mi cro bial gas sys tem and prospectives of hy dro...

Annales Societatis Geologorum Poloniae (2011), vol. 81: 523–548.

MI CRO BIAL GAS SYS TEM AND PROSPECTIVESOF HY DRO CAR BON EX PLO RA TION IN MIO CENE STRATA

OF THE POL ISH AND UKRAI NIAN CARPATHIAN FOREDEEP

Maciej J. KOTARBA1, Tadeusz M. PERYT2 & Yuriy V. KOLTUN3

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: [email protected]

2 Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, Rakowiecka 4, 00-975 War saw, Po land,e-mail: [email protected]

3 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,Naukova 3a, 79060 Lviv, Ukraine

Kotarba, M. J., Peryt, T. M. & Koltun, Y. V., 2011. Mi cro bial gas sys tem and prospectives of hy dro car bonex plo ra tion in Mio cene strata of the Pol ish and Ukrai nian Carpathian Foredeep. Annales Societatis GeologorumPoloniae, 81: 523–548.

Ab stract: Mo lec u lar com po si tion of nat u ral gases ac cu mu lated in autochthonous Mio cene strata of the Pol ish andUkrai nian Carpathian Foredeep is dom i nated by meth ane, which usu ally con sti tutes over 98 vol%. Meth ane wasgen er ated by the car bon di ox ide re duc tion path way of mi cro bial pro cesses. Eth ane was gen er ated both dur ing mi -cro bial and thermogenic pro cesses (“oil win dow”) and pro pane at the ini tial stage of the low-tem per a ture thermo-genic pro cesses, and also by the mi cro bial pro cesses. The rhyth mic and cy clic de po si tion of Mio cene clays andsands as well as the vig or ous gen er a tion of mi cro bial meth ane caused that the gas pro duced in claystone beds wasac cu mu lated in the over lay ing sand stones, and capped, in turn, by the suc ceed ing claystones. Such gen er a tion andac cu mu la tion sys tem of mi cro bial gases gave rise to the for ma tion of multi-hor i zon tal gas fields. Anal y sis of thedis tri bu tion of im ma ture humic dis persed or ganic mat ter in the Up per Badenian and Lower Sarmatian se quencesin di cates that it is prac ti cally ho mo ge neous. A mi gra tion range of mi cro bial gases was in sig nif i cant and lo ca tionsof their ac cu mu la tions would de pend only on the ex is tence of proper type of traps (compactional anticlinessit u ated above base ment up lifts, sealed by the Carpathian Overthrust and/or by faults; strati graphic pinch ing outand strati graphic traps re lated to un con formi ties). An other sit u a tion is en coun tered in the south, be neath theCarpathian Overthrust. The thick ness of the autochthonous Mio cene strata in this area is more than 1,500 metres.Geo chem i cal stud ies re veal that from a depth of 2,500 metres starts the pro cess of low-tem per a ture thermogenichy dro car bon gen er a tion (“oil win dow”). At greater depths, more than 7,500 metres, within the autochthonousLower Mio cene ba sin only the high-tem per a ture meth ane (“gas win dow”) could be pro duced and ac cu mu lated.

Key words: mi cro bial gas, thermogenic gas, source rocks, hy dro car bon po ten tial, or ganic geo chem is try, prospec-tives of hy dro car bon ex plo ra tion, autochthonous Mio cene strata, Bilche-Volytsia Unit, Carpathian Foredeep, SWPo land, west ern Ukraine.

Manu script received 19 May 2011, accepted 13 October 2011

IN TRO DUC TION

The main ob jec tive of our study is to pres ent the hy dro -car bon po ten tial and geo chem i cal char ac ter is tics of dis -persed or ganic mat ter, or i gin of nat u ral gases, model of mi -cro bial gas gen er a tion and prospectives of hy dro car bon ex -plo ra tion for the autochthonous Mio cene strata of the Pol ish (Fig. 1A) and Ukrai nian Carpathian Foredeep (Fig. 1B) be -tween Kraków and Stryi.

Pre vi ous geo chem i cal char ac ter is tics of dis persed or -ganic mat ter were de scribed by Kotarba et al. (1998b, 2005) and pre lim i nary model of mi cro bial and thermogenic gasgen er a tion by Kotarba et al. (1998a). Mo lec u lar and iso to -

pic stud ies of nat u ral gases ac cu mu lated within the auto-chthonous Mio cene strata of the Pol ish and Ukrai nian Car-pathian Foredeep were pre sented by G³ogoczowski (1976),Calikowski (1983), Jawor and Kotarba (1993), Kotarba(1992, 1998, 2011), Kotarba and Jawor (1993), Kotarba and Koltun (2006, 2011), Kotarba et al. (1987, 2005) and Shabo and Mamchur (1984).

GEO LOG I CAL SET TING

The Carpathian Foredeep de vel oped dur ing the earlyand mid dle Mio cene as a pe riph eral flex ural fore land ba sin

524 M. J. KOTARBA ET AL.

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in front of the ad vanc ing Carpathian Overthrust (Oszczypko et al., 2006). The highly com plex epi-Variscan plat form and its Perm ian–Me so zoic cover form the base ment of the Car-pathian Foredeep (Karnkowski, 1974; Danysh et al., 2004;Oszczypko et al., 2006; Bu³a & Habryn, 2011). The epi-Variscan plat form de vel oped on con sol i dated Pro tero zoiccrys tal line rocks and Pro tero zoic–Lower Palaeozoic meta-sed i men tary rocks. In the area ad ja cent to the Pol ish-Ukrai -nian bor der, the Carpathian Foredeep is su per im posed onthe NW–SE trending Teisseyre–Tornquist Zone, a ma jorcrustal bound ary be tween the West- and the East-Eu ro peanCratons (Oszczypko et al., 2006). This com plex struc turecon trolled the sub se quent evo lu tion, and in par tic u lar in thePol ish-Ukrai nian bor der area where dur ing fi nal, Bade-nian–Sarmatian de vel op ment, the in her ited Me so zoic rift-re lated tec tonic zone was re ac ti vated due to the com binedef fect of thrust wedge em place ment and slab-pull of fore -land plate; this zone fo cused Mio cene ex ten sion re lated tothe flex ure of the Carpathian fore land plate (Krzywiec,1999).

The Carpathian Foredeep ba sin can be sub di vided,based on the tec tonic style and type of sed i ments, into twoparts: in ner and outer. The in ner foredeep is lo cated at thefront and be neath the Carpathian Overthrust and is char ac -ter ized by strongly folded Eggenburgian–Badenian strata:Sambir (Stebnik) Nappe and Zg³obice thrust-sheets in Po -land and Boryslav-Pokuttya and Sambir Nappes in theUkraine (Andreyeva-Grigorovich et al., 1997, 2008); itcon tains up to 1,500 m of Lower to Mid dle Mio ceneautochthonous de pos its that are over rid den by thrust sheets(Oszczypko, 2006a, b; Oszczypko et al., 2006). The outerforedeep (termed the Bilche-Volytsia zone in the Ukraine)is filled with gen er ally un dis turbed, flat-ly ing Mid dle Mio -cene (Badenian and Sarmatian) ma rine de pos its, whichrange in thick ness from a few hun dred metres in the north -ern, mar ginal parts of the Foredeep to as much as 3.5 km inSE Po land (Wielkie Oczy graben – Oszczypko et al., 2006)and 5 km in west ern Ukraine (Krukenychy De pres sion –Kurovets et al., 2004). Gen eral struc ture of the Pol ish(Stebnik Unit and the Outer Carpathian Foredeep ba sin) and Ukrai nian (Boryslav-Pokuttya, Sambir and Bilche-Volytsiaunits) parts of the Carpathian Foredeep is pre sented in Fig.2A and Fig. 2B, re spec tively.

The Lower Mio cene de pos its of the Carpathian Fore-deep formed a clastic wedge along the Carpathians, com pa -ra ble with the Lower Fresh wa ter Molasse of the Al pineFore land (Oszczypko et al., 2006); they formed from rel a -tively shal low sub aque ous turbidite cur rents as well as from subaerial de bris flows. The early Badenian re gional trans -gres sion (Oszczypko, 1998) re sulted in vari a tions in thepalaeobathymetry of the early Badenian ba sin be tween up -per bathyal depths in the axis of the ba sin to neritic and lit to -ral ones in the mar ginal parts. The deep est part of the ba sin,which reached midbathyal depths, was lo cated about 20 kmsouth of the pres ent-day po si tion of the Carpathian front and it was dom i nated by de po si tion of dark-grey cal car e ousmudstones with spo radic in ter ca la tions of fine sand stones(Skawina, Balych and Bohorodchany for ma tions; Osz-czypko et al., 2006). Some de pos its re vealed basinal tur-bidite char ac ter is tics, and the south ern slope and its shelf

with small embayments are dom i nated by cal car e ous mud-stones (Oszczypko et al., 2006).

There were three pe ri ods of in tense fore land sub si -dence, namely dur ing the early Mio cene, early Badenian,and late Badenian to Sarmatian times. The Mio cene con ver -gence of the Carpathian wedge with the fore land causedout ward mi gra tion of the depocentre of the foredeep and inits dis tal parts onlapping of suc ces sively youn ger de pos itsonto the fore land (Oszczypko, 2006a). Sub se quent ero sionwas cal cu lated, based on illite/smectite diagenesis, at some400–800 m in SE Po land (Dudek, 1999).

The dis cussed area of the Carpathian Foredeep, sub di -vided by the Rzeszów Palaeoridge into the west ern andeast ern parts (Oszczypko, 1999), con tains a re duced sec tionof the lower and mid dle parts of the Badenian sec tion.

In the west ern part of the Carpathian Foredeep, themost wide spread Lower Badenian unit is the Skawina For -ma tion – deep-wa ter, clayey-marly and clayey-muddy de -pos its with in ter ca la tions of sands and grav els that in creasein abun dance to wards the south (Garlicki, 1979); the thick -ness of the Skawina For ma tion ranges from sev eral tens ofmetres in the north ern part of the foredeep to 800 m in theGdów Bay (Ney et al., 1974; Garlicki, 1979). It is fol lowedby the Wieliczka For ma tion (30–100 m thick) com posed ofchlo rides with siliciclastic in ter ca la tions ac com pa nied bytuffites and bentonites. The Wieliczka For ma tion oc cursalong the Carpathian Overthrust and to wards the north ispassing into the Krzy¿anowice For ma tion, mainly com -posed of lam i nated an hyd rites 10–30 m thick (Peryt, 2000).The evaporites are over lain by the de pos its of the MachówFor ma tion – mostly marly clays with laminae and in ter ca la -tions of fine-grained sand stones. The thick ness of this for -ma tion, that in cludes the Up per Badenian and the Sarma-tian, may reach 2000 m be tween Tarnów and D¹browa Tar-nowska (Ney et al., 1974; Krzywiec et al., 2008).

In the area of the Rzeszów Palaeoridge, the sub strate iscov ered by the Up per Badenian Grabowiec beds (60–300 m thick) – clayey-muddy de pos its with tuffites and bentonitesand lo cal sandy in ter ca la tions, fol lowed by a con sid er ablythicker (1,000–2,000 m) com plex of the Krakowiec bedshav ing a sim i lar li thol ogy but in cluded into the Sarmatian.South of the Rzeszów Palaeoridge, also older Badenianstrata oc cur be low the Grabowiec beds: Chodenice beds (up to 50 m thick), evaporites of the Krzy¿anowice and Wie-liczka for ma tions (up to 30 m thick), and Skawina For ma -tion (0– 150 m thick) (Kotlarczyk, 1991).

East of the Rzeszów Palaeoridge, the Badenian sec tionbe gins with the Piñczów For ma tion in Po land and theZhuriv For ma tion in the Ukraine (both for ma tions in theolder lit er a ture were also termed the Baranów beds) –quartz sand stones, sandy clays and clays, with com monbasal con glom er ates. The thick ness of the Baranów bedsvar ies usu ally from sev eral to 90 m, and in creases to wardsthe north and the east. They are over lain by lam i nated andbrecciated an hyd rites of the Krzy¿anowice For ma tion (inPo land) and Tyras For ma tion (in the Ukraine), a few to over 40 metres thick, and these in turn by the clayey-marly andsandy de pos its of the Machów For ma tion in Po land. Itslower mem ber is the Spirialis Clays Mem ber (100 m thick),pass ing to ward the mar gin of the ba sin into the Pecten Clays

MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 525


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and Marls Mem ber (Ney, 1969). The up per, Sarmatian partof the Machów For ma tion is com posed of the KrakovetsClays (Ney et al., 1974) that in the north ern part of the fore-deep are de vel oped as the Syndesmia Clays Mem ber (Ney,1969). The thick ness of the beds ranges from 200 m nearBi³goraj to 2,400 m in the Przemyœl re gion. The se ries con -sists mostly of clays with sandy laminae and to wards thesouth the fre quency and thick ness of the sandy oc cur rencesin crease: they were in ter preted as wide spread delta de pos its(Jawor, 1983; Karnkowski, 1989). In the Ukraine, the Up -per Badenian strata are in cluded into the Kosiv For ma tion(10–150 m thick) and the Sarmatian strata into the DashavaFor ma tion that is fur ther sub di vided into two parts, thelower part (up to 3,500 m thick) and the up per part (up to1,500 m thick) (see for de tails Kurovets et al., 2004). Themar ginal fa cies of the Dashava For ma tion con tains in ter ca -la tions of lime stones within mostly clayey se ries; it is up to120 m thick and is termed the Volhynian beds.

In some well pro files of the Bilche-Volytsia Unit, var i -ous siliciclastic, 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 thegeo log i cal un pub lished documentations as well as in thesyn the ses based on those documentations (e.g., Vul et al.,1998a; Krups’kyy, 2001) to be of Karpatian (and/or Pala-eo gene) age. These de pos its are re cently in cluded into theSandy-Cal car e ous Se ries and its Karpatian age is con cluded (Vashchenko et al., 2007), but the fos sils quoted as sup port -ers of such an age as sign ment have long ranges and in di cateonly their Mio cene prov e nance. On the other hand, the re -gional strati graphical cor re la tions and palaeogeographic re -con struc tions do not sup port the pres ence of Karpatianstrata in the Bilche-Volytsia Unit (e.g., Andreyeva-Grigo-rovich et al., 1997; Oszczypko et al., 2006) and hence it ispos si ble that the de pos its in cluded into the Sandy-Cal car e -ous Se ries are transgressive Lower Badenian de pos its thatshould be un der lain by, or in cluded into, the Bohorodchany For ma tion (Suite) (Fig. 3).

PE TRO LEUM OC CUR RENCE

The fron tal part of the Pol ish and Ukrai nian Carpa-thians con sti tutes one of the old est pe tro leum-pro duc ing re -gions in the world. The ex ploi ta tion of oil started in 1853 inBóbrka vil lage near Krosno in Po land (in Austro-Hun gar ian Mon ar chy at that time) in the Outer (Flysch) Carpathians,and nat u ral gas pro duc tion in 1920 in Dashava vil lage nearBoryslav in Ukraine (in Po land at that time) in the auto-chthonous Mio cene strata of the Bilche-Volytsia Unit of the Carpathian Foredeep (Vul et al., 1998b; Karnkowski, 1999;Fedyshyn et al., 2001). At the end of the 1930s, pe tro leumex plo ra tion started in the Carpathian Fore land, and in theyears 1945–1955 pe tro leum in dus try in Po land and Ukraine had to be re stored and re or gan ised. The ex plo ra tion ac tiv itywas fo cused mainly on the Outer (Flysch) Carpathians andpartly on the Carpathian Foredeep (Karnkowski, 1994,1999). In ten sive de vel op ment of pe tro leum ex plo ra tiontook place in the mid dle of the 1960s and in the 1970s(Karnkowski, 1999; Fedyshyn et al., 2001; Myœliwiec et al., 2006; Popadyuk et al., 2006). Up to now, twenty-eight andeleven gas and gas-con den sate fields within the Palaeo-zoic–Me so zoic base ment of the Pol ish part of the Carpa-thian Foredeep (from 1948), and within the Me so zoic base -ment of the Ukrai nian part of the Carpathian Foredeep(since 1944), re spec tively, were dis cov ered. Hy dro car bon(mostly gas, and gas-con den sate, and only lo cally oil) ac cu -mu la tions oc cur in the autochthonous Mio cene strata (e.g.,Karnkowski, 1999; Boyko et al., 2004; Fedyshyn et al.,2001; Kurovets et al., 2011) of the outer part of the Carpa-thian Foredeep. Gas ac cu mu la tions oc cur ring in the Mid dleMio cene strata show a clear pat tern, and there are three zo-nes that ac cord ingly ex tend close to the mar gin of the Car-pathian Overthrust, at the front of Carpathian orogen, andalong the north-east ern limit of the outer Carpathian Fore-deep (e.g., Karnkowski, 1999; Krups’kyy, 2001); the larg -est gas de pos its oc cur in the Lower Sarmatian strata close to


Fig. 3. Dis tri bu tion of gas, gas-con den sate and oil de pos its of the lithostratigraphic for ma tions of the Ukrai nian Carpathian Foredeep.L. – Lower, U. – Up per

the Carpathian Overthrust (Karnkowski, 1999; Fedyshyn etal., 2001).

There are 101 gas and gas-con den sate de pos its (firstdis cov ery in 1945) in the autochthonous Mio cene strata ofthe Pol ish Carpathian Foredeep as well as 44 gas and gas-con den sate de pos its and one oil de posit in the Bilche-Volytsia Unit of the Ukrai nian Carpathian Foredeep (firstdis cov ery in 1920). Al though the share of de pos its dis cov -ered in the pre-Mio cene strata has in creased (Kotarba et al.,2011a), the Mio cene de pos its re main the main host for gasde pos its, and their sig nif i cance is re lated to a low de gree oftec tonic dis tur bance and the pres ence of un re cog nised, broadstruc tures with per fect res er voir ho ri zons (Karnkowski,1999). The for ma tion of var i ous traps was sig nif i cantly con -trolled by di ver si fied base ment palaeorelief and vari able sed -i men tary con di tions, which were cru cial for the for ma tion ofvar i ous hy dro car bon traps, such as those related to pinch-outs and fa cies changes. Prin ci pally, lay ered and mas sive types ofde pos its oc cur there (Karnkowski, 1999).

Pol ish part of the Carpathian Foredeep

Pe tro leum de pos its in the Pol ish sec tor of the Carpa-thian Foredeep are ac cu mu lated in both Neo gene com plex(Fig. 1A) and in its base ment (Kotarba et al., 2011b).

The pe tro leum res er voir rocks in the autochthonousMio cene strata of the outer part of the Pol ish CarpathianForedeep are sand stones orig i nated in var i ous depositionalenvironments (Myœliwiec et al., 2006): (i) Lower Badenianglauconitic sand stones – the Baranów beds (Kury³ówka,Rokietnica and Sarzyna gas de pos its), (ii) Mid dle Badenianevaporitic de pos its (RoŸwienica gas ac cu mu la tion), (iii)Up per Badenian–Lower Sarmatian – sand stones de pos itedin sub ma rine fan, deltaic and shal low ma rine en vi ron ments(e.g., Przemyœl, Jod³ówka, Albigowa-Krasne, Husów, ¯o-³ynia, Rudka, Biszcza, Wola Obszañska, Ksiê¿pol, £ukowa, Dzików, Cha³upki Dêbniañskie, Grodzisko Dolne, £azy,£êtowice, Wierzchos³awice, Borek, D¹brówka gas de pos -its), and (iv) Lower Sarmatian – mas sive sand stones (blocky sand stones of the Dzików gas de posit). More over, Eoceneand Oligocene de pos its were dis cov ered in the north-east -ern part of the Carpathian Foredeep. A gas ac cu mu la tion inOli- gocene sand stones was dis cov ered in Luchów 3 well(My- œliwiec et al., 2006). Oligocene sand stones have verygood res er voirs pa ram e ters, with av er age po ros ity of 26%and average per me abil ity equals to 1250 mD.

The sand stones de pos ited in the sub ma rine fan anddeltaic en vi ron ments are the most im por tant and pro duc tiveUp per Badenian–Lower Sarmatian res er voir rocks. Thesubma rine fan sand stone com plex oc curs at a depth in ter valof 1,000–3,500 m in the south ern part of the CarpathianForedeep – an area in front of and un der the CarpathianOverthrust (Myœliwiec et al., 2006). The thick beds of thesub ma rine fan sand stones are the pri mary gas res er voirs inthe Przemyœl, Jod³ówka, Husów, Albigowa-Krasne ar eas(Myœliwiec et al., 2006). The av er age po ros ity of the sand -stone res er voirs, de ter mined from cores, var ies from 9 to21% for the Przemyœl gas field (Karnkowski, 1999). A sim i -lar po ros ity value was eval u ated for the Jod³ówka, Albi-gowa-Krasne, and Husów res er voirs. The per me abil ity is

more di ver gent. For the Jod³ówka area, the max i mum valueis only 200 mD, but in the Husów area it can be up to 800mD (Karnkowski, 1999). Ex am ples of res er voirs con nectedwith deltaic sed i men tary en vi ron ments are the Biszcza,Wola Obszañska, Ksiê¿pol, £azy, £êkawica gas de pos its.The deltaic type of sed i ments oc curs at a depth of 400–800 m in the Biszcza area. Their po ros ity is 15–32% andpermeability 900 mD (Myœliwiec, 2004a). Glauconiticsandstones of characteristic green col our are good qual ityreservoir rocks with po ros ity rang ing from 5 to 25% (Myœli- wiec et al., 2006). The sec ond ary po ros ity of the Mid dleBadenian diage netic an hyd rites in the RoŸwienica gas ac cu -mu la tion ranges be tween 4.3 and 16.1%; the in crease in po -ros ity and per me abil ity was caused by the in ter ac tion be -tween an hyd rites and hy dro car bons (Myœliwiec, 2004a;Myœliwiec et al., 2006). The so-called Dzików – blockysand stones res er voir rocks are lo cated along the fault zoneof the Uszkowce-Lubaczów area. The blocky sand stonesare ex cel lent res er voir rocks, with a po ros ity of 15–35% and per me abil ity of 200 mD (Myœliwiec et al., 2006).

The oc cur rence of traps for gas ac cu mu la tions in theMio cene was de ter mined by two main fac tors: palaeomor-phology of the pre-Mio cene base ment and de for ma tion in -flu ence of the Carpathian Overthrust (Karnkowski, 1999;Myœliwiec, 2004b). The ma jor type of traps in the Mio ceneof the Carpathian Foredeep are: (i) compactional anticlinessit u ated above base ment up lift (e.g., Rudka, Palikówka,Kañczuga de pos its), (ii) traps sealed by the CarpathianOverthrust plane (e.g., Przemyœl, Albigowa-Krasne, Husów and Pilzno fields), (iii) traps sealed by faults (Mo³odycz,Jaros³aw, Radymno and Sarzyna de pos its), (iv) stratigra-phic pinch-out traps (e.g., the Husów field), and (v) strati -graphic traps re lated to un con formi ties (the Tarnów areaand the deep est ho ri zons of the Rudka de posit). In somecases, traps in Mio cene strata may be formed by com binedmech a nism of seal ing (e.g., Husów, Przemyœl, and Rudkagas de pos its).

Ukrai nian part of the Carpathian Foredeep

Gas, gas-con den sate and oil de pos its (Fig. 1B) havebeen dis cov ered within the autochthonous Mio cene strata,com bin ing Mio cene and Me so zoic base ment res er voirs inthe Bilche-Volytsia Unit of the Ukrai nian Carpathian Fore-deep (Vul et al., 1998b; Shcherba et al., 1987).

Seven fields con tain con den sate and/or gas in the com-bined Me so zoic–Mio cene res er voirs (Figs 1B, 3): the Rudkyfield – gas and con den sate in the Up per Ju ras sic-res er voirand five gas ac cu mu la tions in the Lower Dashava For ma -tion of the Lower Sarmatian, the Vereschytsia field – gas inthe Up per Ju ras sic–Lower Badenian Sandy-Cal car e ous Se -ries, the Medenychi field – gas and con den sate in the Up perCre ta ceous–Lower Badenian Sandy-Cal car e ous Se ries, theBilche-Volytsia field – gas and con den sate in the Up perCre ta ceous–Lower Badenian Sandy-Cal car e ous Se ries andfour gas ac cu mu la tions in the Lower Dashava For- mationof the Lower Sarmatian, the Uhers’ko – gas and con den satein the Up per Cre ta ceous–Lower Badenian Sandy- Cal car e -ous Se ries an and five gas ac cu mu la tions in the Up per andLower Dashava For ma tion of the Lower Sarmatian, the


Pivdenne Uhers’ko – gas in the Up per Cre ta ceous– LowerBadenian Sandy-Cal car e ous Se ries and four gas ac cu mu la -tions in the Lower Dashava For ma tion of the Lower Sar-matian, and the Letnia field – gas and con den sate in the Up -per Ju ras sic–Lower Badenian Sandy-Cal car e ous Se ries,Up per Cre ta ceous–Lower Badenian Sandy-Cal car e ous Se -ries and nine gas ac cu mu la tions in the Lower Dashava For -ma tion of the Lower Sarmatian strata. The Orkhovychideposit con tains oil in Up per Ju ras sic–Lower BadenianSandy-Cal car e ous Se ries res er voir (Kotarba et al., 2011b).

The re main ing thirty-seven de pos its con tain gas ac cu -mu la tions in Mio cene res er voirs (Figs 1, 3). The Mala Ho-rozhanna field con tains gas in the Lower Badenian Sandy-Cal car e ous Se ries and Up per Badenian Tyras For ma tion.The Hai field con tains gas and con den sate in the Lower Ba-denian Sandy-Cal car e ous Se ries and in two of the LowerDashava For ma tion of Lower Sarmatian ho ri zons and gas in one Lower Sarmatian ho ri zon. The Zaluzhany field con tains gas and con den sate in one and gas in twelve ac cu mu la tionsin the Up per and Lower Dashava For ma tion. The Hrushivfield con tains gas and con den sate in one and gas in two ac -cu mu la tions in the Lower Dashava For ma tion. The Hru-divka field con tains gas in the Lower Badenian Sandy-Cal -car e ous Se ries and in three of the Lower Dashava For ma -tion, and the Hlynky and Pivdenne Hrabyne fields – in theLower Badenian Sandy-Cal car e ous Se ries res er voirs. TheSvydnytsia field con tains gas in the Up per Dashava For ma -tion of the Lower Sarmatian, and the Vyzhomlia, Nyklo-vychi, Makuniv, Novosilky, Mainych, Susoliv, ShidneDovhe, Opory, Kavske, and Tyniv fields – in the Lower Da- shava For ma tion; the Vyshnia, Khidnovychi, Sadkovychi,Pyniany, Dashava, and Horodok fields con tain gas in theUp per and Lower Dashava For ma tion; the Kadobna, Kosiv, and Chornohuzy fields – in the Up per Badenian and LowerSarmatian; the Turady, Hrynivka, Bohorodchany, Debesla-vtsi, Yabluniv, Kovalivka, Sheremetivka, and Krasnoils’kfields – in the Up per Badenian; and the Cheremkhiv-Strup-kiv and Pylypy fields – in the Lower Badenian res er voirs.They oc cur along the whole Bilche-Volytsia Unit, though ama jor part of them is con cen trated in its north-west ern part(Fig. 1). In these de pos its the traps are lithologically andtecton i cally-sealed. In par tic u lar, of ten the Stebnyk Over-thrust serves as a seal, e.g., in the Uhers’ko, Zaluzhany,Khidnovychi, Sadkovychi, Kavs’ke, and Hrynivka fields.How ever, in ad di tion to this, the ma jor part of the gas fieldsof the Bilche-Volytsia Unit oc cur in the el e vated struc tures,e.g., Svydnytsia, Rudky, Letnia, Bilche-Volytsia, Uher-s’ko, Dashava and other fields.

Com monly, there are sev eral pro duc ing ho ri zons in afield: in the Bilche-Volytsia Unit 175 such ho ri zons ex ist in40 fields, and 165 in the Up per Badenian and Lower Sarma- tian strata (Boyko et al., 2004).

The Mio cene se quence con tains the ma jor part of fieldsin the area and com prises sev eral lev els of res er voir rocks.The Lower Badenian Sandy-Cal car e ous Se ries, oc cur ring at the base of the Mio cene, are gas- and con den sate-bear ing insev eral fields in the north-west ern part of the Bilche-Voly-tsia Unit. The po ros ity of these rocks ranges be tween 6 and31% (mean 13%), per me abil ity is usu ally less than 0.1 mD,but lo cally reaches 270 mD (Shcherba et al., 1987). The

traps in the Lower Badenian Sandy-Cal car e ous Se ries andBaraniv beds are sealed by the Up per Badenian Tyras For ma -tion de pos its. The Badenian strata are gas-bear ing mainly inthe cen tral and south-east ern parts, while the Lower Sar-matian strata con tain the ma jor part of gas fields of the north-west ern part of the Bilche-Volytsia Unit. The res er voir rocksrep re sent sand stone lay ers with po ros ity of 20–30%, lo cally30–40% and per me abil ity from 0.1 mD to 1,595 mD. TheBadenian res er voirs are of ten lithologically sealed within theBadenian se quence, while the Lower Sarmatian ones forman ti cli nal struc tures, sealed from the top by the Lower Sarma- tian clayey rocks, and are of ten fault-sealed lat er ally.

HY DRO CAR BON PO TEN TIALOF OR GANIC MAT TER IN THE POL ISH

AND UKRAI NIAN PARTS OF THE OUTER CARPATHIAN FOREDEEP

The re sults of or ganic geo chem is try anal y ses en able the pre lim i nary as sess ment of the dis persed or ganic mat ter(DOM) con tained in autochthonous Mio cene se quence ofthe Pol ish and Ukrai nian parts of the Outer CarpathianForedeep (Kotarba, 1999; Kotarba & Koltun, 2006; Kotarba et al., 1998b, 2005, 2011a). The study was mainly fo cusedon rec og ni tion of hy dro car bon po ten tial of Up per Badenianand Lower Sarmatian strata due to the con sid er able thick -nesses of those lithostratigraphic units and the en closedgreat claystone-mudstone potential source rocks.

Small thick nesses of the Lower Badenian strata are therea son for poorer geo chem i cal rec og ni tion. In the Lower Ba-denian Sandy-Cal car e ous Se ries, the Lower Badenian Bara-niv beds and the Up per Badenian Tyras For ma tion of theUkrainian Outer Carpathian Foredeep (Bilche-VolytsiaUnit), the to tal or ganic car bon (TOC) con tents are low andrange from 0.00 to 0.76 wt% (av er age 0.28 wt%) (Kotarba& Koltun, 2006, 2011). This Mio cene se quence con sists ofim ma ture, ter res trial, gas-prone and ma rine, oil-prone or -ganic mat ter, which has gen er ated mainly the mi cro bial me-thane and small amounts of mi cro bial eth ane. In the LowerBadenian Baraniv beds and in the Tyras For ma tion, an in put of ma rine or ganic mat ter was dis cov ered. More over, in thevi cin ity of Kokhanivka, the rocks of the Tyras For ma tionwere de pos ited un der hypersaline con di tions, as re vealed by the pres ence of ma rine plank ton (Kotarba & Koltun, 2011).

In the Up per Badenian strata, the to tal or ganic car bon(TOC) con tents vary from 0.02 to 1.48 wt% (av er age 0.75wt%) in the Pol ish Carpathian Foredeep (Kotarba, 1999;Kotarba et al., 1998b, 2005) and from 0.44 to 2.01 wt%(average 0.96 wt%) in the Ukrai nian Carpathian Foredeep(Kotarba & Koltun, 2011). In the Lower Sarmatian strata,the TOC var ies from 0.02 to 3.22 wt% (av er age 0.69 wt%)in the Pol ish Carpathian Foredeep (Kotarba, 1999; Kotarbaet al., 1998b, 2005) and from 0.01 to 1.45 wt% (av er age0.71 wt%) in the Ukrai nian Carpathian Foredeep (Kotarba& Koltun, 2011). Nu mer ous TOC re sults from the Pol ishOil & Gas Com pany lab o ra to ries (un pub lished data) arevery sim i lar to those data and reach up to 5.1 wt% (av er age0.88 wt%) and 3.4 wt% (av er age 0.82 wt%) in the Up perBadenian and the Lower Sarmatian strata, re spec tively


(Kotarba et al., 1998b). Ac cord ing to Dickey and Hunt(1972), the TOC con cen tra tion over 0.5 wt% is re quired toqual ify a rock as hav ing hy dro car bon po ten tial. Dis tri bu -tions of me dian TOC con tents in the ana lysed pro files ofwells in the Up per Badenian and Lower Sarmatian strata are shown in Figs 4 and 5, re spec tively. The dis tri bu tion of me -dian TOC con tents in the Up per Badenian strata gen er allyde creases from the south to the north (Fig. 4), while in theLower Sarmatian strata the low est val ues are ob served inthe south ern zone near the Carpathian Overthrust, increas-ing northwards (Fig. 5).

Geo chem i cal anal y ses (Rock-Eval, biomarkers and sta -ble iso topes) of DOM (Kotarba et al., 1998b, 2005; Kotarba & Koltun, 2006, 2011) in di cate the gen eral dom i nance ofthe ter res trial Type III kerogen in both the Up per Badenianand Lower Sarmatian se quences. This DOM mainly con -sists of vitrinite-group macerals (from 70.9 to 84.0 %) withthe com plete ab sence of the liptinite group macerals(Kotarba et al., 1998b), which also sup ports the con cept ofits ter res trial or i gin. More over, such an or i gin has been alsocon firmed by the re sults of el e men tal anal y ses of the Mio -cene fos sil rem nants (Kotarba et al., 1987). Ter res trialDOM dis persed in claystones/mudstones ca pa ble of gen er -at ing and ex pel ling oil should have HI val ues typ i callyhigher than about 200 (Hunt, 1991). Dis tri bu tion of me dianHI val ues in the Up per Badenian and Lower Sarmatianstrata usu ally does not ex ceed a value of 200 mg HC/g TOC(Figs 6, 7) and only in a small area of the Up per Badenianstrata north of Le¿ajsk this value is in creased up to 213 mgHC/g TOC (Fig. 6). The HI val ues close to that limit werefound in a few sam ples from the Up per Badenian strata inthe Rzeszów-Stryi area (Fig. 8C), which may sug gest the

pres ence of small amounts of al gal (ma rine and/or non-ma -rine) DOM. In the Kraków-Rzeszów area, only gas-proneType III kerogen (Fig. 9C) has been found. The lack of ob -vi ous depth trends of both the Tmax tem per a ture and HI val -ues (Figs 8B, C, 9B, C) ad vo cates the im ma ture gas-pronechar ac ter of the en tire Up per Badenian and Lower Sarma-tian se quences. The im ma ture ter res trial DOM shows RockEval Tmax tem per a ture be low 435°C (Espitalié & Borde-nave, 1993). The Tmax tem per a ture val ues at the bot tom ofthe Up per Badenian and Lower Sarmatian strata var ies from about 390 to 435°C (Figs 8B, 9B). Only in three sam plesfrom the Up per Badenian strata (Jo-4 and Rn-6 wells,depths 3,300–3,500 m), the Tmax 435–438°C (Fig. 8B) wasmea sured. Such Tmax val ues to gether with no ob vi ous depth trends (Figs 8B, 9B) in di cate that down to the depths of 3,300– 3,500 metres the Mio cene ter res trial DOM is im ma ture andgenerates al most ex clu sively mi cro bial meth ane. The ini tialphase of low-tem per a ture thermogenic pro cess pro ceeds be -neath these depths, un der the Outer Carpathian Overthrust.Dis tri bu tion of vitrinite reflectance val ues of the Mio ceneDOM with depth con firms its low ma tu rity (Kotarba et al.,1998b). The ma tu rity of DOM in the Up per Badenian stratagen er ally in creases from the north to the south and the south-east from about 400 to about 435°C (Fig. 10), while in theLower Sarmatian strata Tmax changes only from about 420 toabout 430°C, and de creases to the south-east (Fig. 11)

OR I GIN OF NAT U RAL GASES

The sta ble car bon and hy dro gen isotopic com po si tionof meth ane can be sen si tive in di ca tors of gas or i gin and mi -gra tion. Car bon in meth ane has a wide range of d13C val ues


Fig. 4. Con tour map of me dian val ues of the to tal or ganic car bon (TOC) con tent in sec tions of ana lysed wells within the Up perBadenian strata of the Carpathian Foredeep. Lith. – Lith u a nia


Fig. 5. Con tour map of me dian val ues of the to tal or ganic car bon (TOC) con tent in sec tions of ana lysed wells within the LowerSarmatian strata of the Carpathian Foredeep. Lith. – Lith u a nia

Fig. 6. Con tour map of me dian val ues of the hy dro gen in dex (HI) in sec tions of ana lysed wells within the Up per Badenian strata of theCarpathian Foredeep. Lith. – Lith u a nia

due to a va ri ety of chem i cal, phys i cal and mi cro bial pro -cesses that cause sig nif i cant vari a tions in the iso to pic com -po si tions of the source or ganic mat ter and the daugh tergases. The anal y ses of sta ble car bon and hy dro gen iso topesin meth ane en ables the iden ti fi ca tion of the source or ganicmat ter, from which gases were gen er ated dur ing mi cro bialor thermogenic pro cesses (e.g., Berner & Faber, 1996;Kotarba, 1995). Meth ane gen er ated dur ing thermogenicpro cesses from the sapropelic or ganic mat ter shows d13Cval ues from –55 to –30‰ whereas that pro duced from thehumic or ganic mat ter has d13C from –30 to –20‰. In con -trast, microbially gen er ated meth ane has typ i cally very lowd13C val ues (less than –55‰), even be low –100‰. Mi cro -bial gases can be gen er ated by two pro cesses: meth ane fer -men ta tion and car bon di ox ide re duc tion (Mar tini et al.,1996, 1998; Whiticar et al., 1986). Eth ane, and in in sig nif i -cant quan ti ties, pro pane have been gen er ated from mi cro -bial pro cesses (Claypool, 1999; Gautier & Claypool, 1984;Hinrichs et al., 2006; Jenden & Kaplan, 1986; Lillis, 2007;Oremland et al., 1986; Tay lor et al., 2000). Re sults of sta ble car bon iso tope anal y ses of eth ane, pro pane, butanes andpentanes al lowed the prep a ra tion of more pre cise ge neticclas si fi ca tion of nat u ral gases, i.e. the dis tin guish ing of ge -netic groups and en abled the iden ti fi ca tion of mi gra tion andmix ing of ei ther ge net i cally dif fer ent gases or gases pro -

duced from the same source or ganic mat ter but dur ing thesuc ces sive gen er a tion stages and mi gra tion dis tance (Berner & Faber, 1996; Kotarba et al., 1994; Prinzhofer & Pernaton, 1997; Prinzhofer et al., 2000; Whiticar, 1994). Both the ex -per i men tal data and the o ret i cal cal cu la tion dem on stratedthat sta ble car bon iso tope stud ies of meth ane, eth ane andpro pane are es sen tial for de ter min ing the type and the mat u -ra tion de gree of the source or ganic mat ter in vitrinite reflec-tance scale (Berner & Faber, 1996; Whiticar, 1994). An im -por tant im pli ca tion of the di a grams for ge netic in ter pre ta -tion is that a lin ear de pend ence of car bon iso topes val ues inmeth ane, eth ane, pro pane, butanes and propanes to their re -cip ro cal car bon num ber is not an ex clu sive in di ca tor of theor i gin of nat u ral gas from a sin gle source, as it was as sumed(e.g., Rooney et al., 1995). In stead, Kotarba et al. (2009)sug gested that in such plots a “dog leg” trend, ex em pli fiedby rel a tively 13C-de pleted meth ane and 13C-en riched pro -pane in com par i son to eth ane, in di cates that such nat u ral gas was not gen er ated from a sin gle source rock or that it hasun der went post-gen er a tion al ter ations (e.g., sec ond ary gascrack ing, mi cro bial ox i da tion or thermochemical sul phatere duc tion). More over, rel a tively more 13C-de pleted meth -ane in re la tion to eth ane can be ap plied to eval u ate mix ingpro por tions be tween mi cro bial meth ane and thermogenicgases (Kotarba & Lewan, 2004; Kotarba et al., 2009).


Fig. 7. Con tour map of me dian val ues of the hy dro gen in dex (HI) in sec tions of ana lysed wells within the Lower Sarmatian strata of the Carpathian Foredeep. Lith. – Lith u a nia

Most of the ana lysed gases were col lected from the Up -per Badenian and Lower Sarmatian strata (Kotarba, 1998,2011). Re sults of sta ble car bon iso tope anal y ses of meth ane, eth ane, pro pane, butanes and pentanes, and sta ble hy dro geniso tope anal y ses of meth ane (Figs 12–15) and spatial dis tri -bu tions of both d13C(CH4) and d13C(C2H6) val ues in thesestrata (Figs 16–19) in di cate that the gas eous hy dro car bonswere mainly gen er ated dur ing mi cro bial pro cesses, and spo -rad i cally dur ing low-tem per a ture thermogenic pro cesses.

In sig nif i cant changes in val ues of sta ble car bon iso topera tios of meth ane and eth ane with depth sug gest quite uni -form gen er a tion con di tions of mi cro bial meth ane and eth -ane in the en tire Badenian and Lower Sarmatian se quences

(Figs 8D, E and 9D, E). Thermogenic meth ane oc curs onlyin the BrzeŸnica, £êkawica and Tarnów ac cu mu la tions inthe Pol ish Carpathian Foredeep and Pyniany and Zaluzhany ac cu mu la tions in the Ukrai nian Carpathian Foredeep (Figs8D, E and 9D, E). More over, thermogenic eth ane oc curs inthe Kury³ówka (Ku-3), Hai and Letnia ac cu mu la tions of the Rzeszów-Stryi area (Fig. 8E). Pro pane is mainly thermo-genic in or i gin (Figs 8F, 9F, 14, 15). Mi cro bial pro pane oc -curs in the ¯o³ynia-Le¿ajsk (Zn-25) and Bilche-Volytsia(BV 500) ac cu mu la tions of the Rzeszów-Stryi area (Fig.8F).

In the Pol ish Carpathian Foredeep, in the RoŸwienicafield, gas is ac cu mu lated in the Lower (and partly Up per)


Fig. 8. (A) To tal or ganic car bon, (B) Rock-Eval Tmax tem per a ture, (C) hy dro gen in dex, (D) d13C(CH4), (E) d13C(C2H6) and (F)d13C(C3H8) ver sus depth of (A, B and C) oc cur rence of or ganic mat ter and (D, E and F) nat u ral gas ac cu mu la tions within the Badenianand Lower Sarmatian res er voirs of the Carpathian Foredeep be tween Rzeszów and Stryi. Rock sam ple code (A, B and C) – see Kotarba etal. (1998b, 2005, 2011a), and gas sam ple code (D, E and F) – cf. Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al.(2005)

Badenian se quence; in the Lubaczów and Uszkowce de pos -its it oc curs in the Lower Badenian strata and in the top mostparts of Up per Ju ras sic car bon ates (Kotarba, 1998, 2011).The gas from the Tarnów, £êkawica and BrzeŸnica de pos its mi grated from the Me so zoic base ment to the Mio cene strata and is typ i cally thermogenic. This gas was gen er ated fromthe oil-prone, ma rine or ganic mat ter (Type II kerogen) (Ko-tarba & Jawor, 1993; Kotarba, 2011). In both the RoŸwie-nica and the Lubaczów de pos its, a very small ad mix ture ofdiage netic (and/or low-tem per a ture thermogenic) meth anewas iden ti fied (Kotarba, 1998). In the Lubaczów de posit the gas also filled up the trap in Up per Ju ras sic car bon ates. Ge -net i cally, this gas is a typ i cal mi cro bial meth ane, which mi -grated to the Up per Ju ras sic trap from the autochthonousMio cene strata along a fault zone (Kotarba, 1998). In the

Ukrai nian Carpathian Foredeep, eight fields (Rudky, Vere-schytsia, Medenychi, Mala Horozhanna, Bilche-Volytsia,Uhers’ko, Pivdenne Uhers’ko and Letnia) con tain con den -sate and/or gas, and one Orkhovychi de posit con tains oil incom bined Me so zoic–Mio cene res er voirs. The mi cro bialgases (meth ane, hy dro gen and partly eth ane and pro pane)generated dur ing mi cro bial pro cesses within the Mio cenestrata, 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 mentand to the bot tom most Lower Badenian Sandy-Cal car e ousSeries reservoirs of the traps of the above mentioned de-pos its (Kotarba & Koltun, 2011).

In creased d13C(CH4) and d13C(C2H6) val ues in the Up -per Badenian strata close to Tarnów town (Figs 16, 17) andin Lower Sarmatian strata near Dêbica town (Figs 18, 19) in


Fig. 9. (A) To tal or ganic car bon, (B) Rock-Eval Tmax tem per a ture, (C) hy dro gen in dex, (D) d13C(CH4), (E) d13C(C2H6) and (F)d13C(C3H8) ver sus depth of (A, B and C) oc cur rence of or ganic mat ter and (D, E and F) nat u ral gas ac cu mu la tions within the Badenianand Lower Sarmatian res er voirs of the Carpathian Foredeep be tween Kraków and Rzeszów. Rock sam ple code (A, B and C) – see Kotarba et al. (1998b, 2005), and gas sam ple code (D, E and F) – cf. Kotarba (1998, 2011)


Fig. 10. Con tour map of val ues of Rock-Eval Tmax tem per a ture at the bot tom of the Up per Badenian strata of the Carpathian Foredeep.Lith. – Lith u a nia

Fig. 11. Con tour map of val ues of Rock-Eval Tmax tem per a ture at the bot tom of the Lower Sarmatian strata of the Carpathian Foredeep.Lith. – Lith u a nia

the Pol ish part of the Carpathian Foredeep are caused by mi -gra tion of thermogenic gases from the Me so zoic base ment,while in creased d13C(CH4) and d13C(C2H6) val ues in theLower Sarmatian strata near Boryslav, Drohobych andSambir (Figs 18, 19) in the Ukrai nian part of the CarpathianForedeep are most prob a bly caused by mi gra tion ofthermogenic gases, which were gen er ated within the Mio -cene strata be neath the Outer Carpathians at greater depths,from 3,500 to 12,000 metres (see chap ter be low).

MODEL OF MI CRO BIAL ANDTHERMOGENIC GAS GEN ER A TION

The hy dro car bon gen er a tion con di tions in the auto-chthonous Mio cene were ana lysed in nine se lected test ar -eas (Figs 20, 21). The A (Tarnów), B (Mielec-Le¿ajsk), F(Pidluby) and (G) Rudky ar eas rep re sent the part of theCarpathian Foredeep lo cated north of the pres ent mar gin ofthe Outer Carpathians. The C (Rzeszów), D (Przemyœl) and


Fig. 12. Hy dro car bon in dex (CHC) ver sus d13C(CH4) for nat u ral gases ac cu mu lated in the Mio cene res er voirs of the CarpathianForedeep in (A) Kraków – Rzeszów and (B) Rzeszów – Stryi ar eas. Compositional fields af ter Whiticar (1994). Gas sam ple code – seeKotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005). L. – Lower, U. – Up per

Fig. 13. d13C(CH4) ver sus dD(CH4) for nat u ral gases ac cu mu lated in the Mio cene res er voirs of the Carpathian Foredeep in (A) Kraków– Rzeszów and (B) Rzeszów – Stryi ar eas. Compositional fields af ter Whiticar et al. (1986). Gas sam ple code – see Kotarba (1998, 2011),Kotarba and Koltun (2011) and Kotarba et al. (2005). L. – Lower, U. – Up per

H (Zaluzhany-Sambir) ar eas are lo cated in the autochtho-nous Mio cene outer ba sin along the pres ent edge of theOuter Carpathians (Figs 20, 21). The E (Krosno-Sanok) and I (Skole) ar eas were cho sen as an ex am ple of hy po thet i caldeep est lev els where the autochthonous Lower Mio cene

strata might have ex isted at a depth of about 11–12 km. Thehy dro car bon gen er a tion mod el ling used the time-tem per a -ture (TTI) method (Waples, 1980). The time scale and re -gional Mio cene cor re la tion were based upon Gradstein etal. (2004), Czepiec & Kotarba (1998) and Oszczypko


Fig. 14. d13C(C2H6) ver sus d13C(CH4) (A and C) and d13C(C3H8) (B and D) for nat u ral gases ac cu mu lated in the Mio cene res er voirs ofthe Carpathian Foredeep in the (A and B) Kraków-Rzeszów and (C and D) Rzeszów – Stryi ar eas. Po si tion of vitrinite reflectance curvesfor type III kerogen af ter Berner and Faber (1996). Curves were shifted based on av er age val ues of d13C = –24.8‰ for (A and B) Up perBadenian kerogen and d13C val ues = –25.7‰ for (C and D) Lower Sarmatian kerogen from the autochthonous Mio cene strata (Kotarba etal., 1998a, 2005). Gas sam ple code – see Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005). L. – Lower, U. –Up per

(2006a) whereas the geo ther mal gra di ent – 33°C/10–3 m isthe av er age value ob tained from palaeotemperature cal cu la -tions for the Mio cene ba sin (Szafran, 1990) and cal i bratedwith vitrinite reflectance val ues (Kotarba et. al., 1998a).Thick nesses of strati graphic units were taken from the re -sults of drillings in the analysed parts of the outer ba sin. The es ti mated ero sional re duc tion was 100 metres in A, B, F and G test ar eas and 250 metres in the re main ing ones. TTI mod -el ling in the outer ba sin (test ar eas A to D and F to H) was

re lated to the bot tom of the Up per Badenian suc ces sion asthe da tum sur face. The top of the Lower Badenian Sandy-Cal car e ous Se ries se quence was the da tum sur face in thetest ar eas E and I.

The mod el ling for the test E and I ar eas in di cated thepos si bil ity of gen er a tion of the higher gas eous hy dro car bon(and even liq uid hy dro car bons) be neath the CarpathianOverthrust, at about 7,500 m of depth (Fig. 21). At greaterdepths, within the rel ics of autochthonous Lower Mio cene


Fig. 15. (A and B) Sta ble car bon iso tope com po si tion of meth ane, eth ane, pro pane, butanes and propanes ver sus the re cip ro cal of theircar bon num ber for nat u ral gases ac cu mu lated in (A and C) the Up per Badenian and (B and D) Lower Sarmatian res er voirs of theCarpathian Foredeep (A and B) in the Kraków – Rzeszów and (C and D) Rzeszów – Stryi ar eas. Struc ture of the graph for meth ane, eth ane and pro pane af ter Rooney et al. (1995). Av er age val ues of d13C = –24.8 ‰ for Up per Badenian kerogen (A and C), and d13C val ues =–25.7 ‰ for Lower Sarmatian kerogen (B and D) from the autochthonous Mio cene strata (Kotarba et al., 1998a, 2005). Gas sam ple code – Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005)


Fig. 16. Con tour map of val ues of d13C(CH4) at the bot tom of the Up per Badenian strata of the Carpathian Foredeep. Lith. – Lith u a nia

Fig. 17. Con tour map of val ues of d13C(C2H6) at the bot tom of the Up per Badenian strata of the Carpathian Foredeep. Lith. – Lith u a nia

strata of the in ner ba sin, only the high-tem per a ture meth anecould be pro duced. The north ern range of gen er a tion ofthermogenic hy dro car bons was presented in Fig. 20.

The re sults of mod el ling re vealed that in the A, B, F and G test ar eas (north of the mar gin of the Carpathians) the TTI val ues were 0.05 to 1 (Fig. 21) prov ing that thermogenicgases had not been gen er ated. At the edge of the Carpathianorogen, the max i mum TTI val ues were: 21 at a depth ofabout 3,600 metres in C test area, 4 at a depth of about 2,600 metres in D area, and 480 at a depth of about 5,000 metres in D area. These val ues sug gest the ap pear ance of low-tem per -a ture thermogenic pro cesses within the autochthonous Mio -cene strata be neath the Carpathian Overthrust, i.e. be neath2,500 m depth.

The re sults of mod el ling for all the test ar eas proved thegen er a tion of mi cro bial meth ane in al most en tire thick nessof autochthonous Mio cene of the outer ba sin. Tak ing intoac count the quan ti ta tive cri te ria for mi cro bial gas gen er a -tion (Clay ton, 1992), the in ten sity of meth ane pro duc tionwas cal cu lated for the gen er al ized sed i men ta tion con di tionsof the Mio cene ma rine ba sin. It was found that gen er a tionpro cesses of mi cro bial meth ane were most in ten sive at thedepth in ter val of 700 to 1,500 m be neath the Mio cene seafloor (Fig. 21). The maximum yield of microbially-pro -

duced meth ane was about 5 cu bic metres of CH4 per one cu -bic metre of source rock. The vol ume of pro duced mi cro bial meth ane is spa tially vari able within the ba sin and de pendson the burial his tory. As no dra matic changes of ther malcon di tions have been de duced within the ba sin since theSarmatian, these mi cro bial gen er a tion pro cesses have pre -sum ably con tin ued un til re cent (Kotarba et al., 1995). Eventhe fold ing and up lift of the Carpathian orogen and the con -se quent re gres sion of the Miocene sea at the break of theearly and late Sarmatian did not impede the microbialprocesses.

The re sults of mod el ling re vealed that both in the hy po -thet i cal deep est test ar eas E and I the TTI val ues were 109

and 1010, re spec tively, thus prompt ing a con clu sion thathigh-tem per a ture thermogenic zone started be low depth of7,500 m (Fig. 21).

HY DRO CAR BON AC CU MU LA TIONAND EX PLO RA TION PROSPECTIVES

The composition of nat u ral gases ac cu mu lated in auto-chthonous Mio cene strata of the Pol ish and Ukrai nian Outer Carpathian Foredeep is dom i nated by meth ane, which usu -


Fig. 18. Con tour map of val ues of d13C(CH4) at the bot tom of the Lower Sarmatian strata of the Carpathian Foredeep. Lith. – Lith u a nia

ally con sti tutes over 98 vol%. Meth ane was gen er ated bythe car bon di ox ide re duc tion path way of mi cro bial pro -cesses (Kotarba, 1998; 2011; Kotarba & Koltun, 2006,2011; Kotarba et al., 2005). Higher gas eous hy dro car bons(mainly eth ane and pro pane) are usu ally mi nor con stit u ents(con cen tra tions less than 0.2 vol%). Eth ane was gen er ateddur ing both mi cro bial pro cesses and at the ini tial stage ofthe low-tem per a ture thermogenic pro cesses. Pro pane wasgen er ated mainly at the ini tial stage of the low-tem per a turethermogenic pro cesses and also dur ing mi cro bial pro cesses(Kotarba, 2011). The mi cro bial meth ane is of vi tal eco -nomic im por tance (Rice & Claypool, 1981; Rice, 1992,1993).

Sta ble car bon iso tope com po si tions of meth ane andethane (Figs 16–19) re veal that nat u ral gases in autochtho-nous Up per Badenian and Lower Sarmatian strata of thePol ish and Ukrai nian Carpathian Foredeep north of the Car- pathian Overthrust were mainly gen er ated dur ing mi cro bialpro cesses from dis persed or ganic mat ter in side this Mio -cene complex. The occurrence of thermogenic hy dro car -bons near the bot tom se quence of the Mio cene strata nearTarnów (Figs 16, 17) was caused by an in flow from the Me -so zoic base ment to Mio cene strata (Wiêc³aw, 2011). Iso to -pic anom a lies near Dêbica (Figs 18, 19) in the Pol ish part

and Boryslav, Drohobych and Sambir (Figs 16–19) in theUkrai nian part of the Carpathian Foredeep were most prob -a bly caused by the mi gra tion of thermogenic gases, whichwere gen er ated within the Miocene strata beneath the OuterCarpathians.

The high sed i men ta tion rates to gether with rhyth micand cy clic de po si tion of Mio cene clays and sands as well asthe vig or ous gen er a tion of mi cro bial meth ane caused thatthe gas pro duced in claystone beds was ac cu mu lated in theover lay ing sand stones, and capped, in turn, by the suc ceed -ing claystones. Re sults of the dis tri bu tion of im ma turehumic dis persed or ganic mat ter in the Up per Badenian andLower Sarmatian se quences in di cates that it is prac ti callyho mo ge neous (Figs 4–7, 8A–C, 9A–C). The mi gra tionrange of mi cro bial gases was in sig nif i cant and lo ca tions oftheir ac cu mu la tions would de pend only on the ex is tence ofproper type of traps (compactional anticlines sit u ated abovebase ment up lift, sealed by the Carpathian Overthrust, andsealed by faults, strati graphic pinch-out and strati graphictraps re lated to un con formi ties). Such a gen er a tion and ac -cu mu la tion sys tem of mi cro bial gases gave rise to the for -ma tion of multi-hor i zon tal gas fields, e.g., Przemyœl andHusów gas fields. Re cent in tro duc tion of in ter pre ta tion of2-D and 3-D seis mic sec tions and ap pli ca tion of DHI (di rect


Fig. 19. Con tour map of val ues of d13C(C2H6) at the bot tom of the Lower Sarmatian strata of the Carpathian Foredeep. Lith. – Lith u a nia


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hy dro car bon in di ca tor) on the seis mic data en ables lo ca tionof ar eas of hy dro car bon pros pect ing (Baran & Jawor, 2009;Myœliwiec, 2004a, b; Myœliwiec et al., 2006).

An idea has been put for ward in the geo chem i cal lit er a -ture that mi cro bial meth ane might have been ac cu mu latedas hy drate be neath the Mio cene seafloor (Kotarba, 1992).How ever, the re sults of bathymetric anal y ses (Czepiec &Kotarba, 1998) re vealed that the Badenian sea was pre sum -ably only slightly deeper than the outer shelf, i.e. about300 m, at most. Ini tially, the Sarmatian sea was about 30–50 m deep and has shallowed to only about 10 m. Both theBadenian and Sarmatian seas were the warm bas ins of sur -face wa ter tem per a tures of 17–20°C (Czepiec & Kotarba,1998). Tak ing into ac count these data, the con cept that nat u -ral meth ane hy drate zone formed be neath the Mio cene sea-floor must be re jected.

Geo chem i cal stud ies and TTI mod el ling within theautochthonous Mio cene strata of the Carpathian Foredeepoc cur ring south be neath the Carpathian Overthrust re vealthat the low-tem per a ture thermogenic pro cess starts from adepth of 2,500 m, the high-tem per a ture thermogenic pro -cess be gins from a depth of 7,500 m, and three gen er a tion

zones were es tab lished: mi cro bial, low-tem per a ture thermogenic and high-tem per a ture thermogenic (Figs 20, 21).

All hy dro car bon de pos its dis cov ered in the autochtho-nous Mio cene strata of the Pol ish and Ukrai nian CarpathianForedeep, north and along the pres ent edge of the OuterCarpathians, oc cur within the mi cro bial zone (Figs 1, 20).Gases ac cu mu lated in this zone are mainly of mi cro bial or i -gin. Oil from one Orkhovychi de posit and from pe tro leumin flows near Tarnów mi grated to the Mio cene strata fromthe Me so zoic base ment (Kotarba & Jawor, 1993; Wiêc³aw,2011; Wiêc³aw et al., in press). The en tire sur face of thiszone has a very good seis mic and drill ing re cord. In the Po-lish Carpathian Foredeep, the gas re sources were as sessedto 200–220 bil lion cu bic metres, and about 160 bil lion cu bic metres have been already discovered.

The cur rent in ter pre ta tion (Oszczypko, 2006b; Osz-czypko et al., 2006) as sumes that the Up per Badenian de -pos its south of the Carpathian Overthrust con tinue over adis tance of some 10–20 km, and far ther south only LowerMio cene de pos its oc cur; and the south ern ex tent of LowerSarmatian de pos its is de creased as com pared to the Up perBadenian. The in ferred south ern bound aries of the Up per


Fig. 21. Model of gas gen er a tion within the autochthonous Mio cene ba sin and fam ily of burial-his tory subsurface tem per a ture grid andTTI (Time Tem per a ture In dex) val ues for (A) Tarnów, (B) Mielec-Le¿ask, (C) Rzeszów, (D) Przemyœl, (E) Krosno-Sanok, (F) Pidluby,(G) Rudky, (H) Zaluzhany, and (I) Skole ar eas. Model for A to D ar eas mod i fied af ter Kotarba et al. (1998a). S – Sarmatian, E – early, L – late

Badenian and Lower Sarmatian strata fol low such an in ter -pre ta tion (cf. Figs 4–7, 10, 11, 16–19), how ever, the gen -eral pat tern of dis tri bu tion of Mio cene fa cies be low theCarpathian nappes is very well es tab lished only west of the“Cra cow bolt” (Ney, 1968) where a suf fi cient doc u men ta -tion by wells ex ists (e.g., Oszczypko & Oszczypko-Clowes, 2003; Oszczypko et al., 2006). In other ar eas, the well con -trol is poor and it is not cer tain if the pat tern es tab lished inthe Silesian part of the Carpathian Foredeep is valid for itsother area that is re lated to the Ma³opolska block (Bu³a &Habryn, 2011), con sid er ing the re corded dif fer ences in pa-laeotopography of the west ern and east ern parts of the Car-pathian Foredeep ba sin (Bu³a et al., 2008). Dur ing the Pala-eo gene, the area of the Carpathian Foredeep was sub ject toin ten sive ero sion that re sulted in highly vari able mor phol -ogy. The max i mum height dif fer ence ex ceeded 2,000 m and the val ley depths reached 1,000 m (Karnkowski & Ozim-kowski, 2001); in these deep val leys Palaeogene con glom -er ates and sandy-silty de pos its up to 300 m thick have beenfound in some places (Moryc, 1995). Con sid er ing that thepalinspastic re con struc tion as sumes that bathyal de po si tiontook place in the ma jor part of the early Badenian ba sin(Oszczypko et al., 2006, Fig. 18) and that these deep ba sincon di tions con tin ued dur ing the sub se quent Tyras andKosiv de po si tion in west ern Ukraine (Peryt, 2006), it ishighly prob a ble that the south ern Badenian ex tent, in par tic -u lar in SE Po land and west ern Ukraine, does not dif fer sig -nif i cantly from the Mio cene ex tent as shown in Fig. 20.

North and along the pres ent edge of the CarpathianOverthrust in the Ukraine, the Lower Badenian Sandy-Cal -car e ous Se ries con tains twelve gas and gas-con den sate de -pos its, and one oil de posit (Fig. 1B). As the lithofacies de -vel op ment of the Lower Mio cene strata re sem bles the de -vel op ment of the Up per Badenian and Lower Sarmatianstrata, it can also con tain a sim i lar quan tity and sim i lar ge -netic type of gas-prone kerogen. Their thick ness be neath the Carpathian Overthrust ex ceeds even 1.5 km (Oszczypko,2006b). All these facts may in di cate that in both low- andhigh-tem per a ture thermogenic zones con sid er able quan ti -ties of gases could be generated and accumulated.

CON CLU SIONS

The Carpathian Foredeep de vel oped dur ing the earlyand mid dle Mio cene as a pe riph eral flex ural fore land ba sinin front of the ad vanc ing Carpathian Overthrust. The outerforedeep (termed the Bilche-Volytsia zone in the Ukraine)is filled with gen er ally un dis turbed, flat-ly ing autochtho-nous Mid dle Mio cene ma rine de pos its, which range inthick ness from a few hun dred metres in the north ern, mar -ginal parts of the Foredeep to as much as 3.5 km in SE Po -land and 5 km in western Ukraine.

Since 1920, within the autochthonous Mio cene stratanorth and along the pres ent edge of the Carpathian Over-thrust, 101 gas and gas-con den sate de pos its in the Pol ishCarpathian Foredeep (mainly in the Up per Badenian andLower Sarmatian strata and lo cally in the Lower Badenianstrata), and 44 gas and gas-con den sate de pos its and one oilde posit in the Ukrai nian Carpathian Foredeep (mainly in the

Up per Badenian and Lower Sarmatian strata and lo cally inthe Lower Badenian Sandy-Cal car e ous Se ries) have beendis cov ered. The molecular com po si tion of nat u ral gases ac -cu mu lated in autochthonous Mio cene strata of the Pol ishand Ukrai nian Carpathian Foredeep is dom i nated by meth -ane, which usu ally con sti tutes over 98 vol%. Meth ane wasgen er ated by the car bon di ox ide re duc tion path way of mi -cro bial pro cesses. Eth ane was gen er ated by both mi cro bialand thermogenic pro cesses and pro pane at the ini tial stageof the low-tem per a ture thermogenic pro cesses, and also bymi cro bial pro cesses.

In the Up per Badenian strata, the to tal or ganic car bon(TOC) con tents vary from 0.02 to 1.48 wt% (av er age 0.75wt%) in the Pol ish Carpathian Foredeep and from 0.44 to2.01 wt% (av er age 0.96 wt%) in the Ukrai nian CarpathianForedeep. In the Lower Sarmatian strata, the TOC var iesfrom 0.02 to 3.22 wt% (av er age 0.69 wt%) in the Pol ishCarpathian Foredeep and from 0.01 to 1.45 wt% (av er age0.71 wt%) in the Ukrai nian Carpathian Foredeep. This dis -persed or ganic mat ter in ma rine Mio cene sed i ments is gas-prone, ter res trial (humic) in origin.

The high sed i men ta tion rates to gether with rhyth micand cy clic de po si tion of Mio cene clays and sands as well asthe vig or ous gen er a tion of mi cro bial meth ane caused thatthe gas pro duced in claystone beds was ac cu mu lated in theover lay ing sand stones, and capped, in turn, by the suc ceed -ing claystones. A mi gra tion range of mi cro bial gases wasin sig nif i cant and lo ca tions of their ac cu mu la tions would de -pend only on the ex is tence of proper type of traps (compa-ctional anticlines sit u ated above base ment up lift, sealed bythe Carpathian Overthrust, and sealed by faults, stratigra-phic pinch ing out and strati graphic traps re lated to un con -formi ties). Such gen er a tion and ac cu mu la tion sys tem of mi -cro bial gases gave rise to the formation of multi-horizontalgas fields.

An other sit u a tion is en coun tered in the autochthonousMio cene strata south be neath the Carpathian Overthrust. As the lithofacies de vel op ment of the Lower Mio cene strata re -sem bles the de vel op ment of the Up per Badenian and Lower Sarmatian strata, it can also con tain a sim i lar quan tity andsim i lar ge netic type of gas-prone kerogen. Their thick nessbe neath the Carpathian Overthrust ex ceeds even 1.5 km.Geo chem i cal stud ies re veal that from the depth of 2,500 mstarts the pro cess of low-tem per a ture thermogenic hy dro -car bon gen er a tion (“oil win dow”). At greater depths, morethan 7,500 m, within the autochthonous Lower Mio cene ba -sin ex isted the high-tem per a ture meth ane gen er a tion zone(“gas win dow”). All these facts may ev i dence that in bothlow- and high- tem per a ture thermogenic zones con sid er able quantities of gases might have generated and accumulated.

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 car riedout at the AGH Uni ver sity of Sci ence and Tech nol ogy in Krakówand the Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute in War saw. Sci en tific work was fi nanced from the sci en tific funds inthe years 2007–2010. The de tailed com ments of Paul Lillis, Nestor Oszczypko and Francois Roure were of great as sis tance in our re -


vi sions of this manu script. Tech ni cal work by Joanna Gawêda-Skrok, Marek Hajto, Hieronim Zych, Tomasz Kowalski and Maria Fremel from the AGH Uni ver sity of Sci ence and Tech nol ogy inKraków is grate fully ac knowl edged.

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