Paweł Lis & Anna Wysocka

Annales Societatis Geologorum Poloniae (2012), vol. 82: 239–253.

MID DLE MIO CENE DE POS ITS IN CARPATHIAN FOREDEEP:

FA CIES ANAL Y SIS AND IM PLI CA TIONS FOR HY DRO CAR BON

RES ER VOIR PROS PECT ING

Pol ish Geo log i cal In sti tute, 00-975 War saw. Pres ent ad dress: Bacha St.10/701, 02-743 War saw, Poland, e-mail: pawel.prot.lis@gmail.com

2

Fac ulty of Ge ol ogy, Uni ver sity of War saw, al. ¯wirki i Wigury 93, 02-089 War saw, Poland, e-mail: anna.wysocka@uw.edu.pl

Lis, P. & Wysocka, A., 2012. Mid dle Mio cene de pos its in Carpathian Foredeep: fa cies anal y sis and im pli ca tions for hy dro car bon res er voir pros pect ing. Annales Societatis Geologorum Poloniae, 82: 239–253.

Ab stract: This sedimentological study was based on well cores from the Pol ish and Ukrai nian parts of the Carpa-thian Foredeep. It re vealed gen eral het er o ge ne ity of fa cies in the mid dle Mio cene of the sed i men tary suc ces sion in the ba sin. Four teen sed i men tary fa cies were dis tin guished and their or i gin was in ter preted: mas sive, non-graded sand stones; nor mal-graded, mas sive sand stones, with and with out a strat i fied up per most part; hydroplastically de formed sand stones; pla narpar al lelstrat i fied sand stones; troughcrossstrat i fied sand stones; rip plecrosslam i -nated sand stones; heterolithic de pos its, com posed of thinly interlayered sand stone and mudstone; mas sive and lam i nated mudstones; and basal gyp sum/anhydrite evaporites, of ten in ter ca lated with mudstone. Four main mo dal i ties of ver ti cal fa cies or ga ni za tion were rec og nized and at trib uted to the fol low ing en vi ron ments: (1) the mid-late Badenian, shoal-wa ter, evaporitic en vi ron ment that pre ceded the lat est Badenian–early Sarmatian, main phase of foredeep de vel op ment; (2) a lit to ral, tidal en vi ron ment of the in ner parts of storm-in flu enced, coastal bays and tidal flats or pos si bly spit-shel tered la goons; (3) a wave-dom i nated, lit to ral, sandy en vi ron ment, con sid ered to be shoreface, ex tended by waves, in front of ad vanc ing del tas; and (4) a neritic to subneritic, muddy, off shore slope, char ac ter ized by fre quent in cur sions of tempestite and turbidite sand. The study con trib uted to a better un der stand ing of the mid-Mio cene depositional sys tems in the ba sin, with sig nif i cant im pli ca tions for on go ing hy dro car bon ex plo ra tion. In ter pre ta tions of the or i gins of po ten tial res er voir sand stones pro vided im por tant in for ma tion on their pos si ble strati graphic dis tri bu tion in the ba sin fill. The po ten tial, eco nomic im por tance of strati graphic hy dro car bon traps un der scored the ur gent need for a full-scale fa cies anal y sis and fully cored wells in stra te gic parts of the ba sin.

Key words: fa cies anal y sis, lit to ral, tidal, shoreface, neritic slope, turbidites, Markov-chain anal y sis. Manu script re ceived 11 April 2011, ac cepted 20 December 2012

IN TRO DUC TION

The Carpathian Foredeep Ba sin for many de cades has at tracted geo log i cal in ter est, re lated to pros pect ing for hy -dro car bon re sources, with nu mer ous geo log i cal and geo-physical stud ies (e.g., Dziadzio et al., 1997, 2006; Dziadzio, 2000; Porêbski et al., 2002; Kurovets et al., 2004; Lyzun et

al., 2004; Myœliwiec et al., 2004; Mastalerz et al., 2006;

Oszczypko et al., 2006; Popadyuk et al., 2006; Œl¹czka et

al., 2006; Pietsch et al., 2010; Warcho³, 2011). Struc tural

and sedimentological data sug gest that the area has con sid er able po ten tial for new dis cov er ies through the use of mod -ern ex plo ra tion meth ods.

The Pol ish-Ukrai nian, Carpathian Foredeep evolved from an intracratonic, Paratethyan ba sin that was cut off from the main Paratethys realm in the late Badenian (Osz-czypko et al., 2006; and ref er ences therein). As a re sult,

evaporites formed (Peryt, 2006; and ref er ences therein) as an im por tant unit for cor re la tion in the ba sin. They are over -lain by a suc ces sion of cal car e ous and siliciclastic molasse de pos its (Vyalov, 1965; Oszczypko, 1996), with a thick ness of up to 3 km in the Pol ish part (Ney et al., 1974) and up to 6 km in the Ukrai nian part of the ba sin (Oszczypko, 2006). Sed i men ta tion was con trolled by tec ton ics, re gional cli mate and rel a tive sealevel changes. Dis tinct zones of sed i men ta -tion are rec og niz able in the foredeep ba sin, with the Carpa-thian-de rived, terrigenous sed i ment dom i nat ing the higher-re lief south ern part and with bioclastic lime stones and northerlyde rived, terrigenous sed i ment, de pos ited in the shal -low-wa ter, north ern part of the ba sin (Wysocka, 2002). The deep, subneritic, ax ial zone of the ba sin was dom i nated by east ward-flow ing tur bid ity cur rents.

The pres ent study fo cussed on the mid dle Mio cene depo si tional en vi ron ments and their sed i men ta tion pat terns, as so ci ated with the south ern mar gin of the Carpathian Fore- deep, in the east ern part of the ba sin, which stretches across the Pol ish-Ukrai nian bor der (Fig. 1). The re search pro ject was a re sult of co-op er a tion be tween Pol ish and Ukrai nian in sti tu tions, which pro vided a unique op por tu nity to ex tend the study area eastwards, by us ing well cores from both sides of the bor der. On the ba sis of cores, a sed i men tary fa -cies anal y sis of the mid-Mio cene de pos its was con ducted, shed ding light on their het er o ge ne ity and yield ing sig nif i cant im pli ca tions with re gard to re gional pros pect ing for hy -dro car bon res er voirs.

GEO LOG I CAL SET TING

The Carpathian Foredeep is the lat est part of a pe riph -eral fore land ba sin, formed in front of the Carpathian oro-genic belt. The base ment con sists of var i ous Pro tero zoic and Palaeozoic rocks of the Eu ro pean Plat form, in clud ing its Me so zoic cover (Oszczypko et al., 2006). The asym met ri cal fore land ba sin de vel oped by flex ural base ment sub si -dence in front of the Outer Carpathian Thrust in Eggenbur-gian–Sarmatian time (Oszczypko, 1996). The lithostratigra- phic di vi sions of the ba sin-fill molasse suc ces sion in the Pol ish and Ukrai nian parts of the ba sin are some what dif fer -ent, but broadly cor re la tive (Fig. 2). A key marker unit is the Badenian evaporites (the Krzy¿anowice For ma tion in Po land and the Tyras For ma tion in Ukraine), which cor re -spond to the lower part of nannoplankton zone NN6 (Peryt, 1997, 1999).

Along the front of the Outer Carpathian Thrust (Fig. 1), there is a wide zone of folded, Mio cene de pos its, both in Ukraine and in Po land. These de pos its form the Boryslav-Pokuttya and Sambir nappes in Ukraine and the Stebnik Nappe and Zg³obice im bri cate thrust-sheets in Po land (Osz- czypko et al., 2006). The Stebnik Nappe is con sid ered to be

equiv a lent to the Sambir Nappe, whereas the ex act, lat eral ex ten sion of the Boryslav-Pokuttya Nappe in the Pol ish area re mains un clear (Andreyeva-Grigorovich et al., 2008). All the wells stud ied, ex cept for Lanivka 1 and Jod³ówka 18 (Fig. 1), are lo cated out side this fron tal fold-and-thrust belt. The Pol ish wells are in the east ern part of the Pol ish Carpa-thian Foredeep and the Ukrai nian wells are in the so-called Bilche-Volytsia Zone (Figs 1 and 2).

STUDY MA TE RIAL AND METH ODS

Cores from eight wells were ana lysed in the Pol ish part of the study area: the Pruchnik 22, Jod³ówka 18, Kupno 2, Stanis³awice 2, Witkowice 2, Nowosielce 3, Cha³upki Dêb-niañskie 1 and Wierzchos³awice 13 wells, with a to tal of 194 m of well cores logged (Fig. 3). In ad di tion, cores from ten wells in the Ukrai nian part were stud ied: the Chorno-kuntsi 1, Bortiatyn 1, Nyklovychi 26, Makuniv 1, Mosty 1, Susoliv 5, Pivnichne Hirs’ke 1, Hrudivka 3, Pyatnychany 1 and Lanivka 1 wells, with a to tal of 519 m of cores logged (Fig. 4). Un for tu nately, the Ukrai nian cores were con sid er -ably poorer in qual ity, ow ing to their stor age in un suit able con di tions.

Con ven tional meth ods of strati graphic log ging and standard, de scrip tive, sedimentological ter mi nol ogy were used (Harms et al., 1975; Reineck & Singh, 1975; Collinson & Thomp son, 1982), pay ing par tic u lar at ten tion to such fea tures as grain size, col our, and pri mary and sec ond ary sed i -men tary struc tures (in clud ing soft-sed i -ment de for ma tion), as well as the oc cur rence of or ganic mat ter, intraformational clasts and pro nounced cal cite ce men ta tion. Sed i men tary fa -cies were de fined as the main types of de pos its, dis tin guished on such a de scrip tive ba sis, with a thick ness res o lu -tion of ~5 cm adopted in the pres ent study. Thin (1–3 cm) lay ers of mas sive or lam i nated siltstone or sand stone, in ter -ca lated with mudstone, were con sid ered to be com po nents of a heterolithic de posit, which as a whole might be mud-Fig. 1. Lo cal ity map of study area in re la tion to Carpathian, outer-thrust, orogenic front, with lo ca tion of wells stud ied and lines of their cor re la tion pan els

stone- or sand stone-dom i nated. For eas ier ref er ence in the text and fig ures, fa cies have been la belled with the mod i fied let ter code of Eyles et al. (1983).

The quan ti ta tive method of em bed ded Markov-chain anal y sis (Gingerich, 1969; Krumbein & Dacey, 1969) was used for rec og ni tion of pref er en tial trends in ver ti cal fa cies or ga ni za tion, with an im proved, sta tis ti cal test (Harper, 1984) for the sig nif i cance of the ob served fre quency of ver -ti cal fa cies tran si -tions.

MID DLE MIO CENE FA CIES

Four teen sed i men tary fa cies have been dis tin guished in the well cores of the mid dle Mio cene de pos its in the Pol ish and Ukrai nian parts of the study area. The fa cies, with their key char ac ter is tics and brief in ter pre ta tion, are listed in Ta -ble 1 to give an over view. They are il lus trated with core-sam ple pho to graphs in Fig ure 5, and are fur ther de scribed and in ter preted in de tail in the pres ent sec tion. The strati -graphic dis tri bu tion of fa cies in the main well logs is shown in Fig ures 6 and 7.

Fa cies Sm: mas sive sand stones

De scrip tion. These sand stones are whit ish-grey,

fineto me diumgrained and mas sive, with no rec og niz able, in -ter nal strat i fi ca tion (see Sm in Fig. 5). Their beds have sharp bases and ei ther sharp or gradational tops and oc cur in both the Pol ish (Fig. 6) and the Ukrai nian wells (Fig. 7), but seem to be most com mon in the Pol ish Jod³ówka 18 well (Fig. 3). Bed thick ness ranges from a few centi metres to 2.5 m (e.g., see well log Jod³ówka 18 in Fig. 6, depth 2104.5–

2107 m). The beds lack any sys tem atic, in ter nal grain-size grad ing, al though some of the thicker ones show a sub tle, up ward coars en ing at the base and/or slight, up ward fin ing at the top. Many beds con tain scat tered frag ments of bi valve shells, car bo na ceous plant de tri tus and abun dant mus co vite flakes. Mudstone intraclasts oc cur in this sand stone fa cies in the Nowosielce 3 well (Fig. 3, well depth 1173 m). How -ever, this sand stone fa cies is gen er ally arenitic and con tains lit tle or no dis sem i nated mud ma trix.

In ter pre ta tion. The lack of in ter nal strat i fi ca tion and

bedscale, nor mal grad ing pre cludes the pos si bil ity of de po -si tion from tur bu lent flow and in di cates sand em place ment by a cohesionless de bris flow in the fric tional re gime of Drake (1990). Basal, in verse grad ing in di cates in tense grain col li sions at the base of self-lu bri cated shear ing flow (Campbell, 1989), whereas the nor malgraded bed tops re flect the in ter ac tion of shear ing flow with the over ly ing am bi ent wa ter (Middle ton & Southard, 1978). The arenitic tex -ture and the oc cur rence of plant de tri tus and bi valve shells in di cate sub ma rine resedimentation of river-de rived sand, which most likely would oc cur on the slopes of shelfmar -gin del tas (Porêbski et al., 2002; Porêbski & Steel, 2003). The thicker, mas sive beds prob a bly rep re sent large deltaslope fail ures, whereas the thin ner ones may be due to sec -ond ary col laps ing of the steep snouts of “frozen” de bris flows or to sec ond ary, grav i ta tional slough ing of the slope-scar head slope-scarps, left by large, grav i ta tional fail ures.

Fa cies Sng: nor mal-graded sand stones

De scrip tion. These sand stone are whit ish-grey to light

grey, fine- to me dium-grained and non-strat i fied (mas sive), but their beds show nor mal grad ing and oc ca sion ally also

MID DLE MIO CENE DE POS ITS IN THE CARPATHIAN FOREDEEP

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Fig. 2. Ten ta tive cor re la tion of Mio cene, lithostratigraphic units in Pol ish and Ukrai nian parts of Carpathian Foredeep (based on Andreyeva-Grigorovich et al., 1997; Jasionowski, 1997; and Oszczypko et al., 2006)

MID DLE MIO CENE DE POS ITS IN THE CARPATHIAN FOREDEEP

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pla nar, par al lel strat i fi ca tion at the top. This fa cies oc curs mainly in the Pol ish wells. Beds are 10 to 70 cm thick and most com monly show con sis tent, nor mal grad ing (e.g., the Kupno 2 well in Fig. 6, depth 586 m). Basal, in verse grad ing is less com mon (e.g., well log Cha³upki Dêbniañskie 1 in Fig. 6, depth 687.7 m), whereas pensymmetric or sym met -ric, in verse-to-nor mal grad ing is rare (e.g., well log Cha-³upki Dêbniañskie 1 in Fig. 6, depths 688.8 and 855.5 m). Some beds con tain mudstone intraclasts, frag ments of coalified plant mat ter and/or abun dant mus co vite flakes.

In ter pre ta tion. The nor mal-graded mas sive beds with

or with out a par al lel-strat i fied top part in di cate de po si tion by rapid dump ing from an ex ces sively con cen trated tur bu lent sus pen sion (Lowe, 1988), of ten fol lowed by a trac tional phase of planebed trans port. These beds in a sub ma -rine set ting would be neritic or subneritic turbidites Ta(b) (Bouma, 1962; Allen, 1991), at trib uted to highden sity tur -bid ity cur rents (sensu Lowe, 1982), or a va ri ety of tempes-tites, emplaced in sublittoral en vi ron ment (Myrow & Sou-thard, 1991, 1996). A rel a tively thin (£ 10 cm) basal zone of in verse grad ing may rep re sent turbiditic trac tion car pet (Lowe, 1982; Hiscott, 1994). Beds with a thicker in versely-graded lower part are at trib uted to high-den sity tur bid ity cur rents, gen er ated by the ret ro gres sive slump ing of a delta slope, where the first-mo bi lized, lower-slope sed i ment would be finer-grained than the sub se quently mo bi lized

sed i ment of a higher slope. The head part of the tur bid ity cur rent in such a case then would carry a finer-grained load than that car ried by the flow body. Al ter na tively, the up ward in crease in grain size may re flect wax ing of a deltade -rived, hyperpycnal flow (Kneller & Branney, 1995; Par sons

et al., 2001) or a stormgen er ated, sea ward re lax ation cur

-rent (Myrow & Southard, 1991).

Fa cies Sd: hydroplastically de formed sand stones De scrip tion. These sand stones range from fine to me

-dium/coarse-grained, are grey to light-grey in col our and show hydroplastically de formed, in ter nal, par al lel strat i fi ca -tion and/or rip ple cross-lam i na -tion (see Sd in Fig. 5). Bed thick nesses are in the range of 4 to 20 cm and at least the up per bed bound aries are gen er ally sharp. Some of the sand -stone beds con tain scat tered mud-stone intraclasts, typ i cally rounded in shape and from 0.5 cm (e.g., well log Pruchnik 22 in Fig. 6, depth 668 m) to 6 cm in size (e.g., the Jod³ówka 18 well in Fig. 3, depth 2449.5 m).

In ter pre ta tion. All the thin beds of this fa cies were

con vo luted in situ by such fac tors as sed i ment load ing, par -tial liq ue fac tion, caused by a seis mic tremor or an overpas-sing bot tom cur rent (D¿u³yñski & Walton, 1965), and in some cases by bioturbation (e.g., see Sd in Fig. 5) How ever, at least some of the thicker and sharpbased beds may rep re

-Ta ble 1 Sed i men tary fa cies, dis tin guished in mid dle Mio cene, ba sin-fill suc ces sion, with their key, de scrip tive fea tures and brief,

sent lo cal intraslope slumps, caused by an ex cess ac cu mu -la tion of terrigenous sed i ment on the ba sin-mar gin, gen tle, neritic slope (Lewis, 1971).

Fa cies Sh: pla nar-par al lel-strat i fied sand stones De scrip tion. These sand stones range from very fine- to

me diumgrained, are lightgrey to grey in col our and char -ac ter ized by pla nar par al lel strat i fi ca tion, hor i zon tal or slightly in clined (see Sh in Fig. 5). Strat i fi ca tion is marked by sub tle changes in grain size and/or col our. Bed thick -nesses are from a few centi metres to 80 cm (e.g., well log Pruchnik 22 in Fig. 6, depth 668 m). Mudstone intraclasts and coal field plant de tri tus are rare (e.g., found in the Jod-³ówka 18 well, depth 2403.5 m; Fig. 3).

In ter pre ta tion. This fa cies is at trib uted to the

planebed trans port of sand by a uni di rec tional cur rent in the up -per flow re gime (Allen, 1968; Harms et al., 1975) or by waves with rel a tively high, or bital ve loc i ties (Komar & Mil- ler, 1975; Komar, 1998). The so-called lower stage, plane-bed trans port can be pre cluded, as it would re quire at least coarse sand (grain size >0.7 mm). In clined, pla nar-par al lel

strat i fi ca tion is typ i cally due to the plane-bed trans port of sand over a non-hor i zon tal sur face, such as a gen tle scour, formed by a cur rent, an ero sional sub strate trun ca tion by storm waves, or a preex ist ing, lowre lief, depositional to -pog ra phy (e.g., delta-slope mounds or storm-emplaced shoreface sand patches).

Fa cies St: trough-cross-strat i fied sand stones De scrip tion. The sand stones of this fa cies are fine- and

me dium-grained and light grey in col our, char ac ter ized by trough cross-strat i fi ca tion. Their con cave-up wards sets of tan gen tial strata are 7–10 cm thick, oc cur ring iso lated in as -so ci a tion with the pre vi ous fa cies (Sh) or form ing cross-strata cosets, up to 30 cm thick (as found only in the Pruch-nik 22 well at depths 670.5 and 1422.5 m; Fig. 6).

In ter pre ta tion. The spo radic iso lated trough crossstrata sets are thought to be scourandfill fea tures, the as so ci a tion of which with fa cies Sh sug gests swaley crossstrat i -fi ca tion and thus ep i sodic, storm-gen er ated con di tions, combining uni di rec tional cur rents and waves (Dumas & Ar -nott, 2006). The cosets of trough cross-strata in di cate the

MID DLE MIO CENE DE POS ITS IN THE CARPATHIAN FOREDEEP

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Fig. 5. Well-core pho to graphs, show ing main, sed i men tary fa cies, dis tin guished in mid-Mio cene suc ces sion stud ied; fa cies let ter code, as given in Ta ble 1 and used in text

mi gra tion of small dunes, which im plies de po si tion by a unidi rec tional cur rent in the up per range of the lower flow re gime (Harms et al., 1975). For the dunes to form, the cur -rent could not have been a brief surge and was prob a bly a delta-de rived, hyperpycnal flow, per haps slightly channelized or con veyed by a deltaslope chute (Nemec, 1990; Par -sons et al., 2001).

Fa cies Sr: rip ple-cross-lam i nated sand stones De scrip tion. These sand stones are light grey, range from very fine- to me dium-grained and show rip ple cross-lam i na tion. Well-core sam ples do not al low in all cases a clear dis tinc tion be tween cur rent- and wave-rip ple crosslam i na tion. The crosscrosslaminae sets in the pres ent case in di -cate mainly asym met ri cal rip ples, of ten climb ing at an an gle of 10–70° and oc ca sion ally de formed, but nei ther of these fea tures is discriminative (Allen, 1968; Collinson & Thom-pson, 1982). There fore, this sand stone fa cies in cludes both va ri et ies of rip ple cross-lam i na tion. These sand stones oc cur in most wells and their beds vary in thick ness from a few centi metres to 40 cm (e.g., well-log Pruchnik 22, depth 663 m; Fig. 6). The bed bases are gen er ally sharp, but their tops of ten show the rip ple-form re lief pre served, par tic u larly when cov ered by mudstone fa cies.

In ter pre ta tion. This fa cies rep re sents sand de po si tion

from a uni di rec tional cur rent with flow power in the lower range of the lower flow re gime (Allen, 1968; Harms et al., 1975) or by waves with rel a tively low, or bital ve loc i ties (Komar & Miller, 1975; Komar, 1998). The climb ing of rip -ples in ei ther case would in di cate a high rate of sand fall-out from sus pen sion, rel a tive to the rate of lat eral trans port in bedload trac tion (Allen, 1968). Spo radic de for ma tion may be due to the shear ing ac tion of an overpassing cur rent, stron ger wave strokes, bioturbation or spontaneous, partial liquefaction.

Fa cies H: heterolithic de pos its

De scrip tion. These de pos its con sist of lightgrey sand

-stone and grey silt-stone, thinly interbedded with dark-grey mudstone. The pro por tion of sand stone and siltstone to mud-stone by thick ness var ies and ac cord ingly the de pos its have been cat e go rized as three heterolithic fa cies (Ta ble 1): fa cies HS>F, dom i nated by sand stone and siltstone; fa cies HS»F, with an ap prox i mately equal pro por tion of sand stone/siltstone and mudstone; and fa cies HF>S, dom i nated by mudstone.

Fa cies HS>F (see in Fig. 5) oc curs in most of the wells (Figs 6 and 7). Its coarser-grained com po nent ranges from siltstone to me dium-grained sand stone and is com monly cross-lam i nated. Bidirectional cross-lam i na tion and subho-rizontal, ero sional trun ca tions are com mon. Mudstone forms thin, dis con tin u ous flasers (sim ple or bi fur cated) and sub or di nate, con tin u ous, un du lat ing, wavy interlayers (wavy bed ding sensu Reineck & Singh, 1975). Mud-flame load fea tures, loaded rip ples and pseudonodules (D¿u³yñski & Walton, 1965) are com mon. The units of this fa cies are 1–9 m thick (e.g., see well log Pruchnik 22, depth 1413– 1422 m, Fig. 6), most of ten al ter nat ing with units of the other heterolithic facies.

Fa cies HS»F (see in Fig. 5) sim i larly oc curs in most of the wells (Figs 6 and 7). Its coarser-grained com po nent is pre dom i nantly very fine- to fine-grained sand stone, mainly crosslam i nated. Bidirectional crosslam i na tion and ero sional trun ca tions are com mon. Mudstone forms com pa ra -bly thick, con tin u ous and un du lat ing wavy interlayers (wavy bed ding sensu Reineck & Singh, 1975). The units of this fa cies are mainly 50–70 cm thick, but lo cally reach 3 m (the Jod³ówka 18 well, depth 2404–2407 m, Fig. 6), and are al ter nat ing with the other heterolithic fa cies.

Fa cies HF>S (see in Fig. 5) also oc curs in most of the wells (Figs 6 and 7). It is dom i nated by mudstone, and its coarser-grained com po nent forms mainly lenses of siltstone and very fine- to fine-grained sand stone (len tic u lar bed ding

sensu Reineck & Singh, 1975), al though some sand stone

interlayers are lat er ally more con tin u ous and mudstone locally is re duced to flasers. Loaded rip ples and pseudonodu- les are com mon. The units of this fa cies are mainly about 1 m in thick ness, al ter nat ing with the other, heterolithic fa cies.

In ter pre ta tion. The al ter na tion of sand stone/siltstone

and mudstone and the ev i dence of bidirectional crosslam i -na tion are di ag nos tic of a tidal en vi ron ment (Reineck & Singh, 1975), which in the pres ent case is as cribed to inter-deltaic, nearshore embayments, shel tered from wave ac tion. No ev i dence of coastal bar ri ers was rec og nized, which ren -ders tidal la goons a less likely, al ter na tive in ter pre ta tion. Fa cies HS>F, HS£F and HF>S thus would rep re sent sandflat, mixed-flat and mud flat, tidal en vi ron ments, re spec tively (Reineck & Singh, 1975). The strati graphic al ter na tion of these fa cies re flects vari able, lo cal de liv ery of silt and sand, which may be due to the tidal en vi ron ment’s autogenic changes in sed i mentdis persal pat tern or may pos si bly re -flect mi nor changes in rel a tive sea level.

Fa cies F: mudstones

De scrip tion. These de pos its are dark grey to blackish

grey mudstones, which are com mon in all of the well cores stud ied (Figs 6 and 7) and range from mas sive (fa cies Fm) to lam i nated (fa cie Fl). The lat ter fa cies is char ac ter ized by an al ter na tion of darker and lighter grey, thin bands, 3–5 mm in thick ness (see Fl in Fig. 5), which ap par ently re flects vari able pro por tions of the clay and silt frac tions. Ver ti cal and hor i zon tal bur rows oc cur spo rad i cally.

In ter pre ta tion. The or i gin of this fa cies is at trib uted to

the fall-out of hemipelagic mud sus pen sions, de rived mainly by storms, hyperpycnal flows and other tur bid ity cur rents. The banded mudstones of fa cies Fl prob a bly owe their or i gin to a pul sat ing, sea ward de liv ery of muddy sus -pen sion by storms and delta-de rived, hypopycnal plumes (Nemec, 1995).

Fa cies E: evaporites

De scrip tion. This fa cies con sists of whit ish to light grey evaporites, chiefly gyp sum al ter nat ing with anhydrite. The anhydrite is nod u lar or lam i nated, whereas gyp sum has a coarsecrys tal line tex ture. Some depositional units are com -posed solely of evaporites (fa cies E, see Fig. 5), whereas other evaporitic units are thinly interlayered with sub or di nate

mudstone (fa cies EF, see Fig. 5) or vir tu ally dom i nated by the lat ter (fa cies FE). The inter-evaporitic mudstone lay ers are dark grey and 1–5 mm in thick ness. The units of fa cies EF en coun tered by the wells (Figs 6 and 7) are 120–500 cm thick. Only one unit of fa cies E (~2.8 m thick) and one unit of fa cies FE (>15 m thick, not drilled to its base) were en -coun tered by the well. These evaporitic fa cies are found in the deep est parts of the wells (see the well logs Cha³upki Dêbniañskie 1 in Fig. 6 and Hrudivka 3, Lanivka 1 and Pya- tnychany 1 in Fig. 7).

In ter pre ta tion. These evaporitic de pos its rep re sent the

Badenian Krzy¿anowice For ma tion in Po land and the co -eval Tyras For ma tion in Ukraine (Fig. 2), which are the shal lowma rine sub strate, pre dat ing the late Badenian, flex -ural sub si dence of the foredeep ba sin. The depositional conditions of the evaporites have been dis cussed by Peryt (1997, 1999), among other au thors. The pre cip i ta tion of gyp -sum and anhydrite oc curred prob a bly in coastal sabkhas or shal low, ma rine embayments, iso lated from the Paratethian Sea do main by the fore land up lift. The dif fer ing, vol u met ric pro por tion of the mudstone com po nent in fa cies E, EF and FE re flects the vari able de gree of storm, tidal and deltaic in flu -ence in these pro tected shoal-wa ter en vi ronments.

VER TI CAL FA CIES OR GA NI ZA TION

The in di vid ual sed i men tary fa cies (Ta ble 1) dif fer mark edly in tex ture and depositional struc tures, which ob vi ously has a bear ing on their in ter nal po ros ity and per me abil -ity prop er ties and is ar gu ably the main source of the pri mary het er o ge ne ity of the sed i men tary suc ces sion (e.g., see We-ber, 1982, 1986). The ver ti cal fa cies tran si tions in well logs have thus been stud ied on a sta tis ti cal ba sis, with the use of Markov-chain anal y sis, in or der to rec og nize the pat tern of ver ti cal fa cies or ga ni za tion in the study area. The role of sec ond ary heterogeneities, re lated to faults and/or dif fer en tial ce men ta tion, is not con sid ered here and should be a sub -ject for a sep a rate study.

The re sults of Markov-chain anal y sis are shown in the form of nu mer i cal ma tri ces in Fig ure 8. The ver ti cal fa cies tran si tions that ap pear to be more fre quent than ran dom (see the pos i tive val ues in ma trix D; Fig. 8) are dis played in Fig ure 9A, which thus re veals an over all pat tern of highfre -quency, ver ti cal fa cies changes within the mid-Mio cene sed i men tary suc ces sion of the study area. In met ric terms, these fre quen cies can be scaled fur ther, us ing the thick ness range of the in di vid ual fa cies (Ta ble 1). The po ros ity and per me abil ity prop er ties of the in di vid ual fa cies, once de ter -mined by a lab o ra tory study, can be attached readily to the facies organization model.

The sta tis ti cally pref er en tial trend of ver ti cal fa cies tran si tions, re vealed by the Harper (1984) test (Fig. 8E), is shown in Fig ure 9B and its in ter pre ta tion is given in Fig ure 10. There seem to be four dif fer ent mo dal i ties in the ver ti cal or ga ni za tion of fa cies, rep re sent ing the fol low ing four main, sed i men tary environments (Fig. 10):

(1) A shoalwa ter evaporitic en vi ron ment, with al ter -na tion of fa cies E and EF (Fig. 9B), which formed prior to the late Badenian ul ti mate phase of foredeep de vel op ment;

(2) A lit to ral, tidal en vi ron ment (prob a bly the in ner parts of open bays, coastal, tidal flats and/or spit-shel tered, aer ated la goons), where the de po si tion of “do mes tic”, hete-rolithic fa cies HS>F, HS»F and HF>S was fre quently in ter -rupted by the storm em place ment of fa cies Sm;

(3) A lit to ral, wave-dom i nated, sandy en vi ron ment, probably a shoreface zone, where wave-worked fa cies Sr and Sh al ter nated with each other, ac cord ing to the vary ing, or bital ve loc ity of sea waves;

(4) A neritic to subneritic, muddy, off shore slope en vi -ron ment, fre quently in vaded by sandy tempestites Sh-Sr in its shal lower part and by surge-type turbidites Tad/Tbcd and hyperpycnites Tbcbc…d in the deeper part.

The onedi rec tional, up ward tran si tion of the en vi ron men tal mo dal i ties 1, 2 and 3 into mo dal ity 4 (Fig. 10) in di cate their ces sa tion by ma rine drown ing, which prob a bly re -flects tec toni cally-in duced, rel a tive sea-level rises at the inner foredeep margin.

The pref er en tial, ver ti cal fa cies tran si tions and en vi ron -men tal mo dal ity changes re vealed by the sta tis ti cal anal y sis (Fig. 10) may serve as a use ful, ten ta tive guide for re gional hy dro car bonres er voir pros pect ing. The en vi ron men tal mo -dal ity 3, with its wave-worked sand stone fa cies, is the most prom is ing as a po ten tial hy dro car bon res er voir. Mo dal ity 4 in volves sand stone beds, sep a rated by mudstone caps, but the iso lated sand stone tempestites and turbidites/hy perpycnites may be lat er ally ex ten sive and vol u met ri cally at trac tive as a po ten tial res er voir type. The heterolithic mo -dal ity 2 (Fig. 10) is much less at trac tive, al though many, heterolithic res er voirs of this type have been pro duced suc -cess fully (Martinius et al., 2005; Nordahl et al., 2005; Ringrose et al., 2005), pro vided that the ini tial res er voir pres sure is suf fi ciently high and the pro duc tion wells are well planned. The least at trac tive is fa cies mo dal ity 1 (Fig. 10), which in cludes no po ten tial res er voir rocks and is prac -ti cally a per me abil ity bar rier. How ever, this ap par ent bar rier is bro ken by nu mer ous, mi nor thrusts and youn ger, normal faults, which provide good connections with the deeper-bur ied, potential hydrocarbon source rocks.

Im por tantly, the sta tis ti cal model of ver ti cal fa cies tran -si tions (Fig. 10) sup ports the no tion that the south ern zone of the Carpathian Foredeep ba sin evolved through ep i sodes of tec toni cally-in duced ma rine drown ing and sub se quent shore line progradation. This means that a mudstone-rich, neritic fa cies as sem blage, en coun tered in an ex plo ra tion well, may sig nal an oc cur rence of a more prom is ing sandy, lit to ral facies assemblage, directly below.

DEPOSITIONAL PALAEOENVIRONMENT

The in ter pre tive fa cies anal y sis (Ta ble 1) and rec og ni -tion of fa cies ver ti cal-or ga ni za -tion mo dal i ties (Figs 9B and 10) shed more light on the depositional en vi ron ment of the lat est Badenian–early Sarmatian Machów For ma tion and its Ukrai nian strati graphic coun ter parts (Fig. 2). Prior to its flex ural, tec tonic roll-back that ac com mo dated this thick sed i men tary molasse suc ces sion, the Pol ish-Ukrai nian fore- deep was a shal low-wa ter, evaporitic ba sin (fa cies mo dal ity 1 in Fig. 10). The late Badenian tec tonic down-warp ing

Fig. 8. Nu mer i cal re sults of Markovchain anal y sis. Ma tri ces: A – tran si tion count ma trix, B – prob a bil ity ma trix of ob served tran si -tions, C – prob a bil ity ma trix of ex pected ran dom tran si -tions, D – prob a bil ity diference ma trix) and re lated, sta tis ti cal sig nif i cance test (ma trix E)

ned the ba sin into an asym met ric, synclinal trough (Fig. 11), shal lower to the west, bounded by the Carpathian orogen front from the south and by the outer-fore land pe riph eral bulge from the north.

The ba sin’s north ern mar gin was a broad, lit to ral shelf, dom i nated by waves and in flu enced by tidal cur rents (Wysocka, 2002; Mastalerz et al., 2006), with rel a tively lit -tle of its bioclastic cal car e ous sed i ment re leased to the deep-wa ter, ax ial zone. The main sed i ment sup ply was from the nar row and tec toni cally un sta ble, south ern shelf zone (Fig. 11), host ing del tas, sep a rated by tide-dom i nated coastal embayments, and pass ing seawards into a wave-worked, sandy shoreface zone, swept by longshore cur rents (fa cies mo dal i ties 2 and 3, Fig. 10). The in flu ence of tidal cur rents is well doc u mented in the Mio cene, west ern Paratethys (e.g., Sztano & De Boer, 1995). The ad ja cent, off shore en -vi ron ment was a neritic to subnaritic, muddy slope (Fig. 11), ac cu mu lat ing sandy tempestitites and con vey ing delta-de rived, sand-ladelta-den tur bid ity cur rents (fa cies mo dal ity 4, Fig. 10) to the ba sin’s deep zone. The tur bid ity cur rents in -cluded both brief, col lapse-gen er ated surge flows and river-gen er ated, lon ger-du ra tion, hyperpycnal flows (Porêbski & Warcho³, 2006; Warcho³, 2011).

IM PLI CA TIONS FOR HY DRO CAR BON

EX PLO RA TION

The source rocks for mi cro bial gas in the Carpathian Foredeep are con sid ered to be the Mio cene claystones and mudstones (Kotarba et al., 2011), which vol u met ri cally dom i nate in the ba sin-fill sed i men tary suc ces sion (e.g., Figs 6 and 7). They are in ter ca lated abun dantly with sand stones that act as hy dro car bon res er voirs. The main is sue for hy dro car bon ex plo ra tion in the ba sin is a de tailed un der stand -ing of the paths of pri mary mi gra tion of gas from the source rocks into sand stones and its sec ond ary mi gra tion within the sand stone bod ies. An other cru cial is sue is the rec og ni tion of hy dro car bon traps, some of which may be far more dif fi cult to rec og nize than oth ers.

Struc tural traps. The ev i dence from seis mic data, cores and geo phys i cal well logs and well-pro duc tion tests in di cates that the most im por tant res er voirs in the foredeep ba sin are struc tural traps, such as compactional anticlines, blindthrust anticlines and thrust footwalls, as well as youn -ger, extensional fault zones (Myœliwiec, 2004). Most of the hy dro car bon pro duc tion thus far has been from compactional anticlines, which formed in the Mio cene de pos its in re -sponse to un even bed rock mor phol ogy (i.e., Pre cam brian, Palaeozoic or Me so zoic rocks). Ex am ples of such gas fields, re lated to lo cal bed rock highs, are the Cetynia, Ka-mieñ, Je¿owe, multihorizon Jaros³aw, Kañczuga, Cha³upki Dêbniañskie and Jod³ówka fields (Fig. 1). Gas fields re lated to the Carpathian thrusts are some of the larg est in the Pol -ish part of the ba sin, in clud ing the Przemyœl–Jaksmanice, Husów–Albigowa–Krasne and Pilzno fields (Karnkowski, 1999). The trap in such cases is formed by the thrust hang -ing-wall to the south and the wa ter-gas con tact to the north. The most dif fi cult to rec og nize are traps, re lated to steep fault zones and ex em pli fied by such gas fields as the Rudka,

Ryszkowa Wola, Jaros³aw and Dzików fields (see Karnkowski, 1999). Two main re gional sys tems of faults are rec -og nized in the Carpathian Foredeep. The sys tem of faults, trending NW–SE, is re lated to tec tonic ex ten sion, caused by the lat est ap proach and up lift of the Carpathian orogen (Krzywiec, 1999), and is con sid ered to pro vide the main mi gra tion paths for hy dro car bons (Karnkowski, 1999). The as -so ci ated sys tem of trans verse faults is thought to provide the gas-trapping seals (Karnkowski, 1999).

Strati graphic traps. The other, main type of hy dro car

-bon trap in the Carpathian Foredeep is strati graphic (Myœli-wiec, 2004), al though these are ex tremely dif fi cult to inden- tify from seis mic sec tions, ow ing to the res o lu tion limit and have gen er ally been un der es ti mated in im por tance. The Mio cene ba sin-fill suc ces sion con sists of highly con trast ing fa cies as sem blages (Fig. 10), with the sand stone bod ies, en -vel oped by and pinch ing out in muddy de pos its, which gives in creased po ten tial for nu mer ous, strati graphic traps. On ac count of the great thick ness of the sed i men tary suc ces sion in the ba sin fill, (Figs 3 and 4), even small or “sub -tle” strati graphic traps in to tal may con tain large vol umes of hy dro car bons. If cor rectly rec og nized and ef fi ciently pro -duced as groups, strati graphic traps may be far more impor-tant than presently is believed.

How ever, the rec og ni tion of strati graphic traps re lies heavily on well data, par tic u larly core sam ples, which al low

MID DLE MIO CENE DE POS ITS IN THE CARPATHIAN FOREDEEP

249

Fig. 9. Flow di a grams. A – ver ti cal fa cies tran si tions that ap pear to be more fre quent than ran dom (see pos i tive val ues in ma -trix D in Fig. 8). B – ver ti cal fa cies tran si tions that have an oc cur rence prob a bil ity > 80% ac cord ing to sta tis ti cal test (see val -ues < 0.20 in ma trix E, Fig. 8)

sed i men tary fa cies to be iden ti fied and their spa tial dis tri bu -tion to be pre dicted. In this re spect, the sparse, “eco nomic” cor ing, em ployed to date for wells drilled in the ba sin (e.g., see Figs 3 and 4), does not im prove their sedimentological res o lu tion by com par i son with that of seis mic sec tions, which is far be low what might be re garded as op ti mum . It is cru cial, there fore, that a num ber of fully cored wells be drilled in the most prom is ing parts of the ba sin, rec og nized from seis mic sec tions, in ac cor dance with pres ent-day prac-tice in global petroleum exploration.

CON CLU SIONS

The pres ent sedimentological study, based on well cores from the Pol ish and Ukrai nian parts of the Carpathian Foredeep, re vealed gen eral fa cies het er o ge ne ity in the mid dle Mio cene sed i men tary suc ces sion of the ba sin and rec og -nized four main mo dal i ties of ver ti cal fa cies or ga ni za tion. These dif fer ent fa cies as sem blages rep re sent:

– the midlate Badenian shoalwa ter evaporitic en vi ron

-ment that pre ceded the lat est Badenian–early Sarmatian main phase of foredeep de vel op ment;

– a lit to ral, tidal en vi ron ment, re fer able to the in ner parts of storm-in flu enced, coastal bays and tidal flats and pos si bly some spit-shel tered lagoons;

– a lit to ral, wavedom i nated, sandy en vi ron ment, con -sid ered to be the shoreface zone, ex tended by waves in front of ad vanc ing deltas;

– a neritic to subneritic, muddy, off shore slope en vi ron -ment, with fre quent in cur sions of tempestitic and turbi- ditic sand.

The study has con trib uted to a better un der stand ing of the depositional sys tems of the foredeep ba sin and yielded sig nif i cant im pli ca tions for on go ing hy dro car bon ex plo ra tion. The or i gin of po ten tial res er voir sand stones was rec og -nized, which also sheds light on their ex pected, strati graphic oc cur rence in the ba sinfill suc ces sion. The po ten tial, eco -nomic im por tance of strati graphic hy dro car bon traps was noted. In this con text, the ur gent need for full-scale fa cies anal y sis, with fully cored wells in strategic parts of the basin, was indicated.

Fig. 10. Palaeoenvironmental in ter pre ta tion of ver ti cal, fa cies-tran si tion mo dal i ties, re vealed by Markov-chain anal y sis (Fig. 9B and ma trix E in Fig. 8)

Ac knowl edge ments

The study was con ducted in 2007–2010 as a part of the re search pro ject UKRAINA/193/2006, fi nanced by the Pol ish Min -is try of Sci ence and Higher Ed u ca tion and fa cil i tated by the University of Sci ence and Tech nol ogy (AGH) in Kraków and the Pol -ish Geo log i cal In sti tute in Warszawa. All the help, re ceived from the In sti tute of Ge ol ogy and Geo chem is try of Com bus ti ble Min er -als, Ukrai nian Na tional Acad emy of Sci ence, in Lviv, is grate fully ac knowl edged.

Cor dial thanks are of fered to the re view ers, Szczepan Po-rêbski (Uni ver sity of Sci ence and Tech nol ogy, Kraków) and ASGP ed i tor Wojciech Nemec, whose con struc tive com ments and crit i cal re marks have im proved and en riched this pa per.

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