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

MICRO BIAL GAS SYSTEM AND PROSPECTIVES OF 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 Technol 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, Poland, e-mail: [email protected] 2 Polish Geolog i cal Insti tute – Nati onal Re search Insti tute, Rakowiecka 4, 00-975 Warsaw, Poland, 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 system and prospectives of hy dro car bon ex plo ra tion in Mio cene strata of the Pol ish and Ukrai nian Carpathian Foredeep. Annales Societatis Geologorum Poloniae, 81: 523–548.

Ab stract: Molec u lar com po sition of nat u ral gases ac cu mulated in autochthonous Mio cene strata of the Pol ish and Ukrai nian Carpathian Foredeep is domi nated by methane, which usually con sti tutes over 98 vol%. Meth ane was gen 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 micro bial pro cesses. The rhyth mic and cy clic de posi tion of Mio cene clays and sands as well as the vig or ous gen er a tion of mi cro bial meth ane caused that the gas produced in claystone beds was ac cu mulated in the over lay ing sand stones, and capped, in turn, by the suc ceed ing claystones. Such gen er a tion and ac cu mula tion system of micro bial gases gave rise to the for mation of multi-hor i zon tal gas fields. Anal y sis of the distri bu tion of im mature humic dis persed or ganic mat ter in the Upper Badenian and Lower Sarmatian se quences in 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 tions of their ac cu mula tions would de pend only on the ex istence of proper type of traps (compactional anticlines sit u ated above base ment uplifts, sealed by the Carpathian Overthrust and/or by faults; strati graphic pinching out and strati graphic traps re lated to uncon formities). An other sit u a tion is en coun tered in the south, be neath the Carpathian Overthrust. The thick ness of the autochthonous Miocene 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 thermogenic hy dro car bon gen er a tion (“oil win dow”). At greater depths, more than 7,500 metres, within the autochthonous Lower Mio cene ba sin only the high-temper a ture meth ane (“gas win dow”) could be pro duced and ac cu mulated.

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, SW Po land, west ern Ukraine.

Manu script received 19 May 2011, accepted 13 October 2011

IN TRO DUC TION pic studies of nat ural gases ac cum ulat ed within the auto- chthonous Miocene strata of the Polish and Ukrai nian Car- The main objec ti ve of our study is to present the hy dro- pathian Foredeep were present ed by G³ogoczowski (1976), car bon po ten tial and geo chem i cal char ac ter is tics of dis - Calikowski (1983), Jawor and Kotarba (1993), Kotarba persed organic matter , ori gin of natu ral gases, model of mi- (1992, 1998, 2011), Kotarba and Jawor (1993), Kotarba and cro bial gas gen er a tion and prospectives of hy dro car bon ex- Koltun (2006, 2011), Kotarba et al. (1987, 2005) and Shabo plora ti on for the autochthonous Miocene strata of the Polish and Mamchur (1984). (Fig. 1A) and Ukrainian 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 - GEO LOG I CAL SET TING ganic matter were described by Kotarba et al. (1998b, 2005) and preli m inary model of micro bial and thermogenic gas The Carpathian Foredeep devel oped during the early gen er a tion by Kotarba et al. (1998a). Molec ular and isoto - and middle Miocene as a periph eral flexural foreland ba sin 524 M. J. KOTARBA ET AL. p e e d e r o F

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in front of the advanc ing Carpathian Overthrust (Oszczypko with small embayments are domi nated by cal car eous mud- et al., 2006). The highly com plex epi-Variscan platform and stones (Oszczypko et al., 2006). its Permian–Me so zoic cover form the base ment of the Car- There were three pe riods of intens e foreland subsi - pathian Foredeep (Karnkowski, 1974; Danysh et al., 2004; dence, namely dur ing the early Miocene, early Badenian, Oszczypko et al., 2006; Bu³a & Habryn, 2011). The epi- and late Badenian to Sarmatian times. The Mio cene con ver - Variscan plat form de vel oped on con sol i dated Pro tero zoic gence of the Carpathian wedge with the foreland caused crys talline rocks and Pro tero zoic–Lower Palaeozoic meta- outward migra ti on of the depocentre of the foredeep and in sed im en tary rocks. In the area ad ja cent to the Polish-Uk rai - its dis tal parts onlapping of suc ces sively younger depos it s nian bor der, the Carpathian Foredeep is super im posed on onto the foreland (Oszczypko, 2006a). Subse quent erosion the NW–SE trending Teisseyre–Tornquist Zone, a major was cal cu lated, based on illite/smectite diagenesis, at some crustal boundary betwee n the West- and the East-Euro pean 400–800 m in SE Poland (Dudek, 1999). Cratons (Oszczypko et al., 2006). This com plex struc ture The discusse d area of the Carpathian Foredeep, subdi - con trolled the sub se quent evo lu tion, and in par tic u lar in the vided by the Rzeszów Palaeoridge into the west ern and Polish-Ukr ainian bor der area where during fi nal, Bade- eastern parts (Oszczypko, 1999), contai ns a re duced secti on nian–Sarmatian de vel op ment, the in her ited Me so zoic rift- of the lower and middle parts of the Badenian sec tion. re lated tectonic zone was re acti vated due to the combined In the west ern part of the Carpathian Foredeep, the effect of thrust wedge em placem ent and slab-pull of fore- most widesprea d Lower Badenian unit is the Skawina For- land plate; this zone focuse d Miocene exten sion re lated to mation – deep-water, clayey-marly and clayey-muddy de - the flex ure of the Carpathian fore land plate (Krzywiec, posit s with inter cala ti ons of sands and gravels that increa se 1999). in abundance towards the south (Garlicki, 1979); the thick- The Carpathian Foredeep basin can be subdi vided, ness of the Skawina Form ati on ranges from several tens of based on the tectoni c style and type of sedi m ents, into two metres in the northern part of the foredeep to 800 m in the parts: inner and outer. The inner foredeep is locat ed at the Gdów Bay (Ney et al., 1974; Garlicki, 1979). It is followed front and beneat h the Carpathian Overthrust and is charac - by the Wieliczka Form ati on (30–100 m thick) com posed of ter ized by strongly folded Eggenburgian–Badenian strata: chlo rides with siliciclastic in ter ca la tions ac com pa nied by Sambir (Stebnik) Nappe and Zg³obice thrust-sheets in Po - tuffites and bentonites. The Wieliczka Form ati on occurs land and Boryslav-Pokuttya and Sambir Nappes in the along the Carpathian Overthrust and towards the north is Ukraine (Andreyeva-Grigorovich et al., 1997, 2008); it passing into the Krzy¿anowice Form ati on, mainly com - contai ns up to 1,500 m of Lower to Middle Miocene posed of lam inate d anhyd rit es 10–30 m thick (Peryt, 2000). autochthonous de posit s that are overrid den by thrust sheets The evaporites are overlai n by the depos it s of the Machów (Oszczypko, 2006a, b; Oszczypko et al., 2006). The outer For mation – mostly marly clays with laminae and in ter ca la - foredeep (termed the Bilche-Volytsia zone in the Ukraine) tions of fine-grained sandstone s. The thickness of this for- is filled with gen er ally un dis turbed, flat-ly ing Mid dle Mio - mati on, that include s the Upper Badenian and the Sarma- cene (Badenian and Sarmatian) marine de pos its, which tian, may reach 2000 m betwee n Tarnów and D¹browa Tar- range in thickness from a few hundred metres in the north- nowska (Ney et al., 1974; Krzywiec et al., 2008). ern, marginal parts of the Foredeep to as much as 3.5 km in In the area of the Rzeszów Palaeoridge, the sub strate is SE Poland (Wielkie Oczy graben – Oszczypko et al., 2006) covered by the Upper Badenian Grabowiec beds (60–300 m and 5 km in western Ukraine (Krukenychy Depres sion – thick) – clayey-muddy de posit s with tuffites and bentonites Kurovets et al., 2004). General struc ture of the Polish and lo cal sandy in ter ca la tions, fol lowed by a con sid er ably (Stebnik Unit and the Outer Carpathian Foredeep basin) and thicker (1,000–2,000 m) com plex of the Krakowiec beds Ukrai nian (Boryslav-Pokuttya, Sambir and Bilche-Volytsia having a sim ilar li thol ogy but include d into the Sarmatian. units) parts of the Carpathian Foredeep is present ed in Fig. South of the Rzeszów Palaeoridge, also older Badenian 2A and Fig. 2B, re spec tively. strata oc cur below the Grabowiec beds: Chodenice beds (up The Lower Miocene depos it s of the Carpathian Fore- to 50 m thick), evaporites of the Krzy¿anowice and Wie- deep formed a clastic wedge along the Carpathians, com pa - liczka form ati ons (up to 30 m thick), and Skawina Form a- ra ble with the Lower Fresh wa ter Molasse of the Al pine tion (0– 150 m thick) (Kotlarczyk, 1991). Fore land (Oszczypko et al., 2006); they formed from rel a- East of the Rzeszów Palaeoridge, the Badenian sec tion tively shal low subaque ous turbidite current s as well as from begins with the Piñczów Form ati on in Poland and the subaerial de bris flows. The early Badenian regional trans - Zhuriv Form ati on in the Ukraine (both form ati ons in the gression (Oszczypko, 1998) re sulted in varia ti ons in the older lit er ature were also termed the Baranów beds) – palaeobathymetry of the early Badenian ba sin be tween up - quartz sandstone s, sandy clays and clays, with com mon per bathyal depths in the axis of the basin to neritic and lit to- basal conglom erate s. The thickness of the Baranów beds ral ones in the marginal parts. The deepest part of the basin, varies usuall y from several to 90 m, and increa ses towards which reached midbathyal depths, was locat ed about 20 km the north and the east. They are overlai n by lam inate d and south of the pres ent-day posi ti on of the Carpathian front and brecciated an hyd rites of the Krzy¿anowice For mation (in it was dom i nated by de po si tion of dark-grey cal car e ous Poland) and Tyras For mati on (in the Ukraine), a few to over mudstones with sporadi c inter cala ti ons of fine sandstone s 40 metres thick, and these in turn by the clayey-marly and (Skawina, Balych and Bohorodchany form ati ons; Osz- sandy depos it s of the Machów Form ati on in Poland. Its czypko et al., 2006). Some depos it s reveal ed basinal tur- lower member is the Spirialis Clays Mem ber (100 m thick), bidite charac ter isti cs, and the southern slope and its shelf passing toward the margin of the basin into the Pecten Clays 526 M. J. KOTARBA ET AL. n o i t a c o l

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Fig. 3. Distri bu tion of gas, gas-con den sate and oil de posits of the lithostratigraphic for mations of the Ukrai nian Carpathian Foredeep. L. – Lower, U. – Upper

and Marls Mem ber (Ney, 1969). The upper, Sarmatian part PE TRO LEUM OC CUR RENCE of the Machów Form ati on is com posed of the Krakovets Clays (Ney et al., 1974) that in the northern part of the fore- The frontal part of the Polish and Ukrai nian Carpa- deep are de vel oped as the Syndesmia Clays Mem ber (Ney, thians con sti tutes one of the old est pe tro leum-pro duc ing re- 1969). The thickness of the beds ranges from 200 m near gions in the world. The exploi tati on of oil started in 1853 in Bi³goraj to 2,400 m in the Przemyœl region. The se ries con- Bóbrka vil lage near Krosno in Poland (in Austro-Hungar ian sists mostly of clays with sandy laminae and to wards the Mon ar chy at that time) in the Outer (Flysch) Carpathians, south the frequency and thickness of the sandy occur rence s and nat ural gas produc ti on in 1920 in Dashava vil lage near in crease: they were in ter preted as wide spread delta de pos its Boryslav in Ukraine (in Po land at that time) in the auto- (Jawor, 1983; Karnkowski, 1989). In the Ukraine, the Up- chthonous Mio cene strata of the Bilche-Volytsia Unit of the per Badenian strata are include d into the Kosiv Form ati on Carpathian Foredeep (Vul et al., 1998b; Karnkowski, 1999; (10–150 m thick) and the Sarmatian strata into the Dashava Fedyshyn et al., 2001). At the end of the 1930s, pe troleum Form ati on that is further subdi vided into two parts, the explo ra ti on started in the Carpathian Foreland, and in the lower part (up to 3,500 m thick) and the upper part (up to years 1945–1955 petro leum indus try in Poland and Ukraine 1,500 m thick) (see for detai ls Kurovets et al., 2004). The had to be re stored and re or gan ised. The ex plo ra tion ac tiv ity mar ginal fa cies of the Dashava For mation con tains in ter ca - was focuse d mainly on the Outer (Flysch) Carpathians and la tions of limeston es within mostly clayey se ries; it is up to partly on the Carpathian Foredeep (Karnkowski, 1994, 120 m thick and is termed the Volhynian beds. 1999). In ten sive de vel op ment of pe tro leum ex plo ra tion In some well pro files of the Bilche-Volytsia Unit, var i - took place in the middle of the 1960s and in the 1970s ous siliciclastic, of ten con glomer ates and brec cias, and car - (Karnkowski, 1999; Fedyshyn et al., 2001; Myœliwiec et al., bonate depos it s occur that are com monly regarded in the 2006; Popadyuk et al., 2006). Up to now, twenty-eight and geolog i cal unpub li shed documentations as well as in the eleven gas and gas-con den sate fields within the Palaeo- synthe ses based on those documentations (e.g., Vul et al., zoic–Meso zoic base ment of the Polish part of the Carpa- 1998a; Krups’kyy, 2001) to be of Karpatian (and/or Pala- thian Foredeep (from 1948), and within the Meso zoic base - eogene) age. These de posit s are recent ly include d into the ment of the Ukrai nian part of the Carpathian Foredeep Sandy-Calcar eous Series and its Karpatian age is con cluded (since 1944), re spec tively, were dis cov ered. Hy dro car bon (Vashchenko et al., 2007), but the fossil s quoted as support - (mostly gas, and gas-conden sat e, and only local ly oil) accu - ers of such an age as signm ent have long ranges and indi cat e mula ti ons occur in the autochthonous Miocene strata (e.g., only their Miocene prove nance . On the other hand, the re- Karnkowski, 1999; Boyko et al., 2004; Fedyshyn et al., gional strati graphical corre la ti ons and palaeogeographic re- 2001; Kurovets et al., 2011) of the outer part of the Carpa- construc tions do not support the pres ence of Karpatian thian Foredeep. Gas ac cum ula ti ons occur ring in the Middle strata in the Bilche-Volytsia Unit (e.g., Andreyeva-Grigo- Mio cene strata show a clear pat tern, and there are three zo- rovich et al., 1997; Oszczypko et al., 2006) and hence it is nes that accord ingl y extend close to the margin of the Car- possi ble that the depos it s include d into the Sandy-Cal care - pathian Overthrust, at the front of Carpathian orogen, and ous Serie s are transgressive Lower Badenian depos it s that along the north-eastern limit of the outer Carpathian Fore- should be under lai n by, or include d into, the Bohorodchany deep (e.g., Karnkowski, 1999; Krups’kyy, 2001); the larg- Form ati on (Suite) (Fig. 3). est gas de pos its oc cur in the Lower Sarmatian strata close to 528 M. J. KOTARBA ET AL.

the Carpathian Overthrust (Karnkowski, 1999; Fedyshyn et more di vergent. For the Jod³ówka area, the maxi m um value al., 2001). is only 200 mD, but in the Husów area it can be up to 800 There are 101 gas and gas-conden sat e depos it s (first mD (Karnkowski, 1999). Exam ples of reser voirs connect ed discov ery in 1945) in the autochthonous Miocene strata of with deltaic sed i men tary en vi ron ments are the Biszcza, the Polish Carpathian Foredeep as well as 44 gas and gas- Wola Obszañska, Ksiê¿pol, £azy, £êkawica gas depos its. conden sat e depos it s and one oil deposit in the Bilche- The deltaic type of sedi m ents occurs at a depth of 400– Volytsia Unit of the Ukrai nian Carpathian Foredeep (first 800 m in the Biszcza area. Their poros it y is 15–32% and discov ery in 1920). Al though the share of depos it s dis cov- permeability 900 mD (Myœliwiec, 2004a). Glauconitic ered in the pre-Mio cene strata has in creased (Kotarba et al., sandstones of characteristic green colour are good qualit y 2011a), the Miocene depos it s rem ain the main host for gas reservoir rocks with poros it y ranging from 5 to 25% (Myœli- depos it s, and their signif i canc e is relat ed to a low de gree of wiec et al., 2006). The sec ondary poros it y of the Middle tec tonic dis tur bance and the pres ence of un re cog nised, broad Badenian diage netic an hyd rites in the RoŸwienica gas ac cu - struc tures with per fect res er voir ho ri zons (Karnkowski, mula ti on ranges betwee n 4.3 and 16.1%; the increa se in po - 1999). The form ati on of vari ous traps was signif i cant ly con- rosit y and perm eabil ity was caused by the inter acti on be- trolled by diver sified basem ent palaeorelief and vari able sed - tween an hyd rites and hy dro car bons (Myœliwiec, 2004a; im entar y condi ti ons, which were crucia l for the form ati on of Myœliwiec et al., 2006). The so-called Dzików – blocky vari ous hydro car bon traps, such as those related to pinch-outs sandstone s res ervoir rocks are locat ed along the fault zone and facies changes. Prin cipally, layered and massive types of of the Uszkowce-Lubaczów area. The blocky sandstone s depos it s occur there (Karnkowski, 1999). are excel lent reser voir rocks, with a poros it y of 15–35% and per meabil ity of 200 mD (Myœliwiec et al., 2006). The oc currence of traps for gas accu m ula ti ons in the Polish part of the Carpathian Foredeep Mio cene was deter mined by two main fac tors: palaeomor- Petro leum depos it s in the Pol ish sector of the Carpa- phology of the pre-Miocene base ment and defor m ati on in- thian Foredeep are ac cum ulat ed in both Neogene com plex fluence of the Carpathian Overthrust (Karnkowski, 1999; (Fig. 1A) and in its base ment (Kotarba et al., 2011b). Myœliwiec, 2004b). The major type of traps in the Miocene The pe troleum reser voir rocks in the autochthonous of the Carpathian Foredeep are: (i) compactional anticlines Miocene strata of the outer part of the Polish Carpathian sit u ated above base ment up lift (e.g., Rudka, Palikówka, Foredeep are sandstone s origi nate d in var ious depositional Kañczuga de pos its), (ii) traps sealed by the Carpathian environments (Myœliwiec et al., 2006): (i) Lower Badenian Overthrust plane (e.g., Przemyœl, Albigowa-Krasne, Husów glauconitic sandstone s – the Baranów beds (Kury³ówka, and Pilzno fields), (iii) traps sealed by faults (Mo³odycz, Rokietnica and Sarzyna gas de posit s), (ii) Middle Badenian Jaros³aw, Radymno and Sarzyna depos it s), (iv) stratigra- evaporitic de pos its (RoŸwienica gas ac cu mu la tion), (iii) phic pinch-out traps (e.g., the Husów field), and (v) strati- Upper Badenian–Lower Sarmatian – sandstone s depos it ed graphic traps re lated to uncon form iti es (the Tarnów area in subm arine fan, deltaic and shallow marine envi ron m ents and the deepest hori zons of the Rudka deposit ). In some (e.g., Przemyœl, Jod³ówka, Albigowa-Krasne, Husów, ¯o- cases, traps in Mio cene strata may be formed by combined ³ynia, Rudka, Biszcza, Wola Obszañska, Ksiê¿pol, £ukowa, mech a nism of seal ing (e.g., Husów, Przemyœl, and Rudka Dzików, Cha³upki Dêbniañskie, Grodzisko Dolne, £azy, gas de pos its). £êtowice, Wierzchos³awice, Borek, D¹brówka gas de pos - its), and (iv) Lower Sarmatian – massive sandstone s (blocky Ukrainian part of the Carpathian Foredeep sandstone s of the Dzików gas de posit). More over, Eocene and Oligocene depos it s were discov ered in the north-east - Gas, gas-conden sat e and oil depos it s (Fig. 1B) have ern part of the Carpathian Foredeep. A gas ac cum ula ti on in been discov ered within the autochthonous Miocene strata, Oli- gocene sandstone s was discov ered in Luchów 3 well com bin ing Mio cene and Me so zoic base ment res er voirs in (My- œliwiec et al., 2006). Oligocene sandstone s have very the Bilche-Volytsia Unit of the Ukrai nian Carpathian Fore- good res er voirs pa ram e ters, with av er age po ros ity of 26% deep (Vul et al., 1998b; Shcherba et al., 1987). and average perm eabil ity equals to 1250 mD. Seven fields contai n conden sat e and/or gas in the com- The sandstone s depos it ed in the subm arine fan and bined Meso zoic –Miocene reser voirs (Figs 1B, 3): the Rudky deltaic envi ron m ents are the most im portant and produc ti ve field – gas and conden sat e in the Upper Juras sic -reser voir Upper Badenian–Lower Sarmatian res ervoir rocks. The and five gas accu m ula ti ons in the Lower Dashava Form a- submarine fan sandstone com plex oc curs at a depth inter val tion of the Lower Sarmatian, the Vereschytsia field – gas in of 1,000–3,500 m in the southern part of the Carpathian the Up per Ju ras sic–Lower Badenian Sandy-Cal car e ous Se - Foredeep – an area in front of and under the Carpathian ries, the Medenychi field – gas and conden sat e in the Upper Overthrust (Myœliwiec et al., 2006). The thick beds of the Cre ta ceous–Lower Badenian Sandy-Cal car e ous Se ries, the subm arine fan sandstone s are the prim ary gas reser voirs in Bilche-Volytsia field – gas and conden sat e in the Upper the Przemyœl, Jod³ówka, Husów, Albigowa-Krasne areas Cre ta ceous–Lower Badenian Sandy-Cal car e ous Se ries and (Myœliwiec et al., 2006). The aver age poros it y of the sand - four gas accu m ula ti ons in the Lower Dashava For- mation stone reser voirs, deter mined from cores, varies from 9 to of the Lower Sarmatian, the Uhers’ko – gas and conden sat e 21% for the Przemyœl gas field (Karnkowski, 1999). A sim i- in the Up per Cre taceous– Lower Badenian Sandy- Cal car e - lar poros it y value was eval uate d for the Jod³ówka, Albi- ous Serie s an and five gas accu m ula ti ons in the Upper and gowa-Krasne, and Husów res ervoirs. The perm eabil ity is Lower Dashava Form ati on of the Lower Sarmatian, the MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 529

Pivdenne Uhers’ko – gas in the Upper Creta ceous – Lower traps in the Lower Badenian Sandy-Cal car eous Series and Badenian Sandy-Cal car e ous Se ries and four gas ac cu mu la- Baraniv beds are sealed by the Up per Badenian Tyras For ma - tions in the Lower Dashava Form ati on of the Lower Sar- tion depos it s. The Badenian strata are gas-bearing mainly in matian, and the Letnia field – gas and conden sat e in the Up- the cen tral and south-eastern parts, while the Lower Sar- per Ju ras sic–Lower Badenian Sandy-Cal car e ous Se ries, matian strata contai n the major part of gas fields of the north- Up per Cre ta ceous–Lower Badenian Sandy-Cal car e ous Se - west ern part of the Bilche-Volytsia Unit. The res er voir rocks ries and nine gas accu m ula ti ons in the Lower Dashava For- repre sent sandstone lay ers with poros it y of 20–30%, local ly mati on of the Lower Sarmatian strata. The Orkhovychi 30–40% and perm eabil ity from 0.1 mD to 1,595 mD. The deposit contai ns oil in Upper Juras sic –Lower Badenian Badenian res er voirs are of ten lithologically sealed within the Sandy-Cal car e ous Se ries res er voir (Kotarba et al., 2011b). Badenian se quence, while the Lower Sarmatian ones form The re maining thirty-seven depos it s contai n gas accu - an ticli nal struc tures, sealed from the top by the Lower Sarma- mula ti ons in Miocene reser voirs (Figs 1, 3). The Mala Ho- tian clayey rocks, and are of ten fault-sealed later ally. rozhanna field contai ns gas in the Lower Badenian Sandy- Cal car e ous Se ries and Up per Badenian Tyras For ma tion. The Hai field contai ns gas and conden sat e in the Lower Ba- HY DRO CAR BON PO TEN TIAL denian Sandy-Cal care ous Serie s and in two of the Lower OF ORGANIC MATTER IN THE POLISH Dashava Form ati on of Lower Sarmatian hori zons and gas in AND UKRAI NIAN PARTS OF THE OUTER one Lower Sarmatian hori zon. The Zaluzhany field contai ns CARPATHIAN FOREDEEP gas and conden sat e in one and gas in twelve accu m ula ti ons in the Upper and Lower Dashava Form ati on. The Hrushiv The re sults of or ganic geo chem is try anal y ses en able the field contai ns gas and conden sat e in one and gas in two ac- pre lim i nary as sess ment of the dis persed or ganic mat ter cum ula ti ons in the Lower Dashava Form ati on. The Hru- (DOM) contai ned in autochthonous Miocene sequence of divka field contai ns gas in the Lower Badenian Sandy-Cal- the Polish and Ukrai nian parts of the Outer Carpathian car eous Series and in three of the Lower Dashava For ma - Foredeep (Kotarba, 1999; Kotarba & Koltun, 2006; Kotarba tion, and the Hlynky and Pivdenne Hrabyne fields – in the et al., 1998b, 2005, 2011a). The study was mainly focuse d Lower Badenian Sandy-Cal car e ous Se ries res er voirs. The on rec og ni tion of hy dro car bon po ten tial of Up per Badenian Svydnytsia field contai ns gas in the Upper Dashava Form a- and Lower Sarmatian strata due to the con sid er able thick - tion of the Lower Sarmatian, and the Vyzhomlia, Nyklo- nesses of those lithostratigraphic units and the enclos ed vychi, Makuniv, Novosilky, Mainych, Susoliv, Shidne great claystone-mudstone potential source rocks. Dovhe, Opory, Kavske, and Tyniv fields – in the Lower Da- Small thick nesses of the Lower Badenian strata are the shava Form ati on; the Vyshnia, Khidnovychi, Sadkovychi, reason for poorer geochem ical recog ni ti on. In the Lower Ba- Pyniany, Dashava, and Horodok fields contai n gas in the denian Sandy-Cal car eous Series, the Lower Badenian Bara- Upper and Lower Dashava Form ati on; the Kadobna, Kosiv, niv beds and the Upper Badenian Tyras Form ati on of the and Chornohuzy fields – in the Upper Badenian and Lower Ukrainian Outer Carpathian Foredeep (Bilche-Volytsia Sarmatian; the Turady, Hrynivka, Bohorodchany, Debesla- Unit), the total organic carbon (TOC) content s are low and vtsi, Yabluniv, Kovalivka, Sheremetivka, and Krasnoils’k range from 0.00 to 0.76 wt% (aver age 0.28 wt%) (Kotarba fields – in the Upper Badenian; and the Cheremkhiv-Strup- & Koltun, 2006, 2011). This Miocene sequence consis ts of kiv and Pylypy fields – in the Lower Badenian res ervoirs. im mature , terres tri al, gas-prone and marine , oil-prone or- They occur along the whole Bilche-Volytsia Unit, though a ganic matter, which has gen er ated mainly the mi cro bial me- major part of them is concen tra ted in its north-west ern part thane and small amounts of micro bial ethane. In the Lower (Fig. 1). In these de posit s the traps are lithologically and Badenian Baraniv beds and in the Tyras For mati on, an input tecton i cally-sealed. In par tic u lar, of ten the Stebnyk Over- of marine organic matter was discov ered. More over, in the thrust serves as a seal, e.g., in the Uhers’ko, Zaluzhany, vicin it y of Kokhanivka, the rocks of the Tyras For mati on Khidnovychi, Sadkovychi, Kavs’ke, and Hrynivka fields. were de posit ed under hypersaline condi ti ons, as reveal ed by However, in addi ti on to this, the major part of the gas fields the presence of marine plankton (Kotarba & Koltun, 2011). of the Bilche-Volytsia Unit oc cur in the el e vated struc tures, In the Upper Badenian strata, the total organic carbon e.g., Svydnytsia, Rudky, Letnia, Bilche-Volytsia, Uher- (TOC) content s vary from 0.02 to 1.48 wt% (aver age 0.75 s’ko, Dashava and other fields. wt%) in the Pol ish Carpathian Foredeep (Kotarba, 1999; Com monly, there are several produc ing hori zons in a Kotarba et al., 1998b, 2005) and from 0.44 to 2.01 wt% field: in the Bilche-Volytsia Unit 175 such ho ri zons ex ist in (average 0.96 wt%) in the Ukrai nian Carpathian Foredeep 40 fields, and 165 in the Upper Badenian and Lower Sarma- (Kotarba & Koltun, 2011). In the Lower Sarmatian strata, tian strata (Boyko et al., 2004). the TOC var ies from 0.02 to 3.22 wt% (aver age 0.69 wt%) The Mio cene se quence con tains the major part of fields in the Pol ish Carpathian Foredeep (Kotarba, 1999; Kotarba in the area and comprises sev eral lev els of res er voir rocks. et al., 1998b, 2005) and from 0.01 to 1.45 wt% (aver age The Lower Badenian Sandy-Cal car e ous Se ries, oc cur ring at 0.71 wt%) in the Ukrai nian Carpathian Foredeep (Kotarba the base of the Mio cene, are gas- and con den sate-bear ing in & Koltun, 2011). Num erous TOC result s from the Polish several fields in the north-western part of the Bilche-Voly- Oil & Gas Com pany lab o ra to ries (un pub lished data) are tsia Unit. The poros it y of these rocks ranges be tween 6 and very sim ilar to those data and reach up to 5.1 wt% (aver age 31% (mean 13%), perm eabil ity is usuall y less than 0.1 mD, 0.88 wt%) and 3.4 wt% (aver age 0.82 wt%) in the Upper but local ly reaches 270 mD (Shcherba et al., 1987). The Badenian and the Lower Sarmatian strata, re spec tively 530 M. J. KOTARBA ET AL.

Fig. 4. Contour map of median val ues of the to tal or ganic car bon (TOC) con tent in sec tions of ana lysed wells within the Upper Badenian strata of the Carpathian Foredeep. Lith. – Lith u a nia

(Kotarba et al., 1998b). Accord ing to Dickey and Hunt presence of small amounts of algal (marine and/or non-ma- (1972), the TOC concen tra tion over 0.5 wt% is require d to rine) DOM. In the Kraków-Rzeszów area, only gas-prone qual ify a rock as hav ing hy dro car bon po ten tial. Dis tri bu- Type III kerogen (Fig. 9C) has been found. The lack of ob- tions of median TOC content s in the ana lysed profil es of vious depth trends of both the Tmax tem per ature and HI val- wells in the Up per Badenian and Lower Sarmatian strata are ues (Figs 8B, C, 9B, C) advo cat es the im mature gas-prone shown in Figs 4 and 5, respec tively. The distri buti on of me- char acter of the en tire Up per Badenian and Lower Sarma- dian TOC content s in the Upper Badenian strata gener all y tian se quences. The imm ature ter res trial DOM shows Rock decrea ses from the south to the north (Fig. 4), while in the Eval Tmax tem pera ture below 435°C (Espitalié & Borde- Lower Sarmatian strata the low est val ues are ob served in nave, 1993). The Tmax tem pera ture values at the bottom of the southern zone near the Carpathian Overthrust, increas- the Up per Badenian and Lower Sarmatian strata var ies from ing northwards (Fig. 5). about 390 to 435°C (Figs 8B, 9B). Only in three sam ples Geo chem i cal anal y ses (Rock-Eval, biomarkers and sta- from the Upper Badenian strata (Jo-4 and Rn-6 wells, ble isotopes) of DOM (Kotarba et al., 1998b, 2005; Kotarba depths 3,300–3,500 m), the Tmax 435–438°C (Fig. 8B) was & Koltun, 2006, 2011) indi cat e the general dom inance of measured . Such Tmax values togethe r with no obvi ous depth the terres tri al Type III kerogen in both the Upper Badenian trends (Figs 8B, 9B) indi cat e that down to the depths of 3,300– and Lower Sarmatian se quences. This DOM mainly con - 3,500 metres the Mio cene ter res trial DOM is imma tur e and sists of vitrinite-group macerals (from 70.9 to 84.0 %) with generates al most ex clu sively mi cro bial meth ane. The ini tial the complete ab sence of the liptinite group macerals phase of low-tem pera ture thermogenic proces s procee ds be- (Kotarba et al., 1998b), which also supports the concept of neath these depths, under the Outer Carpathian Overthrust. its terres tri al or igin. Moreover, such an ori gin has been also Dis tri bu tion of vitrinite reflectance val ues of the Mio cene con firmed by the re sults of el em en tal analy ses of the Mio - DOM with depth con firms its low matu rity (Kotarba et al., cene fos sil remnants (Kotarba et al., 1987). Terres tri al 1998b). The matu rit y of DOM in the Upper Badenian strata DOM disperse d in claystones/mudstones ca pable of gener - gener all y increa ses from the north to the south and the south- ating and expel li ng oil should have HI val ues typi cal ly east from about 400 to about 435°C (Fig. 10), while in the higher than about 200 (Hunt, 1991). Dis tri buti on of median Lower Sarmatian strata Tmax changes only from about 420 to HI val ues in the Upper Badenian and Lower Sarmatian about 430°C, and decrea ses to the south-east (Fig. 11) strata usuall y does not excee d a value of 200 mg HC/g TOC (Figs 6, 7) and only in a small area of the Upper Badenian OR I GIN OF NAT U RAL GASES strata north of Le¿ajsk this value is increa sed up to 213 mg HC/g TOC (Fig. 6). The HI val ues close to that limit were The sta ble car bon and hy dro gen isotopic com po si tion found in a few sam ples from the Upper Badenian strata in of methane can be sensi ti ve indi ca tors of gas ori gin and mi- the Rzeszów-Stryi area (Fig. 8C), which may sug gest the grati on. Carbon in methane has a wide range of d13C val ues MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 531

Fig. 5. Contour map of median val ues of the to tal or ganic car bon (TOC) con tent in sec tions of ana lysed wells within the Lower Sarmatian strata of the Carpathian Foredeep. Lith. – Lith ua nia

Fig. 6. Contour map of median val ues of the hy dro gen in dex (HI) in sec tions of ana lysed wells within the Upper Badenian strata of the Carpathian Foredeep. Lith. – Lith u a nia 532 M. J. KOTARBA ET AL.

Fig. 7. Contour map of median 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 due to a vari ety of chem ical , phys ical and micro bial pro- duced from the same source organic matter but during the cesses that cause sig nif icant vari a tions in the iso to pic com- suc ces sive gen er a tion stages and mi gra tion dis tance (Berner posi ti ons of the source organic matter and the daughter & Faber, 1996; Kotarba et al., 1994; Prinzhofer & Pernaton, gases. The anal yses of sta ble carbon and hy drogen isotopes 1997; Prinzhofer et al., 2000; Whiticar, 1994). Both the ex - in methane enable s the identi fica ti on of the source organic per i men tal data and the o ret i cal cal cu la tion dem on strated matter , from which gases were gener ate d during micro bial that sta ble carbon isotope studies of methane, ethane and or thermogenic proces ses (e.g., Berner & Faber, 1996; propane are es senti al for deter mining the type and the matu - Kotarba, 1995). Methane gener ate d during thermogenic ra tion de gree of the source or ganic matter in vitrinite reflec- proces ses from the sapropelic or ganic matter shows d13C tance scale (Berner & Faber, 1996; Whiticar, 1994). An im - values from –55 to –30‰ whereas that produced from the por tant im pli ca tion of the di a grams for ge netic in ter pre ta- humic or ganic matter has d13C from –30 to –20‰. In con- tion is that a linear de pendence of carbon isotopes values in trast, microbially gen er ated meth ane has typ ically very low methane, ethane, propane, butanes and propanes to their re- d13C val ues (less than –55‰), even below –100‰. Mi cro- cip ro cal car bon num ber is not an ex clu sive in di ca tor of the bial gases can be gen er ated by two pro cesses: meth ane fer - ori gin of natu ral gas from a single source, as it was assum ed men ta tion and car bon di ox ide re duc tion (Mar tini et al., (e.g., Rooney et al., 1995). Inste ad, Kotarba et al. (2009) 1996, 1998; Whiticar et al., 1986). Ethane, and in insig nif i - suggest ed that in such plots a “dogleg” trend, exem pli fied cant quanti ties, propane have been gener ate d from micro - by rel a tively 13C-de pleted meth ane and 13C-enric hed pro- bial proces ses (Claypool, 1999; Gautier & Claypool, 1984; pane in com pari son to ethane, in dicat es that such nat ural gas Hinrichs et al., 2006; Jenden & Kaplan, 1986; Lillis, 2007; was not gener ate d from a sin gle source rock or that it has Oremland et al., 1986; Taylor et al., 2000). Re sults of stable under went post-gener a t ion alt erat ions (e.g., second ary gas carbon isotope analy ses of ethane, propane, butanes and cracking, micr obial oxi da t ion or thermochemical sulpha te pentanes allowe d the prepa ra ti on of more precis e geneti c reduc t ion). Moreover, rela ti vely more 13C-de pleted meth - clas si fi ca tion of nat u ral gases, i.e. the disti nguish ing of ge - ane in rela ti on to ethane can be appli ed to evalu ate mixing netic groups and enable d the identi fica ti on of migra ti on and pro por tions be tween mi cro bial meth ane and thermogenic mixing of either genet i cal ly differ ent gases or gases pro- gases (Kotarba & Lewan, 2004; Kotarba et al., 2009). MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 533

13 13 Fig. 8. (A) To tal or ganic car bon, (B) Rock-Eval Tmax tem per a ture, (C) hy dro gen in dex, (D) d C(CH4), (E) d C(C2H6) and (F) 13 d C(C3H8) ver sus depth of (A, B and C) occur rence of organic matter and (D, E and F) nat u ral gas ac cu mu la tions within the Badenian and Lower Sarmatian res er voirs of the Carpathian Foredeep be tween Rzeszów and Stryi. Rock sam ple code (A, B and C) – see Kotarba et al. (1998b, 2005, 2011a), and gas sample code (D, E and F) – cf. Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005)

Most of the ana lysed gases were col lected from the Up - (Figs 8D, E and 9D, E). Thermogenic methane oc curs only per Badenian and Lower Sarmatian strata (Kotarba, 1998, in the BrzeŸnica, £êkawica and Tarnów ac cu mu lations in 2011). Re sults of stable carbon isotope analy ses of methane, the Polish Carpathian Foredeep and Pyniany and Zaluzhany ethane, propane, butanes and pentanes, and stable hy drogen accu m ula ti ons in the Ukrai nian Carpathian Foredeep (Figs isotope analy ses of methane (Figs 12–15) and spatial distri - 8D, E and 9D, E). Moreover, thermogenic ethane oc curs in 13 13 buti ons of both d C(CH4) and d C(C2H6) val ues in these the Kury³ówka (Ku-3), Hai and Letnia ac cum ula ti ons of the strata (Figs 16–19) indi cat e that the gas eous hy drocar bons Rzeszów-Stryi area (Fig. 8E). Pro pane is mainly thermo- were mainly gen erate d during micro bial proces ses, and spo- genic in ori gin (Figs 8F, 9F, 14, 15). Mi crobial propane oc - rad i cally dur ing low-tem per a ture thermogenic pro cesses. curs in the ¯o³ynia-Le¿ajsk (Zn-25) and Bilche-Volytsia In sig nif i cant changes in val ues of sta ble car bon iso tope (BV 500) ac cu mu lations of the Rzeszów-Stryi area (Fig. rati os of methane and ethane with depth suggest quite uni- 8F). form gen er a tion con di tions of mi cro bial meth ane and eth- In the Polish Carpathian Foredeep, in the RoŸwienica ane in the en tire Badenian and Lower Sarmatian sequen ces field, gas is accu m ulat ed in the Lower (and partly Upper) 534 M. J. KOTARBA ET AL.

13 13 Fig. 9. (A) To tal or ganic car bon, (B) Rock-Eval Tmax tem per a ture, (C) hy dro gen in dex, (D) d C(CH4), (E) d C(C2H6) and (F) 13 d C(C3H8) ver sus depth of (A, B and C) occur rence of organic matter and (D, E and F) nat u ral gas ac cu mu la tions within the Badenian and Lower Sarmatian res er voirs of the Carpathian Foredeep be tween Kraków and Rzeszów. Rock sample code (A, B and C) – see Kotarba et al. (1998b, 2005), and gas sample code (D, E and F) – cf. Kotarba (1998, 2011)

Badenian sequence ; in the Lubaczów and Uszkowce de pos- Ukrai nian Carpathian Foredeep, eight fields (Rudky, Vere- its it occurs in the Lower Badenian strata and in the topm ost schytsia, Medenychi, Mala Horozhanna, Bilche-Volytsia, parts of Upper Juras sic carbon ate s (Kotarba, 1998, 2011). Uhers’ko, Pivdenne Uhers’ko and Letnia) contai n conden - The gas from the Tarnów, £êkawica and BrzeŸnica de pos its sate and/or gas, and one Orkhovychi de posit contai ns oil in migrated from the Meso zoic base ment to the Mio cene strata com bined Me so zoic–Mio cene res er voirs. The mi cro bial and is typi cal ly thermogenic. This gas was gener ate d from gases (methane, hy drogen and partly ethane and propane) the oil-prone, marine organic matter (Type II kerogen) (Ko- generated dur ing micro bial proces ses within the Miocene tarba & Jawor, 1993; Kotarba, 2011). In both the RoŸwie- strata, then migrat ed to the Upper Juras sic and Upper Creta - nica and the Lubaczów depos it s, a very small adm ixture of ceous (Cenomanian) res er voirs in the Me so zoic base ment diage netic (and/or low-tem per a ture thermogenic) meth ane and to the bottom most Lower Badenian Sandy-Calcar e ous was identi fied (Kotarba, 1998). In the Lubaczów de posit the Series reservoirs of the traps of the above mentioned de- gas also filled up the trap in Upper Juras sic carbon ate s. Ge- posit s (Kotarba & Koltun, 2011). 13 13 net ically, this gas is a typ ical micro bial meth ane, which mi- In creased d C(CH4) and d C(C2H6) val ues in the Up- grated to the Upper Juras sic trap from the autochthonous per Badenian strata close to Tarnów town (Figs 16, 17) and Miocene strata along a fault zone (Kotarba, 1998). In the in Lower Sarmatian strata near Dêbica town (Figs 18, 19) in MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 535

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

Fig. 11. Contour map of val ues of Rock-Eval Tmax temper a ture at the bottom of the Lower Sarmatian strata of the Carpathian Foredeep. Lith. – Lith u a nia 536 M. J. KOTARBA ET AL.

13 Fig. 12. Hy dro car bon in dex (CHC) ver sus d C(CH4) for natu ral gases accu mu lated in the Miocene reser voirs of the Carpathian Foredeep in (A) Kraków – Rzeszów and (B) Rzeszów – Stryi ar eas. Compositional fields af ter Whiticar (1994). Gas sample code – see Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005). L. – Lower, U. – Up per

13 Fig. 13. d C(CH4) ver sus dD(CH4) for nat ural gases ac cu mulated in the Mio cene res er voirs of the Carpathian Foredeep in (A) Kraków – Rzeszów and (B) Rzeszów – Stryi ar eas. Compositional fields after Whiticar et al. (1986). Gas sample code – see Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005). L. – Lower, U. – Up per

the Polish part of the Carpathian Foredeep are caused by mi- MODEL OF MI CRO BIAL AND grati on of thermogenic gases from the Meso zoic base ment, THERMOGENIC GAS GEN ER A TION 13 13 while in creased d C(CH4) and d C(C2H6) val ues in the Lower Sarmatian strata near Boryslav, Drohobych and The hy dro car bon gen er a tion con di tions in the auto- Sambir (Figs 18, 19) in the Ukrai nian part of the Carpathian chthonous Mio cene were ana lysed in nine selected test ar - Foredeep are most proba bly caused by migra ti on of eas (Figs 20, 21). The A (Tarnów), B (Mielec-Le¿ajsk), F thermogenic gases, which were gener ate d within the Mio - (Pidluby) and (G) Rudky areas repre sent the part of the cene strata be neath the Outer Carpathians at greater depths, Carpathian Foredeep locat ed north of the present margin of from 3,500 to 12,000 metres (see chapter below). the Outer Carpathians. The C (Rzeszów), D (Przemyœl) and MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 537

13 13 13 Fig. 14. d C(C2H6) versu s d C(CH4) (A and C) and d C(C3H8) (B and D) for natu ral gases accu mu lated in the Miocene reser voirs of the Carpathian Foredeep in the (A and B) Kraków-Rzeszów and (C and D) Rzeszów – Stryi ar eas. Po sition of vitrinite reflectance curves for 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 per Badenian kerogen and d13C val ues = –25.7‰ for (C and D) Lower Sarmatian kerogen from the autochthonous Mio cene strata (Kotarba et al., 1998a, 2005). Gas sample 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- strata might have exist ed at a depth of about 11–12 km. The nous Miocene outer basin along the pres ent edge of the hy dro car bon gen er a tion mod el ling used the time-tem per a- Outer Carpathians (Figs 20, 21). The E (Krosno-Sanok) and ture (TTI) method (Waples, 1980). The time scale and re- I (Skole) ar eas were cho sen as an ex am ple of hypo thet ical gional Miocene corre la ti on were based upon Gradstein et deepest levels where the autochthonous Lower Miocene al. (2004), Czepiec & Kotarba (1998) and Oszczypko 538 M. J. KOTARBA ET AL.

Fig. 15. (A and B) Sta ble car bon isotope com posi tion of meth ane, eth ane, pro pane, butanes and propanes ver sus the recip ro cal of their car 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 the Carpathian Foredeep (A and B) in the Kraków – Rzeszów and (C and D) Rzeszów – Stryi ar eas. Structure of the graph for methane, ethane 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 sample code – Kotarba (1998, 2011), Kotarba and Koltun (2011) and Kotarba et al. (2005)

(2006a) whereas the geother mal gradi ent – 33°C/10–3 m is relat ed to the bottom of the Upper Badenian suc ces sion as the av er age value ob tained from palaeotemperature calcu la- the da tum surface . The top of the Lower Badenian Sandy- tions for the Miocene basin (Szafran, 1990) and cali brat ed Calcar eous Series se quence was the datum surface in the with vitrinite reflectance val ues (Kotarba et. al., 1998a). test ar eas E and I. Thicknesse s of stratigraphi c units were taken from the re- The model li ng for the test E and I areas indi cat ed the sults of drillings in the analysed parts of the outer basin. The pos si bil ity of gen er a tion of the higher gas eous hy dro car bon esti mated erosiona l re ducti on was 100 metres in A, B, F and (and even liquid hy drocar bons) beneat h the Carpathian G test ar eas and 250 metres in the rem aining ones. TTI mod - Overthrust, at about 7,500 m of depth (Fig. 21). At greater elli ng in the outer basin (test areas A to D and F to H) was depths, within the rel ics of autochthonous Lower Miocene MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 539

13 Fig. 16. Contour map of val ues of d C(CH4) at the bot tom of the Up per Badenian strata of the Carpathian Foredeep. Lith. – Lith u a nia

13 Fig. 17. Contour map of val ues of d C(C2H6) at the bot tom of the Up per Badenian strata of the Carpathian Foredeep. Lith. – Lith u a nia 540 M. J. KOTARBA ET AL.

13 Fig. 18. Contour map of val ues of d C(CH4) at the bot tom of the Lower Sarmatian strata of the Carpathian Foredeep. Lith. – Lith u a nia

strata of the inner basin, only the high-tem pera ture methane duced methane was about 5 cubic metres of CH4 per one cu- could be produced. The northern range of gener a ti on of bic metre of source rock. The vol ume of produced micro bial thermogenic hydro car bons was presented in Fig. 20. methane is spati ally variable within the basin and depends The re sults of model li ng reveal ed that in the A, B, F and on the burial history . As no dram atic changes of therm al G test ar eas (north of the margin of the Carpathians) the TTI condi ti ons have been de duced within the ba sin since the values were 0.05 to 1 (Fig. 21) proving that thermogenic Sarmatian, these mi cro bial gen er a tion pro cesses have pre - gases had not been gener ate d. At the edge of the Carpathian sum ably con tin ued un til re cent (Kotarba et al., 1995). Even orogen, the maxi m um TTI values were: 21 at a depth of the folding and upli ft of the Carpathian orogen and the con- about 3,600 metres in C test area, 4 at a depth of about 2,600 sequent regres sion of the Miocene sea at the break of the metres in D area, and 480 at a depth of about 5,000 metres in early and late Sarmatian did not impede the microbial D area. These val ues sug gest the ap pear ance of low-temper - processes. ature thermogenic proces ses within the autochthonous Mio - The re sults of model li ng reveal ed that both in the hy po - cene strata be neath the Carpathian Overthrust, i.e. be neath thet ical deep est test ar eas E and I the TTI values were 109 2,500 m depth. and 1010, respec tively, thus prompting a conclu sion that The re sults of model li ng for all the test areas proved the high-tem pera ture thermogenic zone started be low depth of gen er a tion of mi cro bial meth ane in al most en tire thick ness 7,500 m (Fig. 21). of autochthonous Miocene of the outer basin. Taking into ac count the quan ti ta tive cri te ria for mi cro bial gas gen er a - tion (Clayton, 1992), the inten sit y of methane produc ti on HY DRO CAR BON AC CU MU LA TION was cal cu lated for the gen er al ized sed i men ta tion con di tions AND EX PLO RA TION PROSPECTIVES of the Miocene marine basin. It was found that gener a ti on pro cesses of micro bial meth ane were most in ten sive at the The composition of natu ral gases ac cum ulat ed in auto- depth inter val of 700 to 1,500 m beneat h the Miocene sea chthonous Miocene strata of the Polish and Ukrai nian Outer floor (Fig. 21). The maximum yield of microbially-pro- Carpathian Foredeep is dom inate d by methane, which usu- MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 541

13 Fig. 19. Contour map of val ues of d C(C2H6) 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%. Methane was gener ate d by and Boryslav, Drohobych and Sambir (Figs 16–19) in the the car bon di ox ide re duc tion path way of mi cro bial pro- Ukrai nian part of the Carpathian Foredeep were most prob- cesses (Kotarba, 1998; 2011; Kotarba & Koltun, 2006, ably caused by the migra ti on of thermogenic gases, which 2011; Kotarba et al., 2005). Higher gaseous hy drocar bons were gen er ated within the Miocene strata beneath the Outer (mainly ethane and propane) are usuall y minor consti tu ents Carpathians. (concen tra tions less than 0.2 vol%). Ethane was gener ate d The high sedi m enta ti on rates togethe r with rhythm ic during both micro bial proces ses and at the ini tial stage of and cy clic de po sition of Mio cene clays and sands as well as the low-tem per a ture thermogenic pro cesses. Pro pane was the vig or ous gen er a tion of mi cro bial meth ane caused that gen er ated mainly at the ini tial stage of the low-temper a ture the gas produced in claystone beds was accu m ulat ed in the thermogenic proces ses and also during micro bial proces ses overlay ing sandstone s, and capped, in turn, by the succee d - (Kotarba, 2011). The micro bial methane is of vital eco- ing claystones. Re sults of the dis tri bu tion of imma tur e nomic im portanc e (Rice & Claypool, 1981; Rice, 1992, humic disperse d organic matter in the Upper Badenian and 1993). Lower Sarmatian sequen ces in di cates that it is prac tically Sta ble car bon iso tope com po si tions of meth ane and hom oge neous (Figs 4–7, 8A–C, 9A–C). The migra ti on ethane (Figs 16–19) reveal that natu ral gases in autochtho- range of mi cro bial gases was in sig nif i cant and lo ca tions of nous Upper Badenian and Lower Sarmatian strata of the their accu m ula ti ons would de pend only on the exis tenc e of Polish and Ukrai nian Carpathian Foredeep north of the Car- proper type of traps (compactional anticlines situ ate d above pathian Overthrust were mainly gener ate d during micro bial base ment upli ft, sealed by the Carpathian Overthrust, and proces ses from disperse d organic matter inside this Mio - sealed by faults, strati graphic pinch-out and strati graphic cene complex. The occurrence of thermogenic hydro car - traps relat ed to uncon form iti es). Such a gener a ti on and ac - bons near the bottom sequence of the Miocene strata near cum ula ti on sys tem of micro bial gases gave rise to the for- Tarnów (Figs 16, 17) was caused by an inflow from the Me- ma tion of multi-hor i zon tal gas fields, e.g., Przemyœl and so zoic base ment to Mio cene strata (Wiêc³aw, 2011). Iso to - Husów gas fields. Re cent in tro duc tion of in ter pre ta tion of pic anom ali es near Dêbica (Figs 18, 19) in the Pol ish part 2-D and 3-D seism ic secti ons and appli cati on of DHI (direct 542 M. J. KOTARBA ET AL. d n a

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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 and TTI (Time Temper a ture Index) values for (A) Tarnów, (B) Mielec-Le¿ask, (C) Rzeszów, (D) Przemyœl, (E) Krosno-Sanok, (F) Pidluby, (G) Rudky, (H) Zaluzhany, and (I) Skole areas. Model for A to D areas modi fied after Kotarba et al. (1998a). S – Sarmatian, E – early, L – late hy dro car bon in di ca tor) on the seis mic data en ables lo ca tion zones were es tab lished: mi cro bial, low-tem per a ture thermo- of areas of hydro car bon prospect ing (Baran & Jawor, 2009; genic and high-tem pera ture thermogenic (Figs 20, 21). Myœliwiec, 2004a, b; Myœliwiec et al., 2006). All hy dro car bon de pos its dis cov ered in the autochtho- An idea has been put forward in the geochem ical liter a - nous Miocene strata of the Polish and Ukrai nian Carpathian ture that micro bial meth ane might have been accu mu lated Foredeep, north and along the present edge of the Outer as hy drate beneat h the Miocene seafloor (Kotarba, 1992). Carpathians, oc cur within the micro bial zone (Figs 1, 20). How ever, the re sults of bathymetric anal yses (Czepiec & Gases ac cu mu lated in this zone are mainly of micro bial or i- Kotarba, 1998) re vealed that the Badenian sea was pre sum- gin. Oil from one Orkhovychi de posit and from petro leum ably only slightly deeper than the outer shelf, i.e. about inflows near Tarnów migrat ed to the Miocene strata from 300 m, at most. Initi ally, the Sarmatian sea was about 30– the Me so zoic base ment (Kotarba & Jawor, 1993; Wiêc³aw, 50 m deep and has shallowed to only about 10 m. Both the 2011; Wiêc³aw et al., in press). The en tire sur face of this Badenian and Sarmatian seas were the warm basins of sur - zone has a very good seism ic and drilling re cord. In the Po- face wa ter tem per atures of 17–20°C (Czepiec & Kotarba, lish Carpathian Foredeep, the gas re sources were as sessed 1998). Taking into ac count these data, the concept that natu - to 200–220 billi on cubic metres, and about 160 billi on cubic ral meth ane hy drate zone formed be neath the Mio cene sea- metres have been already discovered. floor must be rejected. The cur rent in ter pre ta tion (Oszczypko, 2006b; Osz- Geo chem i cal stud ies and TTI mod el ling within the czypko et al., 2006) assum es that the Upper Badenian de- autochthonous Miocene strata of the Carpathian Foredeep posit s south of the Carpathian Overthrust conti nue over a occur ring south beneat h the Carpathian Overthrust re veal distanc e of some 10–20 km, and farther south only Lower that the low-tem per ature thermogenic pro cess starts from a Miocene depos it s occur; and the southern extent of Lower depth of 2,500 m, the high-tem pera ture thermogenic pro- Sarmatian depos its is de creased as com pared to the Up per cess begins from a depth of 7,500 m, and three gener a ti on Badenian. The inferred southern boundari es of the Upper 544 M. J. KOTARBA ET AL.

Badenian and Lower Sarmatian strata fol low such an inter - Up per Badenian and Lower Sarmatian strata and lo cally in pre ta tion (cf. Figs 4–7, 10, 11, 16–19), however, the gen - the Lower Badenian Sandy-Calcar eous Series) have been eral patter n of distri buti on of Miocene facie s below the dis cov ered. The molecular com po si tion of nat u ral gases ac- Carpathian nappes is very well estab li shed only west of the cum ulat ed in autochthonous Miocene strata of the Polish “Cra cow bolt” (Ney, 1968) where a suffi cie nt docu m enta - and Ukrai nian Carpathian Foredeep is dom inate d by meth- tion by wells ex ists (e.g., Oszczypko & Oszczypko-Clowes, ane, which usuall y con sti tutes over 98 vol%. Methane was 2003; Oszczypko et al., 2006). In other areas , the well con- gen er ated by the car bon di ox ide re duc tion path way of mi- trol is poor and it is not cer tain if the pat tern es tab lished in crobial proces ses. Ethane was gener ate d by both micro bial the Silesian part of the Carpathian Foredeep is valid for its and thermogenic proces ses and propane at the ini tial stage other area that is re lated to the Ma³opolska block (Bu³a & of the low-tem pera ture thermogenic proces ses, and also by Habryn, 2011), consid er ing the recorded differ ence s in pa- mi cro bial pro cesses. laeotopography of the western and east ern parts of the Car- In the Upper Badenian strata, the total organic carbon pathian Foredeep ba sin (Bu³a et al., 2008). During the Pala- (TOC) content s vary from 0.02 to 1.48 wt% (aver age 0.75 eo gene, the area of the Carpathian Foredeep was sub ject to wt%) in the Pol ish Carpathian Foredeep and from 0.44 to inten sive erosion that result ed in highly vari able morphol - 2.01 wt% (aver age 0.96 wt%) in the Ukrai nian Carpathian ogy. The maxi m um height differ ence excee ded 2,000 m and Foredeep. In the Lower Sarmatian strata, the TOC var ies the valley depths reached 1,000 m (Karnkowski & Ozim- from 0.02 to 3.22 wt% (aver age 0.69 wt%) in the Polish kowski, 2001); in these deep valley s Palaeogene conglom - Carpathian Foredeep and from 0.01 to 1.45 wt% (aver age erate s and sandy-silty depos it s up to 300 m thick have been 0.71 wt%) in the Ukrai nian Carpathian Foredeep. This dis - found in some places (Moryc, 1995). Consid er ing that the persed or ganic matter in marine Mio cene sed im ents is gas- palinspastic re con struc tion as sumes that bathyal de po si tion prone, terres tri al (humic) in origin. took place in the major part of the early Badenian basin The high sedi m enta ti on rates togethe r with rhythm ic (Oszczypko et al., 2006, Fig. 18) and that these deep ba sin and cy clic de po sition of Mio cene clays and sands as well as con di tions con tin ued dur ing the sub se quent Tyras and the vig or ous gen er a tion of mi cro bial meth ane caused that Kosiv de posi ti on in western Ukraine (Peryt, 2006), it is the gas produced in claystone beds was accu m ulat ed in the highly proba ble that the southern Badenian extent , in parti c- overlay ing sandstone s, and capped, in turn, by the succee d - ular in SE Poland and west ern Ukraine, does not differ sig - ing claystones. A migra ti on range of micro bial gases was nifi cant ly from the Miocene extent as shown in Fig. 20. in sig nif i cant and lo ca tions of their ac cu mu la tions would de- North and along the pres ent edge of the Carpathian pend only on the exis tenc e of proper type of traps (compa- Overthrust in the Ukraine, the Lower Badenian Sandy-Cal - ctional anticlines situ ated above base ment up lift, sealed by car eous Series con tains twelve gas and gas-con den sate de - the Carpathian Overthrust, and sealed by faults, stratigra- posit s, and one oil deposit (Fig. 1B). As the lithofacies de - phic pinching out and stratigraphi c traps re lated to uncon - vel op ment of the Lower Mio cene strata re sembles the de - formi ties). Such gen er a tion and ac cu mu la tion sys tem of mi- velop m ent of the Upper Badenian and Lower Sarmatian crobial gases gave rise to the formation of multi-horizontal strata, it can also con tain a simi lar quan tity and simi lar ge - gas fields. netic type of gas-prone kerogen. Their thickness be neath the An other sit u a tion is en coun tered in the autochthonous Carpathian Overthrust excee ds even 1.5 km (Oszczypko, Miocene strata south beneat h the Carpathian Overthrust. As 2006b). All these facts may indi cat e that in both low- and the lithofacies devel op ment of the Lower Mio cene strata re - high-tem per a ture thermogenic zones con sid er able quan ti - sem bles the de velop m ent of the Upper Badenian and Lower ties of gases could be generated and accumulated. Sarmatian strata, it can also con tain a simi lar quan tity and sim ilar geneti c type of gas-prone kerogen. Their thickness beneat h the Carpathian Overthrust excee ds even 1.5 km. CON CLU SIONS Geochem ical studies reveal that from the depth of 2,500 m starts the proces s of low-tem pera ture thermogenic hydro - The Carpathian Foredeep devel oped during the early carbon gener a ti on (“oil window”). At greater depths, more and middle Miocene as a periph eral flexural foreland ba sin than 7,500 m, within the autochthonous Lower Miocene ba- in front of the advanc ing Carpathian Overthrust. The outer sin ex isted the high-tem per a ture meth ane gen er a tion zone foredeep (termed the Bilche-Volytsia zone in the Ukraine) (“gas win dow”). All these facts may ev idence that in both is filled with gener all y undis turbe d, flat-lying autochtho- low- and high- tem pera ture thermogenic zones consid er able nous Middle Miocene marine depos it s, which range in quantities of gases might have generated and accumulated. thickness from a few hundred metres in the northern, mar- ginal parts of the Foredeep to as much as 3.5 km in SE Po- land and 5 km in western Ukraine. Ac knowl edge ments Since 1920, within the autochthonous Miocene strata north and along the pres ent edge of the Carpathian Over- The research was under taken as the project No. UKRAINE/ 193/2006 of the Min istry of Sci ence and Higher Ed u ca tion car ried thrust, 101 gas and gas-conden sat e depos it s in the Pol ish out at the AGH Univer sity of Sci ence and Tech nol ogy in Kraków Carpathian Foredeep (mainly in the Upper Badenian and and the Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute in Lower Sarmatian strata and lo cally in the Lower Badenian Warsaw. Scien tific work was financed from the scien tific funds in strata), and 44 gas and gas-conden sat e depos it s and one oil the years 2007–2010. The de tailed comments of Paul Lillis, Nestor deposit in the Ukrai nian Carpathian Foredeep (mainly in the Oszczypko and Francois Roure were of great as sistance in our re - MI CRO BIAL GAS SYS TEM AND PROSPECTIVES OF HY DRO CAR BON EX PLO RA TION 545

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