Or i gin of hy dro car bon gases ac cu mu lated in the Mid dle Cam brian res er voirs of the Pol ish part of the Bal tic region
Maciej J. KOTARBA
Kotarba M. J. (2010) – Or i gin of hy dro car bon gases ac cu mu lated in the Mid dle Cam brian res er voirs of the Pol ish part of the Bal tic re - gion. Geol. Quart., 54 (2): 197–204. Warszawa.
The or i gin of nat u ral gases as so ci ated with oil and con den sate ac cu mu la tions within the Mid dle Cam brian sand stone res er voirs on the Pol ish part of the Bal tic re gion was char ac ter ized by means of mo lec u lar anal y ses, sta ble car bon iso topes of meth ane, eth ane and pro - pane, and sta ble hy dro gen iso topes of meth ane. Gases gen er ated from the Up per Cam brian–Tremadocian source rock suc ces sion by hy - drous py rol y sis at 330°C for 72 h was used to char ac ter ize thermogenic gas and to iden tify the mi cro bial meth ane in put in the nat u ral gas ac cu mu la tions. A in sig nif i cant com po nent of mi cro bial meth ane is only pres ent in gases from the B3 off shore field and from two in flows in the B7-1/91 bore hole. The traps within the Mid dle Cam brian sand stone res er voirs had al ready been formed and sealed be tween the Late Cam brian and the Early Or do vi cian time span when mi gra tion of mi cro bial meth ane took place along the fault sys tem. The traps were suc ces sively sup plied with thermogenic gas eous hy dro car bons, con den sate and oil gen er ated from the same source or ganic mat ter of the Up per Cam brian–Tremadocian source rock suc ces sion at a suc ces sively higher mat u ra tion stage.
Maciej J. Kotarba, Fac ulty of Ge ol ogy, Geo phys ics and En vi ron men tal Pro tec tion, AGH Uni ver sity of Sci ence and Tech nol ogy, Mickiewicza 30, PL-30-059 Kraków, Po land, e-mail: kotarba@uci.agh.edu.pl (re ceived: De cem ber 11, 2009; ac cepted: May 17, 2010).
Key words: Pol ish Bal tic re gion, Up per Cambrian–Tremadocian source rock suc ces sion, thermogenic gases, mi cro bial meth ane, hy - drous py rol y sis, sta ble iso topes.
INTRODUCTION
In the Pol ish on shore part of the Bal tic re gion, the small Żarnowiec oil de posit was first dis cov ered in 1970. Later, in this area two small oil ac cu mu la tions at Dębki in 1971 and Białogóra in 1991, and one gas-con den sate ac cu mu la tion at Żarnowiec-West in 1987 were dis cov ered (Karnkowski, 1999;
Domżalski et al., 2004; Karnkowski et al., 2010). In the Pol ish off shore part of the Bal tic re gion, three oil ac cu mu la tions in the B3 struc ture in 1981, B8 in 1983, and B34 in 1996, and four gas-con den sate ac cu mu la tions in the B4 struc ture in 1991, B6 in 1982, B16 in 1985, and B21 in 1996 were dis cov ered in Mid dle Cam brian sand stone res er voirs (Domżalski et al., 2004; Karnkowski et al., 2010). The Up per Cam brian–Lower Or do vi cian (Tremadocian) suc ces sion con tains the best source-rocks with low-or ganic sul phur and oil-prone Type-II kerogen (Więcław et al., 2010a). The Słupsk Block and the on - shore part of the Łeba Block (Fig. 1) are the main pe tro leum
gen er a tion and ex pul sion ar eas that de vel oped from the end of the Pridoli (lat est Si lu rian) to the end of the Car bon if er ous (Kosakowski et al., 2010; Wróbel and Kosakowski, 2010). The or i gin of oils ac cu mu lated in the Mid dle Cam brian res er voirs in the Pol ish part of the Bal tic re gion is has been de scribed by Więcław et al. (2010b).
De tails of the ge ol ogy and pe tro leum oc cur rence in the Pol - ish part of the Bal tic re gion were pub lished by Witkowski (1989), Karnkowski (1999), Domżalski et al. (2004), Karnkowski et al. (2010), Modliński and Podhalańska (2010), Pokorski (2010), and in ref er ences therein.
The ob jec tive of this study is to de ter mine the or i gin of hy - dro car bon gases dis solved in oil and in con den sate ac cu mu la - tions of the Mid dle Cam brian sand stone res er voirs of the Pol ish part of the Bal tic re gion. The study in volves mo lec u lar and sta - ble-iso tope char ac ter iza tion of nat u ral gases and gases gen er - ated from Up per Cambrian–Tremadocian source rocks by hy - drous py rol y sis (tem per a ture 330°C and time 72 h).
SAMPLE DESCRIPTION
ROCK SAMPLES
Hy drous py rol y sis ex per i ments were con ducted for three ther mally im ma ture sam ples rep re sent ing Up per Cam brian and Tremadocian shales. One Up per Cam brian sam ple was taken from the B4-N1/01 bore hole and the two Tremadocian sam ples orig i nated from the B6-2/85 and B7-1/91 bore holes. All three bore holes are lo cated in the Pol ish Ex clu sive Eco nomic Zone of the Bal tic Sea op er ated by the LOTOS Petrobaltic S.A.
Com pany. The sam ples are ~2 kg com pos ites col lected from drill cores. The lo ca tion of the sam pling sites is shown in Fig - ure 1 and sam ple depth is given in Ta ble 1. The sam ples were crushed and sieved to gravel size (0.5–2.0 cm), and were not pre-ex tracted for the ex per i ments.
NATURAL GAS SAMPLES
Eight nat u ral gas sam ples were col lected from Mid dle Cam brian sand stones in the Pol ish part of the Bal tic re gion (Fig. 1). Four of these sam ples were taken from off shore bore -
holes (B3-4/91, B3-9/95 and two in ter vals in B7-1/91), and four sam ples were col lected from the on shore Dębki 2 (Db 2), Dębki 5K (Db 5K), Malbork IG 1(Mb IG 1) and Żarnowiec 7 (Zn 7) bore holes (Fig. 1). Gases dis solved in oils (B3-4/91, B3-9/95, Db 2 and Db 5K sam ples) and con den sates (Zn 7 sam ple) were col lected from sep a ra tors to glass or metal con - tain ers (~500 and ~1000 cm3, re spec tively). Gas sam ples from
Fig. 1. Sketch map of the Pol ish Bal tic re gion show ing lo ca tion of sam pled rock and gas bore holes Fault sys tem af ter Pokorski (2010)
Sam ple code B4-N1 comp. B7-1 comp. B6-2 comp.
Depth [m] 1192–1197 2320–2323 1432–1438
Age Cm3 O–Tr O–Tr
TOC [wt.%] 13.0 9.6 10.6
Tmax [°C] 436 434 441
HI [mg/g TOC] 364 289 302
Cm3 – Up per Cam brian, O–Tr – Or do vi cian–Tremadocian; TOC – to tal or ganic car bon; Tmax – max i mum tem per a ture; HI – hy dro gen in dex; comp.
– com pos ite (an av er age sam ple from whole in ter val of lithostratigraphic unit)
T a b l e 1 De scrip tion of rock sam ples sub jected to hy drous py rol y sis
and Rock-Eval data
the B7-1/91 and Malbork IG 1 bore holes were col lected to glass con tain ers from gas and oil in flows. Sam ple depth in ter - vals are given in Ta ble 2.
EXPERIMENTAL AND ANALYTICAL PROCEDURES
HYDROUS PYROLYSIS
Hy drous py rol y sis (HP) ex per i ments were con ducted on 400 g of gravel-sized Up per Cam brian and Tremadocian shales. Sam ples were heated iso ther mally in the pres ence of liq uid wa ter in 1-L re ac tors made of Hastelloy-C276 (Parr In - stru ment Com pany, Il li nois, USA). It was cal cu lated that 375 g of dis tilled/deionized wa ter was suf fi cient to main tain liq uid wa ter in con tact with the rock be fore, dur ing, and af ter the ex - per i ments (Lewan, 1993). Af ter load ing and seal ing of the re - ac tor, the re main ing headspace was evac u ated and filled with he lium. The re ac tor was then placed in an elec tric heater and brought to the de sired ex per i men tal tem per a ture within 65 min - utes. Iso ther mal heat ing was con ducted at time 72 h and tem - per a ture 330°C with stan dard er rors less than ±0.5°C. This ex - per i men tal con di tion for lig nite (ini tially 0.34% Ro) has been shown to be equiv a lent to a mean ran dom vitrinite reflectance of 1.28% (Kotarba et al., 2009). Cool-down times ranged from 18 to 24 h. At room tem per a tures, gas pres sures and vol umes were mea sured, and gases were col lected in stain less-steel 304 dual-valve cyl in ders.
ANALYTICAL PROCEDURE
Mo lec u lar com po si tions of the hy drous py rol y sis gases (C1
to C6 sat u rated and un sat u rated hy dro car bons, H2S, CO2, O2, H2, N2, He and Ar) were ana lysed by a set of col umns on a Hewlett Packard 6890 gas chromatograph con fig ured by Wasson Ece In stru men ta tion. Mole per cent ages de ter mined by this anal y sis were con verted to moles with the ideal gas law and the re corded col lec tion vol umes, pres sures, and tem per a tures.
The re sults of anal y ses (Ta ble 3) are given in mmoles/g TOC.
He placed in the re ac tor as an in ter nal stan dard is ex cluded from cal cu la tions. The gen er ated gas anal y ses re fer ex clu sively
to headspace gas and do not in clude gases dis solved in the wa - ter or oils gen er ated by hy drous py rol y sis. Mo lec u lar com po si - tions of nat u ral gases were ana lysed with Hewlett Packard 5890 Se ries II and Chrom 5 gas chromatographs equipped with flame ion iza tion (FID) and ther mal con duc tiv ity (TCD) de tec - tors. The re sults of anal y ses are given in mole% and in clude the nat u rally oc cur ring He (Ta ble 2).
Sta ble iso tope anal y ses were per formed us ing Finnigan Delta Plus and Micromass VG Op tima mass spec trom e ters.
The sta ble car bon and hy dro gen iso tope data are pre sented in the d no ta tion rel a tive to V-PDB and V-SMOW stan dards (Coplen, 1995; Hoeffs, 2007), re spec tively. An a lyt i cal pre ci - sion is es ti mated to be ±0.2 and ±3‰, re spec tively. The sta ble iso tope com po si tion of hy drous py rol y sis and nat u ral gases are pre sented in Ta bles 4 and 5, re spec tively.
Bore hole Sam ple
code Depth [m] Mo lec u lar com po si tion [mole%]
CH4 C2H6 C3H8 iC4 nC4 iC5 nC5 C6+ N2 CO2 He Ar H2
B3-4/91 B3-4 1730–1755 50.88 25.75 14.35 0.82 2.27 n.a. 0.49 0.09 4.90 0.35 0.05 0.03 n.a.
B3-9/95 B3-9 1478–1479.8 45.00 25.10 17.10 1.35 3.01 0.32 0.38 0.15 6.37 0.22 0.01 n.a. 0.03
B7-1/91 B7-1(2) 2365.3–2374.4 67.60 5.86 2.00 0.07 0.19 n.a. 0.02 0.01 13.90 8.85 1.44 0.12 – B7-1/91 B7-1(6) 2336.6–2365.0 34.90 17.26 14.15 0.82 2.48 n.a. 0.75 0.14 21.87 1.61 0.08 0.29 5.64
Dębki 2 Db 2 2733–2753 71.80 14.40 7.60 0.63 1.48 n.a. 0.04 0.06 2.58 0.83 0.12 0.03 0.06
Dębki 5K Db 5K 2763–2785 57.30 19.30 14.60 1.30 3.27 n.a. 1.03 0.37 2.30 0.40 0.06 n.a. n.a.
Malbork IG 1 Mb IG 1 3234–3261 61.49 15.36 7.11 1.49 2.38 n.a. 1.71 0.67 8.45 1.15 0.09 0.11 n.a.
Żarnowiec 7 Zn 7 2752–2775 73.40 13.70 6.43 0.64 1.57 n.a. 0.73 0.27 2.64 0.47 0.12 0.02 –
iC4 – iC4H10; nC4 – nC4H10; iC5 – iC5H12; nC5 – nC5H12; C6+ = C6H14 + C7H16; n.a. – not ana lysed
T a b l e 2 Mo lec u lar com po si tion of nat u ral gases
Com po nent Sam ple code
B4-N1comp. B7-1comp. B6-2 comp.
Meth ane 0.2403 0.2396 0.1714
Eth ane 0.1925 0.2175 0.1161
Ethene 0.0000 0.0005 0.0000
Pro pane 0.1736 0.1862 0.1165
Propene 0.0030 0.0028 0.0044
n-Bu tane 0.0618 0.0662 0.0489
trans-2-Bu tene 0.0015 0.0016 0.0020
i-Bu tene 0.0004 0.0005 0.0008
i-Bu tane 0.0149 0.0161 0.0130
cis-2-Bu tene 0.0013 0.0012 0.0016
i-Pentane 0.0081 0.0093 0.0080
n-Pentane 0.0161 0.0191 0.0156
n-Hex ane 0.0034 0.0042 0.0038
To tal hy dro car bons 0.7168 0.7648 0.5023
* – acet y lene, iso-butanes, propadiene, neo-pentane, 1,3-bu ta diene, 3-methyl-1-bu tene, 2-methyl-2-bu tene, 1-pentene, neo-hex ane, and ben zene con tents are less than 0.1 µmole/g TOC; other ex pla na tions as in Ta ble 1
T a b l e 3 Mo lec u lar com po si tion (mmoles/g TOC)* of gas gen er ated
by hy drous py rol y sis at 330°C for 72 h
Meth ane, eth ane and pro pane were sep a rated chromatographically for sta ble car bon iso tope anal y ses. The gases were combusted sep a rately over hot cop per ox ide (850°C) pro duced by the on-line sys tem and then trans mit ted to a mass spec trom e ter. Wa ter re sult ing from the com bus tion of meth ane for sta ble hy dro gen iso tope anal y ses was re duced to gas eous hy dro gen with zinc (Florkowski, 1985).
The rock sam ples for Rock-Eval anal y ses were pul ver ized to the frac tion less than 0.2 mm. Py rol y sis as say was con ducted on Delsi Model II Rock-Eval in stru ment equipped with an or ganic car bon mod ule, which de ter mined to tal or ganic car bon (TOC).
RESULTS AND DISCUSSION
The gas eous hy dro car bons gen er ated at 330°C for 72 h by hy drous py rol y sis are shown in Ta ble 3. The quan ti ties of gen - er ated gas eous hy dro car bons range from 0.5 to 0.8 mmoles/g TOC and show no cor re la tion to orig i nal con cen tra tion of TOC and hy dro gen in dex (Ta ble 1). These gen er ated quan ti ties do not rep re sent max i mum hy dro car bon yields, and higher ex per i - men tal tem per a tures may re veal better cor re la tions with the ini - tial geo chem i cal pa ram e ters. All the hy drous-py rol y sis gases gen er ally have sim i lar or der of mag ni tude of mo lec u lar proportionalities (C1/C2, C2/C3, C3/C4+, and iC4/nC4) as shown in Ta ble 4, which may in di cate that or ganic mat ter in the ana -
lysed rock sam ples con tains the same Type-II kerogen of sim i - lar ma tu rity.
Al though the dry ness of a gas, as mea sured here by the C1/C2 ra tio, is gen er ally con sid ered to in crease with the ther mal ma tu rity of a source rock, Whiticar (1994) notes that this trend is typ i cal of Type-I and -II kerogens. The greater dry ness ob - served in nat u ral gases rel a tive to py rol y sis gases gen er ated in the lab o ra tory has been doc u mented (Mango, 1992). Price and Schoell (1995) con tend that such dry ness dis crep ancy be tween nat u ral and py rol y sis gases may be the re sult of the for mer be - ing pref er en tially en riched in meth ane dur ing sec ond ary migration after expulsion from a source rock.
Al though the pro por tion al ity of meth ane to eth ane in nat u - ral gas is not sim u lated in the thermogenic gases gen er ated by hy drous py rol y sis and in other py rol y sis meth ods (Mango, 1992; Price and Schoell, 1995), the dis tri bu tions of eth ane, pro - pane and butanes in nat u ral gases is sim i lar to those known from thermogenic gases gen er ated by hy drous py rol y sis. An - other pos si ble rea son for higher C1/C2 ra tios in nat u ral gases (Ta ble 5) than in hy drous py rol y sis gases (Ta ble 4) can be due to the pres ence of a small com po nent of mi cro bial meth ane in nat u ral gases. The tra di tional plot of gas dry ness (C1/[C2+C3]) ver sus d13C of meth ane (Fig. 2) re veals that nat u ral gases in - clude thermogenic gas and vary ing mix tures of thermogenic and mi cro bial gases. Typ i cally, the C2/C3 ra tios for nat u ral gases range from 1.5 to 5.0 (Nikonov, 1972). In the sam ples stud ied the C2/C3 ra tios for ana lysed hy drous py rol y sis vary
Sam ple code
Indices Sta ble iso tope [‰]
CH C H
4
2 6
C H C H
2 6
3 8
C H C H
3 8 4 10+
CH
C H C H
4
2 6+ 3 8
i n
C H C H
4 10 4 10
d13C
(CH4) dD
(CH4) d13C
(C2H6) d13C (C3H8)
B4-N1 comp. 1.25 1.11 1.66 0.66 0.24 –43.9 –328 –39.6 –34.5
B7-1 comp. 1.10 1.17 1.62 0.59 0.24 –42.4 –318 –39.3 –33.4
B6-2 comp. 1.48 0.99 1.30 0.74 0.27 –44.1 –322 –39.2 –35.1
Ex pla na tions as in Ta ble 1; C4H10+ = iC4H10 + nC4H10 + iC5H12 + nC5H12 + C6H14 + C7H16
T a b l e 4 Mo lec u lar pro por tion al ity in di ces and sta ble iso tope com po si tion of hy drous py rol y sis gases (330°C and 72 h)
Sam ple code
In di ces Sta ble iso tope [‰]
CH C H
4
2 6
C H C H
2 6
3 8
C H C H
3 8
4 10+
CH
C H C H
4
2 6+ 3 8
i n
C H C H
4 10 4 10
δ13C (CH4)
dD
(CH4) d13C
(C2H6) d13C (C3H8)
B3-4 1.98 1.79 3.90 1.27 0.36 –50.9 –214 –42.3 –33.0
B3-9 1.79 1.47 3.28 1.07 0.45 –49.7 –260 –42.7 –34.2
B7-1(2) 11.53 2.93 6.94 8.60 0.37 –49.6 –232 –40.8 –30.0
B7-1(6) 2.02 1.22 3.38 1.11 0.33 –50.2 –230 –41.9 –32.8
Db 2 4.99 1.90 3.42 3.26 0.42 –48.1 n.a. –39.0 n.a.
Db 5K 2.97 1.32 2.45 1.69 0.40 –47.1 –222 –38.0 –32.0
Mb IG 1 4.00 2.16 1.14 2.74 0.62 –47.4 –183 –36.2 n.a.
Zn 7 5.36 2.13 2.00 3.65 0.41 –47.1 –187 –38.0 n.a.
Ex pla na tions as in Ta ble 4
T a b l e 5 Mo lec u lar pro por tion al ity in di ces and iso tope com po si tion of nat u ral gases
from 1.00 to 1.17 (Ta ble 4) and those for nat u ral gases from 1.22 to 2.93 (Ta ble 5). The lower C2/C3 ra tios of both nat u ral and py rol y sis gases may in di cate that nat u ral gases ac cu mu - lated within Mid dle Cam brian sand stones were gen er ated from or ganic mat ter of the Up per Cam brian–Tremadocian source rock suc ces sion. The iC4H10/nC4H10 ra tios of hy drous py rol y sis gases gen er ated (from 0.24 to 0.27; Ta ble 4) and nat u ral gases ana lysed (from 0.33 to 0.62; Ta ble 5) gen er ally cor re spond to the pre scribed trend of Oudin (1993) who pre dicted this ra tio to be <1 for thermogenic gases gen er ated dur ing the catagenesis.
There fore, with the ex cep tion of the pro por tion al ity of meth ane to other hy dro car bon gases, hy drous py rol y sis is a rea son able ap proach to sim u lat ing the nat u ral gen er a tion of thermogenic gas. In par tic u lar, the geo chem i cal char ac ter is tics of gas gen er - ated dur ing hy drous py rol y sis (Figs. 2–5) serves as a model for a com par a tive di a gram for thermogenic gases which have not un der gone sec ond ary pro cesses dur ing mi gra tion and ac cu mu - la tion, and mix ing with mi cro bial meth ane.
The d13C val ues of hy dro car bon gases gen er ated dur ing hy - drous py rol y sis are given in Ta ble 4 and dis played in a re cip ro - cal car bon num ber plot in Fig ure 3 to gether with the nat u ral gas data. The sim u la tion of thermogenic gas gen er a tion by hy drous py rol y sis from the Up per Cam brian–Tremadocian source rock suc ces sion shows that the sta ble car bon iso tope com po si tion in creases in the meth ane-eth ane-pro pane sys tem (Fig. 3). In this sys tem (Fig. 3), all the hy dro car bon gases gen er ated show
“dog-leg” trends, which are dif fer ent from the more lin ear trends re ported by Chung et al. (1988). An im por tant im pli ca - tion of these ex per i men tal re sults is that a lin ear re la tion ship of meth ane, eth ane and pro pane car bon iso topes with their re cip - ro cal car bon num ber is not an ex clu sive in di ca tor of a nat u ral
gas from a sin gle source as some times as sumed by e.g., Chung et al. (1988) and Rooney et al. (1995). Zou et al. (2007) sug gest that in this type of plot a “dog-leg” trend, ex em pli fied by rel a - tively 13C-de pleted meth ane and en riched pro pane com pared to eth ane, re sults from the fact that nat u ral gas might have not been gen er ated from a sin gle source rock or that it may have un der went post-gen er a tion al ter ations (e.g., sec ond ary gas crack ing, mi cro bial ox i da tion, thermochemical sul phate re duc - tion). How ever, a “dog-leg” trend for the thermogenic gases gen er ated by hy drous py rol y sis ex per i ments of the Up per Cam - brian–Tremadocian source rock suc ces sion (Ta ble 4 and Fig. 3) as well as of Oligocene Menilite Shales of the Pol ish Outer Carpathians (Kotarba et al., 2009) al low one to pro pose a sin gle source rock.
The Fig ure 4 shows plots of d13C of eth ane ver sus d13C of meth ane and d13C of pro pane along with the em pir i cal ma tu rity trend for Type-II kerogen pro posed by Berner and Faber (1996). The later ma tu rity trend takes into ac count the d13C of the orig i nal kerogen from which gas was gen er ated. The Type-II trend is based on the av er age d13C value (–29.0
±0.02‰) of kerogen from the Up per Cam brian–Tremadocian source rock suc ces sion (Więcław et al., 2010a). The re sults of sta ble car bon iso tope anal y ses of eth ane and pro pane of nat u ral gases ac cu mu lated in the Mid dle Cam brian sand stone res er voir (Ta ble 5 and Fig. 4B) and their com par i son with gas com po - nents gen er ated by hy drous py rol y sis (Ta ble 3 and Fig. 4B) sug gest that these nat u ral gases were gen er ated dur ing low-tem per a ture thermogenic pro cesses from Type-II kerogen of ma tu rity 0.6–0.7% in the vitrinite reflectance scale con tained in the Up per Cam brian–Tremadocian source rock suc ces sion.
d13C(C2H6) val ues de vi ate from the re lated trend (Fig. 4) which
Fig. 2. Hy dro car bon in dex CHC (i.e., CH4/[C2H6+C3H8]) ver sus d13C of meth ane for nat u ral gases ac cu mu lated in the Mid dle Cam brian res er - voir and for hy drous py rol y sis gases gen er ated from kerogen of the Up - per Cam brian–Tremadocian source rock suc ces sion at 330°C and 72 h
Compositional fields are from Whiticar (1994)
Fig. 3. d13C of meth ane, eth ane, and pro pane ver sus the re cip ro cal of their car bon num ber for nat u ral gases ac cu mu lated in the Mid dle Cam brian res er voir com pared with hy drous py rol y sis gases in gray at 330°C and 72 h for the rock sam ple ana lysed
Or der ac cord ing to Rooney et al. (1995)
can be caused by the con tri bu tion of mi cro bial eth ane.
Claypool (1999) doc u ments that eth ane can be also gen er ated dur ing mi cro bial pro cesses.
The dis crep ancy be tween the sta ble car bon iso tope com po - si tion of the gases ana lysed (Fig. 4) and the iso to pic curves of Berner and Faber (1996) can be ex plained in part by dif fer - ences in the ki net ics for re com bi na tion/con den sa tion re ac tions re spon si ble for vitrinite reflectance and the wide vari a tion in ki net ics for free rad i cal crack ing re ac tions re spon si ble for pe - tro leum gen er a tion (Lewan, 1985). On the other hand, this dis - crep ancy can po ten tially pro vide ev i dence that the Berner-Faber plot is not valid for the HP gases.
The dD val ues of meth ane gen er ated by hy drous py rol y sis vary from –328 to –318‰ (Ta ble 4 and Fig. 5). As pre vi ously dis cussed by Kotarba and Lewan (2004) and Kotarba et al.
(2009), 2H-de pleted meth ane can be ex plained by the 2H-de - pleted dis tilled wa ters used in the hy drous py rol y sis ex per i - ments. The abil ity of wa ter to be a source of hy dro gen dur ing the ther mal crack ing of hy dro car bons has been shown ex per i - men tally (Hoering, 1984; Lewan, 1997; Schimmelmann et al., 1999, 2001) and has been ad vo cated in nat u ral meth ane gen er - a tion (Smith et al., 1982; Schoell, 1988). There fore, un like the im por tance of kerogen in flu enc ing the d13C val ues for gen er - ated hy dro car bon gases, dD val ues of hy dro car bon gases can be in flu enced by the for ma tion wa ters pres ent in a source rock dur ing their gen er a tion.
Sta ble car bon iso tope anal y sis of meth ane from nat u ral gas ac cu mu lated in the Mid dle Cam brian res er voirs (Ta ble 5) com - pared with meth ane gen er ated dur ing hy drous py rol y sis in di - cates that a mi cro bial com po nent (Ta ble 4 and Figs. 2–5) also ex ists. A in sig nif i cant com po nent of mi cro bial meth ane is pres - ent only in gases from the B3-4, B7-1 and B3-9 sam ples (Figs. 3 and 5). Con clud ing so far, the traps within the Mid dle Cam brian sand stone res er voirs had al ready been formed and sealed dur ing mi gra tion of mi cro bial meth ane which was gen - er ated from im ma ture or ganic mat ter of the Up per Cam -
brian–Tremadocian suc ces sion. The thermogenic gen er a tion and ex pul sion pro cesses were ini ti ated in the Pridoli (lat est Si - lu rian) and lasted un til the Car bon if er ous (Kosakowski et al., 2010; Wróbel and Kosakowski, 2010). The fault sys tem in the Bal tic re gion has old Neoproterozoic roots and was ac ti vated in the Cam brian and Or do vi cian (Poprawa, 2006; Poprawa et al., 2006). There fore, mi cro bial meth ane and eth ane might have mi grated from the Up per Cam brian–Tremadocian source rock
Fig. 4. d13C of eth ane ver sus A – d13C of meth ane and B – d13C of pro pane for nat u ral gases ac cu mu lated in the Mid dle Cam brian res er voir and for hy drous py rol y sis gases gen er ated from kerogen of the Up per Cam brian–Tremadocian source rock suc ces sion at 330°C and 72 h
Vitrinite-reflectance trend line for Type-II kerogen is based on equa tion re ported by Berner and Faber (1996)
Fig. 5. d13C ver sus dD of meth ane for nat u ral gases ac cu mu lated in the Mid dle Cam brian res er voir and for hy drous py rol y sis gases gen er ated from kerogen of the Up per Cambrian–Tremadocian source rock suc - ces sion at 330°C and 72 h
Compositional fields are from Whiticar et al. (1986)
suc ces sion to the Mid dle Cam brian sand stone traps in the Late Cam brian to Early Or do vi cian in ter val. Iso to pic data also sug - gest that thermogenic gen er a tion and ex pul sion pro cesses them selves were of at least one-phase type, and that traps al - ready con tain ing mi cro bial gases were sup plied by new por - tions of thermogenic gas eous hy dro car bons as well as by con - den sate and oil.
CONCLUSIONS
The trend be tween d13C-val ues of meth ane, eth ane and pro - pane of gases gen er ated by hy drous py rol y sis of a rep re sen ta tive Up per Cam brian–Tremadocian source-rock suc ces sion con tain - ing Type-II kerogen and their re cip ro cal car bon num ber is not al - ways lin ear, as pre scribed by some in ves ti ga tors. Ex per i men tal de ter mi na tion of a “dog-leg” trend of the hy drous py rol y sis gases and how it changes dur ing pe tro leum gen er a tion pro vides a cor re la tion pa ram e ter that can help as sess thermogenic gas end-mem bers and mi cro bial gas in put in nat u ral gases.
Iso to pic char ac ter iza tion of nat u ral hy dro car bon gases ac - cu mu lated in the Mid dle Cam brian sand stone res er voir com - pared to hy drous py rol y sis gases in the Pol ish part of the Bal tic re gion re vealed that these nat u ral gases were mainly gen er ated by thermogenic pro cesses from oil-prone Type-II kerogen con - tained in the Up per Cam brian–Tremadocian source rock succession. In the B3 off shore oil and gas field and in pe tro - leum in flows to the B7-1/91 bore hole an in sig nif i cant mi cro -
bial com po nent was found. The traps within the Mid dle Cam - brian sand stone res er voir had al ready been formed and sealed be tween the Late Cam brian and the Early Or do vi cian time span when mi gra tion of mi cro bial meth ane took place along the fault sys tem. The thermogenic gen er a tion and ex pul sion were at least one-phase pro cesses. Sub se quently, the gases have un der - gone mi gra tion and mix ing. The traps al ready con tain ing mi - cro bial gases were sup plied by new portions of thermogenic gaseous hydrocarbons as well as by con den sate and oil.
Ac knowl edg ments. The re search was un der taken as part of a pro ject of the Min is try of the En vi ron ment (No.
182/2005/Wn-06/FG-sm-tx/D) car ried out at the AGH Uni ver - sity of Sci ence and Tech nol ogy and fi nanced by the Na tional Fund for En vi ron men tal Pro tec tion and Wa ter Man age ment.
Par tic u lar grat i tude is ex pressed to M. D. Lewan from the U.S.
Geo log i cal Sur vey in Den ver for help in hy drous py rol y sis ex - per i ments and valu able ad vice. E. Faber from Bundesanstalt für Geowissenschaften und Rohstoffe in Hannover, J. A.
Bjesen-Koefoed from the Geo log i cal Sur vey of Den mark and Green land in Co pen ha gen and J. B. Curtis from the Col o rado School of Mines in Golden gave very con struc tive re marks and com ments that greatly im proved the dis cus sion and the pos si - ble con se quences of the hy poth e ses pre sented in the manu - script. An a lyt i cal work by H. Zych and T. Kowalski from the AGH Uni ver sity of Sci ence and Tech nol ogy and by A. War den from the U.S. Geo log i cal Sur vey is grate fully ac knowl edged.
REFERENCES
BERNER U. and FABER E. (1996) – Em pir i cal car bon iso tope/ma tu rity re la tion ships for gases from al gal kerogens and terrigenous or ganic mat ter, based on dry, open-sys tem py rol y sis. Org. Geochem., 24:
947–955.
CHUNG H. M., GORMLY J. R. and SQUIRES R. M. (1988) – Or i gin of gas eous hy dro car bons in subsurface en vi ron ments: the o ret i cal con sid - er ations of car bon iso tope dis tri bu tion. Chem. Geol., 71: 91–103.
CLAYPOOL J. E. (1999) – Biogenic eth ane – Where does it come from?
In: AAPG Hedberg Re search Con fer ence “Nat u ral Gas For ma tion and Oc cur rence”, June 6–11, 1999, Durango, Col o rado.
COPLEN T. B. (1995) – Re port ing of sta ble car bon, hy dro gen, and ox y gen iso to pic abun dances. In: Ref er ence and Intercomparison Ma te ri als for Sta ble Iso topes of Light El e ments. Proc. con sul tants meet ing held in Vi enna, December1–3, 1993: 31–34. Internat. Atomic En ergy Agency, Vi enna.
DOMŻALSKI J., GÓRECKI W., MAZUREK A., MYŚKO A., STRZETELSKI W. and SZAMAŁEK K. (2004) – The pros pects for pe tro leum ex plo ra tion in the east ern sec tor of South ern Bal tic as re - vealed by sea bot tom geo chem i cal sur vey cor re lated with seis mic data.
Prz. Geol., 52 (8/2): 792–799.
FLORKOWSKI T. (1985) – Sam ple prep a ra tion for hy dro gen iso tope anal - y sis by mass spec trom e try. Inter. J. Appl. Ra di a tion and Iso topes, 36:
991–992.
HOEFFS J. (2007) – Sta ble iso tope geo chem is try. Springer-Verlag, Berlin.
HOERING T. C. (1984) – Ther mal re ac tion of kerogen with added wa ter, heavy wa ter, and pure or ganic sub stances. Org. Geochem., 5: 267–278.
KARNKOWSKI P. (1999) – Oil and Gas De pos its in Po land. The Geosynoptics Soc. “Geos”, Kraków.
KARNKOWSKI P. H., PIKULSKI L. and WOLNOWSKI T. (2010) – Pe - tro leum ge ol ogy of the Pol ish part of the Bal tic re gion – an over view.
Geol. Quart., 54 (2): 143–158.
KOSAKOWSKI P., WRÓBEL M. and POPRAWA P. (2010) – Hy dro car - bon gen er a tion and ex pul sion mod el ling of the lower Pa leo zoic source rocks in the Pol ish part of the Bal tic re gion. Geol. Quart., 54 (2):
241–256.
KOTARBA M. J., CURTIS J. B. and LEWAN M. D. (2009) – Com par i son of nat u ral gases ac cu mu lated in Oligocene strata with hy drous py rol y - sis gases from Menilite Shales of the Pol ish Outer Carpathians. Org.
Geochem., 40: 769–783.
KOTARBA M. J. and LEWAN M. D. (2004) – Char ac ter iz ing thermogenic coalbed gas from Pol ish coals of dif fer ent ranks by hy drous py rol y sis.
Org. Geochem., 35: 615–646.
LEWAN M. D. (1985) – Eval u a tion of pe tro leum gen er a tion by hy drous py rol y sis ex per i men ta tion. Philos. Trans. Royal Soc., A315: 123–134.
LEWAN M. D. (1993) – Lab o ra tory sim u la tion of pe tro leum for ma tion:
Hy drous py rol y sis. In: Or ganic Geo chem is try (eds. M. Engel and S.
Macko): 419–442. Ple num Publ. Corp., New York.
LEWAN M. D. (1997) – Ex per i ments on the role of wa ter in pe tro leum for - ma tion. Geochim. Cosmochim. Acta, 61: 3691–3723.
MANGO F. D. (1992) – Tran si tion metal ca tal y sis in the gen er a tion of pe - tro leum and nat u ral gas. Geochim. Cosmochim. Acta, 56: 553–555.
MODLIŃSKI Z. and PODHALAŃSKA T. (2010) – Out line of the li thol - ogy and depositional fea tures of the lower Pa leo zoic strata in the Pol - ish part of the Bal tic re gion. Geol. Quart., 54 (2):109–121.
NIKONOV V. F. (1972) – Dis tri bu tion of meth ane homologs in gas and oil fields (in Rus sian). Dokl. Akad. Nauk SSSR, 206: 234–236.
OUDIN J. L. (1993) – Head-space gas anal y sis. In: Ap plied Pe tro leum Geo chem is try (ed. M. L. Bordenave): 271–278. Edi tions Technip, Paris.
POKORSKI J. (2010) – Geo log i cal sec tion through the lower Pa leo zoic strata of the Pol ish part of the Bal tic re gion. Geol. Quart., 54 (2):
123–130.
POPRAWA P. (2006) – De vel op ment of the Cal edo nian col li sion zone along the west ern mar gin of Baltica and its re la tion to the fore land ba - sin (in Pol ish with Eng lish sum mary). Pr. Państw. Inst. Geol., 186:
189–214.
POPRAWA P., ŠLIAUPA S. and SIDOROV V. (2006) – Late Si lu rian and early De vo nian intra-plate com pres sion in the fore land of Cal edo nian orogen (cen tral part of the Bal tic Ba sin) – anal y sis of seis mic data (in Pol ish with Eng lish sum mary). Pr. Państw. Inst. Geol., 186: 215–224.
PRICE L. C. and SCHOELL M. (1995) – Con straints on the or i gins of hy - dro car bons gas from com po si tions of gases at their site of or i gin. Na - ture, 378: 368–371.
ROONEY M. A., CLAYPOOL G. E. and CHUNG H. M. (1995) – Mod el - ing thermogenic gas gen er a tion us ing car bon iso tope ra tios of nat u ral gas hy dro car bons. Chem. Geol., 126: 219–232.
SCHOELL M. (1988) – Mul ti ple or i gins of meth ane in the Earth. Chem.
Geol., 71: 1–10.
SCHIMMELMANN A., BOUDOU J. P., LEWAN M. D. and WINTSCH R. P. (2001) – Experimential con trols on D/H and 13C/12C ra tios of kerogen, bi tu men and oil dur ing hy drous py rol y sis. Org. Geochem., 32: 1009–1018.
SCHIMMELMANN A., LEWAN M. D. and WINSCH R. P. (1999) – D/H iso tope ra tios of kerogen, bi tu men, oil, and wa ter in hy drous py rol y sis
of source rocks con tain ing kerogen types I, II, IIS, and III. Geochim.
Cosmochim. Acta, 63: 3751–3766.
SMITH J. W., GOULD K. W. and RIGBY D. (1982) – The sta ble iso tope geo chem is try of Aus tra lian coals. Org. Geochem., 3: 111–131.
WIĘCŁAW D., KOTARBA M. J., KOSAKOWSKI P., KOWALSKI A. and GROTEK I. (2010a) – Hab i tat and hy dro car bon po ten tial of the lower Pa leo zoic source rocks in the Pol ish part of the Bal tic re gion. Geol.
Quart., 54 (2): 159–182.
WIĘCŁAW D., KOTARBAM. J. and KOWALSKI A. (2010b) – Or i gin of oils ac cu mu lated in the Mid dle Cam brian res er voirs of the Pol ish part of the Bal tic re gion. Geol. Quart., 54 (2): 205–216.
WHITICAR M. J. (1994) – Cor re la tion of nat u ral gases with their sources.
In: The Pe tro leum Sys tem – from Source to Trap (eds. L. B. Magoon and W. G. Dow). Am. Ass. Petrol. Geol. Mem., 60: 261–283.
WHITICAR M. J., FABER E. and SCHOELL M. (1986) – Biogenic meth - ane for ma tion in ma rine and fresh wa ter en vi ron ments: CO2 re duc tion vs. ac e tate fer men ta tion – isotope ev i dence. Geochim. Cosmochim.
Acta, 50: 693– 709.
WITKOWSKI A. (1989) – Palaeogeodynamics and gas-bear ing of the Lower Palaeozoic of the Pomerania and south ern Bal tic Sea (in Pol ish with Eng lish sum mary). Zesz. Nauk. AGH, Geologia, 43: 1–128.
WRÓBEL M. and KOSAKOWSKI P. (2010) – 2-D mod el ling of pe tro leum pro cesses of the lower Palaeozoic strata in the Pol ish part of the Bal tic re gion. Geol. Quart., 54 (2): 257–266.
ZOU Y.-R., CAI Y., ZHANG CH., ZHANG X. and PENG P. (2007) – Vari - a tions of nat u ral gas car bon iso tope-type curves and their in ter pre ta - tion – a case study. Org. Geochem., 38: 1398–1415.
