompressed type 1b gas during Stage II was related to the uplift of the PB (Fig. 18). In contrast, the northernmost wells in the study area were not or only slightly affected by gas souring. Here, prismatic anhydrites, replacing calcite I cement, have entrapped low density decompressed type 1a CH4 gas. 5.4.3

Stage III

The latest gas migration Stage III is characterized by the migration of high density CH4CO2±N2±C2+ coal gas. This gas migration event is recorded by the presence secondary fluid inclusions in prismatic anhydrite, fluorite cements as well as primary and secondary type 1a inclusions in fracture-fillings (Fig. 7G, K-L). Secondary CH4-CO2-N2 inclusions in prismatic 24

Journal Pre-proof anhydrite mark the transition to the higher pressure regime in the northern part of the studied area due to deep burial from the Upper Cretaceous onwards (Fig. 17). Homogenization temperatures of fluorite-hosted gas-saturated aqueous inclusions indicate that during Stage III the highest temperatures of 144-167°C and pressures of 400-645 bars were achieved (Fig. 17). Estimated depths of 2.6-3.6 km are consistent with renewed burial of the PB assuming pressure conditions from lithostatic P to 1.5×hydrostatic P (Fig. 2). This is in line with the major hydrocarbon

TSR temperatures

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5.5

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expulsion from Upper Carboniferous strata during the Late Cretaceous (Fig. 18).

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The coexistence of S-HS- and CH4-H2S-CO2-N2 fluid inclusions in calcites that are related

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to TSR provides compelling evidence for intensive TSR reactions almost entirely consuming

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ethane and vastly altering dry methane. TSR reactions between sulfates and methane took place at temperatures of 146-149°C (Table 4, Fig. 7C) as derived from aqueous inclusions that are co-

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genetic with S-HS- inclusions from one well.

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Variable δ18O values of calcites in the range between 18.8 and 28.8‰ (VSMOW) reflect different formation temperatures for fracture filling calcites. Calcites that host H2S-rich fluid

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inclusions always show negative δ13C values and the lowest measured δ18O values (Fig. 13, Table 6). The δ18O values for some Zechstein Ca2 reservoir formation waters vary between 3.4‰ and 7.7‰ (Industrial data). Assuming a mean δ18O value of 6.1‰ for the calcite-forming fluids temperatures of TSR-related calcite precipitation, (PB-32, PB-22, PB-20) temperatures for calcite formation and thus TSR between 135 and 150°C can be calculated in three wells (O’Neil et al., 1969). This temperature range is in line with the measured ranges of homogenization temperatures of type 2 inclusions in the studied samples (Fig. 11, Table 4). The lowest homogenization temperature of 135°C, yielded by TSR-calcite-hosted gas-saturated aqueous 25

Journal Pre-proof inclusion is in perfect agreement with the lower limit of the calculated temperature range for TSR calcite precipitation. It is concluded that major H2S production fell into this temperature range, which is considerably lower than the so far reported temperature limit for CH 4-dominated TSR (Li et al., 2019). However, even lower temperatures between 100 and 140°C (median: 119°C) for the CH4-dominated TSR are yielded by homogenization temperatures of fluid inclusions hosted in TSR-related calcites form well PB-21 (Fig. 11). Such low temperatures for TSR involving dry

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to super-dry methane-rich gas are reported here for the first time.

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Temperatures of ~120°C are considered as onset temperatures of TSR (e.g. Worden and

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Smalley, 1996; Machel, 2001). However, conclusive temperatures lower than ~161.5°C (Li et al., 2019) were not reported for CH4-dominated TSR so far. Some aqueous inclusions studied here

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provided evidence for the presence of dissolved Mg2+ cations and H2S, which both act as TSR

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catalyzers (He et al., 2019). In some cases, dolomite crystals are entrapped in aqueous and gas

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inclusions in studied samples suggesting that the gas was mixed or was in contact with Mg-rich aqueous solutions. BSR, degradation of Ca2-hosted S-rich organic matter and oxidation of early

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hydrocarbons by native sulfur during early diagenesis (Mittag-Brendel, 2000) may have supplied

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TSR catalyzer such as H2S. These processes most likely contributed to decrease of temperatures required for TSR to proceed. However, the influence of early BSR-derived H2S has been limited because most of it was consumed for the formation of early diagenetic pyrite (Biehl et al., 2016; Orr, 1977).

6. Summary and Conclusions Chemical and isotopic analyses of reservoir and fluid inclusion gases revealed compelling evidence of progressing and probably still ongoing TSR reactions in Zechstein Ca2 carbonate reservoirs in the southernmost part of the PB (Fig. 18). This study constrains minimum onset 26

Journal Pre-proof temperatures for CH4-dominated TSR in the NGB of ~135°C in the presence of H2S and dissolved Mg2+ catalyzers. Fluid inclusions unravelled a complex gas migration history (Fig. 18). Three major stages of gas migration in the Zechstein Ca2 reservoirs can be discriminated. During early Stage I the Zechstein reservoirs were charged with Upper Carboniferous coal-derived CH4-CO2±N dry gas from the Late Triassic on. It cannot be excluded that the hydrocarbon gases were affected by BSR

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during burial (Fig. 18). Major H2S generation in carbonate-hosted reservoirs by TSR due to

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increasing temperature commenced during the Jurassic and lasted to the uplift of the PB in Lower

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Cretaceous times. During Stage II, the formation of fracture-fill mineralization in reservoir

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carbonates and following entrapment of H2S-rich gases at T=100-152°C was related to the uplift of the PB in the Early Lower Cretaceous (Fig. 18). The latest Stage III of gas migration is

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characterized by lateral migration of CH4-CO2±N2 gases sourced from Upper Carboniferous coals

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during renewed pronounced burial in the Late Cretaceous (Fig. 18). Fluid inclusions evidence large-scale lateral migration of Stage III hydrocarbons towards the south.

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This study demonstrates that fluid inclusion analysis is suitable for tracking of H2S

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contents in reservoir gases through time and space. This prognostic tool can help avoiding areas of high concentrations of toxic H2S gas in the reservoirs.

Acknowledgements We thank ExxonMobil Production Germany for providing samples from drill cores. We are indebted to Birgit Plessen (GFZ Potsdam) for carbon and oxygen isotope analysis of calcite samples. Discussions and constructive feedback by Johannes Schoenherr and Alfons M. van den Kerkhof are highly acknowledged. Matthew Steele-MacInnis and Thomas Kirnbauer are warmly thanked for critical and detailed reviews. This study was financially supported by the 27

Journal Pre-proof MinNoBeck joint research project (no. 033R165B) funded by the German Federal Ministry of Education and Research (BMBF) in the frame of the BMBF program r4 “Innovative Technologien für Ressourceneffizienz - Forschung zur Bereitstellung wirtschaftlicher Rohstoffe”.

Figure Captions

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Fig. 1 Geological sketch of major gas reservoirs in the southern Pompeckj Block (PB) and the Lower Saxony Basin (LSB) after LBEG (Landesamt für Bergbau, Energie und Geologie, Deutschland) Report 2014. The basement fault systems are based on Kombrink et al. (2010), PB/LSB boundary after Karnin et al., (2006), boundaries of depositional environments after Strohmenger et al. (1996).

-p

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Fig. 2 Representative burial curves for the Zechstein base and Carboniferous base in the Pompeckj Block and the Lower Saxony Basin (after Bruns et al., 2013).

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Fig. 3 Sampled core material. A-Calcite-fluorite (cal-V+fl) vein following a reverse fault crosscutting bedding-parallel stylolite, hosted by dolomite Ca2, B-anhydrite (anh) associated with stylolite oblique to the bedding, C-vertically oriented fracture-filling containing calcite V (cal-V) and cubical fluorite (fl).

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na

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Fig. 4 Different generations of minerals hosting gas and aqueous fluid inclusions (A-D-transmitted light, E-F-reflected light). A-brown calcite I (cal-I) matrix and calcite II (cal-II) cement filling nodule, B-calcite I (cal-I) cement replaced by prismatic anhydrite (anh), C-late fluorite (fl) cement filling vuggy porosity in calcite I (cal-I) cement, red arrow points to secondary fluid inclusion trail cutting through the cementing fluorite, D-Fracture in dolomite (dol-I) filled with calcite (cal-V) and fluorite (fl), E-reflected light image of two generations of pyrite, porous pyrite I (py-I) occupying the crystal cores is replaced by subhedral to euhedral pyrite II (py-II) and they are intergrown with calcite V (cal-V) in the fracture-filling, F-three generations of pyrite, porous pyrite I (py-I) is replaced by euhedral, cubic pyrite II (py-II), which in turn is overgrown by euhedral pyrite III (py-III), the dotted line indicates growth zone marking boundary between py-II and py-III. Fig. 5 CL images of cements and fracture-fillings. A- Early dolomite (dol-I) cement and dolomite fracturefilling (dol-II) post-dated by fracture-filling calcite (cal-V), B-Early brown calcite cement showing dull red luminescence post-dated by nodule calcite (cal-II) of brighter luminescence, C-Calcite-filled beddingparallel stylolite post-dating dolomite (dol-II), calcite III shows homogeneous orange luminescence, whereas later calcite IV (cal-IV) exhibits characteristic patchy, irregular sectoral zoning, D-vug filled with late calcite (cal-V) cement in dolomite (dol-II) fracture-filling, E-Late calcite (cal-V) cementing interstitial spaces between ooid aggregates and replacing dolomite II, F-late calcite (cal-V) and fluorite cements filling vuggy porosity in dolomite (dol-II) and bedding-parallel stylolite in dolomite (dol-II) G- 5 mm thick fracture filled with calcite (cal-V) showing orange luminescence with prominent sectoral zoning, replaced by later fluorite exhibiting blue luminescence, H-fracture-filling fluorite showing sectoral zoning. Fig. 6 Generalized paragenetic sequence of diagenetic events in Zechstein Ca2 reservoirs in analyzed wells from the Pompeckj Block. 28

Journal Pre-proof

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Fig. 7 Fluid inclusions in minerals from gas exploration wells drilled in the Pompeckj Block. A- coexisting type 1a CH4-CO2 gas and type 2 aqueous 2-phase (L+V) inclusions in calcite cement (cal-I), B-Cfracture-filling calcite V (cal-V) containing three co-existing types of inclusions: type 1b CH4-H2S-CO2-N2 gas, type 2 aqueous as well as type 3 S-HS-, also see Fig.4D, D-primary 1-phase type 1b CH4-H2S-CO2-N2 gas inclusions hosted by fracture-filling calcite V (cal-V), E-type 2 aqueous (L+V±Dol) inclusions with gas-rich vapor bubble and dolomite solids from fracture-filling calcite V (cal-V), which also contains type 1b H2S-rich gas inclusions, F-primary type 1a CH4 gas inclusions distributed along growth zones in prismatic anhydrite (anh), G-secondary type 1a CH4-CO2 sweet gas inclusions and aqueous 2-phase (L+V) inclusions crosscutting growth zones of prismatic anhydrite, H-primary large co-genetic type 2 aqueous 2phase (L+V) inclusions and type 1b CH4-H2S-CO2-N2 gas inclusions along with pseudo-secondary trails of the same inclusion types in fluorite (fl) fracture-filling, I-large type1a CH4-H2S-CO2-N2 gas inclusion (G) wetted by an aqueous rim, fluorite (fl) fracture-filling, J-fluorite-hosted type 2 aqueous 4-phase inclusion containing H2S in the liquid phase (L), CO2-H2S-CH4 gases in the vapor phase (V), dolomite (dol) solid and organic matter-Corg, K-secondary trail of type 1a CH4-CO2±N2±C2+ sweet gas and aqueous (L+V) inclusions in fluorite (fl) cement, also see Fig. 4C, L-primary cluster of type 1a H2S-poor gas inclusions co-existing with type 2 inclusions in fracture-filling fluorite (fl).

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Fig. 8 Raman spectra of hydrocarbon inclusions: a-type 1a CH4-CO2-N2 gas inclusion containing solid bitumen and traces of S compounds (2 mol% H2S) hosted by brown oncolite (cal-I), b-type 1a H2S-poor (6 mol%) CH4-CO2-N2 gas inclusion hosted in fracture-filling fluorite, c-type 1b CH4-H2S-CO2-N2 inclusion containing 25 mol% H2S in fracture-filling fluorite, d-type 1b CH4-H2S-CO2-N2 containing 40 mol% H2S from fracture-filling cal-V.

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Fig. 9 Raman spectra of type 3 S-HS- inclusions in early fracture-filling calcite V: a-1-phase S-HSinclusion, b-S-rich inclusion containing HS-, CH4-CO2-H2S gas phase and liquid H2S. Elemental sulfur spectra were compared with the RRUFF reference.

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Fig. 10 Quaternary diagrams illustrating compositional variability of: a) natural reservoir gases in analyzed gas exploration wells, b) gas inclusions hosted in anhydrite, calcite (I, II, V) and fluorite from individual wells, c) fluid inclusions, meaning gas inclusions and gas phases of aqueous inclusions, hosted in anhydrite, calcite and fluorite, d) gas inclusions of different generations, in the Pompeckj Block. Fig. 11 Diagram presenting microthermometry results of TSR calcite-hosted and different generations of fluorite-hosted inclusions. Fig. 12 Variations of stable carbon isotopic compositions of reservoir gases (a-b) and fluid inclusion dry gases (c-d) in the Pompeckj Block: a) positive trends between δ13CC2H6 and δ13CCH4 values with increasing gas sourness expressed by the gas souring index (GSI), indicating advanced stage of TSR, b) negative trend between decreasing concentrations of methane in reservoir gas with increasing δ 13CCH4 values indicating that it is replaced by non-hydrocarbon gases produced during TSR, c) positive correlation between δ13CCH4 values of fluid inclusion methane and maximum H2S contents in fluid inclusion gases, d) negative trend between decreasing δ13CCO2 values and increasing δ13CCH4 values for fluid inclusion gases indicates that CO2 is TSR-derived. The calculated reference isotopic values for the coal-derived gas are marked by green area.

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Journal Pre-proof Fig. 13 Stable carbon and oxygen isotopic compositions of fracture-filling calcites from the Pompeckj Block. TSR calcites show shift to lower δ13CCAL and δ18OSMOW values compared to typical isotopic compositions of calcites. Fig. 14 Diagram showing positive relationship between reservoir gas sourness expressed as GSI and extreme gas dryness in the Pompeckj Block. Fig. 15 Compositions of natural reservoir gases from gas exploration wells in the Pompeckj Block. The southernmost wells contain the highest concentration of hydrogen sulfide. Fig. 16 Isotopic compositions and relative abundance of methane in reservoir gases (after Schoell, 1980; Stahl, 1977; Tissot and Welte, 1984).

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Fig. 17 Pressure-Temperature conditions of gas migration in the Pompeckj Block: a-Isochoric projection of co-genetically trapped gas and aqueous inclusions hosted in calcite I-II cements, prismatic anhydrites and fracture-filling fluorites, b-Pressure-Depth diagram showing the sampling depth and P-Depth range of gas migration calculated from fluid inclusions assuming P conditions varying from lithostatic P to slightly over-pressured hydrostatic (from 1.3× to 1.5×hydrostatic P) in Ca2 reservoirs.

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Fig. 18 Stages of migration and evolution of hydrocarbon dry gases superimposed on representative burial curves for the Zechstein unit (grey shaded range) in the Pompeckj Block. Solid lines mark stratigraphic boundaries; color code for gas migration/alteration stages is similar as in Fig. 17. During Stages I and III dry coal gas, expelled from Upper Carboniferous during burial, charged Zechstein reservoirs (thick upward arrows). Basin uplift Stage II led to gas decompression and alteration by TSR in Zechstein reservoirs. Lateral migration of dry methane gas towards the south is notable during the latest Stage III (thick rightwards arrow).

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Table 1 Results of multiple analyses of fluid inclusion gases released by crushing of mineral grains from one individual sample 1 from well PB-20, Pompeckj Block. Table 2 Compositional data from gas inclusions and gas phases of aqueous inclusions from anhydrites, calcites and fluorites from the Pompeckj Block. FIA-fluid inclusion assemblage. Table 3 Microthermometric data from gas inclusions, Pompeckj Block. Table 4 Microthermometric data from aqueous inclusions, Pompeckj Block. Table 5 Stable C isotopic compositions of fluid inclusion gases hosted in fracture-filling minerals from Zechstein Ca2, Pompeckj Block. Table 6 Stable C-O isotopic signatures of fracture-filling calcite hosts from the Pompeckj Block.

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Zhao, H., Liu, W., Borjigin, T., Zhang, J., Luo, H., Wang, X., 2019. Study of thermochemical sulfate reduction of different organic matter: Insight from systematic TSR simulation experiments. Mar. Pet. Geol. 100, 434–446. https://doi.org/10.1016/j.marpetgeo.2018.11.009 Ziegler, A.M., 1990. Phytogeographic patterns and continental configurations during the Permian Period. Geol. Soc. Lond. Mem. 12, 363–379. https://doi.org/10.1144/GSL.MEM.1990.012.01.35 Zwahlen, C., Hollis, C., Lawson, M., Becker, S.P., Boyce, A., Zhou, Z., Holland, G., 2019. Constraining the Fluid History of a CO2 -H2S Reservoir: Insights From Stable Isotopes, REE, and Fluid Inclusion Microthermometry. Geochem. Geophys. Geosystems 20, 359–382. https://doi.org/10.1029/2018GC007900 Zwart, E.W., Touret, J.L.R., 1994. Melting behaviour and composition of aqueous fluid inclusions in fluorite and calcite: applications within the system H2O-CaCl2-NaCl. Eur. J. Mineral. 773–786. https://doi.org/10.1127/ejm/6/6/0773

36

Journal Pre-proof

Table 1 13

13

δ CPDB (CO2) ‰ n.d. n.d. n.d. -20.4 -19.2 0.42

ur

na

lP

re

-p

ro

of

δ CPDB (CH4) ‰ -12.5 -12.2 -12.6 -12.5 -12.4 0.07

Jo

Well1-Sample1 Sample 1a Sample 1b Sample 1c Sample 1d Sample 1e Standard Error

37

Journal Pre-proof Table 2

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Fluorite

FIA2

Fluorite

FIA1

Fluorite

FIA5 a

PB-21-2

Calcite

FIA1

PB-21-3

Calcite

FIA1

Fluorite

FIA1

Fluorite

FIA3 a

PB-20-3 PB-20-8 PB-20-8

PB-22-3 PB-22-5

Calcite cement

PB-07

FIA1 FIA1

6

gas

8

gas

primary primary primary primary primary

of

7

FIA1

primary primary

ro

6

Fluorite

primary

FIA1

5

gas

primary

Fluorite

FIA1

6

gas

Calcite Fluorite cement

FIA1

3

FIA1 FIA1

62

33

5

50-66

5-8

34-42

71-73

23-26

4

primary

66-69

0-4

7-9

20-22

gas

primary

65-73

0-2

6-7

18-29

6

gas

secondary

88-91

1-2

9-11

6

gas

primary

63-65

1-2

9-12

22-27

pseudosecondary pseudosecondary

64

2

11

23

58-60

2-3

9-10

28-30

primary

58-59

2-3

9-10

30

gas

Fluorite

FIA1

10

gas

Fluorite

FIA1

5

gas

Calcite

FIA1

1

gas

primary

Calcite

FIA2

4

gas

primary

Fluorite

FIA1

5

gas

primary

PB-20-7

Calcite

FIA1

4

gas

primary

PB-20-8

Fluorite

FIA2

4

gas

primary

Fluorite

FIA2 a

2

gas

pseudosecondary

Fluorite

FIA3

4

gas

primary

Fluorite

FIA3 b

1

gas

secondary

Fluorite

FIA4

5

gas

pseudosecondary

Fluorite

FIA5

1

gas

primary

gas

pseudosecondary

FIA6

5-10

9-11

5

Fluorite

33-51

1-2

FIA1

PB-20-8

44-61

87-89

Fluorite

PB-20-8

15

11-16

Fluorite

PB-20-8

40

84-89

primary

PB-20-8

46

11

gas

PB-20-8

12-36

89

4

PB-20-8

15-38

primary

FIA1

PB-20-7

49-54

26-36

Calcite

PB-20-6

10-12

6-11

PB-13-4167

PB-20-6

26-27

0-4

gas

PB-20-6

62-63

53-64

1

PB-20-4

44

primary

FIA1

PB-20-3

44

14-23

Calcite

PB-20-3

12

9-16

PB-13-4129

Calcite

H2S (mol% )

2

Calcite

PB-16-4295 PB-16_thno.5

CO2 (mol% )

67-71

PB-13-4115

PB-13 PB-164295(2)

N2 CH4 (mol% (mol%) )

primary

-p

5

PB-13-4138

FI origin

re

4

FIA

lP

3

Host Mineral

na

2

PB13 PB20 PB20 PB20 PB21 PB21 PB22 PB22 PB07 PB13 PB13 PB13 PB13 PB16 PB16 PB16 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20 PB20

Sample

ur

1

Well

Jo

N o

No. of flui FI type d incl . aqueous, gas 1 phase aqueous, gas 2 phase aqueous, gas 6 phase aqueous, gas 1 phase aqueous, gas 5 phase aqueous, gas 1 phase aqueous, gas 9 phase aqueous, gas 2 phase

2

62

3

10

25

62

1-2

10-11

25-28

62-63

2-3

10-11

25-26

49-50

2-3

7-8

39-40

58-66

2-3

10-12

21-28

62

3

11-12

25-26

62-63

2

9-11

25-28

11

6

83 62

3

10

25

60

2

9

28

64-66

2-3

10-11

21-23

38

Journal Pre-proof FIA1

10

gas

Fluorite

FIA1

6

gas

Fluorite

FIA2

5

gas

Calcite

FIA2

11

gas

primary

Fluorite

FIA3

4

gas

primary

Fluorite

FIA4

6

gas

pseudosecondary

PB-26

Calcite

FIA1

4

gas

PB-26

Calcite

FIA1

3

PB-26

Anhydrite

FIA1

PB-26

Anhydrite

PB-29-12

PB-22-5

PB-32-2 PB-32-2 PB-32-2 PB-32-2 PB-32-2 PB-40-2 PB-40-6

24-28

64-65

2-4

8-9

23-24

secondary

76-77

1-2

19-20

3

57-65

1-2

9-14

21-34

63-64

2-3

11

23-24

63-64

2-3

10-12

22-24

primary

92-93

0-2

6-7

gas

primary

2-3

5-9

5

gas

primary

87-94 91100

FIA2

4

gas

secondary

91-94

2-3

5-6

Fluorite

FIA1

5

gas

primary

86-88

0-5

3-6

2-7

Anhydrite

FIA4

3

gas

primary

61-69

3-4

6

21-29

Calcite

FIA3

5

gas

69-70

3-4

8-9

18-19

Calcite

FIA4

2

gas

9-10

20-22

Fluorite

FIA1

5

gas

Fluorite

FIA2

3

gas

pseudosecondary

Fluorite

FIA1

5

gas

primary

Fluorite Fluorite

PB-40-3

Fluorite

3

FIA1

4

FIA2 SUM :

6

primary

pseudosecondary primary

68-71

1-2

69

4

7

21

71

3

8

17

92-97

gas

primary

gas

primary

94-95

gas

primary

78-95

91-95

3-8 3

5-6 4-6

0-10

3-8

2-5

224

ur

PB-40-3

FIA2

ro

PB-22-5

8-12

-p

PB-22-5

0-2

re

PB-21-3

61-65

lP

PB-21-3

primary pseudosecondary

of

Calcite

na

PB-20-8

Jo

PB32 20 PB33 21 PB34 21 PB35 22 PB36 22 PB37 22 PB38 26 PB39 26 PB40 26 PB41 26 PB42 29 PB43 32 PB44 32 PB45 32 PB46 32 PB47 32 PB48 40 PB49 40 PB50 40 PB51 40

39

Journal Pre-proof Table 3 Inclusion number 6

Well

Sample

Host

Origin

1

PB-16

PB-16

fluorite cement

2

PB-20

PB-20-7

fluorite in fracture-filling

3

3 4

PB-20 PB-26

PB-20-8 PB-26

2 2

5

PB-26

PB-26

4

primary

6

PB-26

PB-26

8

secondary

7

PB-40

PB-40

fluorite in fracture-filling calcite cements I-II anhydrite replacing calcite I anhydrite replacing calcite I fluorite in fracture-filling

secondary pseudosecondary primary primary

5

primary

-70.7 to -69.9

Th mode LV→L

-78.8 to -77.6

-40.4 to -33.9

LV→V

-93.1 to -90.1

-33.3 to -28.6 -75.8 to -76

LV→V LV→L

-96.5 to -89.7 -92

-76 to -75.5

LV→L

-90.3

-76.1 to -74.4

LV→L

-91.5 to -90.1

-83 to -77.4

LV→L

-101.5 to -95.5

Th

Tm

Jo

ur

na

lP

re

-p

ro

of

No

40

Journal Pre-proof Table 4

8 9 1 0 1 1 1 2 1 3 1 4 1 5

PB-26

calcite cements I-II

PB-26 PB40-2 PB40-3

anhydrite replacing cal-I fluorite in fracturefilling fluorite in fracturefilling

12

primary

4

pseudosecondary

8

primary

12

pseudosecondary

2

primary

10

primary

11

primary

11

primary

8

primary

12

pseudosecondary

11

primary

4

secondary

5

primary

6

primary

144151 124149 132136 137141 121134 146149 100133 101131 105140 136152 121138 102127 125135 149167 153158

Tfm, approx.

149 137 133 138 128

-58 to 48 -54 to 53 -49 to 47 -55 to 49

Salinity/ wt. % NaCl equiv. Medi Range an 23.723.9 24 24.524.6 24.8 24.524.5 24.6 24.424.5 24.6 24.324.5 24.7

dolomi te solid

147

of

7

fluorite in fracturefilling fluorite in fracturefilling fluorite in fracturefilling fluorite in fracturefilling calcite in fracturefilling calcite in fracturefilling calcite in fracturefilling calcite in fracturefilling fluorite in fracturefilling fluorite in fracturefilling

secondary

Medi an

123 112

ro

6

PB20-7 PB20-7 PB20-8 PB20-8 PB20-8 PB21-1 PB21-2 PB21-3 PB21-3 PB21-3

6

-p

5

fluorite cement

Range

re

4

PB-16

Th (LV→L) Origin

lP

3

Host

-50 to 47

128 150 128

-59 to 48 -49 to 47

24.525.1

24.7

+/-

24.5-25

24.8

+/-

24.624.9

24.7

+/-

24.8

24.8

24.224.5

24.5

110 127 159 154

-54 to 48

25.125.4 24.525.6

25.1 25.1

na

2

PB16 PB20 PB20 PB20 PB20 PB20 PB21 PB21 PB21 PB21 PB21 PB26 PB26 PB40 PB40

Inclusi on numbe r

Sampl e

ur

1

Well

Jo

N o

41

Journal Pre-proof Table 5

calcite anhydrite calcite, white calcite, brown calcite calcite calcite calcite fluorite fluorite fluorite fluorite fluorite fluorite fluorite fluorite fluorite fluorite calcite calcite fluorite calcite fluorite fluorite fluorite fluorite fluorite fluorite calcite calcite calcite in stylolite calcite

n.m. 10.6 9.4 n.m.

12.0

n.m.

Int. mV 1079 1651 151 131 4050 5978 717 4540 2433 606 374 1169 254 265 164 615 288 632 1124 461 382 2529 1328 2059 5529 2837 2411 2736 703 1174 1377 1220

δ13CPDB (CO2), ‰ 8.4 7.0 -1.5 -3.9 2.4 1.6 n.m. -0.9 -22.4 -18.9 n.m. -19.0 n.m. -20.2 n.m. n.m. n.m. n.m. n.m. 2.2 1.0 2.7 3.9 2.5 2.2 3.6

n.m.

na

lP

traces traces traces traces 97 128 traces 126 70 traces traces traces traces traces traces traces n.d. traces traces traces traces traces traces traces 102 traces traces 60 traces traces traces traces

of

PB-01 PB-01 PB-13-4b PB-13-4b PB-15-2 PB-15-3 PB-15-5 PB-15-7 PB-20-3 PB-20-4 PB-20-6 PB-20-6 [2] PB-20-7 PB-20-7 [2] PB-20-7 [3] PB-20-8 PB-21-3 PB-21-3 [2] PB-32-2 PB-38-1 PB-38-2 PB-38-3 PB-38-FLU PB-40-1 PB-40-2 PB-40-3 PB-40-5 PB-40-6 PB-40-7 PB-40-8 PB-40-9 PB-40-10

δ13CPDB (CH4), ‰ -25.0 -25.4 -20.0 -21.5 -23.6 -25.3 -24.4 -22.6 -8.7 -12.5 -13.3 -13.4 -12.0 -10 -11.8 -13.1 -18.7 -15.2 -14.8 -23.7 -24.6 -26.1 -24.8 -25.2 -25.9 -25.7 -25.1 -24.5 -24.3 -25.0 -25.2 -24.5

ro

PB-01 PB-01 PB-13 PB-13 PB-15 PB-15 PB-15 PB-15 PB-20 PB-20 PB-20 PB-20 PB-20 PB-20 PB-20 PB-20 PB-21 PB-21 PB-32 PB-38 PB-38 PB-38 PB-38 PB-40 PB-40 PB-40 PB-40 PB-40 PB-40 PB-40 PB-40 PB-40

Int. mV

-p

Mineral

re

Sample No.

ur

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

δ15N, ‰

Well

Jo

No.

42

Int. mV 99 119 127 168 250 440 66 497 372 109 74 183 traces 59 traces 102 53 94 86 92 90 511 208 117 355 201 171 179 traces traces traces 77

Journal Pre-proof Table 6 δ18OVSMOW‰ 22.3 24.6 28.8 23.1 26.7 22.1 21.7 26.7 19.8 19.3 19.2 20.7 18.8 19.9 24.3 21.9 21.8 21.3

δ13CCAL ‰ 5.0 6.0 6.1 4.5 3.9 3.7 2.7 5.4 -4.6 -9.0 -8.9 5.0 -7.3 -10.4 5.8 5.4 4.0 5.7

of

Mineral white calcite white calcite brown calcite white calcite brown calcite white calcite white calcite brown calcite colorless calcite colorless calcite colorless calcite white calcite white calcite white calcite white calcite white calcite white calcite white calcite

na

lP

re

-p

ro

Sample PB-01 PB-13-3 PB-13-4 PB-13-4a PB-13-4 PB-15-2 PB-15-3 PB-15-7 PB-20-6 PB-21-1 PB-21-2 PB-22-1 PB-22-2 PB-32-2 PB-38-3 PB-40-7 PB-40-8 PB-40-9

ur

Well PB-01 PB-13 PB-13 PB-13 PB-13 PB-15 PB-15 PB-15 PB-20 PB-21 PB-21 PB-22 PB-22 PB-32 PB-38 PB-40 PB-40 PB-40

Jo

No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

43

Journal Pre-proof Declaration of competing interest

Jo

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

44

Journal Pre-proof Highlights  Detailed study of C-O-H-N-S fluid inclusions from Zechstein Ca2 gas reservoirs in the Pompeckj Block (PB)  Three stages of gas migration in Ca2 carbonate were deciphered by fluid inclusions  High H2S accumulations in reservoirs are due to in situ thermochemical sulfate reduction-TSR of CH4

Jo

ur

na

lP

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of

 TSR onset at low temperatures (~135°C) not reported for dry coal gas alteration so far

45

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13

Figure 14

Figure 15

Figure 16

Figure 17

Figure 18