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Deep gases discharged from mud volcanoes of Azerbaijan: Newgeochemical evidence

Marco Bonini a,*, Franco Tassi a,b, Akper A. Feyzullayev c, Chingiz S. Aliyev c,Francesco Capecchiacci a, Angelo Minissale a

aConsiglio Nazionale delle Ricerche (CNR), Istituto di Geoscienze e Georisorse, UOS Firenze, via G. La Pira 4, 50121 Firenze, ItalybDipartimento di Scienze della Terra, Università degli Studi di Firenze, via G. La Pira 4, 50121 Firenze, ItalycGeology Institute of the Azerbaijan National Academy of Sciences, H. Javid pr., 29 A, Baku AZ1143, Azerbaijan

a r t i c l e i n f o

Article history:Received 3 May 2012Received in revised form11 December 2012Accepted 12 December 2012Available online 26 December 2012

Keywords:Mud volcanoesAzerbaijanFluid geochemistryGas source depthStructural controls

a b s t r a c t

This paper presents new geochemical data of hydrocarbon-rich gases released from some mud volcanoesof Azerbaijan. Methane is considerably the most abundant component of all the sampled gases, whichshow dD-CH4 and d13C-CH4 values likely related to a dominant thermogenic source. These gases arecharacterized by the presence of more than 20 different cyclic compounds with concentrations up toseveral mmol/mol. A similar gas composition has recently been found to characterize many mud volca-noes of the Northern Apennines and Sicily (Italy). The data of the Azerbaijan mud volcanoes corroboratethe notion that cyclic compounds can be considered reliable tracers for hydrocarbon gas production atconsiderable depths and temperatures up to 120e150 �C, which correspond to a 6.5e8.3 km depth rangeassuming an average geothermal gradient of 18 �C/km. This depth interval is consistent with both thedepth of potential source rocks imaged seismically beneath some mud volcanoes, and the results ofprevious estimates that used the 13C/12C values of methane and ethane. Such deep-sourced gases andmaterial (fluidized clayey mass and rock fragments) ascend into the core of anticlines and accumulate atshallower reservoirs, where fold-parallel outer-arc faults or fold-orthogonal fractures may penetrate andtransfer the fluids to the surface. Finally, the basically equivalent composition of the different hydro-carbon groups (C2eC10 alkanes, aromatics and cyclic) determined in the gases sampled in bothAzerbaijan and Italy manifests the lack of evident relationships between the chemistry of light hydro-carbons and the type of source rock.

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1. Introduction

Mud volcanoes mostly occur in sedimentary environments inrelations to the presence of subsurface pressurized fluids. The gasphase (often represented by methane) is the most important indriving the ascent of subsurface mudefluid mix and variably sizedrock fragments (Jakubov et al., 1971; Brown, 1990; Milkov, 2000;Dimitrov, 2002; Kopf, 2002). Extrusion of this material yields thedevelopment of a number of scenic morphological features, ofwhich the conical-shaped mud volcanoes represent the mosttypical outcome. Mud volcanoes may vary greatly in size, from lessthan 1m to the gigantic submarine (‘Conical Seamount’) serpentine

mud volcanoes of the Mariana forearc that may exceed 25 km indiameter and 2 km in height (Fryer and Pearce, 1992; Fryer et al.,1999). These features usually mark zones of active tectonic short-ening, where sediments are affected by increasing stresses andtemperatures leading to the maturation of organic matter (Higginsand Saunders, 1974; Brown, 1990; Kopf et al., 2001; Kopf, 2002;Deville et al., 2006). Mud volcanoes are particularly frequentalong the AlpineeHimalayan collision zone, where they punctuatesubmerged or exposed accretionary prisms (e.g., Gulf of Cadiz,Somoza et al., 2003; Mediterranean Ridge, Kopf et al., 2001; Mak-ran, Delisle et al., 2002), and the front of fold-and-thrust belts (e.g.,Northern Apennines, Bonini, 2007; Carpathians, Baciu et al., 2007;Caucasus, Kopf et al., 2003; Black Sea, Herbin et al., 2008; CaspianSea area, Jakubov et al., 1971; Yusufzade and Guliyev, 1995).

Mud volcanoes are thus closely associated with petroleumsystems, and commonly release hydrocarbon gases dominantlyconsisting of methane, with significant concentrations of CO2 and

* Corresponding author. Tel.: þ39 055 2757541; fax: þ39 055 290312.E-mail addresses: mbonini@igg.cnr.it, mbonini@geo.unifi.it (M. Bonini).

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C2þ organic compounds (e.g. Valyaev et al., 1985; Stamatakis et al.,1987; Lavrushin et al., 1996; Blinova et al., 2003; Schmidt et al.,2005). Investigations on the origin of hydrocarbons dischargedfrom emissions in different natural environments generally rely onthe chemical and stable isotopic compositions of C1eC3 alkanes(Bernard et al., 1978; Schoell, 1980, 1983, 1988; Chung et al., 1988;Whiticar, 1999). Genetic characterization carried out on isotopicbasis on worldwide mud volcanoes has allowed the compilation ofa global data-set including more than 140 onshore mud volcanoes.This analysis documents a dominant thermogenic character, the gasphase being thermogenic in the 76% of cases, mixed thermogenic/microbial in the 20%, and only the 4% is entirely microbial (Etiopeet al., 2009a,b). The original molecular and isotopic compositionof reservoir gas in mud volcanoes may be strongly affected by post-genetic processes, such as molecular fractionation during advectivefluid migration and secondary methanogenesis. Therefore, theorigin of gases from these natural systems cannot be univocallyassessed on the only basis of the above listed geochemicalparameters.

A detailed analysis of chemical composition of light hydrocar-bons (alkane, cyclic and aromatic compounds) discharged frommud volcanoes and CH4-rich emissions in Italy has evidenced twodistinct groups of thermogenic gases, particularly (i) aromatic-richgases related to significant hydrothermal fluid contribution, and (ii)cyclic-rich gases expelled from mud volcanoes marking theexternal compressive front of the Northern Apennines and Sicily(Tassi et al., 2012). We focus on the latter features, which arerepresentative of the most typical worldwide mud volcanoes.Thermodynamic and geological/structural considerations haveallowed to propose that the cyclic compounds are likely formed byi) thermal cracking of heavier organic molecules and/or ii) catalyticreforming process consisting of incomplete aromatization of thelight alkanes. In particular, these emissions were estimated tooriginate from organic sources located at depths �3000 m, ina genetic environment characterized by high pressure andtemperatures in the 100e120 �C range, or not exceeding 120e150 �C (Tassi et al., 2012). These conditions may be common toother mud volcanoes, which may be as many as more than 8e900onshore, 500 on the continental shelves, and even some 5000 mudvolcanoes are speculated to occur in deep waters (Guliyev andFeizullayev, 1997; Dimitrov, 2002; Milkov et al., 2003; Etiope andMilkov, 2004).

According to these considerations, we aim to test thetemperatureedepth conditions of light hydrocarbon gas gener-ation (determined in the Northern Apennines) in other areas ofmud volcanism. More specifically, this study aims to find possiblerelations between the chemistry of the light hydrocarbon frac-tion and the thermodynamic conditions of the gas source regionthat may be generally valid in mud volcano systems. We havetherefore extended our geochemical investigation to Azerbaijan,which has more onshore mud volcanoes than any other localityon Earth (Jakubov et al., 1971; Guliyev and Feizullayev, 1997).Mud volcanism in Azerbaijan is demonstrably coupled witha number of closely interrelated parameters, specifically (1)subsurface hydrocarbon accumulations (Inan et al., 1997), (2)abnormal fluid overpressures (Feyzullayev and Lerche, 2009),and (3) suitable tectonic structures (Jakubov et al., 1971). Themud volcanoes of Azerbaijan provide ideal conditions because ofthe accessibility of accurate information regarding parametersthat are fundamental to this study, in particular the nature anddepth of the main source rock, the regional geothermal gradient,and pore fluid pressure conditions (e.g., Guliyev et al., 2011). Wethus use this area as a case study for testing the information thatchemistry of the light hydrocarbon gases, in integration with theregional/local geological and tectonic setting, may reveal about

the chemicalephysical conditions acting at the fluid sourceregion.

2. Terminology and activity of mud volcanoes

‘Mud volcano’ is a generic term to indicate the variousmorphologic features associated with the extrusion of subsurfacefluids and solid material. Mud volcanoes show impressivemorphologic similarities with the magmatic counterpart, and forthis reason a number of terms used for mud volcanism are bor-rowed from magmatic volcanism. Besides ‘volcano’, other termsshared by both types are ‘crater’ to indicate the sub-circularcollapsed areas topping the variously sized extrusive edifices,‘caldera’ to indicate the depressions forming from the withdrawalof subsurface material, and ‘vent’ to indicate generic openingsthrough which fluids and material are released. The typical conicalconstructional edifices are termed ‘gryphons’ and ‘mud cones’when tall less than 3 m and 10 m, respectively (see Planke et al.,2003). According to Jakubov et al. (1971) mud cones are up to40e50 m high, and the term ‘mud volcano’ should be restricted tothe larger edifices, which in Azerbaijan may rise even more than300e400 m above the surrounding subtle topography and theirbase may attain widths up to 4e5 km (e.g., Bonini and Mazzarini,2010). Other manifestations are represented by mud pools (or‘salses’) characterized by bubbling gas centers.

Regarding their activity, mud volcanoes normally displaya dormant behavior, which most commonly consists in the quietand continuous expulsion of mud breccia, fluids, and gases at bothconical edifices and mud-water pools. Such a dormant activitycharacterized by gas bubbles rising and popping up the muddywater may be viewed as analog to the ‘strombolian’ behavior ofsome igneous volcanoes, where gas bubbles rise through themagma column and burst near the surface in response to pressuredecrease. In some cases, the dormant periods are interrupted bysporadic eruptive events that may be characterized by effusive orexplosive activity, with large flaming eruptions caused by the self-ignition of the releasing methane (e.g., Mellors et al., 2007).

3. Mud volcanoes of Azerbaijan

Mud volcanoes of Azerbaijan are strongly coupled with thetectonic evolution of the WNW-trending Greater Caucasus fold-and-thrust belt, which has resulted from the still ongoing colli-sion of the Arabian Plate with Eurasia (e.g., Philip et al., 1989; Axenet al., 2001). More than 200 active mud volcanoes have beenidentified onshore and even more offshore Azerbaijan (Jakubovet al., 1971; Dadashev et al., 1995; Guliyev and Feizullayev, 1997;Dimitrov, 2002; Etiope et al., 2004; Yusifov and Rabinowitz,2004; Feyzullayev, 2012). The Azerbaijan mud volcanoes distin-guish for their remarkable dimensions, as well as for the impressiveeruptions they may experience occasionally (Guliyev andFeizullayev, 1997; Aliyev et al., 2001; Mellors et al., 2007).

3.1. Geologic and stratigraphic setting

The Greater Caucasus fold-and-thrust belt is characterized bya dominant southesouthwestern structural vergence (e.g., Koppand Shcherba, 1985). This range exposes a core of Paleozoic base-ment in its the central part, while its southeastern (Azerbaijan)sector is characterized by SeSSW-verging tight to isoclinal folds,commonly associated with thrusts, deforming Mesozoic andPaleogeneeMiocene strata (Rogozhin and Sholpo, 1988; Allen et al.,2003; Alizadeh, 2008; Fig. 1a). The elevation of this rangeprogressively decreases south-eastwards, as this approaches theAbsheron Peninsula and the South Caspian Sea. The line of

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anticlines forming the Absheron Sill in the Caspian Basin representsthe offshore continuation of this range, and separates the shallownorthern Caspian from the deeper South Caspian Basin (e.g.,Jackson et al., 2002; Fig. 1a,b).

A very thick package of NeogeneeQuaternary synorogenicsediments filled a large foreland basin system that formed ahead(i.e., southwest) of this range, and that comprises both the exposedKura Basin and part of the submerged South Caspian Basin (Fig. 1b).Deposition of the 20e25-km-thick South Caspian basin successionis thought to have started during Paleocene or earlier (Zonenshainand Le Pichon,1986; Abrams and Narimanov,1997). This successionrecords a dramatic acceleration in sedimentation rate sincew5.5 Ma, which coincided with the onset of the deposition of thelatest Mioceneeearly Pliocene ‘Productive Series’ consisting offluvialedeltaic sand bodies interbedded with mudstones depositedduring short-lived lacustrine events (Allen et al., 2002; Hinds et al.,

2004). This succession is thick as much as 5e7 km, and affords themajority of hydrocarbon reservoirs (Guliyev and Feizullayev, 1996;Abrams and Narimanov, 1997; Reynolds et al., 1998; Fowler et al.,2000; Planke et al., 2003; Stewart and Davies, 2006). Sedimenta-tion of the Productive Series was followed by the deposition of thelate Pliocene marine mudrocks of the Akchagyl Series, and after-ward by the Absheron Series and younger units, which weredeposited in nonmarine, commonly brackish conditions. Interest-ingly, the PlioceneeQuaternary (post-5.5 Ma) succession reachesw10km in thickness in the central deep buried part of the SouthCaspian basin, defining an extremely high average sedimentationrate ofw1.8 mm/yr, with peaks up to 3 mm/yr (Guliyev et al., 2003,2011) and 5e6 mm/yr (Soto et al., 2011).

The strata underlying the Productive Series consist of middleelate Miocene shales and marls with interbeds of sandstones andsiltstones, including the Diatom Suite. The above succession rest

Figure 1. Mud volcanoes of Azerbaijan. (a) Simplified structural sketch map of the Greater Caucasuseeastern Caspian Basin (modified after Jackson et al., 2002). (b) Regional crosssection through the Absheron Sill and the South Caspian Basin (vertical exaggeration 2�; simplified from Stewart and Davies, 2006). (c) Main structures and mud volcano fieldsaround the Greater Caucasus front and Absheron area (adapted from Jakubov et al., 1971; Jackson et al., 2002; Bonini and Mazzarini, 2010); these features are superposed ona shaded digital terrain model (U.S. Geological Survey, available from: http://www.gisweb.ciat.cgiar.org/sig/90m_data_tropics.htm).

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above the OligoceneeEarly Miocene sediments of the MaykopSeries, which is a 200e1200 m thick (up to 3000 m offshore)regionally continuous layer of anoxic fine-grained, organic-richsediments (Abrams and Narimanov, 1997; Feyzullayev et al., 2001;Hudson et al., 2008). The highest burial depths of the top of theMaykop Series occur in the central (deep water) part of the SouthCaspian Basin, where may reach w12 km; shallower burial depths(w4e6 km) occur at the Absheron Sill and onshore easternAzerbaijan (Gobustan area; Abrams and Narimanov, 1997; Fowleret al., 2000; Knapp et al., 2004) (Fig. 1b). The Maykop Series iswidely cropping out on the western part of the Absheron Peninsulaand the southeastern slope of the Great Caucasus.

3.2. Structural patterns and controls on mud volcanism

Trains of elongated N-to-NW-trending tight anticlines deformthe northeastern margin of the Kura Basin, thereby manifesting theinvolvement of this foreland basin in the thrusting. These foldsoften expose the Productive Series up to the Absheron sediments,and are commonly assumed to detach in overpressured shales ofthe Maykop Series (Devlin et al., 1999; Jackson et al., 2002; Sotoet al., 2011). These folds often show complex surface patternscharacterized by marked changes in orientation; in this regard,

remarkable are the fold trains bending from wWNWeESE, at theGreater Caucasus range, to wNNWeSSE along the South Caspianbasin margin (e.g., Jackson et al., 2002; Fig. 1a,c).

Likewise other worldwide provinces, mud volcanoes ofAzerbaijan are intimately associated with hydrocarbon-bearingfaulted anticlines, and typically puncture the crests of thoseexposed in the Kura Basin and eastern Azerbaijan (Jakubov et al.,1971; Guliyev and Feizullayev, 1996), as well as those underwaterin the Absheron Sill (Fowler et al., 2000; Cooper, 2001; Stewart andDavies, 2006; Fig. 1). The Maykop Series plays a primary role in theprocess, as this forms the main décollement level for the folds (e.g.,Jackson et al., 2002; Soto et al., 2011), as well as the principalregional source rock for hydrocarbons (Guliyev and Feizullayev,1996; Jones and Simmons, 1997; Feyzullayev et al., 2001) and themud volcano systems, which often bring to surface fragments ofthis rock unit (Inan et al., 1997; Kopf et al., 2003). Excellent sourcerock properties have also been determined in sediments directlyoverlying the Maykop Series (i.e., the middle-late Miocene DiatomSuite), as well as beneath this series at depth > 10 km (Guliyev andFeizullayev, 1996; Cooper, 2001; Feyzullayev et al., 2001). Theinitiation of onshore and offshore anticlines during the EarlyeLatePliocene (e.g., Devlin et al.,1999; Yusifov and Rabinowitz, 2004) andthe nearly synchronous inception of most of mud volcanoes around

Figure 2. (a) Geologic setting of the Ayran-Tekan mud volcano field, showing the typical location over the crest of an anticline (image extracted from Google Earth�; http://earth.google.it/download-earth.html; location in Fig. 1c). (b) Bubbling crater at the top of a gryphon; circle shows hammer for scale. (c) Lateral view of the Ayran-Tekan mud caldera (23May 2010).

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3.5 Ma (Yusifov and Rabinowitz, 2004) also suggest a closeconnection between folding and the mud volcanism (Kadirov et al.,2003).

The prolific mud volcanism appears strongly linked to the ratherexceptional association of parameters that characterized the SouthCaspian region, namely: (1) the abnormally thick (up to w25 km)package of clastic sediments; (2) the extremely high rate of sedi-ment accumulation, especially during PlioceneeQuaternary times(up to 5e6 mm/yr; Soto et al., 2011); (3) important hydrocarbongeneration that favors the development of relevant fluid over-pressures up to lithostatic levels (Tagiyev et al., 1997), and (4) thevery low geothermal gradient (20e15 �C/km), with temperature ofapproximately 105e110 �C at 6 km depth (e.g., Bredehoeft et al.,1988; Buryakovsky et al., 1995; Feyzullayev and Lerche, 2009),which may define an average geothermal gradient of w18 �C/km.

4. Study areas

Gases emitted from mud volcanoes were strategically sampledin two distinct regions, one group nearby the front of the GreaterCaucasus (hereinafter referred to as ‘Gobustan mud volcanogroup’), at the boundary with the Kura Basin, and the other group(referred herein to as ‘Absheron mud volcano group’) in a moreinternal position (i.e., northeastward) (Fig. 1c). The Gobustan mudvolcano group includes (from west to east) the Shikhzagirli (boththe smaller cone and larger volcano), Ayran-Tekan, Dashgil, KichikBahar and Bahar mud volcanoes, while the Absheron mud volcanogroup has considered the Pirekyushkul, Buransyz-Julga, and Uch-tape mud volcanoes (Fig. 1c). Organic matter of different aged rocksfrom mud volcanoes of the Shamakhy-Gobustan, Absheron andLower Kura regions are type IIeIII (Guliyev et al., 2005); the organic

Figure 3. (a) Geologic setting of the Dashgil mud volcano (image extracted from Google Earth�; http://earth.google.it/download-earth.html; location in Fig. 1c). (b) Oil seep at thefoot of this volcano; lens cap (circled) for scale. (c) Row of gryphons and (d) bubbling pool at the summit of this mud volcano (23 May 2010); notice in (b) the ridge in thebackground showing a reddish color that likely originated from the 1926 large self-igniting eruption (Kopf et al., 2010). Photo viewpoints are indicated in (a). (e) Lateral view of thegryphon field topping the Bahar mud volcano (location in Fig. 1c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of thisarticle.)

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matter of Shikhzagirli and Bahar mud volcano areas is referredmostly to type III (gas prone).

4.1. Geological and structural setting

4.1.1. The Gobustan mud volcano groupMud volcanoes exhibit the typical coincidence with anticline

structures (Jakubov et al., 1971). In particular, the Ayran-Tekan mudvolcano field is situated over a main anticlinal ridge running nearbyand sub-parallel to the Greater Caucasus front (Figs. 1c and 2a). Thisanticline folds Early Pliocene (Productive Series) to Early Pleisto-cene (Absheron) deposits, and shows a clear bending of its axialtrace from NW to WNW (Fig. 2a). The Ayran-Tekan field consists ofvarious mud volcanic elements situated along the anticlinal crest,such as rows of gryphons and small vents, as well as a large mudcaldera (Fig. 2aec). The caldera is extruding two mud breccia flowsexceeding 1 km in length, and extending downwards to the foot ofthe anticline forelimb. Rock fragments attributable to the MaykopSeries have been identified in the extruded mud.

Moving to the east, the Dashgil mud volcano is an approximately50 m tall edifice located in a similar structural position over thecrest of an EeW-trending anticline (Jakubov et al., 1971; Plankeet al., 2003; Roberts et al., 2011; Figs. 1c, 3a). Oil seepage occursat the base of the main volcano (Fig. 3b), while the main active ventzone is situated over the summit of this edifice; the crest of thevolcano is a smooth area dotted by 2e4 m tall gryphons, a numberof which define a w100 m-long row that trends in NWeSE direc-tion (Fig. 3c). The summit of Dashgil is also characterized by twomud-water pools 20e30 m in diameter showing some bubblingsites (Fig. 3d). A row of coalescent gryphons defines aw200m-longridge delimiting the mud pools southward; interestingly, themudstone is at places reddish to black in color owing to the burningproduced by violent flaming eruptions that repeatedly occurred atthis site (the latest occurred in 1902, 1908, 1926 and 1958; see Kopfet al., 2010). A considerable part of the material extruded at Dashgilis originating from the Maykop Series, as testified by the nature ofthe variably size rock fragments (Kopf et al., 2010).

The Bahar mud volcano is anotherw50 m tall composite edificethat is located further east along the axis of the same fold (Fig. 1c).This volcano is topped by a nearly flat area from which discreteclusters of gryphons erect (Fig. 3e).

Though the Shikhzagirli mud volcano is relatively far from theprevious mud volcanoes, it can be ascribed to the same group(Jakubov et al., 1971; Fig. 1c). This volcano has experienced severaleruptions (Aliyev et al., 2002). It occurs again over an antiform

crest, and is characterized by a wNNE-trending elongated shape inplan-view. The summit area of the Shikhzagirli mud volcano isnearly flat, sub-circular, and dotted by a few gryphon fields; a mudcone occurs at the foot, in the southern portion of the main edifice(Fig. 1c).

4.1.2. The Absheron mud volcano groupThis group of mud volcanoes occurs in the western part of the

Absheron peninsula (Fig. 1a,c). Mud volcanism is less widespreadand the dimensions are normally smaller in comparison to those ofsome mud volcanoes settled around the Greater Caucasus front.The structural position is however equivalent, as the Absheronmudvolcanoes coincide again with the crest of fold anticlines (Fig. 1c).Fromwest to east, the sampled features are the Buransyz-Julga andPirekyushkul gryphon fields, and Uchtape mud volcano (Fig. 1c).Uchtape is a w20 m tall mud volcano topped by a caldera that hasoriginated a mud flow; likewise Dashgil, oil seeps punctuate thebase of this edifice (Fig. 4a). It is worth noting that rocks of theMaykop Series (i.e., the main source rock) are exposed at the core oftight anticlines, such as that settled only 5e6 km northwest ofUchtape (Figs. 1c and 4b). In this area, a number of mud volcanoeslie directly over rocks of the Maykop Series, such as the mentionedShikhzagirli. This may suggest a deeper source for the dischargedhydrocarbon gases, even though the thrust faults can often producethe repetition of stratigraphic units in the subsurface of the mudvolcanoes.

4.2. Source depth of gases and connections with the structuralsetting

Seismic reflection lines represent a useful tool for unraveling therelationships between the potential source depth of hydrocarbongases and the structural setting. Mud volcanoes are often clearlyimaged on seismic sections to be associated with buckle foldsdetached in the Maykop Series (e.g., Stewart and Davies, 2006;Graham and Pepper, 2009; Guliyev et al., 2011; Soto et al., 2011).In some cases, the anticlinal cores are characterized by low signal-to-noise ratios closely associated with the presence of shale thrustsheets and overpressured fluid conditions (Duerto and McClay,2011).

Seismic sections crossing the mud volcanoes can thus be used toconstrain the depth of the Maykop Series, which represents themain source rock for the mud volcanoes situated onshore theCaspian Sea (Feyzullayev and Aliyeva, 2003). On this basis, the topof the Maykop Series may be considered a reliable estimate for the

Figure 4. (a) Lateral view of the Uchtape mud volcano; notice the oil seeps at the base of the western flank of the volcano (24 May 2010). (b) Vertical strata of the Maykop Seriesexposed near the core of a tight fold (location in Fig. 1c).

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minimum depth of hydrocarbon gas formation for the mud volca-noes sampled in Gobustan (Fig. 1c). We use therefore an availableseismic section intercepting along-strike the fold anticline hostingthe Ayran-Tekan mud volcano field south of Gobustan (Figs. 1c and2). It is interesting to note that both the Ayran-Tekan system andother (unsampled) mud volcanoes occur in correspondence ofanticlines that are clearly detectable on this seismic line despitetheir core is often characterized by poorly reflective zones (cf.Figs. 1c and 5). Similarly to other settings, such a low signal-to-noise response is likely associated with shale thrust sheetsaccompanied by fluid expulsion; this may create localized over-pressuring into the fold core, a condition that is evidently favorableto the development of the mud volcanism (Duerto and McClay,2011).

Previous and novel investigations carried out on the materialejected from onshore mud volcanoes settled around Gobustan(Dashgil, Kyanizadag, Otmanbozdag, Ayran-Tekan) have corrobo-rated the well established idea that the Maykop Series affords themain source layer for hydrocarbons (Kopf et al., 2010; Guliyevet al., 2011). In particular, the top of the Maykop Series in thatarea has been attributed to a 5150e5400 m depth range (Guliyevet al., 2011). This estimate accords well with the top of thisorganic-rich succession that is imaged seismically at w2.8e3.8 stwo-way traveltime (twt) (Fig. 5). Along the considered seismicsection, the shallowest depth (w2.8 s twt) occurs at the anticlinebeneath the Ayran-Tekan mud volcano. However, the MaykopSeries top represents only a conservative estimate, given that theproduction of light hydrocarbon gases occur at deeper levels ofthe stratigraphic section (in the area, the base of the MaykopSeries lies in the 6100e6600 m depth range; Guliyev et al., 2011),as well as in sectors depressed by synclines. In particular, thegases could flow up from adjacent synclines into the cores ofanticlines in response to the lateral pressure gradient driven bythickness variations of overlying overburden, as proposed for themud volcanoes of the Emilia Apennines (Tassi et al., 2012). In sucha way, the rising gases may collect at anticlines’ crests, and thismight explain the common occurrence of mud volcanoes dis-charging gases at these structures (Fig. 5).

4.3. Structural controls on mud volcanism

The preferred structural evolutionary model considers the oiland light hydrocarbon gases produced mainly in the organic-richMaykop Series (as well as in deeper stratigraphic levels). Thenthese compounds may migrate up-through fault-controlled steep

fluid conduits into the core of anticlines, where they are trapped inreservoirs within the Productive Series (see Jakubov et al., 1971;Fig. 6a). Seismic reflection profiles allow imaging these anticlines astight buckle folds detached on, and cored by the overpressuredMaykop shales (e.g., Fowler et al., 2000; Stewart and Davies, 2006;Soto et al., 2011; Fig. 6a). In some cases, the fluids may eventuallyrise along normal faults resulting from the stretching of the crestalarea of the fold anticlines. This process is referred to as ‘tangentialelongitudinal strain folding’ and consists in the development ofnormal faults parallel to the fold axis, above a neutral surface thatforms during the buckling of stratigraphic units e i.e., whenisotropic layers are sandwiched between much less competentmaterial (or the surfaceeair interface), and the resulting detach-ment levels accommodate folding by flexural slip (Ramberg, 1961;Ramsay, 1967; Ramsay and Huber, 1987). The Akhtarma-Karadagmud volcano field, settled over the NE-trending anticline near theCaspian coast (25 km southwest of Baku), provides a clear exampleof outer-arc fault control on mud volcanism (Fig. 6b,c). Many othermud vent alignments parallel to fold axes and elongated mudcalderas in Azerbaijan are thought to reflect subsurface faultingassociated with outer-arc extension (Bonini and Mazzarini, 2010;Roberts et al., 2011). This kind of faults is expected to exploitsource layers or fluid reservoirs settled above the neutral surface,which usually occurs at relatively shallow depths in the cores ofanticlines (see Fig. 6a). However, fold-axis orthogonal faults/frac-tures e like cross-fold joints or transverse faults e are alsodemonstrably efficient in controlling the ascending fluids feedingthe mud volcano systems (Bonini and Mazzarini, 2010; Robertset al., 2011). Such fold-axis orthogonal structures are indeedpotentially capable of exploiting relatively deep fluid reservoirs,especially when folds strike approximately perpendicular to theregional maximum horizontal stress (SH) and these brittle elementsare favorably oriented for dilating in the direction of the minimumhorizontal stress (Sh).

5. Gases discharged from mud volcanoes

5.1. Gas sampling and analysis

Gases were collected using a plastic funnel up-side-downpositioned above emission and connected through silicone/tygontubes to pre-evacuated 250mL glass flasks equipped with Thorion�

valves (Vaselli et al., 2006). The analysis of themain gas compounds(CH4, CO2, N2, Ar, O2 and He) was carried out using a Shimadzu 15AGas Chromatograph (GC) equipped with Thermal Conductivity

Figure 5. Interpreted line-drawing of a seismic section crossing the mud volcano area and the Greater Caucasus front south of Gobustan (location in Fig. 1c). The dashed red arrowsschematically illustrate the inferred fluid migration path. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Figure 6. (a) Conceptual setting of Azerbaijan mud volcanism associated with a tight buckle fold detached on the overpressured shales of the Maykop Series (inspired from Sotoet al., 2011). Aqueous fluids, gases (mostly light hydrocarbons) and oil produced in the organic-rich Maykop Series, as well as in deeper stratigraphic layers, supposedly migratedinto the fold core and collected in reservoirs within the Productive Series. From there, the fluids may eventually ascend along normal faults forming in the outer-arc of the anticline.(b) Anticline folding Productive Series deposits at Akhtarma-Karadag, where outer-arc normal faults (red) affect the fold crest and control mud volcanism (satellite image extractedfrom Google Earth�; http://earth.google.it/download-earth.html; location in Fig. 1c). (c) Detail of a recent, SE-dipping, outer-arc normal fault displacing the flank of a w8e10 m-tallmud cone (photo viewpoint in b); the vertical scarp is up tow1.5 m high (25 May 2010). (For interpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

Table 1Chemical and isotopic (d13C-CO2, d13C-CH4 and dD-CH4) composition of gases discharged frommud volcanoes of Azerbaijan. Gas concentrations are in mmol/mol; d13C (in CO2

and CH4) and dD-CH4 ratios are expressed as & V-PDB and & V-SMOW, respectively. Acronyms of sampled mud volcanoes are: ShV, Shikhzagirli volcano; ShC, Shikhzagirlicone; AT, Ayran-Tekan; Da, Dashgil; KB, Kichik Bahar, Ba, Bahar (Gobustan mud volcano group); Pi, Pirekyushkul; BJ, Buransyz-Julga; Uc, Uchtape (Absheron mud volcanogroup).

Shikzagirlivolcano

Shikhzagirlicone

Ayran-Tekan Dashgil KichikBahar

Bahar Pirekyushkul Buransyz-Julga

Uchtape

Acronym ShV ShC AT Da KB Ba Pi BJ Uc

Latitude 40�2901300N 40�2805000N 39�5903800N 39�5904700N 39�5905300N 39�5905600N 40�2805100N 40�2705900N 40�2704500NLongitude 49�0200300E 49�0105400E 49�1803200E 49�2401000E 49�2701900E 49�2801500E 49�2605300E 49�2801200E 49�3403200E

CH4 982 969 978 979 984 948 981 976 968CO2 8.46 15.6 8.5 11.7 7.80 21.6 8.46 10.7 12.5N2 8.80 14.5 11.7 8.46 7.47 28.5 9.49 10.7 16.5Ar 0.23 0.34 0.26 0.20 0.19 0.68 0.24 0.26 0.39O2 0.65 0.96 1.52 0.96 0.89 1.45 0.77 1.98 2.12He 0.021 0.036 0.016 0.015 0.013 0.042 0.024 0.015 0.008d13C-CH4 �41.8 �41.5 �42.1 �45.5 �43.4 �45.6 �41.1 �46.2 �45.5dD-CH4 �193 �192 �189 �193 �189 �185 �190 �195 �188d13C-CO2 17.8 2.0 16.5 11.8 7.1 5.7 21.3 14.2 30.6

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Detector (TCD), whereas the C2eC4 alkanes were analyzed witha Shimadzu 14A gas-chromatograph equipped with a Flame Ioni-zation Detector (FID). The analytical error for the GC analysis is<5%.

The C4þ hydrocarbons were extracted from the gas mixturesusing a divinylbenzene (DVB)eCarboxenepolydimethylsiloxane(PDMS), 50/30 mm, 2 cm long fiber assembly (Supelco; Bellefonte,PA, USA) and analyzed with a Thermo Trace GC Ultra gas chro-matograph coupled with a Thermo DSQ Quadrupole Mass oper-ating in full scan mode, in the mass range 40e400 m/z. Analyteswere desorbed from the absorbing fiber through direct exposure at230 �C for 2 min in the GC injection port. Peak separation wasobtained adopting the following GC setup: a 30 m � 0.25 mm i.d.1.4 mm film thickness TR-V1 fused silica capillary column (Thermo);

helium as carrier gas at a flow-rate of 1.3 mL/min in constantpressure mode; column oven temperature set at 35 �C (hold:10 min), increased at 5.5 �C/min to 180 �C (hold: 3 min), increasedat 20 �C/min up to 230 �C (hold: 6 min). The organic compoundsdetected by the quadrupole detector were identified accordingboth to their retention time of the chromatographic peak and totheir mass spectra using the NIST05 library (NIST, 1995) forcomparison. The calibration procedure for quantitative analyseswas performed using Accustandard� standard mixtures ofcompounds pertaining to the following functional groups: alkanes(12 C4eC10 alkanes, at concentrations ranging from 100 to200 ppbv), cyclic compounds (5 C5eC6 species, at 20 ppbv),aromatic compounds (15 species at concentrations ranging from

Table 2Chemical composition of C2eC10 hydrocarbons of gases discharged from mud volcanoes of Azerbaijan. Gas concentrations are in mmol/mol; n.d.: not detected.

Mud volcano Shikzagirlivolcano

Shikhzagirlicone

Ayran-Tekan

Dashgil KichikBahar

Bahar Pirekyushkul Buransyz-Julga

Uchtape

Acronym ShV ShC AT Da KB Ba Pi BJ Uc

Ethane 2990 2520 5170 2910 3850 1850 3510 2350 2420Propane 215 235 247 274 233 184 311 234 2562 Methyl propane 74 75 94 75 88 84 88 64 91Normal butane 29 35 44 26 28 33 33 26 412 Methyl butane 19 16 19 17 19 17 22 18 153 Methyl butane 14 16 17 16 18 15 19 13 18Normal pentane 13 12 19 14 15 11 15 10 162 Methyl pentane 13 11 10 8.9 9.8 12 9.8 11 9.23 Methyl pentane 6.1 5.8 5.9 6.4 5.6 7.5 4.9 4.8 5.62.3 Dimethyl butane 2.6 2.8 3.8 3.1 2.8 3.3 2.8 2.7 5.5Normal hexane 8.1 6.5 6.2 5.5 4.9 7.8 7.7 5.6 7.22 Methyl hexane 7.4 6.1 6.5 6.2 5.4 8.2 8.8 4.8 6.93 Methyl hexane 2.4 1.6 1.8 2.1 2.3 1.9 2.6 1.1 2.62.2.3 Trimethylbutane 0.41 0.69 0.33 0.48 0.39 0.65 0.35 0.84 0.452.3 Dimethyl pentane 0.56 0.65 0.45 0.28 0.41 0.71 0.41 0.74 0.662.4 Dimethyl pentane 0.51 0.49 0.33 0.36 0.35 0.36 0.29 0.28 0.47Normal eptano 2.1 1.3 2.2 1.1 1.5 1.5 1.4 1.2 1.62.3.4 Trimethyl pentane 0.56 0.71 0.65 0.47 0.47 0.86 0.69 0.61 0.552.3 Dimethyl hexane 0.45 0.55 0.52 0.26 0.25 0.35 0.51 0.45 0.362.5 Dimethyl hexane 0.28 0.41 0.32 0.24 0.31 0.26 0.44 0.38 0.352 Methyl eptano 0.32 0.36 0.21 0.26 0.25 0.28 0.36 0.39 0.19Normal octane 0.45 0.54 0.39 0.49 0.45 0.48 0.39 0.51 0.622 Methyl octane 0.29 0.33 0.19 0.24 0.23 0.35 0.21 0.28 0.51Normal nonane 0.13 0.21 0.13 0.17 0.15 0.23 0.09 0.13 0.242.5 Dimethyl octane n.d. 0.17 0.11 0.09 0.13 0.15 0.08 0.15 0.153 Methyl nonane 0.08 0.11 0.09 0.11 0.14 0.11 n.d. 0.11 0.19Normal decane 0.11 0.15 0.08 0.09 0.08 0.13 0.08 0.11 0.15

Benzene 8.5 8.7 16 13 15 6.2 13 7.4 6.5Toluene 2.9 4.1 11 6.7 8.7 2.8 6.4 2.8 2.8Ethylbenzene 0.21 0.51 0.39 0.22 0.44 0.48 0.29 0.33 0.26m,p Xylene 1.1 1.5 4.4 2.4 3.9 1.6 2.6 1.4 2.1

Cyclopentane 3.8 3.1 9.5 4.8 4.9 3.3 4.9 2.1 3.11 Methyl cyclopentane 2.1 2.7 6.6 2.8 3.3 2.1 3.9 1.5 2.51.3 Dimethyl cyclopentane 1.8 2.6 2.8 1.4 1.7 2.1 2.2 0.87 1.21.2.4 Trimethyl cyclopentane 1.5 1.3 3.6 2.2 1.6 1.1 1.9 1.2 1.51.1.3.4 Tetramethyl cyclopentane 1.4 0.85 1.4 1.1 0.45 1.2 0.89 0.61 1.1Pentamethyl cyclopentane 0.56 0.24 0.45 1.2 0.26 0.26 0.66 0.51 0.882 Propyl cyclopentane 0.41 0.25 0.28 0.81 0.21 0.15 0.45 0.39 0.422 Methylbutyl cyclopentane 0.21 0.15 0.31 0.56 0.14 0.16 0.69 0.71 0.26trans 1.3 Diethyl cyclopentane 0.23 0.21 n.d. 0.23 0.15 n.d. 0.51 n.d. 0.15Cyclohexane 6.2 4.6 2.4 3.9 0.96 5.4 7.8 4.4 2.81 Methyl cyclohexane 5.7 3.7 1.8 3.4 0.75 4.8 6.9 3.1 2.71.3 Dimethyl cyclohexane 4.4 5.1 1.8 3.3 0.88 3.1 3.8 1.1 1.51.5 Dimethyl cyclohexane 2.4 3.4 0.59 1.8 0.39 2.8 3.5 0.55 0.451.3.5 Trimethyl cyclohexane 0.45 0.87 0.51 1.3 0.34 1.1 1.7 0.59 0.281.3.4 Trimethyl cyclohexane 1.1 2.2 0.44 0.55 0.29 1.6 2.3 0.61 0.361 Ethyl 3 methyl cyclohexane 0.68 1.9 0.23 0.28 0.15 1.4 1.4 0.36 0.22trans 1 Ethyl 1.4 dimethyl cyclohexane 0.15 0.36 0.17 0.31 0.26 0.23 0.61 n.d. 0.29trans 1,1,3,5 Tetramethyl cyclohexane 0.15 0.15 n.d. 0.28 n.d. n.d. 0.55 n.d. 0.15cis 1.1.3.5-Tetramethyl cyclohexane 0.21 0.26 n.d. 0.26 0.15 0.15 0.49 n.d. 0.15Cyclooctane 1.7 2.6 0.87 1.7 0.24 1.8 2.3 1.7 1.11.2 Dimethyl cyclooctane 1.4 1.1 0.44 1.1 0.22 0.87 1.8 1.1 0.561.3 Methylpropyl cyclooctane 1.1 0.71 0.31 0.29 0.16 0.56 1.6 0.21 0.451 Ethyl cyclooctane 0.27 0.66 0.26 0.34 0.13 0.41 0.66 n.d. 0.29

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100 to 5000 ppbv). The values of the Relative Standard Deviation(RSD), calculated from five replicate analyses of the gas mixture inwhich the compounds of interest are present at a concentration of50 ppbv, were <5%. The limit of quantification (LOQ) was deter-mined by linear extrapolation from the lowest standard in thecalibration curve using the area of a peak having a signal/noise ratioof 5.

The analyses of the 13C/12C ratios of CO2 (expressed as d13C-CO2& V-PDB) were carried out with a Finningan Delta S mass spec-trometer after purification of the gas mixture by standard proce-dures (Evans et al., 1998). Internal (Carrara and San Vincenzomarbles) and international (NBS18 and NBS19) standards wereused in order to estimate external precision. Analytical error andthe reproducibility were �0.1&. The analyses of the 13C/12C and2H/1H ratios of CH4 (expressed as d13C-CH4& V-PDB and dD-CH4,&V-SMOW, respectively) were performed by mass spectrometry(Varian MAT 250) according to the procedure described by Schoell(1980). Analytical error was �0.15&.

5.2. Chemical and isotopic composition

The concentrations (inmmol/mol) of themain compounds (CH4,CO2, N2, Ar, O2 and He) constituting the 9 mud volcano gasescollected for the present study are reported in Table 1. Methane isthe most abundant compound (from 948 to 984 mmol/mol), fol-lowed by N2 and CO2 (from 7.47 to 28.5 and from 7.80 to 12.5 mmol/mol, respectively). Oxygen and Ar show significant concentrations(up to 2.17 and 0.38 mmol/mol, respectively), whereas Heis < 0.042 mmol/mol.

The d13C-CO2 values (Table 1) are in a wide range, from 2.0 to30.6& V-PDB. Both d13C-CH4 and dD-CH4 values are clustered ina narrow range, from �46.2 to �41.1& V-PDB and from �195 to�185& V-SMOW, respectively. The concentrations of hydrocarbons(up to 54 different species in mmol/mol) are reported in Table 2. Weidentified and quantified 27 alkanes, 4 aromatics and 23 cyclics. TheC2þ hydrocarbon total concentrations range from 2287 to5717 mmol/mol, and they are mainly constituted by alkanes (>98%).Cyclic compounds (from 0.41 to 1.30%) are slightly more abundantthan aromatic compounds (from 0.37 to 0.65%). Ethane concen-trations are up to one order of magnitude higher than the sum of allother compounds, and benzene shows the highest concentrationsamong aromatics. Cyclics having C5, C6 and C8 rings are mostlymethylated, although not-branched compounds (cyclopentane,cyclohexane and cyclooctane) have the highest concentrations oftheir Cx group. Few cyclics showing ethyl, propyl and butylbranches were detected and their concentrations are lower thanthose of not-branched and methylated compounds.

6. Discussion: origin of discharged gases

The N2/Ar ratios of the investigated samples (ranging from 38 to45; see data in Table 1) closely resemble that of ASW (air saturatedwater) suggesting that the origin of these two gases is related tointeraction between shallow meteoric aquifers and the ascendingmudvolcanofluids. The relatively lowO2/Ar ratios, significantly lowerthan thatof ASW, are likely producedbyO2-consumptionduringgasewatererock redox reactions occurring at shallow depth. Heliumshows concentrations up to three orders of magnitude higher thanthose expected for background air, therefore itmostly derives fromanextra-atmospheric source. Unfortunately, 3He/4He data, the onlygeochemical parameter that can unambiguously discriminatemantleand crustal helium, are not available for these gases.

Both the concentrations and isotopic features of CH4 and CO2shown in Table 1 are consistent with those reported by previouspapers for the samemud volcanoes considered in the present study

(Valyaev et al., 1985; Guliyev et al., 2005; Mazzini et al., 2009).According to the dD-CH4 vs. d13C-CH4 ‘Schoell’ diagram (Schoell,1983), the methane in the Azerbaijan gases is produced bydecomposition of organic matter buried in sediments, i.e. the gas isoriginated from thermogenic processes (Fig. 7). However, the‘Bernard’ diagram (Bernard et al., 1978) shows that all the sampledgases have CH4/(C2H6 þ C3H8) ratios (>190) significantly higherthan those typical of thermogenic gases (<100) (Fig. 8). Previousstudies (Deville et al., 2003; Etiope et al., 2007, 2009a,b; Milkov and

-100 -50

methyl- ferment.

microbial carbonate reduction

bacterial

geothermal

thermogenic

-20

-40

-60

-80

-100

-120

-400 -350 -300 -250 -200 -150δ13

C-C

H4

δD-CH4

Figure 7. d13C-CH4 vs. dD-CH4 plot of gases from some mud volcanoes of Azerbaijan(location in Fig. 1c and Table 1).

-80 -70 -60 -40 -30

101

102

103

104

δ13 C-CH4

microbial

thermogenic

-50 -20

CH 4

2H

6+

C 3H

8)/(C

Figure 8. CH4/(C2H6 þ C3H8) vs. d13C-CH4 plot of gases from some mud volcanoes ofAzerbaijan (location in Fig. 1c and Table 1).

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Dzou, 2007; Waseda and Iwano, 2008) suggested that molecularfractionation during the ascending fluids from reservoir to surfaceand secondary anaerobic biodegradation may lead to a significantincrease of CH4 concentrations with respect to those of C2þ alkanes.The occurrence of significant concentrations of C3þ hydrocarbons(Table 2), which cannot be produced by microbial activity but aretypically related to thermal cracking of organic matter at temper-atures >80e100 �C (Hunt, 1984; Mango, 1997, 2000), suggests thatthe Azerbaijan mud volcanoes discharge thermogenic gases that inmost cases are affected by secondary processes that mask theoriginal reservoir composition.

Isotopically heavy CO2, such as that discharged frommost of theinvestigated mud volcanoes (Table 1), are likely produced byoxidative and fermentative destruction of saturated hydrocarbons(Zartman et al., 1961;Wasserburg et al., 1963;Wilhelms et al., 2001;Feyzullayev and Movsumova, 2010). This confirms that our gaseshave suffered the influence of processes occurring at relativelyshallow depth. Summarizing, the apparent inconsistency betweenmethane isotopes (Fig. 7) and C1eC3 alkane concentrations (Fig. 8)is caused by secondary processes. Therefore, the classical approach(i.e. the Bernard and Schoell diagrams) used to investigate theorigin of mud volcano gases may lead to misleading information, asalso suggested by Etiope et al. (2009b) on the basis of a largechemical and isotopic dataset of gases from mud volcanoes locatedin different regions, including Azerbaijan.

A recent investigation (Tassi et al., 2012) has shown that thecomposition of C2eC10 hydrocarbons may provide useful insightsinto the origin of mud volcano gases. Thermogenic gases dis-charged from mud volcanoes and seeps located in the Emilia

Apennines (northern Italy) and central Sicily (southern Italy),which were produced from sedimentary formations atdepth > 3000 m (Riva et al., 1986; Pieri, 2001; Capozzi and Picotti,2002, 2010; Granath and Casero, 2004; Etiope et al., 2007; Bertelloet al., 2008), were found to be enriched in cyclic compounds withrespect to CH4-dominated gas emissions located in 1) RomagnaApennines, produced at shallow depth by microbial activity, and 2)southeastern Sicily, influenced by hydrothermal contribution fromthe nearby Mount Etna volcanic system (Grassa et al., 2004). Thepresence of significant concentrations of cyclic compounds wasattributed to (i) thermal cracking of heavier organic molecules and(ii) incomplete aromatization, which is a reforming process (i.e. thereverse of the previous one) involving light alkanes (Davis, 1999).The latter process is favored at high pressure and temperatures upto 120e150 �C, i.e. the thermodynamic conditions proper of a deepenvironment in a sedimentary domain characterized by a relativelylowgeothermal gradient (w25 �C/km), such as those characterizingboth the Emilia Apennines and central Sicily (Zattin et al., 2000;Capozzi and Picotti, 2002; Grassa et al., 2004). At hydrothermalconditions e i.e. at temperatures rapidly increasing with depth upto 200e250 �C as occurs in the area influenced by heat and fluidsreleased at the Mount Etna volcano (Chiodini et al., 1996) e cata-lytic reforming proceeding through dehydrocyclization of alkanes(Mango, 1994; Mèriaudeau and Naccache, 1997) and thermaldecomposition of alkylated aromatics efficiently produce mono-aromatic compounds instead of cyclics (Smith and Savage, 1991;Kissin, 1998). Accordingly, hydrothermal gases are considered tobe typically enriched in aromatic compounds (Capaccioni et al.,1993, 1995, 2004).

Table 3Total hydrocarbon concentrations (mmol/mol), and concentrations and percentages of alkanes, aromatics and cyclics in gases released from mud volcanoes of Azerbaijan andItaly (Emilia Apennines and central Sicily).

Tot Alkanes Aromatics Cyclics Alkanes % Aromatics % Cyclics %

Azerbaijan 3561 3510 18.5 32.6 98.57 0.52 0.92Italy 1897 1859 6.5 31.0 98.00 0.34 1.63

0

3

6

9

12

15

18

21

24

27

30

m e t h y l a t e d C H

m e t h y l a t e d C H m e t h y l a t e d C H

C H

C H

C H

Range of compositions of cyclic compounds in gases from mud volcanoes of the Emilia Apennines and central Sicily (Italy) A T

D a K B B a

S h V S h C

P i B J U c

%(Σ

cycl

ics)

Figure 9. Relative concentrations (in %) of each cyclic compound with respect to the total cyclics in gases from mud volcanoes of Azerbaijan. The range of compositions of cycliccompounds in gases from mud volcanoes of Italy is reported from Tassi et al. (2012). Acronyms of sampled mud volcanoes are: ShV, Shikhzagirli volcano; ShC, Shikhzagirli cone; AT,Ayran-Tekan; Da, Dashgil; KB, Kichik Bahar, Ba, Bahar (Gobustan mud volcano group); Pi, Pirekyushkul; BJ, Buransyz-Julga; Uc, Uchtape (Absheron mud volcano group).

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The amounts of C2eC10 hydrocarbons measured in theAzerbaijan mud volcanoes (present study) are higher than those ofthe Italian thermogenic gases from Emilia Apennines and centralSicily (Tassi et al., 2012), but the relative proportions of the differenthydrocarbon groups (alkanes, aromatics and cyclics) are funda-mentally equivalent (Table 3). The compositional similaritybetween the Azerbaijan and the Italian thermogenic gases is evenmore evident considering the composition of the cyclic compounds(Fig. 9). This observation thus strengthens the contention that thesource rock does not exert a relevant influence on the finalcomposition of these groups of hydrocarbons expelled at mudvolcanoes. As shown by the distribution of C2eC10 alkanes inFigure 10, the Azerbaijan gases show a significant depletion of theC5þ compounds with respect to the Italian thermogenic gases. Thisdifference is likely caused bymolecular fractionation, a process thatis particularly efficient for gas seeps with relatively low flux and/orfed by extremely deep reservoirs (Etiope et al., 2007, 2009a; Chaoet al., 2010), as well as by hydrocarbon biodegradation (Pallasser,2000). These secondary processes seem to not significantly alterthe composition of cyclic compounds, likely because they havesimilar chemical stability and molecular size.

7. Conclusions

The analysis of the collected data has allowed us to define therelationships between gas chemistry, reservoir conditions andphysicalechemical processes affecting the ascending gases alongstructurally controlled pathways. In particular, this study demon-strates that tectonic structures have the ability to transfer to thesurface hydrocarbon gases produced at chemicalephysical condi-tions proper of a deep environment. Our data corroborate the ideathat cyclic compounds can be considered reliable tracers forhydrocarbon gas production at considerable depths and 120e150 �C. The composition of these species is in fact not significantlydependent on secondary processes, such as molecular fractionationand biodegradation. In the Emilia Apennines and central Sicily(Italy) the temperature conditions favoring the formation of cycliccompounds correspond to a 4.8e6 km depth range considering an

average geothermal gradient of 25 �C/km (see Tassi et al., 2012). InAzerbaijan, this depth canbe extended to approximately6.5e8.3 kmassuming an average geothermal gradient of 18 �C/km. In addition,the Azerbaijan gases are strongly depleted in C5þ alkanes withrespect to the Italian mud volcanoes, suggesting that the formerhave been undergoing more advanced secondary processes. Thisaccords well with the deeper source region inferred for theAzerbaijan mud volcanoes by the presence of cyclic compounds.These results also agree with previous estimates of the gas sourcedepth, attributed to the 7e10 km depth range on the basis of the13C/12C values of methane and ethane (Feyzullayev et al., 2002;Kadirov et al., 2005; Guliyev et al., 2005, 2011; Feyzullayev, 2012).

A last corollary concerns the lack of evident relationshipsbetween the type of source rock and the hydrocarbon gas compo-sition. Despite the variation between types II and III of the organicmatter of the Gobustan and Absheron areas (Guliyev et al., 2005),the compositions of C2eC10 alkanes, aromatics and cyclics arebasically invariable. These compositions are also remarkablycomparable to those obtained from the Italianmud volcanoes (Tassiet al., 2012), thereby suggesting that the chemistry of light hydro-carbon gases is essentially independent upon the source rock.

Acknowledgments

We thank Bruno Capaccioni and an anonymous reviewer forcomments and suggestions. Article developed within the frame-work of the scientific cooperation between Azerbaijan (ANAS) andItaly (CNR) e two year programme 2012/2013.

Appendix A. Supplementary material

Supplementarymaterial associatedwith this article can be found,in the online version, at http://dx.doi.org/10.1016/j.marpetgeo.2012.12.003.

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1

10

100

C 1 0 C 8 C 6

C 4 C 2

0.01

0.001

0.1

Range of compositions of C2−C10 alkane compounds in gases from mud volcanoes of the Emilia Apennines and central Sicily (Italy) A T

D a K B B a P i B J U c

S h V S h C

%(Σ

alk

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