Geo log i cal Quar terly, 2010, 54 (2): 183–196

Hydr ocar bon gene ra tion/e xpul sion model ling of the lower Paleozoic poten tial source rocks in the Gryfice and Kołobrzeg blocks (NW )

Paweł KOSAKOWSKI, Magdalena WRÓBEL and Paweł POPRAWA

Kosakowski P., Wróbel M. and Poprawa P. (2010) – Hy dro car bon gen er a tion/ex pul sion mod el ling of the lower Pa leo zoic po ten tial source rocks in the Gryfice and Kołobrzeg blocks (NW Poland) . Geol. Quart., 54 (2): 183–196. Warszawa.

The lower Paleo zoic source rocks in the offshore part of the Gryfice and Kołobrzeg blocks (NW Poland) were studied through geochem i- cal data and nu meri cal mod el ling. The geo chem i cal study re vealed a pres ence of ef fec tive source rocks in Caradocian strata, but with low hydro car bon poten tial . The rem aining lower Pa leozoic source rock hori zons were not docu m ented by core sam ples. The tim ing of hydro - car bon gener a tion and expul sion was modelle d for the K1-1/86 and L2-1/87 boreholes locate d in the Gryfice and Kołobrzeg blocks, re- spectivel y. 1-D and 2-D model ling indi cate d that in the Kołobrzeg Block the onset of petro leum gener a tion occurr ed at the end of the Silu ria n and the be ginning of the Devo nian. Source rocks in the Gryfice Block reached the early stage of oil gener a tion at the begin ning of the Perm ian and gener a tion pro cesses were com pleted by the end of the Trias sic. Migra tion of hy drocar bons from source rocks be gan in the Carbon if er ous in both the Gryfice and Kołobrzeg blocks and lasted to the end of the Meso zoic. During hydro car bon migra tion, an inten sive disper sion process was observe d, caused by leaking along the fault planes. The model ling reveale d that hy drocar bons may have ac cu mu lated in De vo nian res er voirs. The lack of any dis cov ered ac cu mu la tions could be the re sult of hy dro car bon dis per sion caused by tec tonic de for ma tion and in tense ver ti cal fault block move ments.

Paweł Kosakowski and Magdalena Wróbel, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron mental Pro tec tion, AGH Uni ver sity of Sci ence and Tech nol ogy, Mickiewicza 30, PL-30-059 Kraków, Poland, e-mail: [email protected]; Paweł Poprawa, Pol ish Geo log i cal In sti tute – National Research In stitute, Rakowiecka 4, PL-00-975, Warszawa, Poland, e-mail: [email protected] (re ceived: De cem ber 21, 2009; ac cepted: April 8, 2010).

Key words: Gryfice and Kołobrzeg blocks, lower Paleo zoic, source rock, burial history , therm al evolu tion, petro leum processe s.

INTRODUCTION shore L2-1/87 and K1-1/86 bore holes were used for model - ling of the hy dro car bon gen er a tion and ex pul sion pro cesses (Fig. 1). Mod el ling of hy dro car bon mi gra tion and ac cu mu la- tion was perform ed along the regional 81032K-820 cross-sec - Char acter istics of lower Paleozoic source rocks in the Pom - tion (Fig. 1). era nian segm ent of the Trans-Euro pean Suture Zone (TESZ) To deter mine the thickness of strata rem oved during the late and the petro leum proces ses in the offshore part of this area Paleozoic and Me so zoic, mea sured pa ram e ters of kerogen ther - were ex am ined. mal matu rity were used. The same data were used to model cal- The study area is the offshore parts of the Gryfice and ibra ti on of heat flow changes through time. the therm al Kołobrzeg blocks, locat ed in the Pom era nian segm ent of the and burial his tory con strained, the fol low ing ba sic gen er a tion Trans-Euro pea n Suture Zone (TESZ). Owing to few geologi c condi ti ons were estab li shed for each borehole: data avail able from the lower Pa leo zoic suc ces sion, as well as – time and depth identi fica ti on of the organic matter ther - a lack of rep re sen ta tive re sults of geo chem i cal anal y ses for mal ma tu rity in ter vals; the poten ti al source rocks (only Caradocian shales were sam- – po si tion of gen er a tion stages and de gree of kerogen pled), the in ves ti ga tion has the char ac ter of a gen er a tion test. trans for ma tion; Char acter istics of source rocks of the lower Pa leo zoic strata – amount of gen er ated and ex pelled hy dro car bon mass; and the model of kineti c transfor m ati ons were adopted from – time and range of hy drocar bon migra ti on; the pe tro leum anal ysis of co eval strata in the Bal tic re gion fur - – time and amount of accu m ula ti on. ther east. Re con structed lithostratigraphic sec tions in the off- 184 Paweł Kosakowski, Magdalena Wróbel and Paweł Poprawa

Fig. 1. Geolog ical map of the Pom era nian part of the Cal edo nian Plat form without Perm ian and youn ger de pos its (af ter Pokorski and Modliński, 2007, modified) with lo ca tion of the boreholes ana lysed

GEOLOGICAL SETTING that already recog nize d in the southern Skania outcrops. The base ment of the Rugia, , Gryfice and Kołobrzeg blocks is either re lated to the East Eu ro pean Craton or to terrains The study area is a part of the northern segm ent of the accreted to the TESZ dur ing the Cal edo nian col li sion. TESZ, lo cated be tween two major fault zones, the Trzebiatów The geo log ical de vel op ment of the area in early Pa leo zoic in the west and the in the east (Fig. 1). The lower Pa- time was recon struc ted mainly based on studies of the lower leozoic strata are tectoni cally deform ed, folded and most prob- Pa leo zoic sed i men tary rocks in the nearby Koszalin–Chojnice ably also detac hed from their origi nal basem ent (Dadlez, Zone (see Znosko, 1965; Teller and Korejwo, 1968; Dadlez, 2000). Ac cord ing to Pokorski and Jaworowski (2002), in the 1978; Podhalańska and Modliński, 2006; Poprawa, 2006). In Kołobrzeg Block, beneat h the overthrusted lower Paleo zoic the study area and its east ern vi cin ity the Or do vi cian strata, allochthonous strata, an autochthonous sedi m entar y cover of most fre quently the Llandeillian–lower Caradocian, were the East Euro pean Craton (EEC) might be ex pected, simi lar to found in the bottom part of the boreholes : 2, Brda 3, Hydrocarbon generation/expulsion modelling of the lower Paleozoic potential source rocks in the Gryfice and Kołobrzeg blocks 185

Chojnice 5, L2-1/87, IG 1, Jamno IG 2, Karsina 1, THERMAL EVOLUTION Kościernica 1, Miastko 1, Nowa Karczma 1, Nowa Wieś 1, Sarbinowo 1, Skibno 1 and Wyszebórz 1 (Dadlez, 1978; Podhalańska and Modliński, 2006; Fig. 1). The folded Silu ria n The late Permian–Me so zoic stage of the study area’s ther - strata were encoun ter ed in the boreholes : Chojnice 3, mal his tory is rel a tively well un der stood, while the late Pa leo - Klosnowo IG 1, Kłanino 3, Lutom 1, Nicponie 1, Stobno 1 zoic ther mal his tory is less well doc u mented, de spite the ex is - (S 1), Stobno 2, Stobno 3 (S 3), Wierzchocina 1 and tence of geo chemi cal data. The early Paleozoic stage is poorly Wierzchocina 4 (Dadlez, 1978; Podhalańska and Modliński, rec og nized with little in for ma tion. 2006; Fig. 1). Recon struc tion of the heat flow during the geolog i cal evo- The depth of re cent burial of the lower Paleozoic strata lu tion of the ba sin started with anal ysis of the re cent ther mal ranges betwe en 4000 and 7000 m. The lower Paleozoic strata, regim e. The input data consis ted of a map of tem pera ture s for folded during the Caledo nian orogeny (Znosko, 1965; Dadlez, the top of the Zechstein Main Do lo mite car bon ates, i.e. the 1978; Żaba and Poprawa, 2006), is overlai n by a Devo - best recog nize d strati graphic hori zon of the study area nian-Car bonif er ous cover which is presentl y over 1000 m thick (Karwasiecka, 2000). The recon struc tion was based on di rect (Matyja, 2006) and which was also was de formed during the mea sure ments in bore holes in ther mal equi lib rium con di tions Variscan orogeny . In the vicin it y of the Sarbinowo and west of sim ilar to steady-state. Where there was an ab sence of such the Dygowo–Białogard Fault Zone, lower Perm ian extru sive mea sure ments, the downhole tem per a ture was as sessed be low rocks are pres ent, being a final result of the Variscan orogeny , the observe d hori zon on the basis of publi shed data on the and which are covered by strata of the upper Rotliegend. Perm - magni tude of the heat flow (Karwasiecka and Bruszewska, ian rocks are depos it ed uncon form ably on the Devo nian and 1997). The dis tri bu tion of re cent tem per a tures ob tained at the Car bonif er ous succession. Their total thickness in the specif ic top of the Main Do lo mite strata in the area an alysed re vealed profil es varies from a few to about 350 m. Perm ian de posi ti on values pre dom inantl y in the range of 70–90°C. Geother mal in the northern, Pom era nian segm ent of the Polish re gion con- gra di ents cal cu lated for late Permian and Me so zoic time in the tinued through the Meso zoic . There is a succes sion about study area on the ba sis of the distri buti on of downhole tem- 1800 m thick near Sarbinowo and over 3000 m in the south- pera ture s range betwe en 2.00 and 2.25°C/100 m. These tem - west ern part of (Dadlez, 1974; Dadlez et al., 1995; per a tures in di cated that the lower Pa leo zoic po ten tial source Stephenson et al., 2003). Analo gous to the Pol ish Lowland, the rock, 1000–2000 m lower in the suc ces sion, was certai nly topmost part of the sedi m entar y cover is com posed of Ceno - able to gen er ate hy dro car bons. Re con struc tion of this pro cess, zoic uncon sol i date d depos it s up to 200 m thick. how ever, re quires de ter mi na tion of ther mal changes over the As the lower Paleo zoic strata is poorly recog nize d in the enti re time inter val, from the de posi ti on of the source rocks study region, the downhole condi ti ons of the lower Paleo zoic analy sed to their current depth. are in di rectly in ter preted from seis mic sec tion anal y sis (see Current ly available knowledge on changes in the therm al Pokorski, 2010). Only on this ba sis one can infer the evolu ti on field com es only from regional studies . Several basic models of of the lower Pa leo zoic suc ces sion at the pre-De vo nian stage of ther mal evolu ti on exist , and these were proposed by devel op m ent of the ba sin. That the late Paleo zoic stage of the Majorowicz et al. (1984), Karnkowski (1999, 2003a), Poprawa area’s evo lu tion fin ished with tec tonic de for ma tion and up lift, and Grotek (2004), and Poprawa and Andriessen (2006). and the late Perm ian–Me so zoic stage, ter minated with in ver - Though Majorowicz et al. (1984) in their model covered only sion of the Pol ish Basin, has been gener all y well recog nize d the Lublin area and northern-wes tern Fore-Sudetic Monocline, and described in the liter ature (e.g., Dadlez, 1978; Karnkowski, this ma te rial re mains sig nif i cant for the re maining part of the 1999; Krzywiec, 2002; Lamarche and Schenk-Wenderoth, Polish Lowland. Based on ana ly sed gra dients of therm al matu - 2005a, b; Matyja, 2006). rity, Majorowicz et al. (1984) suggest ed that the Variscan The tectoni c structure of the part of the Cal edonian plat- orogen and its fore land were heated in late Paleozoic time and form studied is a re sult of multiphase de for ma tion re lated after wards progressively cooled. This proces s conti nues till the mainly to contractional and transpressional re gimes (Znosko, pres ent. 1965; Dadlez, 1978; Poprawa et al., 1999; Poprawa, 2006). A Karnkowski (1999, 2003a) suggest ed that the magni tude of few phases of upli ft and erosion accom panied the tectoni c de- the heat flow within the main depocentre of the Pol ish Ba sin form ati on, the main ones being: Early De vonian, late to was consta nt, with a pos iti ve therm al anom aly during the Perm - post-Car bon if er ous, and post-Cre ta ceous. In the off shore part ian, Tri as sic and Ju ras sic only, within the Fore-Sudetic of study area the Early De vonian erosion event led to parti al re- Monocline. Poprawa and Grotek (2004) and Poprawa and moval of Ordo vi cia n and Si luria n strata with a total thickness Andriessen (2006) proposed two al ter nati ve models . In first, of approx i m ately 2000 m (Fig. 2). Af ter the Car bon if er ous to they proposed steady heat flow in the Creta ceous , and variable early Perm ian phase of erosion, the Devo nian and Car bonif er - heat flow in the Perm ian and Trias sic . The other model in - ous strata were parti ally de nuded, with the thickness of the re- cluded a lo cal in crease of heat flow in the Permian and Tri as sic moved secti on deter mined to be up to 1200 m thick. The lat est fol lowed by a pro gres sive de crease, with rel atively lower val - to post-Cre taceous erosion parti ally re moved the Upper Creta - ues in Late Cre taceous time. ceous and Juras sic strata. The thickness of the eroded Meso zoic The model li ng perform ed reveal ed that the observe d ther - sec tion was esti mated at approx i m ately 1300 m in the Gryfice mal matu rity of the lower Pa leo zoic suc ces sion analysed, with Block (Fig. 2A) and 2300 m in the Kołobrzeg Block (Fig. 2B). its up per Pa leo zoic and Perm ian-Me so zoic cover, re quires 186 Paweł Kosakowski, Magdalena Wróbel and Paweł Poprawa

Fig. 2. Burial his tory curves for se lected lithostratigraphic suc ces sion with therm al matu rity zones in pro files: A – K-1/86 bore hole in the Gryfice Block and B – L2-1/87 bore hole in the Kołobrzeg Block Pr – Precam brian, Cm – Cam brian, Cm1 – Lower Cam brian, Cm2 – Middle Cam brian, Cm3 – Upper Cam brian, O – Ordo vi cian, Oar – Or do vi cian (Arenigian), Oln – Or do vi cian (Llanvirnian), Ok – Ordo vi cian (Caradocian), S – Silu ria n, D – Devo nian, D2 – Mid dle De - vonian, D3 – Upper Devo nian, C – Car bonif er ous, C1 – lower Carbon if er ous, C2 – upper Carbon if er ous, P – Perm ian, P1 – lower Perm - ian, P2 – up per Permian, T – Tri as sic, T1 – Lower Tri as sic, T2 – Mid dle Tri as sic, T3k – Up per Tri as sic (Keuper), T3r – Up per Tri as sic (Rhaetian), J – Juras sic, J1 – Lower Ju rassic, Cr – Creta ceous, Pg – Paleogene, N – Neo gene, Q – Quaternary lower heat flow during Meso zoic burial as com pared to recent (Kosakowski et al., 2006) indi cat ed that Variscian heat flow heat flow. This has been descri bed previ ously with refer ence to was slightly higher in com pari son to re cent heat flow. This dif- the onshore part of the Mid-Polish Trough (Poprawa and fer ence grad u ally disap peared to wards the marine part of these Grotek, 2004; Poprawa and Andriessen, 2006). blocks. As in the Bal tic Ba sin (Karnkowski, 2003b; Poprawa Avail able data al lowed re con struc tion of the Variscian and Grotek, 2005; Kosakowski et al., 2010; Poprawa et al., ther mal and burial re gime. Anal ysis of the ther mal history on 2010), the tim ing of the main phase of matu ra ti on in the off- the onshore part of the Kołobrzeg and Gryfice blocks shore part of the Kołobrzeg and Gryfice blocks could not be de- Hydrocarbon generation/expulsion modelling of the lower Paleozoic potential source rocks in the Gryfice and Kołobrzeg blocks 187 ter mined with any preci sion. It is assum ed that suc ces sive in- creases in heat flow took place from the be ginning of De vonian to the late Car bon if er ous, i.e. to the time of maxi m um burial. The pres ent sur face heat flow was cal cu lated on the ba sis of heat con duc tiv ity val ues cal cu lated from lithological pro por- tions for each spe cific strati graphic hori zon of the secti on ana - lysed. The recon structed recent heat flow ranges from ca. 45 mW/m2 in the western part of the ana ly sed platform (Gryfice Block) to ca. 60 mW/m2 in the east ern part (Kołobrzeg Block).

GENERAL CHARACTERISTICS OF THE LOWER PALEOZOIC SOURCE ROCKS

Poten ti al source rock hori zons in the east ern part of the study area have been geochemically doc um ented only within the Caradocian strata. There were only 49 core samples col - lected from this hori zon from the L2-1/87 bore hole in the off- shore part of study area, as well as from the Kłanino 3, Kościernica 1, Sarbinowo 1, and Skibno 1 boreholes in the onshore part (Fig. 1). Quan ti ta tive anal y sis re veals that the to - Fig. 3. Rock-Eval hydro gen in dex ver sus Tmax tem per a ture for Caradocian (Up per Or do vi cian) source rocks of the Koszalin–Chojnice Zone Ta ble 1 Ma tu rity paths of in di vid ual kerogen types af ter

Geo chem i cal char ac ter is tics and hy dro car bon Espitalié et al. (1985a, b); Ro – vitrinite reflectance scale po ten tial of the Caradocian strata of the Pom er a nian part of Cal edo nian plat form tal organic carbon (TOC) content is low, usuall y be low 0.3 wt.%, and the hy drocar bon poten ti al does not excee d 250 mg HC/g TOC. Mea sured Tmax val ues are vari able and in the onshore part are within the initi al phase of the low-tem per- a ture thermogenic proces ses (“oil window”), while in the off- shore part they are within the late phase of the “oil window” (Fig. 3). There fore, geo chem i cal stud ies re veal the pres ence of ef fec tive source rocks in Caradocian strata, but with low hy dro car bon po ten tial. How ever, by anal ogy with the Bal tic region, the gener a ti on model li ng was also com pleted for the Mid dle Cam brian and Up per Cam brian–Lower Or do vi cian strata as a syn thetic sec tion with geo chem i cal char ac ter is tics of the boreholes from the Słupsk Block (Kosakowski et al., 2010; Więcław et al., 2010a).

MODELLING PROCEDURE

The princi pal aim of the 1-D num eri cal model li ng was to de ter mine the depth and tim ing of hy dro car bon gen er a tion and expul sion for secti ons of the L2-1/87 and K1-1/86 bore holes (Fig. 1). These pro cesses were recon structed by means of BasinMod™ 1-D (BMRM 1-D, 2006). To assess the amount of hy dro car bons gen er ated and ex pelled from the lower Pa leo zoic

TOC – to tal or ganic carbon, Tmax – tem per a ture of max i - source rocks, the fol lowing data were quanti fied: present and mum of S2 peak, Ro – mean ran dom reflectance of the orig i nal thick nesses of in di vid ual strati graphic units, thick- vitrinite-like macerals, S2 – re sid ual pe tro leum po ten tial nesses of miss ing sec tions, li thol ogy of in di vid ual strati graphic (mg HC/g rock), S1 – oil and gas yield (mg HC/g rock), HI – hydro gen index (mg HC/g TOC). Range of geo chem ical units, as well as pres ent and palaeo-heat flow. Input data for pa ram e ters is given as nu mer a tor; me dian val ues in de- mod el ling also in clude geo chem i cal data such as re con structed nom i na tor. In pa ren the ses: num ber of sam ples from bore- TOC and geneti c type of kerogen. Ma tu rity was cal cu lated us - holes (num era tor) and num ber of sam pled boreholes ing the EASY%R model (Swee ney and Burnham, 1990). Gen- (de nom i na tor) o 188 Paweł Kosakowski, Magdalena Wróbel and Paweł Poprawa er a tion and ex pul sion of hy dro car bons were cal cu lated by the 0.5 wt.% TOC, while the Upper Cam brian source rock with LLNL (Law rence Livermore Na tional Lab o ra tory) model 10 wt.% TOC is only 1 m thick. The Caradocian source rock is (Ungerer et al., 1988; Forbes et al., 1991; BMRM 1-D, 2006) 60 m thick with 1.5 wt.% TOC. Poros it y and perm eabil ity data and the use of kinetic pa rame ters Ea = 57.5 (kcal/mol) and A0 = were not avail able, so these pa rame ters were cal cu lated from 2.508E + 28 (1/m.y.) calcu lat ed by Więcław et al. (2010b) for the lithological type of each in ter val. These rock types were ad - source rocks in the Bal tic re gion. Ma tu rity mod el ling was cal i - equate ly define d in the region, in the west from the K1-1/86 brated with Rock-Eval Tmax tem per a ture and vitrinite boreholes , and in the east from the L2-1/87 bore holes. The reflectance. same proce dure was used for the therm al data input. Mi gra tion and ac cu mu la tion of hy dro car bons was ana lysed with BasinModTM 2-D software (BMRM 2-D, 2006) along the cross-sec tion 81032K-820 sit u ated be tween bore holes THERMAL MATURITY EVOLUTION K1-1/86 and L2-1/87 (Fig. 1). The follow ing hori zons were in - OF THE LOWER PALEOZOIC STRATA put to the 2-D model based on seism ic secti on inter preta ti on: top of the Ordo vi cia n (Caradocian), top of the Devo nian, top of the Carbon if er ous, the bottom and top of the Zechstein, the bot- Mod el ling of ther mal ma tu rity, de ter mined by the heat flow tom of the Middle Trias sic , the top of the Upper Tri assic , and and burial condi ti ons descri bed above, reveal ed that the early the bot tom of the Juras sic inter vals. More over, two addi ti onal stage of therm al matu rit y (0.5–0.7% Ro) was obtai ned by po - hori zons were input, the Middle and Upper Cam brian source tenti al Middle Cam brian source rocks in the offshore part of the rocks. These are “syn thetic” ho ri zons because this succes sion Gryfice and Kołobrzeg blocks at the end of the Ordo vi cia n and is be yond the res olu ti on of seism ic data, and not drilled in any begin ning of the Si luria n. This occurred at burial depths of bore hole (Fig. 4). The thickness of the Middle Cam brian 1400–1900 m and tem pera ture s over 80°C (Figs. 2 and 5). The source rock is assum ed to be approx i m ately 45 m, with increa se of burial depth of source rocks result ed in their enter -

Fig. 4. The 81032K-820 cross-sec tion along which 2-D mod elling of gen era tion, ex pulsion, migra tion and ac cum ula tion was completed

Dep. S – depo si tion of Si luria n strata; Dep. D-C – depo si tion of De vonian and Carbon if er ous strata; Dep. J-Cr – depo si tion of Juras sic and Creta ceous strata; for lo cation see Fig ure 1 Hydrocarbon generation/expulsion modelling of the lower Paleozoic potential source rocks in the Gryfice and Kołobrzeg blocks 189

ing the main ther mal ma tu rity stage within the “oil win dow” in ter val (0.7–1.0% Ro) prior to the Early De vo nian up lift (Fig. 5). The erosion connect ed with the upli ft hindered any increa se of matu rit y of the kerogen. The de posi ti on of Middle to Upper Devo - nian and Car bonif er ous sedi m ents led to burial of the poten ti al Middle Cam brian source rocks to the ther- mal condi ti ons of the late stage of the “oil window” (1.0–1.3% Ro) (Figs. 2 and 5). Fur ther de po si tion, es- pe cially of the Permian and Lower Tri as sic sed i- ments, caused that poten ti al source rock in part of the area to enter the “gas window” (1.3–2.6% Ro). The Mid dle Cam brian strata reached max im um therm al matu rit y prior to inver sion at the end of the Creta - ceous, at the maxi m um burial depth of 7000–7500 m and tem per a tures of 160–170°C (Fig. 2). Po ten tial source hori zons of Upper Cam brian age enter ed the early stage of the “oil window” at the end of the Or- dovi cia n and in the early Silu ria n (Figs. 2 and 5). The Up per Cam brian–Lower Or do vi cian source rocks reached the main stage of the “oil window” in the Kołobrzeg Block in the Ludlow and the final stage at the end of the Visean. In the Gryfice Block, the main stage was reached in the Famennian and the final stage in the early Permian (Fig. 5). An even bigger time dif fer ence be tween en ter ing com pa ra ble mat u - rati on stages as regards the Middle and Upper Cam - brian poten ti al source rocks can be observe d for ther- mal matu rit y in the “oil window”. The Upper Cam - brian in the Kołobrzeg Block en tered this ma tu rity in ter val in the Mid dle Tri as sic, i.e. ca. 60 m.y. later than the Middle Cam brian, at a depth of burial of about 4500 m and a tem pera ture over 160°C (Fig. 2). The Up per Cam brian–Lower Or do vi cian source rocks in the Gryfice Block enter ed the stage of the “oil window” much later, in the Norian, at a burial depth be low 6000 m and tem pera ture over 150°C (Fig. 2). In the Kołobrzeg and Gryfice blocks the poten ti al Caradocian source ho ri zons en tered the early stage of the “oil window” in Wen lock–Pridoli time, at burial depths of 1400–1900 m and tem pera ture s over 80°C (Figs. 2 and 5). The Early De vonian and late Carbon - ifer ous upli ft and erosion events stopped further in- crease of matu rit y. Only de posi ti on of Perm ian–Me- so zoic strata, which caused a con sid er able in crease of burial depth and tem pera ture , led to enter ing the main stage of the “oil window”. In the Kołobrzeg Block, this stage was reached by the Caradocian rocks at the begin ning of the Tournaisian, and in the Gryfice Block at the turn of the Early and Middle Tri- as sic (Figs. 2 and 5). The matu ra ti on history of the Caradocian strata also varies across both blocks. In the Kołobrzeg Block, the begin ning of the “oil win - dow” took place in the Middle Trias sic , at a depth of Fig. 5. Ma turity evo lu tion in lower Paleozoic source rocks in pro files ° of the K1-1/86 and L2-1/87 bore holes, analysed for A – Middle Cam brian, burial near 4000 m and tem pera ture over 150 C, B – Up per Cam brian and C – Up per Ordo vi cian (Caradocian) while in the Gryfice Block this took place at the turn of the Juras sic and Creta ceous at a burial depth of R – vitrinite reflectance scale; for other ab bre vi ations see Fig ure 2 o 5850 m and tem pera ture over 130°C (Fig. 2). Further 190 Paweł Kosakowski, Magdalena Wróbel and Paweł Poprawa

in crease of burial to ca. 6300 m in the L2-1/87 bore- hole and ca. 6150 m in the K1-1/86 borehole did not re sult in sig nif i cant in crease in ma tu rity or en ter ing of the stage of high-tem pera ture gases (above 1.3% Ro) (Fig. 2).

GENERATION AND EXPULSION PROCESSES IN THE LOWER PALEOZOIC SUCCESSION

Mod el ling of hy dro car bon gen er a tion for the Mid - dle Cam brian strata showed that the poten ti al source rocks reached the en tire gen er a tion po ten tial range from the early to the final stage. In the Kołobrzeg Block, in the L2-1/87 bore hole, the lowest parts of the poten ti al Middle Cam brian source rocks reached the early stage (10–25% of gener a ti on poten ti al) at the end of the Si luria n and begin ning of the Devo nian at a depth of ca. 2700 m and tem pera ture over 130°C (Fig. 6). The rem aining part of the Middle Cam brian sec tion reached that stage in a wide time range from the De vo nian to the be gin ning of the Car bon if er ous. During the same time inter val the source rocks reached the main stage (25–65% of gener a ti on poten - tial), at a depth of burial below 2700 m (Fig. 6). The Early De vonian upli ft and erosion inter rupted the gen - era ti on proces s. Depo si ti on of a thick Middle and Up- per Devo nian succes sion led to resum pti on of this proces s and, in the early Carbon if er ous, to the final stage of gener a ti on (65–90%) (Fig. 6). The poten ti al Middle Cam brian source hori zons in the area around the L2-1/87 bore hole exhaust ed their gener a ti on po - ten tial at the end of the Westphalian, reach ing a to tal de gree of or ganic mat ter trans for ma tion. In the Gryfice Block, in the K1-1/86 bore hole, the Middle Cam brian po ten tial source rocks reached the early stage only at the begin ning of the Perm ian, at a depth of burial below 3600 m and tem pera ture over 120°C (Fig. 6). The main stage was reached in the Early Tri - as sic at a burial depth of ca. 5100 m and tem pera ture over 130°C, and the final stage on the turn of the Mid and Late Trias sic . The poten ti al source rocks of the Mid dle Cam brian ex hausted their gen er a tion po ten tial at the end of the Tri as sic (Fig. 6). The spe cific gen er a - tion stages of the poten ti al Upper Cam brian–Lower Ordo vi cia n source rocks were reached in a broad time inter val from the be ginning of the Devo nian to the end of the Trias sic . The source rocks in the Kołobrzeg Block reached the early stage of hy drocar bon gener a - tion at the begin ning of the Devo nian, and total ly ex- hausted their gener a ti on poten ti al be fore the end of Fig. 6. Trans for ma tion ra tio of kerogen in lower Paleozoic source rocks the Visean (Fig. 6). The source rocks in the Gryfice in the K-1/86 and L2-1/87 bore holes for A – Mid dle Cambrian, Block enter ed the early stage at the begin ning of B – Up per Cam brian and C – Up per Ordo vi cian (Caradocian) Perm ian, and the main stage on the turn of the Early and Mid Trias sic . The fi nal stage of hydro car bon gen- For other ab bre vi a tions see Fig ure 2 er a tion in the Gryfice Block was en tered in the Late Tri as sic, at the end of the Carnian and begin ning of Hydrocarbon generation/expulsion modelling of the lower Paleozoic potential source rocks in the Gryfice and Kołobrzeg blocks 191 the Norian (Fig. 6). De spite the time and depth dif fer ences of TOC, for the Upper Cam brian it was 400 mg/g TOC, and for the reach ing specific gen er ation stages in the Kołobrzeg and Caradocian 300 mg/g TOC (Fig. 7). The total amount of gas gen- Gryfice tectonic blocks, the or ganic matter was completely erate d was on aver age 50 mg/g TOC for all three source rocks. trans formed into hy dro car bons (Fig. 6). A sim ilar range of time Expul sion of hy drocar bons was observe d mainly from the Upper and depth was observe d in ki netic model li ng of Caradocian Cam brian–Lower Or do vi cian in ter val, as the Caradocian ex - source rocks. In the Kołobrzeg Block, in L2-1/87 borehole, the pelled only resid ual amounts of hy drocar bons. Oil expul sion source rocks reached the enti re gener a ti on inter val from the from the Up per Cambr ian in ter val was ini tiated in Namurian early to the fi nal stage. The early and the main stages of hy dro - time in the Kołobrzeg Block and Trzebiatów Fault Zone, while car bon gen er ation were reached in the Westphalian, at a depth in the Gryfice Block expul sion oc curred dur ing the Late Trias sic . of burial of 2700–3200 m and tem pera ture s of 130–140°C The end of oil expul sion from the Upper Cam brian–Lower Or- (Fig. 6). The fi nal gen er ation stage was reached by the do vi cian in ter val was in the Early Ju ras sic in the Trzebiatów Caradocian organic matter only on the turn of the Early and Fault Zone, in the Creta ceous in the Kołobrzeg Block and in the Mid Tri as sic, with to tal trans for ma tion. In the Gryfice Block, in Ju ras sic in the Gryfice Block. Expul sion from the Caradocian borehole K1-1/86, the Caradocian source rocks also enter ed the source rocks took place in the Tri assic . The vol ume of expell ed en tire in ter val from the early to the fi nal stage. The early and oil varies from 0.03 m3/m3 rock for the Caradocian to 0.12 m3/m3 the main gen er ation stages were en tered in a nar row time in ter - rock for the Upper Cam brian–Lower Ordo vi cia n source inter - val of the Early Tri as sic (Carnian). The trans for ma tion de gree vals. Gas expul sion from the Upper Cam brian source rocks in ob tained for kerogen was 90% (Fig. 6). the Gryfice and Kołobrzeg blocks took place during the time in- ter val be tween the end of the Carbon if er ous and the begin ning of the Trias sic . Expul sion of gas from the Caradocian started at the MIGRATION AND ACCUMULATION end of the Mid Trias sic in the eastern part of the study area and in OF HYDROCARBONS the Late Tri as sic in the western part. It lasted un til the Early Cre - IN THE LOWER PALEOZOIC STRATA taceous . The vol ume of expell ed gas varied from 0.006 m3/m3 rock for the Caradocian to 0.016 m3/m3 rock for the Upper Cam - Mod el ling of hy dro car bon mi gra tion and ac cu mu la tion per- brian–Lower Or do vi cian source in ter vals. Hy dro car bon mi gra - formed along the 81032K-820 cross-sec tion for the the lower tion and ac cum ula ti on model li ng carrie d out for the lower Pa leo- Paleo zoic succes sion allowe d constra int of the tim ing and lateral zoic poten ti al source rocks reveal ed the possi bil ity of hy drocar - extent of these proces ses in the region discusse d. The Cam brian bon ac cu mu la tion in the study area. Hy dro car bons, ex pelled source rocks achieved total transfor m ati on rati o at the end of the mainly from the Upper Cam brian–Lower Ordo vi cia n source Car bon if er ous in the Kołobrzeg Block and in the Trzebiatów rocks, may have accu m ulat ed in the Devo nian reser voirs in both Fault Zone. At the same time the Caradocian source rocks in that the Gryfice and Kołobrzeg blocks near anti cli nal struc tures along region achieved betwe en 30 and 50% of kerogen trans for ma tion. faults. Ac cu mu la tion be gan at the end of the Car bon if er ous. In In the Gryfice Block at the end of the Car bonif er ous, the trans- the Kołobrzeg Block, faults betwe en 45 and 50 km of the form ati on rati o for the Middle Cam brian and Upper Cam - cross-sec tion (Fig. 8), were pathway s for dis persion of exist ing brian–Lower Ordo vi cia n source rocks was 40%, and for the hydro car bons. This dis persion stopped at the end of the Perm ian Caradocian about 20%. At the end of the Tri assic the Cam brian and be gin ning of the Tri as sic and ac cu mu la tion be gan again in source rocks were com pletely trans formed. At the same time the the Devo nian reser voir near the L2-1/87 borehole (Fig. 8). Mi - Caradocian source rock was also total ly transform ed, except for grati on of oil in the Kołobrzeg Block lasted un til the Paleogene, a few places in the Kołobrzeg Block and near the K1-1/86 bore- however only a part of the migrat ed hy drocar bons were accu m u - hole, where the fi nal transfor m ati on rati o of 90% was achieved lated and some were disperse d. In the traps involv ing De vonian 3 3 at the end of the Cre taceous. In the area be tween nine and twelve reser voirs in the Kołobrzeg Block 0.095 m /m rock of oil ac cu- kilom etres of the cross-secti on analy sed, the Caradocian source mu lated (Fig. 8). In the Gryfice Block, ac cum ula ti on took part in rocks did not ex ceed 30% trans for ma tion. Hy dro car bon gen er a - east from the K1-1/86 bore hole and began at the end of the Car - tion from the Cam brian source rocks in the Kołobrzeg Block and bonif er ous and lasted to the end of the Perm ian. At the end of the in the Trzebiatów Fault Zone began in the Visean. During the Perm ian and begin ning of the Tri assic some dis persion oc curred 3 3 Namurian the Caradocian source rocks started hy drocar bon gen- there, and finall y about 0.02 m /m rock of oil ac cum ulat ed era ti on in the Kołobrzeg Block. In the Gryfice Block hy drocar - (Fig. 8). In the Gryfice and Kołobrzeg blocks, gas migra ti on and bon gener a ti on from the Cam brian source rocks be gan at the end ac cu mu la tion oc curred in De vo nian strata, mainly in the of Car bon if er ous, and from the Caradocian rocks in the Mid Tri - Trzebiatów Fault Zone and in the Kołobrzeg Block. This accu - as sic (Fig. 7). The end of hydro car bon gener a ti on from the Cam - mu la tion in the Trzebiatów Fault Zone began at the end of the brian source hori zons in the Kołobrzeg Block and Trzebiatów Car bon if er ous and lasted un til Tri as sic time, while in the Fault Zone took place at the end of Carbon if er ous, and for the Kołobrzeg Block it took place at the begin ning of the Juras sic . Caradocian rocks in the Late Tri as sic. The gen er ation pro cess in Some dis persions of accu m u lated gas were observe d, mainly due the Gryfice Block was ter minated at the end of Tri as sic for the to ver ti cal migra ti on along the faults. The volum e of ac cum u - 3 3 Cam brian source rock and at the end of the Juras sic for the lated gas it the Trzebiatów Fault Zone achieved 0.008 m /m 3 3 Caradocian rocks (Fig. 7). The total amount of oil gener ate d rock, and in the Kołobrzeg Block 0.027 m /m rock (Fig. 9). from the Middle Cam brian source rocks was up to 350 mg/g 192 Paweł Kosakowski, Magdalena Wróbel and Paweł Poprawa

Fig. 7. Amount of gen er ated hy dro car bons along the cross-sec tion 81032K-820: A – af ter Car bon if er ous sed i men ta tion, B – af ter Trias sic sed im enta tion and C – pres ent

Dep. C-P – de po si tion of Car bon if er ous and Perm ian strata; for other ab bre vi a tions see Fig ure 4

The expelled hy drocar bons migrat ed from the source rocks CONCLUSIONS into De vo nian sand stones. The con nec tion be tween older source rocks and younger res ervoir rocks was facil itat ed by a The anal y sis of hy dro car bon gen er a tion and ex pul sion pro- num ber of faults. However, it is im portant to note that the Mid - cesses in the lower Paleozoic strata in the Gryfice and dle and Upper Cam brian source rocks were include d in the Kołobrzeg blocks showed that: model as a synthet ic succes sion, and have not been docu - 1. Middle Cam brian poten ti al source rocks reached the en- mented in boreholes, and are poorly doc um ented by seism ic tire hy dro car bon gen er a tion in ter val, from early to late anal ysis ; therefore, changes in the exact posi ti on of the succes - phases, in both the Kołobrzeg and Gryfice blocks and the tim - sion within the secti on might result in changes in time and ing of reach ing of those phases was prac tically the same. The range of the petro leum proces ses discusse d. early stage of hy drocar bon gener a ti on was reached by source rocks at the end of the Si luria n and begin ning of the Devo - Hydrocarbon generation/expulsion modelling of the lower Paleozoic potential source rocks in the Gryfice and Kołobrzeg blocks 193

Fig. 8. Oil ac cu mu lated vol ume ra tio along the cross-sec tion 81032K-820: A – af ter Car bon if er ous sed i men ta tion, B – af ter Trias sic sed im enta tion and C – pres ent

Ab bre vi a tions as in Figures 4 and 7 nian, and the main stage in the Lochkovian. The Early Devo - dro car bon gen er a tion at the be gin ning of the De vo nian. Prior nian ero sion event in ter rupted hy dro car bon gen er a tion. The to the end of the Westphalian they en tered the final stage of succes sive increa se in depth and tem pera ture during the Mid hy dro car bon gen er a tion, com pletely de plet ing their gen er a - and Late Devo nian and early Carbon if er ous caused the source tion po ten tial. In the Gryfice Block, the source rocks reached rocks to enter the final stage of hydro car bon gener a ti on at the their early gener a ti on stage at the end of the Car bonif er ous be gin ning of the Tournaisian. The po ten tial Mid dle Cam brian and the main gener a ti on stage at the turn of the Early and Mid source rocks de pleted their gener a ti on poten ti al at the end of Tri as sic. The fi nal stage of hy dro car bon gen er a tion in that tec - the Westphalian, reach ing a complete de gree of or ganic mat- tonic block was reached during the Late Trias sic . Hydro car - ter trans for ma tion. In the Gryfice Block, the po ten tial Mid dle bon expul sion in both blocks be gan during the Namurian, and Cam brian source rocks reached the early stage of hy dro car - finishe d in Juras sic time. In the Kołobrzeg Block at some lo - bon gener a ti on at the begin ning of the Perm ian, the main ca tions, ex pul sion of hy dro car bons con tin ued un til the Cre ta- stage be ing reached dur ing Late Tri as sic time, and the late ceous. stage at the turn of the Mid and Late Trias sic. Po ten tial Mid - 3. Caradocian source rocks in the Kołobrzeg Block also dle Cam brian source rocks de pleted their gener a ti on poten ti al reached the en tire hy dro car bon gen er a tion in ter val from the at the end of the Tri assic and begin ning of the Juras sic . early to the fi nal stage. The early and main stages of hydro car - 2. Up per Cam brian–Lower Or do vi cian po ten tial source bon gener a ti on were reached by the Caradocian rocks during rocks in the Kołobrzeg Block reached the early stage of hy - the Westphalian, and the fi nal stage on the turn of the Early and 194 Paweł Kosakowski, Magdalena Wróbel and Paweł Poprawa

Fig. 9. Gas ac cu mu lated vol ume ra tio along the cross-sec tion 81032K-820: A – af ter Car bon if er ous sed i men ta tion, B – af ter Trias sic sed im enta tion and C – pres ent

Ab bre vi a tions as in Figures 4 and 7

Mid Trias sic. Or ganic matter of the Caradocian source rocks mi gra tion path ways. Dur ing hy dro car bon mi gra tion in tense was com pletely transform ed by the end of the Mid Trias sic . In hy dro car bon dis per sion was ob served. The hy dro car bons ac- the Gryfice Block, the Caradocian source rocks also reached cum ulat ed mostly during the Perm ian and Trias sic , while dur- the en tire gen er a tion in ter val from the early to the final stage. ing the Juras sic and Creta ceous some dis persion of that accu - The early and main stages of gen er ation were reached in a nar - mula ti on was observe d. On the cross-sec tion analy sed, migra - row time in ter val of the Early Tri as sic, while the fi nal stage was tion took place mainly along the Trzebiatów Fault Zone and in reached on the turn of the Mid and Late Juras sic . The proces ses the Kołobrzeg Block, in the an ticli nal zones of near-fault el e - of hy dro car bon ex pul sion from the Caradocian took place in va tions. the Kołobrzeg Block in the time inter val of Mid and Late Trias - Lack of dis cov ered ac cu mu la tions within De vo nian res er- sic, and in the Gryfice Block during the Late Trias sic . voirs in the study area is re lated to hy drocar bon disper sion 4. Mi grati on of expell ed hy drocar bons from the Upper caused by tec tonic upli ft and defor m ati on. More over, high tem - Cam brian–Lower Or do vi cian and the Caradocian source pera ture s and pressures caused oil cracking and formed a gas rocks to De vo nian res er voirs started dur ing Car bon if er ous phase that readily migrat ed from the reser voir and was time in both the Gryfice and Kołobrzeg blocks and lasted unti l dispersed. the end of the Meso zoic . Faults enhance d the good verti cal Hydrocarbon generation/expulsion modelling of the lower Paleozoic potential source rocks in the Gryfice and Kołobrzeg blocks 195

Ac knowl edge ments. The research was un der taken as part very construc tive rem arks and com ments that greatly im - of a project of the Minis try of the Envi ron m ent (No. proved the qual ity of the manuscri pt. 182/2005/Wn-06/FG-sm-tx/D). The authors thank L. Mali - nowski from the Uni ver sity of Silesia in Katowice and J. Curtis from the Colo rado School of Mines in Golden for

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