Maciej J. Kotarba & Yuriy V. Koltun

Annales Societatis Geologorum Poloniae (2011), vol. 81: 425–441.

OR I GIN OF NAT U RAL GASES IN THE AUTOCHTHONOUS

MIO CENE STRATA OF THE UKRAI NIAN CARPATHIAN

FOREDEEP AND ITS ME SO ZOIC BASE MENT

1

AGH Uni ver sity of Sci ence and Tech nol ogy, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, Al. Mickiewicza 30, 30-059 Kraków, Po land, e-mail: kotarba@agh.edu.pl

2

Na tional Acad emy of Sci ences of Ukraine, In sti tute of Ge ol ogy and Geo chem is try of Com bus ti ble Min er als, 3a Naukova Street, 79060 Lviv, Ukraine

Kotarba, M. J. & Koltun, Y. V., 2011. Or i gin of nat u ral gases in the autochthonous Mio cene strata of the Ukrai nian Carpathian Foredeep and its Me so zoic base ment. Annales Societatis Geologorum Poloniae, 81: 425–441. Ab stract: Meth ane con cen tra tions in nat u ral gases ac cu mu lated in the Lower and Up per Badenian and Lower Sarmatian res er voirs of the Bilche-Volytsia Unit in the west ern part of the Ukrai nian Carpathian Foredeep usu ally ex ceed 96 vol%. Meth ane was gen er ated by mi cro bial re duc tion of car bon di ox ide in the ma rine en vi ron ment. Mi cro bial meth ane and eth ane were pro duced mainly dur ing sed i men ta tion of Mio cene clays and muds. It is pos si ble that this mi cro bial pro cess con tin ues to day. Higher light hy dro car bons (eth ane in part, and mainly pro pane, butanes and pentanes) were gen er ated dur ing the diagenesis and the ini tial stage of the low-tem per a ture, thermogenic pro cesses from Type III and III/II kerogen de pos ited in Mio cene strata and/or Mid dle and Up per Ju ras sic base ment rocks. Lim ited vari a tions in the val ues of geo chem i cal hy dro car bon in di ces and sta ble iso tope ra tios of meth ane, eth ane and pro pane with the depth in di cate sim i lar gas gen er a tion con di tions within the whole Mio cene suc ces sion. The mi cro bial gases (meth ane and partly eth ane) gen er ated dur ing mi cro bial pro cesses within the Mio cene strata later mi grated to the Up per Ju ras sic and the Up per Cre ta ceous (Cenomanian) res er voirs of the Me so zoic base ment, and to the bot tom most Lower Badenian res er voirs of the ana lysed Letnia, Orkhovychi, Rudky and Vereshchytsia fields. The low hy dro gen con cen tra tions within the Mio cene strata as well as within the Up per Ju ras sic and the Up per Cre ta ceous (Cenomanian) res er voirs of the Me so zoic base ment, and within the bot tom most Lower Badenian res er voirs are also re lated to mi cro bial pro cesses. Car bon di ox ide and ni tro gen, which are com mon mi nor con stit u ents, were gen er ated by both mi cro bial and low-tem per a ture thermogenic pro cesses. More over, CO2 also un der went sec ond ary pro cesses, mainly dis so lu tion in wa ter, dur ing mi gra tion. At least part of the ni tro gen ac cu mu lated in the Rudky field, which is re mark ably high in N2 (96.9 vol%), is prob a bly of at mo spheric or i gin and was in tro duced to the res er voir by sec ond ary re cov ery meth ods.

Key words: mi cro bial gases, thermogenic gases, sta ble car bon iso topes, sta ble hy dro gen iso topes, car bon di ox ide, ni tro gen, Mio cene strata, Me so zoic strata, Ukrai nian Carpathian Foredeep.

Manu script received 19 May 2011, accepted 13 October 2011

IN TRO DUC TION

The aim of this study is to ana lyse the con di tions of gen er a tion, mi gra tion and ac cu mu la tion of nat u ral gases within the autochthonous Mio cene strata of the Bilche-Volytsia Unit and the Me so zoic base ment of the Ukrai nian Carpathian Foredeep sec tor be tween the Pol ish-Ukrai nian state bor der and Stryi (Fig. 1). The mo lec u lar com po si tion of the gases, the sta ble car bon iso tope ra tios of meth ane, eth ane, pro pane, butanes, pentanes and car bon di ox ide, the sta ble hy dro gen iso tope ra tios of meth ane, and the iso to pic com po si tion of the mo lec u lar ni tro gen oc cur ring in these gases are pre sented in the pa per. These re sults are re ferred to the geo log i cal set ting and the geo chem i cal char ac ter is tics of the dis persed or ganic mat ter con tained in the

autochtho-nous Mio cene, Mid dle and Up per Ju ras sic and Or do vi cian– Si lu rian strata of the Carpathian Foredeep be tween Tarno-gród and Stryi (south east ern Po land and west ern Ukraine) (Kotarba et al., 2011b; Kosakowski et al., in press; Wiêc³aw

et al., in press).

Pre vi ous mo lec u lar and iso to pic stud ies of nat u ral gases ac cu mu lated within the autochthonous Mio cene strata of the Pol ish and Ukrai nian Carpathian Foredeep have re vealed that the meth ane-dom i nated com po nent was gen er ated by mi cro bial pro cesses (G³ogoczowski, 1976; Shabo & Mam-chur, 1984; Kotarba et al., 1987, 2005; Kotarba, 1992, 1998, 2011; Jawor & Kotarba, 1993; Kotarba & Jawor, 1993; Kotarba & Koltun, 2006). Nat u ral gases ac cu mu lated

within the Palaeozoic–Me so zoic base ment of the Pol ish Carpathian Foredeep can be at trib uted to var i ous ge netic types. Stud ies re vealed the pres ence of mi cro bial and low-tem per a ture, thermogenic va ri et ies as so ci ated with oil and con den sate, and a non-as so ci ated, high-tem per a ture, ther-mogenic va ri ety (Kotarba & Jawor, 1993). More over, in the Up per Ju ras sic car bon ate res er voir form ing one of the tec -tonic blocks of the Lubaczów gas field and in the Cenoma-nian sand stone res er voir of the Brzezowiec gas field typ i cal mi cro bial gases were found, which mi grated from the auto-chthonous Mio cene strata (Jawor & Kotarba, 1991, 1993; Kotarba & Jawor, 1993). Or i gin of nat u ral gases in the Boryslav-Pokuttya Unit of the Ukrai nian Outer Carpathians

was ex plained by Kotarba et al. (2009) and Sechman et al. (2009). Un for tu nately, geo chem i cal stud ies of nat u ral gases ac cu mu lated within the Me so zoic base ment of the Ukrai -nian part of the Carpathian Foredeep, which would ex plain their or i gin, have not been car ried out to date.

GEO LOG I CAL SET TING

AND PE TRO LEUM OC CUR RENCE

The Ukrai nian Carpathians form the mid dle seg ment of the Carpathian Arc lo cated be tween the Pol ish and the Ro -ma nian Carpathians (Fig. 1). This part of the Carpathian

Fig. 1. Sketch map show ing the ma jor tec tonic units of the Ukrai nian Carpathian re gion and lo ca tions of gas sam pling sites. EEP – East Eu ro pean Plat form, B-V – Bilche-Volytsia Unit (outer part of the Carpathian Foredeep), SA – Sambir (Stebnik) Unit, B-P – Boryslav-Pokuttya Unit, OC – Outer (Flysch) Carpathians

orogen con sists of sev eral tec tonic units overthrust north -east ward (Dolenko, 1962; Vialov, 1965; Glushko, 1968; Kruglov et al., 1985; Œl¹czka et al., 2006).

In this part of the Carpathian orogen, the ma jor part of both the oil and gas fields are lo cated within the fron tal tec -tonic units, mainly in the Carpathian Foredeep. The Fore-deep in cludes three tec tonic units, which dif fer in both the geo log i cal struc ture and the oil and gas po ten tial. The Bory- slav-Pokuttya Unit is the fron tal nappe of the Outer Carpa-thian Belt. This is the main oil-bear ing unit in the Ukrai nian Carpathians. It is cov ered by overthrust folded molasse sediments be long ing to the Sambir (Stebnik) Unit. No hy dro -car bons have been found in the Sambir Unit to date. The out er most, Bilche-Volytsia Unit of the Carpathian Foredeep host ing the main gas fields in the area be tween Pol ish-Ukrainian bor der and Stryi (Fig. 1) is the sub ject of this study. To the south west it un der lies the Sambir and Bory-slav-Pokuttya units and to the north east it cov ers a frag ment of the East-Eu ro pean Plat form.

The Bilche-Volytsia Unit shows di ver sity of stratigraphic se quences and vari able thick nesses of Mio cene de pos -its (Shcherba et al., 1987). The Mio cene strata reach their max i mum thick ness of over 5 km in the Krukenychy De -pres sion near the Ukrai nian-Pol ish bor der (Kurovets et al., 2004). In the north west ern part of the unit, the Mio cene strata cover the Palaeozoic and Me so zoic for ma tions. Gas fields oc cur not only in the Mio cene se quence but also in both the Up per Ju ras sic and the Up per Cre ta ceous res er -voirs. In the south east ern part of the Bilche-Volytsia Unit, the Palaeozoic and Me so zoic de pos its also oc cur un der -neath the Mio cene se quence but gas is ac cu mu lated only in the Mio cene rocks.

The Mio cene suc ces sion of the Bilche-Volytsia Unit con sists of the Lower Badenian Sandy-Cal car e ous Se ries and Baraniv beds, Up per Badenian Tyras and Kosiv for ma -tions and Lower Sarmatian Dashava For ma tion (An dre-yeva-Grigorovich et al., 1997, 2008). In the bot tom part of the Bilche-Volytsia Unit, var i ous siliciclastic units, of ten con glom er ates and brec cias and car bon ate de pos its oc cur that are com monly re garded in the geo log i cal un pub lished documentations as well as in the syn the ses based on those documentations (e.g., Vul et al., 1998; Krups’kyy, 2001) to be Karpatian and/or Palaeogene in age. Based on re cent re gional strati graphic cor re la tions and palaeogeographic re -con struc tions, these de pos its are in cluded into the transgres- sive Lower Badenian SandyCal car e ous Se ries (e.g., An -dreyeva-Grigorovich et al., 1997; Oszczypko et al., 2006). These de pos its are mainly sand stones, up to sev eral tens of metres thick. The Lower Badenian Baraniv beds, up to 80 m thick, are rep re sented by argillites, sand stones and lime -stones. At the base of the Up per Badenian suc ces sion, up to 50 m thick Tyras For ma tion oc curs. It is made up mainly of evap o rate sed i ments, cov ered by the Kosiv For ma tion, up to 1,500 m thick, com posed mainly of clayey sed i ments with sand stones in ter ca la tions.

The Lower Sarmatian sed i ments are rep re sented by the Dashava For ma tion, which are sand stones with argillitic in -ter ca la tions. They are sub di vided into the lower part of the Dashava For ma tion with a max i mum thick ness of about 3,000 m and the up per part of the Dashava For ma tion with a

max i mum thick ness of about 1,900 m (Shcherba et al., 1987). A char ac ter is tic fea ture of the whole Lower Sarma-tian suc ces sion is the pres ence of a num ber of sand stone lay ers ex tend ing over the vast area. These lay ers are eas ily iden ti fied in well-logs and are best cor re lated in the north-west ern part of the Bilche-Volytsia Unit. These sand stone ho ri zons are gas res er voirs and the seal is pro vided by clayey lay ers. Such a de vel op ment of the Lower Sarmatian suc ces sion al lowed Vishniakov et al. (1979) to sub di vide them into a num ber of cy cles, which in clude sand stone ho ri -zons and en clos ing clayey lay ers. In to tal, 17 such cy cles were dis tin guished in the Lower Dashava For ma tion and an other 14 were found in the Up per Dashava For ma tion. Each cy cle has its own in dex (Kurovets et al., 2004). Gas fields oc cur in all sand stone ho ri zons, ex cept for the up per four. More over, gas fields were dis cov ered in all other Mio -cene strati graphic units in the Lower and Up per Badenian. Gas res er voirs in the Badenian and Sarmatian suc ces sions are sand stone and siltstone lay ers, usu ally from 0.1 to 2 m (some times up to 5 m) thick. The po ros ity of sand stones usu ally ranges from 20 to 30%, but of ten is up to 40%. The sand stones of the Lower Badenian Sandy-Cal car e ous Se ries com monly form mas sive gas res er voirs to gether with the un der ly ing Cre ta ceous sand stones. The po ros ity of the Lower Badenian sand stones ranges from 6 to 30% (Vul et

al., 1998).

The ma jor part of the Bilche-Volytsia Unit is un der lain by Ju ras sic and Cre ta ceous rocks. The north west ern part of the Bilche-Volytsia Unit rests upon the Lower, Mid dle and Up per Ju ras sic strata. The Lower Ju ras sic se quence, up to 1,000 m thick, com prises mainly terrigenous sed i ments whereas the Mid dle Ju ras sic se quence is dom i nated by black shales of the Kokhanivka For ma tion, reach ing a thickness of 500 m (Dulub et al., 1986). The Up per Ju ras sic strata un con form ably over lie the Me so zoic rocks or the pre- Me so zoic base ment. These are pre dom i nantly lime stones over 800 m thick. The out line of tec ton ics and lithostrati-graphy of the Palaeozoic–Me so zoic base ment in the study area can be found in Bu³a and Habryn (2011) and in Kra-jewski et al. (2011). The ma jor part of the south west ern mar gin of the EastEu ro pean Plat form is cov ered by Cre ta ceous sed i ments, Neocomian to Maastrichtian in age, reach -ing a thick ness of over 500 m (Vul et al., 1998). The general struc ture of the Ukrai nian Carpathian Foredeep is pre sented in Fig. 2B in Kotarba et al. (2011a). The Up per Cre ta ceous sand stones and the Up per Ju ras sic car bon ates pro vide res er -voirs for oil and gas in sev eral fields lo cated in the base ment of the Bilche-Volytsia Unit.

Fourty-four gas and gas-con den sate de pos its and one oil and gas de posit were dis cov ered in the Mio cene strata of the Bilche-Volytsya Unit of the Ukrai nian Carpathian Fore-deep (Kotarba et al., 2011a). These fields are scat tered within the whole Bilche-Volytsia Unit, though they are mainly clus tered in the north west ern part. These fields com -prise over 160 ac cu mu la tions. In both the Badenian and Sar- matian fields the traps are lithologically and tec toni cally sealed. Com monly, the Sambir Overthrust plays the role of the seal. The ma jor part of the gas fields hosted in the Bilche-Volytsia Unit oc cur in el e vated struc tures. In 1920, one of the first gaspro duc ing wells in the world started pro

duc tion from the Dashava de posit hosted in the Bilche-Volytsia Unit (Vul et al., 1998). Eleven oil, gas-con den sate and gas fields were dis cov ered within the Me so zoic base -ment of the Ukrai nian part of the Carpathian Foredeep (Kotarba et al., 2011c). The Mio cene rocks of the ad ja cent tec tonic blocks of ten serve as seals for the Me so zoic fields. Our study pro vides geo chem i cal char ac ter is tics of gases from res er voirs of dif fer ent ages and types (Ta ble 1), and at tempts to ex plain the or i gin of these gases.

METH OD OL OGY

Sam pling pro ce dure

The gas sam ples were col lected from 15 pro duc ing wells drilled into the autochthonous Mio cene (Lower and Up per Badenian and Lower Sarmatian) res er voirs of the Bilche-Volytsia Unit of the Ukrai nian Carpathian Foredeep and five wells ac cess ing the Up per Ju ras sic and Up per Cre -ta ceous (Cenomanian) res er voirs of the Me so zoic base ment Gas sam ple sites in the autochthonous Mio cene strata and Me so zoic base ment of the Carpathian Foredeeep

Well Field Sample code Lithology of

reservoir Lithostratigraphy Age of reservoir Depth (m)

Autochthonous Miocene strata (Bilche-Volytsya unit)

Hlynky-1* Hlynky Hl-1*

Sandstones

Sandy-Calcareous

Series Lower Badenian

1,192-1,222

Mala Horozhanna-5 Mala Horozhanna MH-5 430-463

Pivdenne Hrabyne-10 Pivdenne Hrabyne PH-10 1,336-1,344

Turady-1 Turady Tu-1 Tyras Formation Upper Badenian 254-275

Bilche Volytsia-9*

Bilche-Volytsia BV-9*

Dashava

Formation Lower Sarmatian

841-940

Bilche Volytsia-500* BV-500* 636-650

Dubanevychi-1 Makuniv Du-1 1,570-1,600

Hai-4, -5,-6, -30, -32 Hai Ha 1,512-1,650

Kavs'ke-40 Kavs'ke Ka-40 488-491

Letnia-2* Letnia Le-2* 1,245-1,267 Letnia-21 Le-21 1,098-1,152 Letnia-38 Le-38 1,686-1,694 Letnia-52 Le-52 1,168-1,181 Letnia-60 Le-60 1,283-1,302 Letnia-64 Le-64 1,643-1,660 Makuniv-10 Makuniv Mk-10 2,138-2,155 Opory-22a Opory Op-22a 380-422 Opory-40 Op-40 296-298

Shidne Dovhe-10 Shidne Dovhe SD-10 1,660-1,675

Khidnovychi-133^ Khidnovychi Khi-133^ 719-830

Dashava-65^ Dashava Dh-65^ 754-770 Hrynivka-17, -51^ Hrynivka Hk^ 1,100-1,300 Pyniany-1^ Pyniany Py-1^ 2,000-2,200 Pyniany-25^ Py-25^ 1,712-1,735 Svydnytsia-54^ Svydnytsia Svy-54^ 350-450 Svydnytsia-48, -51, -61^ Svy-48, -51, -61^ 640-644 Svydnytsia-55, -62^ Svy-55, -62^ 660-667

Uhers'ko-98a^ Uhers'ko Uh-98a^ 365-369

Zaluzhany-12^

Zaluzhany Zh-12^ 1,400-1,600

Zaluzhany-14^ Zh-14^ 3,200-3,400

Mesozoic basement

Orkhovychi-2 Orkhovychi Oh-2

Carbonates & Sandstones

Upper Jurassic & Lower Badenian

1,881-1,915

Rudky-228 Rudky Ru-228 1,293-1,380

Vereshchytsia-4 Vereshchytsia Ve-4 1,419-1,490

Letnia-13

Letnia

Le-13 Carbonates Upper Jurassic 1,590-1,595

Letnia-65 Le-65 Sandstones Cenomanian &

Lower Badenian 1,578-1,593 * – af ter Kotarba and Koltun (2006), ^ – af ter Shabo and Mamchur (1984)

and the bot tom most Lower Badenian res er voirs. The names of the wells and fields are listed in Ta ble 1 and their lo ca -tions are shown in Fig. 1. Free gases were col lected di rectly at the pro duc ing well heads in metal con tain ers (vol ume ~1,000 cm3) and gases dis solved in oils were taken from sep a ra tors to glass con tain ers (vol ume ~500 cm3) (Ta ble 1). For in ter pre ta tion pur poses, we also used ear lier mo lec u lar and iso tope com po si tions of four nat u ral gases (Hl-1, BV-9, BV-500 and Le-2) from Mio cene strata pub lished by Kotarba and Koltun (2006), and sta ble car bon iso tope com -po si tion of eleven nat u ral gases from the Lower Sarmatian Dashava For ma tion of Dashava, Khidnovyhchi, Pyniany, Svidnycia, Uhers’ko and Zaluzhany gas fields pro vided by Shabo and Mamchur (1984).

An a lyt i cal pro ce dure

An a lyt i cal meth ods of mea sure ments of the mo lec u lar and iso to pic com po si tions of nat u ral gases were de scribed in an other pa per in this vol ume (Kotarba, 2011).

RE SULTS AND DIS CUS SION

Nat u ral gases from the autochthonous Mio cene res er voirs

The gases col lected from the autochthonous Mio cene (Lower and Up per Badenian and Lower Sarmatian) strata (19 sam ples) and gases from the Lower Sarmatian strata (Shabo & Mamchur, 1984) of the Bilche-Volytsia Unit of the Ukrainian Carpathian Foredeep (11 sam ples) vary in their mo lec u lar and iso to pic com po si tions. Mo lec u lar and iso to pic com -po si tions, and hy dro car bon (CHC) [CHC = CH4/(C2H6 +

C3H8)], car bon di ox ide-meth ane (CDMI) {CDMI = [CO2/

(CO2 + CH4)] 100 (%)} and iC4H10/nC4H10 gas in di ces of

ana lysed gases are sum ma rized in Ta bles 2 and 3.

For clas si fi ca tion of the ana lysed hy dro car bon gases in terms of or i gin and gen er a tion mech a nism, the ge netic di a

-grams de scribed by Whiticar et al. (1986), Schoell (1988), Whiticar (1994) and Berner and Faber (1996) were used (Figs 2–4). Fig ure 5 shows the plots of the car bon iso topes val ues of meth ane, eth ane, pro pane, the butanes and penta-nes vs. their re cip ro cal car bon num bers. As pro posed, by

e.g., Chung et al. (1988) and Rooney et al. (1995), lin ear

trends of these plots are in dic a tive of a sin gle source for thermogenic gases. Zou et al. (2007) and Kotarba et al. (2009) sug gest that in this type of plot a “dog leg” trend, ex -em pli fied by rel a tively 13C-de pleted meth ane and 13Cen riched pro pane as com pared to eth ane, re sults from the pres -ence of nat u ral gas that was not gen er ated from a sin gle sou- rce rock or that un der went post-gen er a tion al ter ation (e.g., sec ond ary gas crack ing, mi cro bial ox i da tion or thermoche-mical sul phate re duc tion). More over, the in creased 13Cde -ple tion of meth ane in re la tion to eth ane has been ap plied used to eval u ate the mix ing pro por tion be tween mi cro bial meth ane and thermogenic gases (Kotarba & Lewan, 2004; Kotarba et al., 2009).

The di ag nos tic plots of sta ble car bon and hy dro gen iso -tope data in Figs 2 and 3 in di cate that the meth ane in the Lower and Up per Badenian and Lower Sarmatian res er voirs was mainly gen er ated by mi cro bial car bon di ox ide re -duc tion with oc ca sional ad mix ture of low-tem per a ture, thermogenic gases. The mi cro bial car bon di ox ide re duc tion pro cess oc curs mainly in the ma rine en vi ron ment (Whiticar

et al., 1986; Rice, 1992). The sta ble car bon iso tope com po

-si tions of eth ane, pro pane, butanes and pentanes (Figs 4, 5) sug gest that eth ane re sults mainly from mi cro bial pro cesses whereas pro pane, butanes and pentanes were pro duced dur -ing diagenesis and/or the early stages of low-tem per a ture, thermogenic pro cesses. Com par i son of sta ble car bon iso -tope com po si tion of pro pane, butanes and pentanes with that of kerogen (Fig. 5A–C) in di cates the thermogenic gas com po nents were sourced by type III kerogen of the Mio cene strata or at least partly from the Mid dle and Up per Ju -ras sic strata (Kotarba et al., 2011b; Kosakowski et al., in press). Eth ane was gen er ated in in sig nif i cant quan ti ties by

OR I GIN OF GASES IN MIO CENE AND ME SO ZOIC STRATA (WEST ERN UKRAINE)

429

Fig. 2. Hy dro car bon in dex (CHC) ver sus d13C(CH4) for nat u ral gases ac cu mu lated in (A) Mio cene and (B) Up per Ju ras sic, Up per Cre ta ceous (Cenomanian) and Lower Badenian res er voirs of the Ukrai nian Carpathian Foredeep and its base ment. Compositional clas si fi ca -tion fields mod i fied af ter Whiticar (1994)

mi cro bial pro cesses, but more than the pro posed ethanoge-nesis rate (Oremland et al., 1986) of one mol e cule of eth ane per one thou sand mol e cules of meth ane. Mi cro bial eth ane with 12C en rich ment (–61.2 to –52.5‰) has been re ported in pro duc ing mi cro bial gas ac cu mu la tions (Lillis, 2007) and

mi cro bial pro pane in some deep ma rine sed i ments (Hinrichs

et al., 2006).

The depth of the sam pled gas ac cu mu la tions in the Lower and Up per Badenian and Lower Sarmatian res er voirs var ied from 254 to 3,400 m (Ta ble 1, Fig. 6). In sig nif i -Mo lec u lar com po si tion of nat u ral gases pro duced from the autochthonous Mio cene strata and Me so zoic base ment

Sample code Molecular composition (vol%)

N2 CO2 He Ar H2 CH4 C2H6 C3H8 iC4H10 nC4H10 iC5H12 nC5H12

Autochthonous Miocene strata ( Bilche-Volytsya unit)

Hl-1* 2.00 0.08 0.04 0.013 n.a. 92.6 2.26 1.65 0.36 0.52 n.a. n.a.

MH-5 1.61 0.27 0.02 0.011 0.016 97.8 0.20 0.06 0.016 0.01 0.007 0.003

PH-10 2.83 0.14 0.04 0.019 0.000 94.2 0.98 0.81 0.27 0.31 0.17 0.12

Tu-1 2.22 0.25 0.03 0.013 0.000 96.9 0.25 0.14 0.07 0.06 0.05 0.02

BV-9* 1.21 0.14 0.013 0.008 n.a. 98.4 0.18 0.05 0.015 0.006 n.a. n.a.

BV-500* 0.95 0.14 0.008 0.008 n.a. 98.8 0.11 0.010 0.005 0.001 n.a. n.a.

Du-1 0.68 0.25 0.013 0.004 0.000 98.4 0.33 0.14 0.10 0.03 0.05 0.01

Ha 0.90 0.13 0.007 0.008 0.000 95.1 1.66 1.23 0.25 0.36 0.13 0.12

Ka-40 0.57 0.09 0.005 0.007 0.000 99.2 0.11 0.012 0.005 0.001 0.002 0.000

Le-2* 1.09 0.08 0.007 0.008 n.a. 98.6 0.16 0.05 0.02 0.007 n.a. n.a.

Le-21 2.26 0.09 0.000 0.011 0.004 97.4 0.12 0.03 0.010 0.005 0.006 0.004 Le-38 0.94 0.13 0.006 0.007 0.000 94.6 1.73 1.32 0.27 0.40 0.16 0.17 Le-52 0.69 0.06 0.007 0.20 0.11 98.6 0.28 0.11 0.02 0.04 0.02 0.03 Le-60 0.95 0.38 0.003 n.a. 0.08 98.4 0.09 0.07 0.04 0.02 0.02 0.008 Le-64 0.45 0.02 0.009 0.008 0.03 98.5 0.32 0.22 0.11 0.08 0.08 0.04 Mk-10 1.03 0.47 0.013 0.003 0.000 95.9 0.61 0.66 0.74 0.25 0.10 0.09 Op-22a 0.55 0.01 0.004 0.006 0.000 99.3 0.10 0.007 0.004 0.0000 0.000 0.000 Op-40 0.34 0.04 0.005 0.02 0.000 99.5 0.11 0.011 0.002 0.0000 tr. 0.000 SD-10 0.78 0.14 0.006 0.07 0.000 98.3 0.26 0.17 0.09 0.06 0.06 0.03

Khi-133^ n.a. 0.10 n.a. n.a. n.a. 98.6 0.35 0.09 0.12

Dh-65^ n.a. 0.20 n.a. n.a. n.a. 99.4 0.23 0.09 0.10

Hk^ n.a. 0.10 n.a. n.a. n.a. 99.3 0.07 0.03 0.04

Py-1^ n.a. 0.75 n.a. n.a. n.a. 98.0 0.08 0.02 0.02

Py-25^ n.a. 0.40 n.a. n.a. n.a. 94.9 1.83 1.42 0.42

Svy-54^ n.a. 0.08 n.a. n.a. n.a. 99.3 0.09 0.14 0.10

Svy-48, -51, -61^ n.a. 0.08 n.a. n.a. n.a. 99.4 0.04 0.07 0.06

Svy-55, -62^ n.a. 0.10 n.a. n.a. n.a. 99.3 0.10 0.17 0.12

Uh-98a^ n.a. 0.20 n.a. n.a. n.a. 97.6 0.32 0.08 0.07

Zh-12^ n.a. 4.80 n.a. n.a. n.a. 92.9 0.52 0.05 0.10

Zh-14^ n.a. 0.89 n.a. n.a. n.a. 95.1 0.97 0.54 1.76

Minimum value 0.34 0.01 0.000 0.003 0.000 92.6 0.04 0.007 0.002 0.0000 0.000 0.000 Maximum value 2.83 4.80 0.04 0.20 0.11 99.5 2.26 1.65 0.76 0.52 0.17 0.17 Mesozoic basement Oh-2 2.57 0.79 0.01 0.02 0.000 92.9 1.17 1.24 0.35 0.57 0.19 0.15 Ru-228 96.9 0.19 tr. n.a. 0.20 2.71 0.017 0.014 0.006 0.42 0.007 0.004 Ve-4 3.05 0.40 0.01 0.008 0.006 94.0 0.85 0.71 0.24 0.27 0.15 0.11 Le-13 1.98 0.19 0.03 0.008 0.005 97.0 0.46 0.23 0.03 0.04 0.02 0.014 Le-65 6.60 0.49 0.03 0.05 0.05 86.7 2.60 1.99 0.42 0.55 0.21 0.18 Minimum value 1.98 0.19 tr. 0.008 0.000 2.71 0.017 0.014 0.006 0.04 0.007 0.004 Maximum value 96.9 0.79 0.03 0.05 0.20 97.0 2.60 1.99 0.42 0.57 0.21 0.18

cant vari a tions in the val ues of geo chem i cal hy dro car bon in di ces (Fig. 6A) and the sta ble car bon iso tope ra tios in meth ane (Fig. 6D) with depth sug gest quite uni form gen er a -tion con di -tions of mi cro bial meth ane in the Mio cene strata down to about 1,200 m depth (Fig. 5D). A de crease in the hy dro car bon (CHC) in dex (Fig. 6A) and an in crease in sta

-ble car bon iso tope ra tios in meth ane and eth ane (Fig. 6D, E) sug gest the oc cur rence of sig nif i cant amounts of diagenetic and/or low-tem per a ture, thermogenic gases be neath 1,200 m depth. More over, the ab sence of a depth trend of sta ble car -bon iso tope com po si tion of pro pane with depth (Fig. 6F) also in di cates sim i lar, thermogenic gen er a tion con di tions within the Mio cene suc ces sion. Most prob a bly the thermogenic com po nents were gen er ated be neath the Outer Carpathians and mi grated from the south (Kotarba et al., 2011a).

In the Pol ish Carpathian Foredeep, the rhyth mic and cy clic de po si tion of clays and sands in the Mio cene ma rine ba -sin and very high sed i men ta tion rates, which ex ceeded 1,500 m/mil lion years and 5,000 m/mil lion years in the late Badenian and the early Sarmatian, re spec tively, fa cil i tated the in tense gen er a tion of mi cro bial meth ane and eth ane, as well as the for ma tion and fill ing of multihorizon traps within the autochthonous Mio cene strata. Mi cro bial gases gen er ated in a par tic u lar clay-mud ho ri zon mi grated to the over ly ing sand ho ri zon which, in turn, is cov ered by an other claymud layer (Kotarba, 1998; Kotarba et al., 1998). Mi -cro bial gen er a tion of meth ane and eth ane was ap par ently most in ten sive in the depth in ter val from 900 to 1,500 metres be neath the Mio cene sea floor (Kotarba et al., 1998). Mi cro bi o log i cal stud ies re vealed the pres ence of con sid er able quan ti ties of methanogenic and methylotrophic bac te ria in wa ters con nected with gas ac cu mu la tions in the Mio

cene strata and that mi cro bial methanogenesis may still con -tinue (Kotarba et al., 1995).

The hy dro gen con cen tra tions in the ana lysed Mio cene gases vary from 0.000 to 0.11 vol% (Ta ble 2). Nat u ral hy dro gen is gen er ated in var i ous biogenic and abiogenic pro cesses: mi cro bial fer men ta tion of sed i men tary or ganic mat ter, mi cro bial car bon di ox ide re duc tion, ther mal de com po -si tion of sed i men tary or ganic mat ter, hy dro ly -sis, wa ter radiolysis (dis so ci a tion of wa ter mol e cules bom barded by al pha par ti cles) and nat u ral nu clear re ac tions (Zobell, 1947; Zinger, 1962; Hawkes, 1972; Dubessy et al., 1988; Whiti-car et al., 1986; Savary & Pagel, 1997). Hy dro gen is a very re ac tive and mo bile gas, hence, its re ten tion in pe tro leum traps and in sed i men tary rocks is rather ephem eral. Con se quently, its pres ence in nat u ral gases in di cates that it was ei -ther gen er ated re cently by mi cro bial pro cesses (Balabane et

al., 1987) in sec ond ary re ac tions within the res er voirs and/or in ad ja cent source beds, or it has as cended from deep-seated sources (Hunt, 1996). As stated be fore, in the Mio cene strata con sid er able quan ti ties of methanogenic and methylotrophic bac te ria were found (Kotarba et al., 1995). There fore, it seems very probable that the presence of hy -drogen within the Miocene strata is related to recent mi cro-bial processes.

The car bon di ox ide con cen tra tions and the val ues of the car bon di ox idemeth ane (CDMI) in dex in the ana lysed nat -u ral gases from the Lower and Up per Badenian and Lower Sarmatian res er voirs vary from 0.01 to 4.80 vol% and from 0.01 to 4.9, re spec tively (Ta bles 2 and 3). The d13

C(CO2)

val ues range from –21.2 to –8.2‰ (Ta ble 3). The d13

C(CH4)

ver sus d13C(CO2) (Fig. 7) in di cate that car bon di ox ide was

gen er ated ex clu sively by mi cro bial pro cesses. The ver ti cal

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431

Fig. 3. d13C(CH4) ver sus dD(CH4) for nat u ral gases ac cu mu lated in (A) Mio cene and (B) Up per Ju ras sic, Up per Cre ta ceous (Cenomanian) and Lower Badenian res er voirs of the Ukrai nian Carpathian Foredeep and its base ment. Compositional clas si fi ca tion fields af ter Whiticar et al. (1986)

Iso to pic com po si tion and gas ra tios of nat u ral gases pro duced from the autochthonous Mio cene strata and the Me so zoic base ment

Sample code

Stable isotopes (‰) Ratios

d13_{C dD d}13_{C d}13_{C d}13_{C d}13_{C d}13_{C d}13_{C d}13_{C d}13_C

CHC CDMI

iC4H10

(CH4) (CH4) (C2H6) (C3H8) (iC4H10) (nC4H10) (iC5H12) (nC5H12) (CO2) (N2) nC4H10

Autochthonous Miocene strata ( Bilche-Volytsya unit)

Hl-1* -59.8 -197 -30.1 -27.5 n.a. n.a. n.a. n.a. n.a. n.a. 24 0.09 0.69

MH-5 -67.6 -193 -44.2 -31.0 -28.8 -28.9 n.a. n.a. -17.6 n.a. 390 0.28 1.90

PH-10 -63.7 -201 -33.6 -30.8 -30.4 -28.8 -26.8 -27.4 -13.2 -3.3 53 0.14 0.89

Tu-1 -69.9 -198 -40.1 -31.7 -30.3 -29.6 -27.8 -23.6 -21.2 -3.0 245 0.26 1.16

BV-9* -64.4 -198 -39.5 -29.4 n.a. n.a. n.a. n.a. n.a. n.a. 424 0.14 2.50

BV-500* -66.4 -196 -51.4 -29.2 n.a. n.a. n.a. n.a. n.a. n.a. 823 0.14 5.00

Du-1 -66.9 -194 -38.0 -31.4 -31.7 -30.7 -29.0 -28.0 -13.1 n.a. 209 0.26 3.78

Ha -62.5 -198 -31.2 -28.0 -28.7 -26.1 -26.8 -25.5 -14.3 n.a. 33 0.13 0.69

Ka-40 -67.6 -198 -50.7 -23.8 n.a. n.a. n.a. n.a. -13.2 n.a. 820 0.09 5.00

Le-2* -65.6 -198 -42.8 -29.6 n.a. n.a. n.a. n.a. n.a. n.a. 481 0.08 2.86

Le-21 -66.8 -202 -44.0 -29.0 -28.0 -26.6 -26.1 -27.5 -15.1 -0.3 654 0.10 2.02 Le-38 -61.6 -206 -31.1 -28.0 -28.9 -26.3 -26.5 -25.5 -13.5 n.a. 31 0.13 0.67 Le-52 -65.6 -207 -40.9 -31.5 -30.2 -29.3 -27.5 -27.7 -15.4 n.a. 249 0.06 0.63 Le-60 -66.6 -198 -40.9 -30.0 -30.2 -27.5 -28.2 -26.0 -8.2 n.a. 642 0.38 2.12 Le-64 -65.8 -202 -38.9 -31.3 -31.3 -29.8 -28.4 -28.5 -14.9 n.a. 182 0.02 1.30 Mk-10 -66.6 -204 -35.9 -33.0 -31.7 -31.0 -28.2 -28.4 -15.5 n.a. 76 0.49 2.93

Op-22a -68.1 -202 -53.5 n.a. n.a. n.a. n.a. n.a. -13.8 n.a. 895 0.01 0.00

Op-40 -68.8 -195 -54.6 n.a. n.a. n.a. n.a. n.a. -15.8 n.a. 822 0.04 0.00

SD-10 -67.0 -205 -41.4 -31.8 -31.3 -29.5 -28.3 -26.4 -13.6 n.a. 232 0.14 1.54

Khi-133^ -72.0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. -12.5 n.a. 224 0.10 n.a.

Dh-65^ -64.3 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 311 0.20 n.a.

Hk^ -56.6 n.a. n.a. n.a. n.a. n.a. n.a. n.a. -18.1 n.a. 993 0.10 n.a.

Py-1^ -54.0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 980 0.76 n.a.

Py-25^ -57.5 n.a. n.a. n.a. n.a. n.a. n.a. n.a. -17.1 n.a. 29 0.42 n.a.

Svy-54^ -61.4 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 432 0.08 n.a.

Svy-48, -51, -61^ -60.7 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 904 0.08 n.a.

Svy-55, -62^ -59.4 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 368 0.10 n.a.

Uh-98a^ -58.0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 244 0.20 n.a.

Zh-12^ -53.1 n.a. n.a. n.a. n.a. n.a. n.a. n.a. -13.6 n.a. 163 4.90 n.a.

Zh-14^ -50.0 n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 63 0.93 n.a.

Minimum value -72.0 -207 -54.6 -33.0 -31.7 -31.0 -29.0 -28.5 -21.2 -3.3 24 0.01 0.00

Maximum value -50.0 -193 -30.1 -23.8 -28.0 -26.1 -26.1 -23.6 -8.2 -0.3 993 4.90 5.00

Mesozoic basement

Oh-2 -63.9 -207 -36.6 -34.8 -31.6 -32.3 -30.4 -30.7 -12.6 n.a. 39 0.84 0.61

Ru-228 -65.9 -191 n.a. n.a. n.a. n.a. n.a. n.a. -16.6 0.4 89 6.39 0.81

Ve-4 -64.3 -205 -34.6 -31.7 -31.1 -29.2 -28.0 -27.1 -13.5 -2.2 61 0.42 0.92

Le-13 -62.8 -204 -37.1 -32.4 -30.4 -30.1 -28.5 -27.8 -3.8 n.a. 141 0.19 0.73

Le-65 -59.5 -207 -30.8 -28.1 -28.0 -26.5 -25.8 -25.9 -12.5 n.a. 19 0.56 0.77

Minimum value -65.9 -207 -37.1 -34.8 -31.6 -32.3 -30.4 -30.7 -16.6 -2.2 19 0.19 0.61

Maximum value -59.5 -191 -30.8 -28.1 -28.0 -26.5 -25.8 -25.9 -3.8 0.4 141 6.39 0.92

* – af ter Kotarba and Koltun (2006), ^ – af ter Shabo and Mamchur (1984), n.a. – not analysed CHC = CH4/(C2H6+C3H8); CDMI = [CO2/(CO2+CH4)]100 (%)

dis tri bu tions of the car bon di ox ide-meth ane (CDMI) in dex and of the d13C(CO2) val ues are pre sented in Fig. 6B, C.

The vari a tions ob served in these di a grams in di cate the oc -cur rence of mi cro bial gas gen er a tion through out the whole Mio cene se quence and/or the in flu ence of sec ond ary pro -cesses, mainly CO2 dis so lu tion in wa ter (e.g., Ha³as et al.,

1997; Leœniak & Zawidzki, 2006), that caused iso to pic frac -tion a-tion dur ing mi gra -tion.

Ni tro gen is pro duced dur ing mi cro bial pro cesses and the thermogenic trans for ma tion of or ganic mat ter (Kotarba, 1988; Krooss et al., 1995). For in stance, dur ing coalifica-tion of 1 kg of humic coal with the vol a tile mat ter (VMdaf) con tent be tween 40 and 4%, about 3.5 dm3 of N2 are pro

-duced (Kotarba, 1988). As sapropelic or ganic mat ter is

richer in ni tro gen com po nents, much more mo lec u lar ni tro -gen can be pro duced from it than from the humic mat ter (Maksimov et al., 1982). The pro cess of mo lec u lar ni tro gen pro duc tion from or ganic mat ter was also doc u mented by pyrolytic ex per i ments (Gerling et al., 1997). The d15

N val -ues of mo lec u lar ni tro gen from nat u ral gases range from –15 to 18‰ (Gerling et al., 1997). This iso to pic frac tion ation re sults from both the pri mary ge netic fac tors and the sec ond -ary pro cesses op er at ing dur ing gas mi gra tion through the gas-rock and gas-res er voir flu ids in ter faces (Stahl, 1977; Gerling et al., 1997; Zhu et al., 2000; Ballentine & Sharwood Lollar, 2002; Krooss et al., 2005). Ni tro gen con cen tra tions and d15N(N2) val ues in the ana lysed nat u ral gases vary from

0.34 to 2.83 vol% and from –3.3 to –0,3‰, re spec tively (Ta

-OR I GIN OF GASES IN MIO CENE AND ME SO ZOIC STRATA (WEST ERN UKRAINE)

433

Fig. 4. d13C(C2H6) ver sus (A and C) d13C(CH4) and (B and D) d13C(C3H8) for nat u ral gases ac cu mu lated in (A and B) Mio cene res er -voirs and (C and D) in Up per Ju ras sic, Up per Cre ta ceous (Cenomanian) and Lower Badenian res er -voirs of the Ukrai nian Carpathian Foredeep and its base ment. Po si tion of vitrinite reflectance curve for Type III kerogen af ter Berner and Faber (1996). Curve was shifted based on av er age val ues of (A and B) d13

C = –24.6 ‰ for Mio cene kerogen from Bilche-Volytsia Unit (Kotarba et al., 2011b), and (C and D) d13C val ues = –24.4‰ for Mid dle and Up per Ju ras sic kerogen (Kosakowski et al., in press)

bles 2, 3). Ab sence of cor re la tion be tween d15N(N2) and N2

con cen tra tion (Fig. 8) sug gests that ni tro gen was gen er ated dur ing both the mi cro bial pro cesses and the ther mal trans -for ma tion of or ganic mat ter.

Nat u ral gases from the Up per Ju ras sic, Cenomanian and bot tommost Lower Badenian res er voirs

The gases col lected from both the Up per Ju ras sic and Up per Cre ta ceous (Cenomanian) res er voirs of the Me so zoic

base ment, and from the bot tom most Lower Badenian res er -voirs of the Ukrai nian Carpathian Foredeep show vari a tions in their mo lec u lar and iso to pic com po si tions. Mo lec u lar and iso to pic com po si tions, hy dro car bon (CHC) [CHC = CH4/

(C2H6 + C3H8)], car bon di ox ide-meth ane (CDMI) {CDMI

= [CO2/(CO2 + CH4)] 100 (%)} and iC4H10/nC4H10 gas in

-di ces of ana lysed gases (5 sam ples) vary within the ranges given in Ta bles 2 and 3.

The sta ble car bon and hy dro gen iso tope com po si tions of meth ane in this se quence (Figs 2, 3) in di cate that, sim i lar

Fig. 5. Sta ble car bon iso tope com po si tion of meth ane, eth ane, pro pane, butanes and pentanes ver sus re cip ro cal car bon num bers for nat -u ral gases from (A) the Letnia field and (B) other fields with Lower Sarmatian res er voirs, and (C) gases ac c-u m-u lated in Lower and Up per Badenian res er voirs and (D) in Up per Ju ras sic, Up per Cre ta ceous (Cenomanian) and Lower Badenian res er voirs of the Ukrai nian Carpathian Foredeep and its base ment. Struc ture of the or der for meth ane, eth ane and pro pane af ter Rooney et al. (1995). Av er age val ues of d13_{C = –29.8 ‰ for Or do vi cian and Si lu rian kerogen (Wiêc³aw et al., in press), d}13

C = –24.4 ‰ for Mid dle and Up per Ju ras sic kerogen (Kosakowski et al., in press), and d13C val ues = –24.6 ‰ for Mio cene kerogen from Bilche-Volytsia Unit (Kotarba et al., 2011b)

OR I GIN OF GASES IN MIO CENE AND ME SO ZOIC STRATA (WEST ERN UKRAINE)

435

Fig. 6. (A) Hy dro car bon in dex, (B) car bon di ox ide-meth ane in dex, (C) d13C(CO2), (D) d13C(CH4), (E) d13C(C2H6) and (F) d13

C(C3H8) ver sus depth of nat u ral gas ac cu mu la tions within Mio cene and Me so zoic res er voirs of Ukrai nian Carpathian Foredeep and its base ment

to the Mio cene gases, this meth ane was gen er ated by mi cro -bial car bon di ox ide re duc tion and was oc ca sion ally mixed with lowtem per a ture, thermogenic gases. The sta ble car -bon iso tope com po si tions of eth ane, pro pane, butanes and pentanes (Figs 4, 5) sug gest that eth ane was gen er ated mainly by mi cro bial pro cesses whereas pro pane, butanes and propanes were formed dur ing diagenesis and/or early stage of lowtem per a ture, thermogenic pro cesses. Com par i -son of sta ble car bon iso tope com po si tions of pro pane, butanes and pentanes with kerogen (Fig. 5D) in di cates that the source of these thermogenic gas com po nents is type III kerogen from the Mid dle and Up per Ju ras sic (Kosakowski

et al., in press) and/or from Mio cene strata (Kotarba et al.,

2011b). More over, type II kerogen oc cur ring in the Or do vi -cian–Si lu rian sed i ments (Wiêc³aw et al., in press) obviously is not the source of these gases.

The depths of the sam pled gas ac cu mu la tions in the Up per Ju ras sic and the Up per Cre ta ceous (Cenomanian) res er -voirs of the Me so zoic base ment, and in the bot tom most Lower Badenian res er voirs var ies from 1,293 to 1,915 metres (Ta ble 1, Fig. 6). In sig nif i cant changes in val ues of geo chem i cal hy dro car bon in di ces (Fig. 6A) and sta ble car -bon iso tope ra tios in meth ane (Fig. 6D) with depth sug gest quite uni form gen er a tion con di tions of mi cro bial meth ane, very sim i lar to those in the whole autochthonous Mio cene se quence. In creas ing sta ble car bon iso tope ra tios in eth ane (Fig. 6E) im ply that sig nif i cant amounts of this gas oc cur

be neath 1,000 m depth. More over, the lack of vari a tion in sta ble car bon iso tope com po si tion of pro pane with depth (Fig. 6F) also in di cates sim i lar, diagenetic and/or lowtem per a ture, thermogenic gen er a tion con di tions for this com po -nent. Most prob a bly, the mi cro bial gases gen er ated within the Mio cene strata then mi grated into the Me so zoic res er -voirs. Pos si ble mi gra tion path ways of mi cro bial gases from autochthonous Mio cene strata to Me so zoic res er voirs in the Rudky and the Bilche-Volytsia fields, and in the Opory and the Letnia ones are doc u mented in Figs 9 and 10, re spec -tively. The thermogenic com po nent (Fig. 5) may have been gen er ated within both from the Mio cene kerogen be neath the Carpathian Overthrust and from the Up per and Mid dle Ju ras sic kerogens (Kosakowski et al., in press). The con den sates oc cur ring in the Me so zoic–Lower Badenian res er -voirs, e.g., in the Bilche-Volutsia, Rudky (Fig. 9) and Letnia de pos its (Fig. 10) were also gen er ated mainly in the Up per and Mid dle Ju ras sic and/or Mio cene strata most prob a bly be neath the Outer (Flysch) Carpathian Overthrust and mi -grated from the south (Kotarba et al., 2011a). Nat u ral gas ac com pa ny ing oil (Oh-2 sam ple) in the Up per Ju ras sic– Lower Badenian res er voir of the Orhonovichi de posit con-tains sig nif i cant mi cro bial com po nents (Figs 2 to 5).

The hy dro gen con cen tra tions in the ana lysed nat u ral gases in the Up per Ju ras sic and Up per Cre ta ceous (Cenomanian) res er voirs of the Me so zoic base ment and in the bot -tom most Lower Badenian res er voirs vary from 0.000 to

Fig. 7. d13C(CH4) ver sus d13C(CO2) for nat u ral gases from (A) Mio cene and (B) Up per Ju ras sic, Up per Cre ta ceous (Cenomanian) and Lower Badenian res er voirs of the Ukrai nian Carpathian Foredeep and its base ment. Compositional clas si fi ca tion fields mod i fied from Gutsalo and Plotnikov (1981) and Kotarba (1988)

0.20 vol% (Ta ble 2). As shown be fore, con sid er able quan ti -ties of re cent methanogenic and methylotrophic bac te ria were found in the Mio cene strata (Kotarba et al., 1995). There fore, it seems very prob a ble that hy dro gen was gen er -ated dur ing mi cro bial pro cesses within the Mio cene strata and then it migrated into the Mesozoic reservoirs.

The car bon di ox ide con cen tra tions and the val ues of car bon di ox ide-meth ane (CDMI) in dex in the nat u ral gases from the Up per Ju ras sic and Up per Cre ta ceous (Cenoma-nian) res er voirs of the Me so zoic base ment, and from the bot tom most Lower Badenian res er voirs vary from 0.19 to 0.79 vol% and from 0.01 to 0.49, re spec tively (Ta bles 2, 3). The d13C(CO2) val ues range from –16.6 to –3.8‰ (Ta

-ble 3). The d13C(CH4) ver sus d13C(CO2) (Fig. 7) in di cate

that car bon di ox ide was also gen er ated by mi cro bial pro -cesses. Ver ti cal dis tri bu tions of the car bon di ox ide-meth ane (CDMI) in dex and d13

C(CO2) val ues are pre sented in Fig.

6B and C. Such vari a tions in con cen tra tion and sta ble iso -tope com po si tion of car bon di ox ide with depth in di cate that, most prob a bly, car bon di ox ide has mi grated to gether with mi cro bial meth ane, eth ane and hy dro gen from the

auto-OR I GIN OF GASES IN MIO CENE AND ME SO ZOIC STRATA (WEST ERN UKRAINE)

437

Fig. 8. d15N(N2) ver sus N2 con cen tra tion of nat u ral gases ac cu mu lated in Up per Ju ras sic and Mio cene res er voirs of the Ukrai -nian Carpathian Foredeep and its base ment. Ma tu rity of source rock af ter Gerling et al. (1997)

Fig. 9. Sche matic geo log i cal cross-sec tion through the Rudky and Bilche-Volytsia fields (mod i fied af ter Shcherba et al., 1987) with mo lec u lar and iso to pic ra tios for gas (Ta ble 3). For lo ca tion, see Fig ure 1. S.-C. – Sandy-Cal car e ous, Fm. – For ma tion, fms – formations

chthonous Mio cene strata and that it was in flu enced dur ing mi gra tion by sec ond ary pro cesses, mainly CO2 dis so lu tion

in wa ter.

The ni tro gen con cen tra tions in the nat u ral gases vary from 1.98 to 6.60 vol% (4 sam ples) and 96.9 vol% in Ru-228 sam ples. The d15

N(N2) val ues are –2.2 and 0.4‰

(Ta bles 2, 3). Ab sence of cor re la tions be tween d15

N(N2)

and N2 con cen tra tion (Fig. 8) sug gests that ni tro gen was

gen er ated dur ing both mi cro bial pro cesses and ther mal transformation of or ganic mat ter.

The no ta bly high N2 con tent in the Ru-228 sam ple

(96.9 vol%, Ta ble 2) is prob a bly re lated with the sec ond ary re cov ery meth ods, when the at mo spheric air was in jected into the Rudky field. The d15

N(N2) mea sured in this gas

sam ple was 0.4 ‰ (Ta ble 3), which is in dic a tive of at mo -spheric ni tro gen. Ox y gen as very re ac tive agent re acted quickly. This gas sam ple also con tains a sig nif i cant per cent -age of H2 (0.20 vol%) in re la tion to the other gas sam ples

(Ta ble 2). This may be at trib uted to se quen tial, sec ond ary mi cro bial ox i da tion and fer men ta tion in the res er voir by mi crobes that were in jected with the chem i cals in sep a rate re -cov ery pro cess than at mo spheric air in jec tion. A sim i lar phe nom e non was ob served in the Wañkowa field, in the Pol ish Outer Carpathians (Kotarba et al., 2009). How ever, it can not be ex cluded that at mo spheric gases have been in -tro duced into the source re gions or into the pas sage of gases to the sur face, sim i lar as in nat u ral gases in the Up per Ju ras -sic res er voir in Tarnów de posit in the Pol ish Carpathian Foredeep (Kotarba & Nagao, 2008).

CON CLU SIONS

The meth ane con cen tra tions in nat u ral gases ac cu mu -lated in the Lower and Up per Badenian and Lower Sarma-tian res er voirs of the Bilche-Volytsia Unit of the Ukrai nian Carpathian Foredeep (be tween the Pol ish-Ukrai nian state bor der and Stryi) usu ally ex ceed 96 vol%. Meth ane was

gen er ated by mi cro bial re duc tion of car bon di ox ide in the ma rine en vi ron ment. Pre sum ably meth ane and eth ane were pro duced mainly dur ing the sed i men ta tion of Mio cene clays and muds. It is pos si ble that this microbial process conti-nues today.

Both the gen er a tion and the ac cu mu la tion of mi cro bial meth ane and eth ane as well as the for ma tion and the load ing of mul ti ple-stacked Mio cene res er voirs of the Ukrai nian Carpathian Foredeep were fa cil i tated by rhyth mic and cy -clic de po si tion of clays, muds and sands at very high sedi-mentation rates.

The higher light hy dro car bons (mainly pro pane, buta-nes and pentabuta-nes) were gen er ated dur ing diagebuta-nesis and the ini tial stage of the low-tem per a ture, thermogenic pro cesses from type III kerogen ac cu mu lated in the Mio cene strata of the Foredeep and/or in the Mid dle and Up per Ju ras sic base ment sed i ments. Ab sence of vari a tions in the val ues of geo -chem i cal hy dro car bon in di ces and sta ble iso tope ra tios of meth ane, eth ane and pro pane with depth in di cate sim i lar gas gen er a tion con di tions within the whole Miocene succe-ssion.

Meth ane and in part eth ane were gen er ated by mi cro -bial pro cesses within the Mio cene strata then mi grated to the Up per Ju ras sic and Up per Cre ta ceous (Cenomanian) res er voirs in the Me so zoic base ment and to the bot tom most Lower Badenian res er voirs of the ana lysed Letnia, Orkho-vychi, Rudky and Vereshchytsia fields.

Hy dro gen (con cen tra tions to 0.20 vol%) en coun tered within the Mio cene strata as well as in the Up per Ju ras sic and the Up per Cre ta ceous (Cenomanian) res er voirs of the Me so zoic base ment, and also within the bot tom most Lower Badenian res er voirs are also at trib uted to mi cro bial pro cesses. Car bon di ox ide and ni tro gen, which are mi nor com -po nents, were gen er ated in both mi cro bial and the low-tem-perature, thermogenic pro cesses. How ever, at least part of the ni tro gen ac cu mu lated in the no ta bly high-ni tro gen Rudky field (96.9 vol% N2) is of at mo spheric or i gin injected to the

res er voir dur ing the sec ond ary re cov ery op er a tions.

Fig. 10. Sche matic geo log i cal cross-sec tion through the Letnia and Opory fields (mod i fied af ter Vul et al., 1998) with mo lec u lar and iso to pic ra tios for gas (Ta ble 3). For lo ca tion, see Fig ure 1. Key for lithostratigraphy and geo chem i cal data – see Fig ure 9

Ac knowl edge ments

The re search was un der taken as the Pro ject No. UKRAINE/ 193/2006 of the Min is try of Sci ence and Higher Ed u ca tion and car ried out at the AGH Uni ver sity of Sci ence and Tech nol ogy in Kraków and at the Pol ish Geo log i cal In sti tute – Na tional Re search Institute in War saw. Sci en tific stud ies were fi nanced in the years 2007–2010. The de tailed com ments of Bernd Krooss and Eckhard Faber were of great as sis tance in the re vi sions of this manu script. We also thank Paul Lillis of the U.S. Geo log i cal Sur vey in Den ver for im prove ment of Eng lish manu script. An a lyt i cal work by Zofia Stecko and Tomasz Kowalski from the AGH Uni ver sity of Sci -ence and Tech nol ogy in Kraków is grate fully ac knowl edged, as well.

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