Origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in Dodan Field (SE-Turkey)

Accepted Manuscript Origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in dodan field (SE-Turkey) H. Hoşgörmez, M.N. Yalçın, C. Soylu, İ. Bahtiyar PII:

S0264-8172(14)00195-0

DOI:

10.1016/j.marpetgeo.2014.05.012

Reference:

JMPG 1970

To appear in:

Marine and Petroleum Geology

Received Date: 15 April 2013 Revised Date:

21 May 2014

Accepted Date: 23 May 2014

Please cite this article as: Hoşgörmez, H., Yalçın, M.N., Soylu, C., Bahtiyar, İ., Origin of the hydrocarbon gases carbon dioxide and hydrogen sulfide in dodan field (SE-Turkey), Marine and Petroleum Geology (2014), doi: 10.1016/j.marpetgeo.2014.05.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

CO2 generation H2S generation Hydrocarbon generation

ACCEPTED MANUSCRIPT ORIGIN of the HYDROCARBON GASES CARBON DIOXIDE and HYDROGEN SULFIDE in DODAN FIELD (SE-TURKEY)

a,*

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H. Hoşgörmez , M. N. Yalçın , C. Soylub, İ. Bahtiyarc Istanbul University, Engineering Faculty, Geology Dept,TR-34850, Avcılar, Istanbul, TURKEY

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TPAO International Projects Department, Söğütözü Mah. 2180 Cad. No:86, 06100 Çankaya, Ankara, TURKEY

c

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TPAO Exploration Department, Söğütözü Mah. 2180 Cad. No:86, 06100 Çankaya, Ankara, TURKEY

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*Corresponding author. E-mail: [email protected]

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Telephone number: 00905325924653

Keywords: origin of gases, hydrocarbon gases, CO2, H2S, thermochemical sulfate reduction, BSR, basin modelling, Dodan Field, southeastern Turkey Abstract

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Gas occurrences consisting of carbon dioxide (CO2), hydrogen sulfide (H2S), and hydrocarbon (HC) gases and oil within the Dodan Field in southeastern Turkey are located in Cretaceous carbonate reservoir rocks in the Garzan and Mardin Formations. The aim of this study was to determine gas composition and

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to define the origin of gases in Dodan Field. For this purpose, gas samples were analyzed for their molecular and isotopic composition. The isotopic composition of CO2, with values of -1.5‰ and -2.8‰,

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suggested abiogenic origin from limestone. δ34S values of H2S ranged from +11.9 to +13.4‰. H2S is most likely formed from thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) within the Bakuk Formation. The Bakuk Formation is composed of a dolomite dominated carbonate succession also containing anhydrite. TSR may occur within an evaporitic environment at temperatures of approximately 120-145oC. Basin modelling revealed that these temperatures were reached within the Bakuk Formation at 10 Ma. Furthermore, sulphate reducing bacteria were found in oil-water phase samples from Dodan Field. As a result, the H2S in Dodan Field can be considered to have formed by BSR and TSR. 1

ACCEPTED MANUSCRIPT As indicated by their isotopic composition, HC gases are of thermogenic origin and were generated within the Upper Permian Kas and Gomaniibrik Formations. As indicated by the heavier isotopic composition of methane and ethane, HC gases were later altered by TSR. Based on our results, the Dodan gas field may have formed as a result of the interaction of the following processes during the last

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7-8 Ma: 1) thermogenic gas generation in Permian source rocks, 2) the formation of thrust faults, 3) the lateral-up dip migration of HC-gases due to thrust faults from the Kas Formation into the Bakuk Formation, 4) the formation of H2S and CO2 by TSR within the Bakuk Formation, 5) the vertical

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migration of gases into reservoirs through the thrust fault, and 6) lateral-up dip migration within reservoir

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rocks toward the Dodan structure.

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ACCEPTED MANUSCRIPT 1. Introduction The most important petroleum fields in Turkey are located within the southeastern Anatolian region that forms the northernmost portion of the Middle East Petroleum Province (Fig. 1). Oil occurrences are generally related to petroleum systems of Cretaceous age (Salem et al., 1986; Soylu et al., 2005) although

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limited and local oil fields related to Paleozoic or Jurassic age source rocks are also present (Gürgey, 1991; Yılmaz and Duran, 1997; Connan et al., 2006). Differences in oil fields are due to different source rock facies or maturity, or due to the degree of biodegradation. The various oils have an API gravity

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ranging from heavy to light (12 to 36 API). Although limited in number, some gas fields also exist in southeastern Anatolia. The fields in this area (Silivanka, Dodan, and Camurlu) are mainly located east of

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this region and, in addition to hydrocarbon (HC) gases, contain carbon dioxide (CO2) and hydrogen sulfide (H2S).

The Dodan oil and gas field near Batman in southeastern Turkey originates from Cretaceous carbonate reservoir rocks, namely from the Garzan Formation and the Mardin group (Fig. 2). Gas production in

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different wells within Dodan Field consist mainly of CO2, significant amounts of H2S, some HC gases, and a high sulfur content (4.3–4.6%) oils (Gürgey, 1991). For recovery purposes, since 1986, 5.172 million m3 of CO2 have been produced and pumped into oil reservoirs of the heavy oil Raman Field.

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HC, CO2, and H2S gases in Dodan Field likely have different formation mechanisms and origins. For hydrocarbon gases, alterations of organic substances by methanogenic bacteria, the thermal degradation

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of kerogen and coal, and the cracking of oil in reservoirs are the primary formation mechanisms (Tissot and Welte, 1984; Hunt, 1996). Hydrogen sulfide is generally an undesirable component of natural gas and is highly toxic and corrosive to production equipment. Different mechanisms have been suggested for H2S formation (Krouse et al., 1988; Machel et al., 1995; Worden and Smalley, 1996; Worden et al., 1996; 2000; Heydari, 1997; Manzano et al., 1997; Belenitskaya, 2000; Machel, 2001; Cai, et al., 2003; 2004; Zhang et al., 2005; Zhu et al., 2005; 2007; 2011). Bacterial sulfate reduction (BSR) and thermochemical sulfate reduction (TSR) are the two main processes resulting in high H2S production in oil and gas fields

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ACCEPTED MANUSCRIPT although mantle and volcanic outgassing, kerogen decomposition, oil thermal cracking, and asphaltene thermal degradation may also contribute. Under appropriate temperatures, salinities, oxygen content, and nutrient levels, H2S can be formed in small amounts by bacteria (BSR) (Hose et al., 2000; Paul and Baosuhin, 2000; Cai, et al., 2001). H2S can also be of organic origin and upon thermal maturation formed

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from the sulfur in kerogen or oil (Orr, 1974, 1977; Hunt 1996). As a result of the reaction of anhydrite with petroleum fluids or methane at temperatures of 120–145°C, H2S is additionally formed by TSR (Cai et al., 2001).

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CO2 formation can be related to the degradation of organic matter, the thermal decarbonation of calcite at 200–250°C, and/or the decomposition of carbonate or reactions between certain minerals at 230–320°C

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(Petrucci, et al., 1994); it can also be of mantle origin (Javoy et al., 1986). Zhu et al., (2011) proposed a mixed origin for CO2 from TSR and carbonate decomposition through acidolysis. The aim of this study was to determine the formation mechanisms and the origin of hydrocarbon gases, carbon dioxide, and hydrogen sulfide in Dodan Field on the basis of molecular and isotopic composition,

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as well as the geological evolution of the region. Gases from the wells Dodan 16M, Dodan 17M, Dodan 6G, Dodan 6M, Dodan 17G, and Dodan 1A were investigated. Data obtained from the analyses were used to characterize and classify the gases and to distinguish their origin, as well as possible secondary

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processes impacting isotopic composition and ratios.

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ACCEPTED MANUSCRIPT 2. Geological Setting The Dodan oil and gas field is located near Batman in southeastern Turkey. The region consists of a sedimentary sequence that ranges from Paleozoic to Quaternary. The oldest unit encountered in the wells and/or in seismic sections is the Ordovician aged Bedinan Formation composed mainly of sandstone,

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siltstone, and shale alternations (Fig. 2). Devonian aged Yığınlı and Köprülü Formations unconformably overlie the Bedinan Formation. The Yığınlı Formation consists of quartzite and dolomite. The Köprülü Formation is composed of a siltstone and sandstone succession. The Upper Permian-aged Kas and

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Gomaniibrik Formations (Stolle et al., 2011) unconformably overlie these older units. Whereas the Kas

shale, coal, sandstone, and limestone.

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Formation consists of shale, sandstone, and a few thin coal beds, the Gomaniibrik Formation consists of

The Lower Triassic is represented by the Uludere Formation that is composed of sandstone and shale. The Middle Triassic Bakuk Formation unconformably overlies the Uludere Formation and consists of shale, anhydrite, carbonate, and laminated marl that contains pyrite minerals. The Areban Formation that is

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composed of medium-to-thick bedded, cross-laminated fine to very fine-grained quartz sandstone, siltstone, shale, and sandy dolomite is the first unit deposited at the base of the Cretaceous sequences (Yılmaz and Duran, 1997). The Sabunsuyu Formation that conformably overlies the Areban Formation is

al., 1997).

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dominated by dolomite. The succeeding Turonian Derdere Formation is composed of carbonate (Çoruh et The unit is conformably overlain by the Karababa Formation containing organic rich

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carbonate. The Karababa Formation is unconformably overlain by the Beloka Formation that is also dominated by carbonate. The Beloka Formation is overlain by the shale and sand of the Kıradağ Formation which, in turn, is overlain by the Garzan Formation that consists of reefoidal limestone (Salem et al., 1986). The Garzan unit is overlain by the Lower Germav Formation, the Lower Sinan Formation, and again, due to lateral facies changes, by the Lower Germav Formation succession. The Lower Germav Formation is composed of a marl and shale alternation deposited under deeper marine conditions. The Lower Sinan Formation is composed of shelf carbonate.

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ACCEPTED MANUSCRIPT The base of the Cenozoic is the Paleocene aged Upper Germav Formation that is composed of a marl and shale alternation. The Upper Sinan Formation conformably overlies the Upper Germav Formation and consists of shelf limestone. An angular disconformity seperates the Eocene to Early Miocene Midyat Group, consisting principally of limestone, the older sedimentary formations (Lustrine et al., 2012).

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The Eocene to Early Miocene Midyat Group can be subdivided into 4 formations. The Gercüş Formation that unconformably overlies the Upper Sinan Formation is composed of terrigenous shale and sandstone which is unconformably overlain by the Eocene-aged Hoya Formation consisting of shelf limestone. The

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overlying Oligocene aged Germik Formation is composed of limestone, dolomite, sandstone, and evaporites deposited in a shelf and continental environment. The youngest formation within the study area

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is the continental Şelmo Formation composed of conglomerate, sandstone, and shale alternations (Fig. 2). Due to collision of the Arabian Plate to the south and the Taurid-Anatolian Plate to the north, the entire region to the south of the Zagros-Bitlis Orogenic Belt is impacted by strong compressional tectonism (Yılmaz, 1993). Compressional deformation resulted in the typical thrust tectonics represented by

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overthrusts, high angle thrust faults, and asymmetric folds. According to regional subsurface geology, high angle thrust faults also exist in the area of Dodan Field. Such high angle thrust faults were formed at approximately 7-8 Ma, causing stratigraphic juxtapositioning of the entire sequence (see the cross section

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in Fig. 1). The Karacadag volcano in southeastern Anatolia is located near the Dodan gas field. Volcanic

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activity lasted from 11 Ma to 0.1 Ma, during which a large amount of magma erupted to the surface covering an area of 10,000 km2 (Lustrino et al., 2012). The distance between the Karacadag volcano and the Dodan gas field is approximately 50 kilometer. In terms of the petroleum system of the Dodan area, the main potential source rocks are the Bedinan, Kas, Gomaniibrik, Derdere, and Karababa Formations (Gürgey, 1991).

Potential reservoir rocks are

limestones that form some portion of the Sabunsuyu, Derdere, Beloka, Garzan, and Sinan Formations (Fig. 2) (Gürgey, 1991).

The Karababa, Kıradag, Germav, and Germik Formations are cap rocks

consisting of impermeable rocks.

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3. Sampling and Methods Gas samples were taken from the Dodan 16M, Dodan 17M, Dodan 6G, Dodan 6M, Dodan 17G, Dodan 1A wells in the Dodan field (Fig. 1) from different depth intervals using gas cylinders.

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A agilent 6890N gas chromatograph, equipped with a Flame Ionization Detector – Thermal Conductivity Detector (FID-TCD) detector, was used to determine types of hydrocarbons present and their molecular composition. Furthermore, amount of CO2 and N2 gases are determined by gas chromatography. Gases

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were separated on TCD detector by means of a Gas Chromatograph (GC), equipped with a HP-Plot/Q (30

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m; 0.542 mm; 40.00 µm) and HP-molesieve (30m; 0.539mm; 50.00µm) column. Hydrocarbon gases were deeply devised by a FID detector with HP-AL/S (50m; 0.534mm; 15.00µm) column. The measurement results were evaluated by GC Chemstation software. Helium is used as a carrier gas. For the determination of molecular composition of the gases, a correction is carried out in order to eliminate admixtures of air. Air correction was calculated according to the equation

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N2corrected = N2measured – (O2measured x 3.7)

(1)

Stable isotope analyses of gases were performed using a GV-IsoPrism High Performance instruments

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with a continuous flow Gas Chromatography -Isotope Ratios-mass spectrometer at TÜBITAK Marmara Research Center. Analytical precision was ± 0.3 ‰ for methane and ± 0.5 ‰ for higher components (C2

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and C3). The isotopic results are reported in δ13C notation in parts per mil (‰), relative to the Peedee Belemnite (PDB) standard.

 RSAMPLE  − 1 × 1000 (‰)  RSTANDARD 

δ SAMPLE = 

(2)

where R denotes the ratio of C13/C12 or D/H, respectively. Sulfur isotope analyses of the H2S were performed on a Finnigan Delta S at Isotech Labs Inc. (Illinois, USA). Sulfur isotopic compositions can be carried out on the hydrogen sulfide (H2S) fraction in a gas mixture by ZnS precipitation followed by Elemental Analysis - Isotope Ratio Mass Spectrometry

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ACCEPTED MANUSCRIPT analysis. Canyon Diablo Troilite (CDT) was used as an international reference for S isotopic (δ34S) values with an analytical precision of ±0.2‰. The geological evolution and temporal development of hydrocarbon generation from potential source

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rocks is determined by the 1-D modelling of Dodan 10/M well using the basin modelling software

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(PetroMod) of IES (Schlumberger)-Germany.

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ACCEPTED MANUSCRIPT 4. The Molecular and Isotopic Composition of Gases Six gas samples from the Dodan gas field were analyzed for their molecular and isotopic composition (Table 1). Percentages of the hydrocarbon gases (HC) methane and ethane, and non-hydrocarbon gases CO2, H2S, and nitrogen (N2) were determined.

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Hydrocarbon gas concentrations within Dodan gas samples ranged from 4.1 to 5.6%. CO2 concentrations ranged from 88.7 to 93.2%, and H2S concentrations comprised up to 1.4% of total gas. Two gas samples from the Dodan 6/G and Dodan 6/M wells were analyzed for their δ13C and δD methane values, which

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ranged from –27.2 to –26.8‰ and –133.5 to –128.4‰, respectively.

For additionally analyzed samples, the δ13C values of methane, ethane, and propane ranged between –

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31.9 to –25.8‰, –31.9 to –26.3‰, and –30.2 to –28.6‰, respectively. The δ13C values of CO2 for Dodan Field ranged between 2.80 to -1.59‰. The δ34S values of H2S from the three production wells ranged between 11.9 and 13.4‰ (Table 1).

5. Temperature and Hydrocarbon Generation History

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Basin modelling, based on a numerical simulation of diverse processes taking place within a basin during its evolution, has been satisfactorily used in conventional hydrocarbon exploration (Welte and Yalçın, 1988; Yalçın, 1991; Yalçın et al., 1997). The approach has also been used to reconstruct temperature

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history, to determine the temporal development of maturation, and to assess hydrocarbon generation from

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potential source rocks within the area surrounding Dodan Field. A simulation of basin evolution within Dodan Field was performed using data from the Dodan 10/M well as well as aspects of regional geology. The simulation began at 268 Ma and continued until the present day (Fig. 3).

A comparison of simulation results using measured values for maturity, temperature, thickness, and porosity within the Dodan 10/M well indicated that a calibration of the so-called conceptual model was required. Therefore, as the main controlling factor of maturity and temperature, heat flow values and the heat flow history were adjusted. The previously assumed heat flow history constant over time, considered

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ACCEPTED MANUSCRIPT as typical for a passive continental margin, was modified to slightly higher values and to an additional increase over the last 7-8 Ma (Fig. 4). A more recent increase can be justified by the regional volcanic activity of Karacadag. Success of calibration was tested via a reasonable match between computed and present day measured temperature and vitrinite reflectance values (Fig. 5). The hydrocarbon generation

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history for potential source rocks was then computed using a predetermined thermal history and kinetic data for hydrocarbon (oil and gas) generation (Tissot et al., 1987 in Waples, 1994). Results obtained from modelling were used to analyze the temporal development of respective processes.

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The burial history diagram for the Dodan 10/M well indicated that several burial and subsequent uplifterosion episodes have occurred since the Permian. Two caused remarkable uplift and subsequent erosion.

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The first period, at the end of Jurassic, was related to epirogenic uplift. The second that commenced at 5Ma and that is still continuing is caused by Alpine Orogeny. In Dodan Field, maximum paleoburial temperatures were attained during the Miocene. Temperatures at the base of the Kas Formation reached

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values of up to 202°C (Fig. 3). In Fig. 6, gas generation potential values superimposed on the burial history diagram indicate that gas generation is limited to the Permian Kas and Gomaniibrik Formations and that a gas generation potential of only 60 mg HC/gTOC could be reached. Temporal development of

Ma (Fig. 7).

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this potential indicates that gas generation in the Kas Formation commenced at 9 Ma and continued until 1

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Since most studies suggest that thermochemical sulphate reduction begins at temperatures in the range of 120oC, depending on the hydrocarbon type within the reservoir, it was necessary to check whether or not such values were reached within the Bakuk formation, the only unit with evaporitic rocks and, therefore, where the TSR process likely occurred. At the end of the Miocene (5 Ma), the base of the Bakuk formation was buried up to a depth of 4,200 m and temperatures up to 163oC were reached (Fig. 3). Critical temperatures for TSR at 120oC were already reached at 10 Ma (Fig.8).

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ACCEPTED MANUSCRIPT 6. The Origin of Hydrocarbon Gases The origin of hydrocarbon gases and possible processes of gas generation were first investigated with the help of a Bernard diagram (Bernard et al., 1978) that incorporates molecular composition, namely the C1/C2+C3 ratio and δ13C values of methane. Within the Bernard diagram (Fig. 9a), all of the data points

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for samples were within the zone of thermal gas generation. Since both the isotopic and molecular composition of gases can be influenced by the type of organic matter, migration, or the mechanism of gas generation (Hunt, 1996; Prinzhofer and Huc, 1995; Whiticar et al., 1986, Whiticar, 1994) another

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classification that uses genetic plots for methane δ13C and δD, according to Schoell (1980), was also applied. As shown in Fig. 9b, methane gas from Dodan Field also plotted within the thermal zone

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indicating that Dodan Field hydrocarbon gases were of thermogenic origin (Fig. 9 a, b). To test the possibility of mixing or degradation, the approach of Chung et al. (1988) was also utilized. The approach uses the stable carbon isotopic composition of methane, ethane, and propane vs. 1/n, where n is the respective number of carbon atoms in each gas molecule. If the relationship between the data points for

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each gas molecule is linear, it is assumed that the respective gas was thermally generated from a single source rock with no mixing from other sources and/or was not degraded by any alteration process. If the

of gas is inferred.

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relationship is not linear, mixing of at least two gases with different isotopic composition or an alteration

Gases from six different wells in Dodan Field demonstrated a shift in the direction of isotopically heavier

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methane and ethane (Fig. 10), resulting in a stable carbon isotopic sequence of δ13C1 > δ13C2 > δ13C3 within gas samples Dodan 16/M and Dodan 1A (Table 1). Such a negative isotope sequence was interpreted by Des Marias et al. (1981), Yuen et al. (1984), Jenden et al. (1993), and Dai et al. (2005) as possibly resulting of secondary processes including migration, mixing, and the oxidation of gases. Heavier methane and ethane isotopes in Dodan Field indicate that these gases are depleted by 12C, which could be due to a mixture of methane and ethane formed from more mature source rocks and/or to the alteration of these gases (Schoell, 1980; Whiticar, 1994).

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ACCEPTED MANUSCRIPT Alterations of methane and ethane may also be related to an alteration by TSR where only a portion of hydrocarbons are reduced to produce H2S. Accordingly, isotopic values of methane and ethane are heavier due to the early stage of a TSR process (Zhu et al., 2011). For a mixture with a more mature gas that has isotopically heavier methane, only the methane component

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becomes heavier, not the ethane. However, ethane from Dodan Field HC gases is even isotopically heavier than methane. Furthermore, according to the results of basin modelling, the Permian Kas and Gomaniibrik Formations are the only possible source rocks for remarkable amounts of thermogenic gas

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within Dodan Field (Fig. 6). Organic geochemical analyses conducted both within the framework of previous studies (Stolle et al., 2012) and in this study using samples from the Çelikli well (see Figure 1

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for location) indicate that the Kas Formation contains coal and organic rich shale intervals with TOC amounts of more than 0.9%. At present, younger potential source rocks are still too immature for gas generation (Fig.6). Hence, a mixture with gases from more mature source rock can be excluded. As a result, we assumed that heavier methane and ethane isotopes for HC gases were caused by an alteration of

6.1. The Origin of CO2

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thermogenic gas during TSR.

Different mechanisms have been suggested for CO2 formation. Mechanisms and their controlling

The bacterial or thermal degradation of organic matter; for this case the δ13C of the formed CO2 is

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1.

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parameters are summarized in the following:

less than -8‰ (Hunt, 1996). 2.

Degassing from the mantle; for this case the δ13C of the formed CO2 is between -4 to -7‰ (Hunt, 1996).

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The decomposition of carbonate or reactions between minerals at 230-320oC (Petrucci et al., 1994) or the thermal decarbonation of calcite at high temperatures (<250oC) (Boz et al., 2004). The δ13C of the formed CO2 is between 0 to -2‰ (Dai et al., 2005).

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Carbonate decomposition through acidolysis during TSR; CO2 is heavier than -6.1‰ (Zhu et al., 2011).

The CO2 content for the Garzan Formation and Mardin Group reservoir rocks in Dodan Field is 92.093.2% and 88.7-91.8%, respectively. The isotopic values of δ13CCO2 varied from -2.8 to -1.6‰. Such

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isotopic values for CO2 gases indicate that these gases are not formed by bacterial or thermal degradation of organic matter; values for such a case should be much heavier than -8‰. Furthermore, the amount of CO2 formed from organic origin in reservoirs is generally low (Hunt, 1996), in contrast to the high

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amounts of CO2 in Dodan Field. Therefore, from both the isotopic and quantitative point of view, the CO2 in Dodan Field cannot be of organic origin. On the other hand, the amount of CO2 of organic origin is

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much less than the CO2 formed by other mechanisms, so no significant deviation was encountered in regards to isotopic composition. If organically originated CO2 was equivalent or close to the amount formed by another mechanism, CO2’s isotopic composition would be close to the isotopic composition of organic matter. Dodan Field CO2 isotopic values suggest that CO2 was also not formed as a result of the

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bacterial or thermal degradation of organic matter, or that the contribution resulting from CO2 of organic origin is so small that it does not cause an isotopic deviation. The isotopic composition of CO2 within the study area also differed from the mantle origin CO2 isotopic

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composition for δ13C between –4 to –7‰, and was much heavier. Therefore, CO2 within the Dodan gas field is not expected to be of magmatic origin.

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In previous studies, CO2 occurrences with isotopic values between δ13C -5 to 4‰ have been reported to be of limestone origin (δ13C -2 to 2‰, Javoy et al., 1986; δ13C -1 to 2‰, Minissale, et al., 1997; δ13C -5 to 4‰, Hunt, 1996). According to the isotopic values of gases in Dodan Field, CO2 could have a limestone origin. As a result of chemical reactions between clinozoisite, quartz, muscovite, or similar minerals with calcite at high temperatures (230–320oC), K-feldspar, clinozoisite, and CO2 may form (Petrucci et al., 1994). Hutcheon and Abercrombie (1989) indicated that as a result of kaolinite and carbonate reactions, chlorite and CO2 forms. However, these reactions take place at temperatures over

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ACCEPTED MANUSCRIPT 200°C. According to the geological history of the region and basin modelling results, limestone reservoirs have not been buried deep enough to reach the necessary temperatures required for such mineral–fluid reactions (Fig. 3). In volcanically active regions, where igneous intrusive rocks intersect limestone, CO2 is formed due to the

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thermal decarbonation of limestone as a result of high temperatures (>250 oC). The C-isotopic values of the CO2 formed by thermal decarbonation will be similar to that of limestone. The basalts of young Karacadag volcanism in the region are eruptions, limited to fissures. Furthermore, the volcanic rocks that cut

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through limestone have not been encountered in any of the wells in the region. Therefore, CO2 formed by decarbonation can also be excluded. Acidolysis was suggested by Zhu et al. (2011) as a possible mechanism

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for CO2 formation. In limestone reservoirs, where TSR alteration takes place, CO2 is formed by the decomposition of calcium carbonate molecules with acid.

The above discussed formation mechanisms - the bacterial and thermal degradation of organic matter, degassing from the mantle, the decomposition of carbonate or reactions between minerals, and the

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thermal decarbonation of calcite - cannot be considered as the formation mechanism of CO2 in Dodan Field. The origin of CO2 in Dodan wells could result from the mixing of two different gases. As discussed in detail in the subsequent section, the mixing of CO2 formed by TSR and carbonate

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decomposition through acidolysis during TSR are the most plausible mechanisms.

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ACCEPTED MANUSCRIPT 6.2. The Origin of H2S H2S can be formed by various processes such as bacterial sulfate reduction (BSR), the thermal decomposition of kerogen or oil containing organic sulfur, reactions of oil and gas with anhydrite under certain circumstances, and thermochemical sulfate reduction (Cross et al., 2004). In the following

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discussion, these processes are summarized and the formation mechanism of H2S within the study area is evaluated.

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6.2.1. Bacterial Sulfate Reduction (BSR)

BSR is a reduction of sulfates in order to synthesize sulfur-containing cell components caused by bacteria

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(such as Desulfovibro and Desulfotomaculum) at the sediment/water interface or at shallow depths and low temperatures (5 to 85°C) (Krumbein, 1983; Manahan, 1993; Cai et al., 2004). Temperatures within Dodan Field’s Garzan and Mardin group reservoirs are suitable for bacterial sulfate reduction. To determine the abundance, distribution, and activities of SRB (sulphate reducing bacteria) in the reservoirs

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of Dodan Field, Sungur et al. (2011) collected petroleum samples. Two replicates of each sample were obtained and one was used to determine the water phase of oil. At the end of the incubation period, SRB were observed in the oil and oil-water phase samples (Sungur et al., 2011). The amount of H2S formed by

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aerobic bacteria due to BSR was reported to be less than 3% of total gas (Cai et al., 2001). However, the quantity of H2S within the Dodan Field’s reservoir is greater than the H2S that would be formed from

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BSR. Therefore, BSR alone cannot be the responsible for the entire production of H2S.

6.2.2 The Thermal Decomposition of Kerogen or Petroleum Another origin for H2S is organic matter. As a result of thermal maturation, H2S can be formed from the sulfur in kerogen or oil. The formation of organic origin H2S begins at approximately 120°C following the main oil formation phase (Worden and Smalley, 1996; Bildstein et al., 2001). Furthermore, the formation of significant amounts of H2S requires higher temperatures (of at least 170°C) (Hunt, 1996;

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ACCEPTED MANUSCRIPT Aplin and Coleman, 1995). H2S formed due to thermal degradation of oils in reservoirs is described with the formula expressed in (3). However, it has been reported that only half of the sulfur content of oil can be transformed into H2S and that it can only be 3% of the total amount (Hunt, 1996). C100H170S1.4
23 CH4 + 0.7 H2S + C77H77S0.7 (Hunt, 1996)

(3)

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Furthermore, according to the stoichiometric balance, the H2S quantity formed subsequent to the thermal degradation of oil within a reservoir is less than 1/23rd of methane. As far as the molecular composition percentages of Dodan gases are concerned, methane was less than 5% of total gas. If H2S had been

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formed as a result of the thermal degradation of oil within the reservoir, the amount of methane should have been higher.

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The source rock (Mardin Group) of oils within the study area has a maturity of 0.6 to 0.8% Ro (Gürgey, 1991). Therefore, due to their low maturity, the reactions that are expected to form H2S at high temperatures cannot take place in these source rocks. Additionally, the δ34S and δ13C isotopic values of H2S and CO2 forming subsequent to the oxidation or decomposition of oil will be close to the isotopic

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values of the respective oil. The isotopic composition of Dodan Field CO2 gases (δ13C -2.8 to -1.6‰) is not close to the isotopic values of oils in these regions (δ13C -29 to -28‰) (Gürgey, 1991, Connan et al.,

present.

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2006). Due to these indicators, a significant amount of H2S from the sulfur of oil is not expected to be

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At the end of the Miocene, the base of the Kas Formation reached temperatures of 200oC. Therefore, some H2S may have been formed due to the thermal maturation of kerogen within the Kas Formation. However, the amount of H2S formed by this mechanism was low (Hunt, 1996) and cannot be considered as the only source of H2S in Dodan Field. Furthermore, H2S is not present in other gas fields in the region. Since H2S is only present in Dodan Field, a process other than the thermal maturation of kerogen which is not responsible for the large amounts of CO2 and H2S in the studied wells is at work.

6.2.3. Thermochemical Sulfate Reduction

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ACCEPTED MANUSCRIPT In studies performed within reservoirs consisting of carbonate-evaporate interbedding, H2S was found to be higher than 5%. Worden and Smalley (1996) reported that sulfate reduction in anhydrite layers in contact with petroleum fluids is responsible for this process, as follows: anhydrite + petroleum fluid or CH4(g)
calcite + H2S(g) + CO2(g) + altered petroleum (or bitumen) +

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H2O + S

(4)

The reaction is defined as thermochemical sulfate reduction (TSR) and requires a temperature of 120-145 °C to allow anhydrite to react with petroleum fluid or gas to form CO2 and H2S (Bildstein et. al., 2001).

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The quantity of H2S formed by this reaction is high (Cai et al., 2001). The H2S formed by thermochemical sulfate reduction has been reported to react with petroleum fluid and may also form

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sulfur rich altered oils. Worden and Smalley (1996) indicated that this reaction could also take place with CH4 instead of with petroleum fluids. For this case, methane within the reservoir should be richer in δ13C and isotopically heavier.

Investigations indicate that the sulfur isotope in H2S (δ34S) is generally over +10.0‰ in oil and gas fields

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with a high content of H2S (Orr, 1974; Machel, 2001; Liu et al., 2012).

The Bakuk Formation is located below the Mardin group and contains anhydrite beds of remarkable thickness (Yılmaz and Duran, 1997, Erik et al., 2005). According to the interpretation of Erik et. al.

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(2005), the Bakuk Formation contains 67% dolomite and 22% anhydrite. Porosity values range between

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5-8% and permeabilities between 0.03-7 mD. According to the geological history of the region and basin modelling results, the Bakuk Formation was buried up to a depth where the temperatures required for TSR were reached (Fig. 8). Hence, the Bakuk Formation provides the necessary criteria, such as temperature and a suitable lithology, for TSR. Furthermore, the δ34S values of H2S from three production wells from the Dodan gas field (11.9 to 13.4 ‰) are coherent with former studies for oil and gas fields with a high H2S content. Therefore, TSR is the most likely mechanism responsible for the occurrence of H2S and CO2 in the Mardin and Garzan Formations in Dodan Field.

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ACCEPTED MANUSCRIPT 7. Conclusions Based on organic and isotopic geochemical data and assessments, as well as the regional geological framework and evolution, the following scenario could be inferred regarding hydrocarbon gases, CO2, and H2S in Dodan Field:.

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1. As indicated both by the molecular and isotopic composition, hydrocarbon gases in Dodan Field are thermogenic. Relatively heavy isotopic values for methane and ethane indicate a possible secondary alteration process.

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2. According to basin modelling results, the organic rich Kas and Gomaniibrik Formations were the source rocks for hydrocarbon gases. The generation of HC-gases in these source rocks began at

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approximately 10 Ma at a depth of 4,100 m.

3. The high-angle thrust fault formed almost contemporarily by compressional tectonics, allowing both a lateral-up dip migration from Kas and Gomaniibrik Formations into the Bakuk Formation and vertical migration through the fault zone. Hence, HC-gases could partly be accumulated

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within the Bakuk Formation likely due to the created fracture porosity at a temperature of 140oC, as inferred from modelling. 4.

As for the Bakuk Formation, suitable conditions (hydrocarbons, anhydrite, temperatures of

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approximately 120-145oC) existing for thermochemical sulfate reduction (TSR) could have taken place, resulting in remarkable amounts of H2S being formed from the alteration of HC gases.

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5. As indicated by the isotopic composition, carbonates of the Bakuk Formation were impacted both by TSR processes and carbonate decomposition through acidolysis during TSR, resulting in the formation of CO2 of limestone origin. 6.

Some of the formed H2S and CO2 within the Bakuk Formation continued to migrate through the fault upward until reaching reservoir rocks of the Mardin group and the Garzan Formation.

7. The lateral-up dip migration of gases in reservoirs toward the Dodan high led to the formation of the Dodan gas field.

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ACCEPTED MANUSCRIPT 8. Some H2S was also formed by bacterial sulfate reduction (BSR) that took place in reservoir rocks of the Dodan gas field. However, the H2S formed by TSR within the Bakuk Formation and the H2S formed by BSR within the reservoirs cannot be differentiated. 9. All of the above processes - thermogenic gas generation in Permian source rocks, the formation of

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thrust faults, the lateral-up dip migration of HC-gases from the Kas Formation into the Bakuk Formation, the formation of H2S and CO2 by TSR within the Bakuk Formation, vertical migration of all gases into reservoirs through the thrust fault, the lateral-up dip migration of these gases

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toward the Dodan structure - took place during the last 7-8 Ma period.

19

ACCEPTED MANUSCRIPT Acknowledgements The authors are grateful for the financial support by the TUBITAK (The Scientific and Technical Research Council of Turkey; project no: 107Y205) and University of Istanbul Research Fund, (Project numbers ONAP-24702, 22337). This study was also supported by the Turkish Petroleum Corporation (TPC) by providing the gas and rock samples. These contributions are kindly acknowledged. We thank

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Dr. Mustapha Boujana, Dr. Robert Ondrak and an anonymous reviewer for their insightful reviews and

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constructive criticism and comments, which greatly improved the manuscript.

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ACCEPTED MANUSCRIPT REFERENCES

Aplin, A.C., Coleman, M.L., 1995. Sour gas and water chemistry of the Bridport sands

RI PT

reservoir Wytch Farm, UK. In J. M. Cubit & W. A. England, The Geochemistry of Reservoirs 86, 303-314.

M AN U

former Soviet Union. Petrol. Geosci. 6, 175–187.

SC

Belenitskaya, G.A., 2000. Distribution pattern of hydrogen sulfide-bearing gas in the

Bernard, B.B., Brooks, J.M., Sackett, W.M., 1978. Light hydrocarbons in recent Texas continental shelf and slope sediments. Journal of Geophysical Research 83, 4053-4061

TE D

Bildstein, O., Worden, R.H., Brosse, E., 2001. Assessment of anhydrite dissolution as the rate-limiting step during thermochemical sulfate reduction. Chemical Geol. 176,

EP

173-189.

Boz, I., Pozan, G.S., Gürkaynak, M.A., 2004. Tetradecanol Dehydrogenation over CuO

AC C

catalyst supported on BaO, CaO and MgO. React. Kinet. Catal. Lett. 83, 137-146

Cai, C., Hu, W., Worden, R., 2001. Thermochemical sulphate reduction in CambroOrdovician carbonates in Central Tarim. Marine and Petroleum Geol. 18, 729-741.

1

ACCEPTED MANUSCRIPT Cai, K.P., Wang, Y.R., Yang, Y.M., Zhang, Z.R., 2003. Evaluation of Permian and Triassic hydrocarbon source rock and gas source analysis in Guang-Wang region of

RI PT

northwestern Sichuan Basin. Nat. Gas Ind. 23, 10–14.

Cai, C., Xie, Z., Worden, R.H., Hu, G., Wang, L., He, H., 2004. Methane-dominated thermochemical sulphate reduction in the Triassic Feixiangun Formation East Sichuan

SC

Basin, China: towards prediction of fatal H2S concentration. Marine and Petroleum

M AN U

Geol. 21, 1265-1279.

Chung, H. M., Gormly, J. R. and Squıres, R. M., 1988. Origin of gases hydrocarbons in subsurface environments: Theoretical Considerations of Carbon Isotope distribution.

TE D

Chemical Geol. 71, 97-103.

Cross, M.M., Manning, D.A.C., Bottrell, S.H., Worden, R.H., 2004. Thermochemical sulphate reduction (TSR): experimental determination of reaction kinetics and

AC C

35, 393-404.

EP

implications of the observed reaction rates for petroleum reservoirs. Organic Geochem.

Connan, J., Kavak, O., Akin, E., Yalçın, M.N., Imbus, K., Zumburge, J., 2006. Identification and origin of bitumen in Neolithic artefacts from Demirköy Höyük (8100 BC): Comparison with oil seeps and crude oils from southeastern Turkey. Organic Geochem. 37, 1752-1767.

2

ACCEPTED MANUSCRIPT Çoruh, T., Yakar, H., Ediger, V.Ş., 1997. Biostratigraphy Atlas of the Autochthonous Sequence of Southeast Anatolia Region. TPAO Research Center Education Series, 30

RI PT

(in Turkish).

Dai, J., Yang, S., Chen, H., Shen, X., 2005. Geochemistry and occurrence of inorganic

SC

gas accumulations in Chinese sedimentary basins. Organic Geochem. 36, 1664-1688.

Des Marais, D.J., Donchin, J.H., Nehring, N.L., Truesdell, A.H., 1981. Molecular

M AN U

carbon isotopic evidence for the origin of geothermal hydrocarbons. Nature 292, 826– 828.

Erik, N.Y., Özçelik, O., Altunsoy, M., İlleez, İ., 2005. Source-Rock Hydrocarbon potential of the Middle Triassic- Lower Jurassic Cudi Group Units, Eastern Southeast

TE D

Turkey. Int. Geol. Rew. 47, 398-419.

Gürgey, K., 1991. Genetic classification of the SE Turkey oils and delineation of source

AC C

EP

rocks types with the use of biological markers. METU, PhD. Thesis, 320 p.

Heydari, E., 1997. The role of burial diagenesis in hydrocarbon destruction and H2S accumulation, upper Jurassic Smackover Formation, Black Creek Field, Mississippi. AAPG Bull. 81, 25–45.

Hose, D.L., Palmer, A.N., Palmer, M.V., Northup, D.E., Boston, P.J., DuChene, H.R., 2000. Microbiology and geochemistry in a hydrogen-sulphide-rich karst environment. Chemical Geol. 169, 399-423.

3

ACCEPTED MANUSCRIPT

Hunt, J. M., 1996. Petroleum Geochemistry and Geology. 2nd Ed. San Francisco:

RI PT

Freeman, New York.

Hutcheon, I., Abercombie, H.J., 1989. The role of silicate hydrolysis in the origin of CO2 in sedimentary basins. Proceeding of the Sixth International Symposium on Water-

SC

Rock Interaction. Rotterdam: Balkema, 321-324

Chemical Geol. 57, 41-62.

M AN U

Javoy, M., Pineau, F., Delorme, H., 1986. Carbon and nitrogen isotopes in the mantle.

Jenden, P.D., Hilton, D.R., Kaplan, I.R., Craig, H., 1993. Abiogenic hydrocarbons and

TE D

mantle helium in oil and gas fields. In: Howell, D.G. (Ed.), The Future of Energy Gases (US Geological Survey Professional Paper 1570). United States Government Printing

EP

Office, Washington, pp. 31–56.

Krouse, H.R., Viau, C.A., Eliuk, L.S., Ueda, A., Halas, S., 1988. Chemical and isotopic

AC C

evidence of thermochemical sulfate reduction by light hydrocarbon gases in deep carbonate reservoirs. Nature 333, 415-419.

Krumbein, W.E., 1983. Microbial Geochemistry. Blackwell, Oxford, (Krumbein W. E., Ed.) 125-157.

4

ACCEPTED MANUSCRIPT Liu, M., Deng, Q., Zhao, F., Liu, Y., 2012. Origin of hydrogen sulfide in coal seams in China. Safety Science 50, 668-673.

RI PT

Lustrino, M., Keskin, M., Mattioli, M., Kavak, O., 2012. Heterogeneous mantle sources feeding the volcanic activity of Mt. Karacadag (SE Turkey). JAES 46, 120-139.

SC

Machel, H.G., 2001. Bacterial and thermochemical sulfate reduction in diagenetic

M AN U

settings—old and new insights. Sediment. Geol. 140, 143–175.

Machel, H.G., Krouse, H.R., Sassen, R., 1995. Products and distinguishing criteria of bacterial and thermochemical sulfate reduction. Appl. Geochem. 10, 373-389.

TE D

Manahan, S.E., 1993. Fundamentals of Environmental Chemistry. CRC press.

Manzano, B.K., Fowler, M.G., Machel, H.G., 1997. The influence of thermochemical

EP

sulfate reduction on hydrocarbon composition in Nisku reservoirs, Brazeau River area,

AC C

Alberta, Canada. Organic Geochem. 27, 507–521.

Minissale, A., Evans, W., Magro, G., Vaselli, O., 1997. Multiple source components in gas manifestations from north-central Italy. Chemical Geol. 142, 175-192.

Orr, W.L., 1974. Changes in sulfur content and isotopic ratios of sulfur during petroleum maturation-study of Big Horn Basin Paleozoic Oils. Part1. AAPG Bull. 58, 2295-2318

5

ACCEPTED MANUSCRIPT

Orr, W.L., 1977. Geologic and geochemical controls on the distribution of hydrogen sulfide in natural gas. In R. Campos and J. Goni (eds.), Advances in organic

RI PT

geochemistry. Madrid: empressa nacional adaro de investigaciones mineras, 571-597.

Paul, A., Boashun, F., 2000. Microbial sulfate reduction rates and sulfur and oxygen

SC

isotope fractionations at oil and gas seeps in deep water Gulf of Mexico. Geochimica et

M AN U

Cosmochimica Acta 64, 233-246.

Petrucci, E., Gianelli, G., Puxeddu, M., Iacumin, P., 1994. An oxygen study of silicates in the Larderello Gethermal Field, Italy. Geothermics 23, 327-337

TE D

Prinzhofer, A., Huc, A., 1995. Genetic and Post – Genetic molecular and isotopic fractionations in natural gases. Chemical Geol. 126, 281-290

EP

Salem, R., Eren, A., Özbahçeci, H., Araç, M., Öncü, H., İşbilir, M., Üngör, A., Biçer, Z., Yılmaz, Z and Yılmaz, E., 1986. Geologic and hydrocarbon evaluation of

AC C

Maastrichtian sediments in Central District X, Southeast Turkey. TPAO Research Center Internal Report No 1009.

Schoell, M., 1980. The Hydrogen and carbon isotopic composition of methane from natural gases of various origins. Geochimica et Cosmochimica Acta 44, 649 – 661. Soylu, C., 1991, Oil source rocks in the Adiyaman area, Southeast Turkey. JAES 5, 429-434.

6

ACCEPTED MANUSCRIPT

Soylu, C., Yalçın, M.N., Horsfield, B., Schenk, H.J., Mann, U., 2005, Hydrocarbon generation habitat of two Cretaceous carbonate source rocks in SE Turkey. Journal of

RI PT

Petroleum Geol. 28, 67-82.

Stolle, E., Yalçın, M.N., Kavak, O., 2011, The Permian Kas Formation of SE Turkey-

SC

palynological correlation with strata from Saudi Arabia and Oman. Geol. J. 46, 561-

M AN U

573.

Stolle, E., Yalçın, M.N., Kozlu, H., 2012, Palynofacies and Bulk Organic Geochemistry of Permian Clastics in the Eastern Taurides: Implications for Hydrocarbon Potential.

TE D

Turkish Association of Petroleum Geologists Special Publication 6,119-122.

Sungur, İ. E., Güngör, N., Hoşgörmez, H., Çotuk, A., 2011, Investigation of Sulphate Reducing Bacteria in Petroleum Reservoirs of Dodan, Garzan ve Silivanka Fields (SE-

AC C

Turkey, p.172

EP

Turkey), 18th International Petroleum and Natural gas Congress and Exhibition of

Tissot, B. P., Welte, D. H., 1984. Petroleum Formation and Occurrence. Springer, Berlin.

Tissot, B.P., Pelet, R., Ungerer, P., 1987, Thermal history of sedimentary basins, maturation indices, and kinetics of oil and gas generation: AAPG Bull. 71, 1445–1466.

7

ACCEPTED MANUSCRIPT Waples, D.W., 1994. Maturity Modeling: Thermal Indicators, Hydrocarbon Generation and Oil Cracking. AAPG Memoir 60, 285–306.

RI PT

Welte, D. H., Yalçın, M. N., 1988. Basin Modelling- A New Comprehensive Method in Petroleum Geology. Organic Geochem. 13, 141-152.

SC

Whiticar, M. J., Faber, E., Schoell, M., 1986. Biogenic methane formation in marine and freshwater environments: CO2 Reduction vs. Acetate Fermentation – Isotope

M AN U

Evidence. Geochimica et Cosmochimica Acta 50, 693-709.

Whiticar, M.J., 1994, Correlation of Natural Gases with Their Sources. AAPG Memoir

TE D

60, 261-283

Worden, R.H., Smalley, P.C., 1996. H2S-producing reactions in deep carbonate

EP

reservoirs: Khuff Formation, Abu Dhabi. Chemical Geol. 133, 157–171.

Worden, R.H., Smalley, P.C., Oxtoby, N.H., 1996. The effects of thermochemical

AC C

sulfate reduction upon formation water salinity and oxygen isotopes in carbonate reservoirs. Geochim. Cosmochim. Acta 60, 3925-3931.

Worden, R.H., Smalley, P.C., Cross, M.M., 2000. The influence of rock fabric and mineralogy on thermochemical sulfate reduction: Khuff Formation, Abu Dhabi. J. Sediment. Res. 70, 1210–1221.

8

ACCEPTED MANUSCRIPT Yalçın, M.N., 1991. Basin Modelling and Hydrocarbon Exploration. Journal of Petroleum Science and Engineering 5, 379-398.

RI PT

Yalçın, M.N., Littke, R., Sachsenhofer, R.F., 1997. Thermal History of Sedimentary Basins; In: D. H. Welte, B. Horsfield, D. R. Baker. (eds.) Petroleum and Basin

SC

Evolution, 71-167, Springer Verlag, Heidelberg.

Yılmaz, Y., 1993. New evidence and model on the evolution of the southeast Anatolian

M AN U

orogen. The Geological Society of America 105, 251-271.

Yılmaz, E., Duran, O., 1997. Stratigraphic Nomenclature of Autochthonous and Allochthonous Units in Southeast Anatolia Region "Lexicon". TPAO Research Center

TE D

Education Series, 31 (in Turkish).

Yuen, G., Blair, N., Des Marais, D.J., Chang, S., 1984. Carbon isotope composition of low molecular weight hydrocarbons and monocarboxylic acids from Murchison

EP

meteorite. Nature 307, 252–254.

AC C

Zhang, S.C., Zhu, G.Y., Liang, Y.B., Dai, J.X., Liang, H.B., Li, M.W., 2005. Geochemical characteristics of the Zhaolanzhuang sour gas accumulation and thermochemical sulfate reduction in the Jixian Sag of Bohai Bay Basin. Organic Geochem. 36, 1717–1730.

Zhu, G.Y., Zhang, S.C., Liang, Y.B., Dai, J.X., Li, J., 2005. Isotopic evidence of TSR origin for natural gas bearing high H2S contents within the Feixianguan Formation of

9

ACCEPTED MANUSCRIPT the Northeastern Sichuan Basin, southwestern China. Sci. China 48, 1037–1046.

Zhu, G.Y., Zhang, S.C., Liang, Y.B., Zhou, G.Y., Wang, Z.J., 2007. Origin mechanism

Sichuan Basin. Acta Geol. Sin. 81, 805–817.

RI PT

and controlling factors of natural gas reservoir of Jialingjiang Formation in Eastern

SC

Zhu, G.Y., Zhang, S.C., Huang, H., Liang, Y.B., Meng, S., Li, Y., 2011. Gas genetic type and origin of hydrogen sulfide in the Zhongba gas field of the western Sichuan

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EP

TE D

M AN U

Basin, China. Appl. Geochem. 26, 1261-1273.

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Sample Number

Reservoirs

C1/(C2+C3)

CO2

N2

H2S

CH4

C2H6

C3H8

n-C4H10

%

%

%

%

%

%

%

1

JK1185

Mardin Gr.

140

88.7

3.7

1

5.6

0.03

0.01

Dodan 17M

2

JK1183

Mardin Gr.

96

91.5

2.1

1.4

4.8

0.04

0.01

Dodan 6G

3

JK1179

Garzan

153

92.7

2.3

0.4

4.6

0.02

0.01

Dodan 6M

4

JK1176

Mardin Gr.

108

91.8

2.1

0.6

5.4

Dodan 17G

5

JK1177

Garzan

136

93.2

2.1

0.4

4.1

Dodan 1A

6

JK1182

Garzan

102

92.0

1.9

0.7

5.1

δ13C1

δ13C2

δ13C3

δ34SH2S

δ13CO2

δDC1

%

(‰ vs.PDB)

(‰

(‰

(‰)

(‰

(‰

vs.PDB)

vs.PDB)

vs.PDB)

vs.SMOW)

-25.20

-26.31

-28.58

11.9

-1.97

0.001

0.002

0.002

-28.93

-28.12

-29.00

13.4

-1.59

0.001

0.001

-29.03

-31.93

-28.90

n.d.

-2.80

-128.4 -133.5

0.04

0.01

0.002

0.001

-31.91

-26.31

-30.22

n.d.

-1.78

0.02

0.02

0.001

0.005

-29.93

-26.41

-30.14

12.1

-2.00

0.04

0.01

0.001

0.001

-25.83

-26.96

-29.58

n.d.

-2.80

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* (n.d.) not determined

0.002

n-C5H12

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Dodan 16M

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Well Name

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Table 1. Molecular and isotopic composition of the gases from Dodan and Silivanka gas fields in SE Anatolia.

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Figure 2. The generalized stratigraphy of Dodan Field and the petroleum geological properties of various units (modified from Gürgey, 1991).

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Figure 3. A burial history diagram superimposed using the calculated temperature history obtained from the 1-D modeling of the geological evolution of the Dodan 10/M well within

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the Dodan gas field. The red line indicates the main temperature isolines (50, 100, 150 oC).

Figure 4. The heat flow history prior to calibration (stippled red line) and changes conducted following the calibration notice that the increase of heat flow at 8 Ma was due to Karacadag

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volcanism.

Figure 5. a) A comparison between the calculated (solid line) and measured (circles): (a) the maturity (vitrinite reflectance), the maturity line (dotted line) prior to calibration; and (b) the

10/M well.

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uncorrected borehole temperature measured during logging operation values within the Dodan

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Figure 6. A burial history diagram superimposed by the gas generation potential from Type III kerogen obtained from 1-D modeling of the Dodan 10/M well.

Figure 7. The temporal development of gas generation in the Kas Formation in well Dodan 10/M obtained from 1-D basin modeling.

Figure 8. The temporal development of temperature in the Bakük Formation for well Dodan 10/M obtained from 1-D basin modeling. The red line indicates the critical temperature for TSR (120oC).

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Figure 10. An application of the model of Chung et al. (1988) which predicts the source(s) of gases according to the isotopic composition of methane, ethane, and propane components versus 1/n values to the hydrocarbon gases of Dodan field. It indicates that the methane and ethane is formed by mixing with another gas from a more mature source rocks and/or due to

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the degradation of the gases.

a

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b

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b Figure 5.

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Figure 6.

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(mgHC/gTOC)

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Figure 7.

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120 oC

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Figure 8.

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b Figure 9.

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Figure 10.

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Hydrocarbon gases are formed by thermal degradation of organic matter Kas and Gomaniibrik formations are the most probable source rock of the hydrocarbon gases The origin of H2S is the thermochemical sulfate reduction (TSR) process

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A part of H2S could also be formed by the bacterial sulfate reduction (BSR)

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CO2 has a limestone origin and is formed both by TSR and acidolysis during TSR

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•