Przegląd Geologiczny, vol. 46, nr 8/2, 1998
Model of gaseous hydrocarbon
generation
in the Miocene
strata
of
the Polish part of the
Carpathian
Foredeep
Maciej
J.
Kotarba*,
WacławBurzewski*, Tadeusz
Wilczek**,
Kazimierz
Słupczyński*, PawełKosakowski*,
Dariusz
Botor*
The ITI modelling demonstrated that the autochthonous Miocene strata located in the outer Miocene basin ojthe Carpathian Foredeep, north oj the present edge oj the Carpathian overthrust have generated only the microbial gas. In the part oj the Miocene basin covered recently by the Carpathian overthrust the low-temperature thermogenic gases were generated down to 7,000 ojmeters depth. Finally, in the hypothetical zone ojthe Lower Miocene molasse located recently at depth intervaI7,500-11,000 meters only the high-temperature thermogenic gases could be jormed. Alternatively, at the site oj the Lower Miocene molasse the depression filled with Upper Carboniferous coal-bearingjormation may occur. Therejore, at depth beneath 7,500 meters, i.e. beneath the Carpathian overthrust the gas deposits can be expected. The maximum yield ojthe microbial methane generation within the autochthonous Miocene calculatedjor depth interval 900 to 1,500 meters is about 5 cubic meters per cubic meter oj source rock. It is possible that the generation process oj microbial methane stil! continues. Accumulation oj microbial methane within the autochthonous Miocene strata was jacilitated by high sedimentation rate and rhythmic and cyclic deposition oj clays and sands.
Key words: petroleum exploration, Miocene, basin analysis, energy sources, natural gas, genesis, methane, biogenic processes, thermal history, models, Carpathian Foredeep, Poland
Introduction
The Carpathian Foredeep is the largest gas basin among the alI foredeeps of the Alpine orogenie system in Europe. In the autochthonous Miocene strata of the Polish part of the Carpathian Foredeep only methane-dominated deposits have been discovered up to date. Since 1945 about 70 gas fields have been found of total resources around 200 billion cubic meters.
In the following paper the problem of gas generation is
discussed within the autochthonous Miocene sequence of the Polish part of the Carpathian Foredeep.
The selection of analytical material for geochemical studies on the dispersed organie matter (OM) colIected from the drill cores considered the spatiallithofacial development of the autochthonous Miocene molasse in the Polish part of the Carpathian Foredeep (Kotarba et al., 1998). Consequent-ly, the four test areas representative for hydrocarbon gener-ation conditions have been chosen in the Miocene outer basin (Fig. 1).
The two test areas A and B rellect the hydrocarbon generation conditions in the part of the Carpathian Foredeep north of the recent edge of the Carpathian orogen. The test areas C and D represent the conditions at the edge of the Carpathian orogen. FinalIy, the test area E includes the hydrocarbon generation conditions in that part of the inner basin which is recently covered by the overthrust (Fig. 1). For each of the test areas the TTI modelling has been carried out (e.g., Waples, 1980, 1985).
The study has been undertaken as a part of research projects of the Carpathian Foredeep financed by the State Committee for Scientific Research in Warsaw (grant No. 9 9214 92 03) and National Fund for Environmental
Protec-tion and Water Management (grant No. 2.14.0100.00.0).
*University of Mining and Metallurgy, Department of Fossil Fuels, al. Mickiewicza 30,30-059 Kraków, Poland,
e-mail: kotarba@uci.agh.edu.pl
**Polish Oil and Gas Company, Geonafta Office, ul. Jagiellońska 76,03-301 Warszawa, Poland
Outline of basin development
Taking into account the genetic c1assification of sedi-mentary basins (MialI, 1990), the Carpathian Foredeep bel-ongs to the peripheral foreland basins of the Alpine-age tectonic system of the Carpathians. The Carpathian Fore-deep is divided into the two basins: outer and inner (Ney et al.,
1974; Oszczypko, 1996, 1997).
The formation of outer Miocene basin in the Polish part of the Carpathian Foredeep (Oszczypko, 1982; Oszczypko & Ślączka, 1985, 1989; Oszczypko & Tomaś, 1985; Kotlar-czyk, 1988; Oszczypko, 1996, 1997) is c10sely connected with the multiphase, northward thrust of the Carpathian llysch orogen onto the fore1and platform and the deposition of succeeding suites of Badenian and Lower Sarmatian molasses at the front of orogenie belt (Fig. 1). Depositional space of the Polish part of the foredeep was provided by the subsidence of polygenetic basement of the Miocene basin bordered from the south by migrating llysch nappes and from the north by stable slope of the platform. The southern-most part of this space was occupied by the Lower Miocene inner basin filled with molasse sequences ranging in age from Eggenburgian to Karpatian. These sediments were subsequently folded together with the overthrusting llysch nappes and displaced to the north where they are recently known as the Stebnik and the Zgłobice units located along the northern edge of the Flysch Carpathians (N ey, 1968; N ey et al., 1974; Oszczypko, 1997).
In the depositional space of the outer basin the Badenian transgression has invaded the foreland of just developing llysch orogen in the Lower Moravian. Tectonic migration of the Carpathians which has commenced in the Moravian
(Kotlarczyk, 1988) caused folding of Lower Miocene mo
-lasse in the inner basin which resulted in: (i) formation of the Stebnik Unit in the east and (ii) displacement of the llysch orogen together with the Zgłobice Unit in the west. In the Middle Badenian (Wielician) the front of this over-thrust has reached the position some 10-15 kilometers south ofthe recent edge ofthe Carpathians (Wdowiarz, 1983). At the front ofthis overthrust the asymmetric foredeep has be en formed (outer basin) where the thicknesses of Badenian and
Przegląd Geologiczny, vol. 46, nr 8/2, 1998
o
20 40km1t===:::j' _ _ IIIIi'
SLOVAKIA
D
outer part of theCarpathian Foredeep
D
autoehthonous Mioeene northern range of
strata ~ Mioeene strata .---7~ of Mioeene strata (km) isobaths of the basal surfaee inner part of the
Carpathian Foredeep L...;;... ...
Mioeene strata
on the Carpathians
.
@)
representative zones of
Infer~ed southern range
C
geoehemieal eharaeteristies ... of Mloeene strata of organie matterD
the Carpathians margin of the inferred range of reeentI
II
l'~ Carpathian overthrust ~ remnants of inner basin 1-. - - - I eross-seetion see Fig. 3
Fig. 1. Sketch map of the central and eastern parts of the Polish Carpathian Foredeep and location of studied test areas Lower Sarmatian molasses vary from 2,500 to 3,000 meters
at the edge of the Carpathian overthrust to adozen of meters at the northem margin of the basin (Fig. 1). Thickness of autochthonous Miocene strata beneath the Carpathian over-thrust ranges from some tens to some hundreds of meters (Jucha, 1985). Thickness of Lower Miocene suite (Eggen-burgian-Karpatian) of the inner basin reaches maximum of 2,400 meters in the area of the Stebnik Unit (N ey et al., 1974) and amounts to some ten s of meters in the area of the
Zgłobice Unit (Oszczypko, 1996, 1997).
Recent depth of autochthonous Miocene strata in the outer basin, under the flysch nappes varies from 2,500-3,000 meters at the edge of orogenie belt to 7,000-8,000 meters in central part of the Flysch Carpathians. Relics of the autochthonous Lower Miocene of inner basin in Kros-no-Sanok and Nowy Sącz zones are presumably located at depths interval 7,000-11,000 meters (Figs l and 3).
The existing genetic link between Lower Miocene mo-lasses of the inner basin and Upper Miocene momo-lasses of the outer basin allows the application of uniform chronostrati-graphy of the Central Paratethys to lithofacial complexes of
the whole Miocene sequence in the foredeep (Łuczkowska,
1967; Steiniger et al., 1990; Berggren et al., 1995) (Fig. 2). The general lithostratigraphy inc1udes the two main molasse formations: Lower Miocene in the inner basin and Upper Miocene in the outer basin. The Lower Miocene formation of the Stebnik and Zgłobice unit s comprises the Worotyszcze (Eggenburgian), Stebnik (Ottnangian), Balice (Karpatian) and Przemyśl (Lower Moravian) sedes of total thickness up to 2,400 meters (Ney et al., 1974). These sedes are barren from the point of view of petroleum exploration.
The Upper Miocene formation of outer basin which contains Badenian littoral, neritic and deltaic facies (Mora-vian, Wielician, Kosovian) as weB as the Lower Sarmatian strata is gas-prone. Up to date 70 natural gas deposits have been discovered in these rocks and reserves of about 200 billi on cubic meters have been quoted. The eastem part of the outer basin (east of Kraków) is filled with Badenian and Lower Sarmatian sediments of the foBowing thicknesses:
Lower and Middle Badenian - from O to 300 m, Upper Badenian - from O to 1,700 m, and Lower Sarmatian -from O to 2,900 m (Ney et al., 1974). Most ofthe known gas
I STRATIGRAPHY TIME O TIME (Ma) O (Ma) o... w MEDITERRANEAN PARATETHYS ,w M+L C/) z -w ~~ w CALABRIAN 2- w w GELASIAN 1.77 Z ~ ROMANIAN w - l PIACENZIAN o 3.57 4- O ~ ZANCLEAN :J o::: DACIAN o... <x: w 5.3 6- MESSINIAN PONTIAN W 7.5 8-
t:
---1 TORTONIAN PANNONIAN 10 - 10.5 WSARMATIAN ~ BES VOL 10.8
11.3
12 - Z -' KOS.
W SERRA- 12.5
W ---1 VALLlAN BADE- ~ WIEL.
O 14- O NIAN 13.7 () ~ w MOR. LANGHIAN 15.2 16 - O KARPATIAN
18 -~ BUROlGALIAN OTTNANGIAN ~ 20-o:::
EGGENBUR-«
GIAN W 22- AQUITANIAN EGGERIAN24-OllGO LATE CHATTIAN
CENE
Fig. 2. Generalized stratigraphic section and region al Miocene correlation (after Berggren et al., 1995; Steininger et al., 1990; Czepiec & Kotarba, 1998)
fields are reservoired within the Upper Badenian and Lower Sarmatian strata. Both the Upper Badenian and the Lower Sarmatian are represented by clay-sandy, mainly deltaic facies (Karnkowski, 1989). Sedimentation rate of Upper Badenian sediments was maximum 1,500 mIMa, and that of the Lower Sarmatian reached 5,000 mIMa. On the other hand, the Lower and Middle Badenian strata comprise shal-low-water psammitic, argillaceous and chemical sediments.
The maximum depth of Badenian sea was 300 meters. The warm-temperate climate during this time suggests a surficial-water temperature range of 17-20°C. In the Sarma-tian the sea depth was initialIy about 30-50 meters. Then, progressing shalIowing to about 10 meters took place. The Sarmatian sea was a warm basin with temperatures roughly corresponding to those of the Late Badenian ones (Czepiec
& Kotarba, 1998).
GaseOlls hydrocarbon generation system Composition of natural gases accumulated in Miocene sequence of the Carpathian Foredeep, is dominated by me-thane which usualIy constitutes over 98 vol. %. Meme-thane was generated during carbon dioxide reduction pathway of microbial processes (Fig. 4) (Kotarba, 1998). Higher ga-seous hydrocarbons (mainly ethane and propane) which are usualIy minor constituents (concentrations less than 0.2 vol. %), were generated during diagenetic processes and at
Przegląd Geologiczny, vot. 46, nr 8/2, 1998
the initial stage of the low-temperature thermogenic pro-cesses (Kotarba, 1998).
Geochemical analyses of the OM contained in the auto-chthonous Miocene of the Carpathian Foredeep indicated the domination ofhumic, gas-prone type III kerogen in both the Upper Badenian and the Lower Sarmatian strata and only sporadic occurrence of algal, type II kerogen (Fig. 5) (Ko-tarba et al., 1998). The TOC contents vary from 0.02 to 3.22 wt. % (mean 0.68 wt. %). Down to the present depth 3,200 meters the OM is immature. Its transformation degree is very low and corresponds to the vitrinite retlectance Ro from 0.25
to 0.6% (dominating values around 0.4%) and to the Rock Eval Tmax temperatures from 415 to 438°C (Kotarba et al.,
1998).
The hydrocarbon generation conditions were analyzed in the 4 typical test areas. The A (Tarnów) and B (Mielec-Leżajsk) areas represent the part ofthe Carpathian Foredeep located north of the present edge of the Flysch Carpathians. The C (Rzeszów) and D (Przemyśl) areas are located in the autochthonous Miocene outer basin along the present edge oftheFlysch Carpathians (Fig. 1). AdditionalIy, theE (Kros-no-Sanok) area was chosen as an example of hypothetical deepest levels where the relics of autochthonous Lower Miocene strata of the inner basin might have existed (Fig. 1). The hydrocarbon generation modelIing applied the time-temperature (TTI) method (Waples, 1985). The time scale was based upon Berggren et al. (1995); Steininger et al. (1990), Czepiec & Kotarba (1998) (Fig. 2) whereas the geothermal gradient - 33°C/l0-3 m is the average value obtained from paleotemperature ca1culations for the Mioce-ne basin (Szafran, 1990) and correlated with the vitrinite retlectance values (Kotarba et al., 1998). Thicknesses of stratigraphic units were taken from the results of drillings in the analyzed parts of the outer basin. The estimated erosional reduction was 100 meters in A and B test areas and 250 meters in the remaining ones (Oszczypko, personal commu-nication). TTI modellings in the outer basin (test areas A to D) were related to the bottom of Badenian succession as datum surface. In the test area E (inner basin) the datum surface was the top of Karpatian sequence.
The results of modelling revealed that in both the A and B test areas (north of the egde of the Carpathians) the TTI values were 0.1 and 1, respectively (Fig. 6) which proves that thermogenic gases have not been generated. At the edge of the orogen the maximum TTI values were: 21 at depth about 3,600 meters in C test area and 4 at depth about 2,600 meters in D one. These values suggest the appearance of low-temperature thermogenic processes (Fig. 6) within the autochthonous Miocene strata beneath the Carpathian over-thrust (i.e. beneath 2,500 meters depth).
The modelling for the test area E indicated the possibility of generation of the higher gaseous hydrocarbon (and even liquid hydrocarbons) beneath the Carpathian overthrust, at about 7,500 meters of depth. At greater depths, within the relics of autochthonous Lower Miocene strata of the inner basin only the high-temperature methane could be produced. The results of modelIing for aU the test areas proved the generation of microbial methane in almost fulI thickness of autochthonous Miocene of the outer basin. Taking into account the quantitative criteria for microbial gas generation (Clayton, 1992), the intensity of methane production was ca1culated for the generalized sedimentation conditions of the Miocene marine basin. It was found that generation processes of microbial methane were most intensive at depth
Przegląd Geologiczny, vol. 46, nr 8/2, 1998
sw
SANOK PRZEMYŚLl'
NE -100 O---~~--~~~~~~~~~~~~ . -oe:..
-80 3000 6000 9000 (m)D
Flysch CarpathiansD
folded Miocene (Stebnik Unit)o
10 20 30 40km ! ID
remnants of inner basin . -"<t I ~ -60 U C<) ~ -40 -20 -350 -250 -150D
autochthonous Miocene • Precambrian 80 (CH4 )(%0)
Fig. 3. Schematic cross-section through the Carpathian Foredeepo
600 O I-O)U
500 I O)-Ś
400 X UJ O Z 300 Z UJg
200o::
O >-I 100 f- I-T ! I ! Q::0 200!r0
~o*
-<t) li) o / 'i i/\
I -o I ~'\
I 150 -~ I MIOCENE'l\ \ \
ORGANIC 1'i-/;)10J
MATTER ~~ -I I\
I I I 50 -I I xx X VyV'~
t\
! I I6Y
;()~ Q::0 420 430 440~l~
~ "x y x I '-, ! 420 440 460 480 500 520 T max TEMPERATURE CC) Fig. 5. Rock Eval hydrogen index versus Tmux temperature for organic matter in the autochthonous Miocene strata of the Carpa -thian Foredeep after Kotarba et al. (1998) interva1900 to 1,500 meters beneath the Miocene sea floor. Maximum yield of microbially-produced methane were about 5 cubic meters of CłL per cubic meter of a source rock. The volume of produced rnicrobial methane is spatially variable within the basin and depends on the burial history . As no dramatic changes of thermal conditions have been deduced within the basin since the Sarmatian, this microbial generation processes have presumably continued until re-cent. Even the folding and the uplift of the Carpathian orogen, and the consequent regression of the Miocene sea at the break ofLower and Upper Sarmatian did not impede the microbial processes. In geochemical literature the idea has been presented that microbial methane might have been accumulated as hydrate beneath theMiocene seafloor(Głogoczowski, 1980; Fig. 4. Genetic characterization 01' natural gases accumulated in the autochthonous Miocene strata of the Carpathian Foredeep in terms of (513c(CH4) versus D(CH4). Compositional fields from Whiticar et al. (1986) 4000 . --Ś 5000 Ib=
6000 UJ O 7000 8000 9000 10000 11000 - - - -- - -- - - 140 --- - - --- - - 160 - - --- - -- --- - -- 180 - - - --- - -- -- 200 -- - - --- -- 220 - - - --- - - 240 - - -- - --- -- --- 260 --- - - ---- -- 280 -- - - - ---- ---- 300 - -- - - - -- - - --- 320 -- - - --- - - 340 - - - 360E - Early; M - Middle; L - Late S - Sarmatian 900 1500 w o::: => ~ o::: w El. 2 w l-5: o ...J w o::: =>
~
w El. 2 w l-I C> I CI) w CI) CI) w o O o::: a.. S:2 z w (.9 O 2 o::: W II-Fig. 6. Model 01' gas generation within the autochthonous Miocene basins and family ofburial-history curves, subsurface temperature grid and Time-Temperature Index values for: (A) Tarnów, (B)
Mielec-Le-żajsk, (C) Rzeszów, (D) Przemyśl, and (E) Krosno-Sanok areas
Kotarba, 1992). However, the results of new bathymetric analyses (Czepiec & Kotarba, 1998) revealed that the Bade-nian sea was presumably only slight1y deeper than the outer shelf, i.e. about 300 meters, at most. Depth of the Sarmatian sea was initially about 30-50 meters and has shallowed to
Przegląd Geologiczny, vot. 46, nr 8/2, 1998
sw
NE
200 (m) O 400 1000 ..--. 600 E '-'" I 2000 I- 800 a...w
O 3000 1000 1200 1400D
Flysch Carpathians O ! 3 4km I 2 -5 O 5 10 15D
folded Miocene (Stebnik Unit) • Precambrian
TEMPERATURE (C)
D
autochthonous Miocene ~ gas accumulationFig. 7. Impossibility of formation of methane hydrate in Mio- Fig. 8. Schematic cross-section through the Przemyśl gas accumulation cene sea. Phase boundary for methane hydrate after Kvenvol- zone after Czernicki (1977)
den & MacMenarnin (1988) and Kvenvolden (1988)
onlyabout 10 meters. Both the Badenian and the Sarmatian seas were the warm basins of surface water temperatures 17-20°C (Czepiec & Kotarba, 1998). Taking into account these data, the formation of natural methane hydrate zone beneath the Miocene seafloor must be rejected (Fig. 7).
The high sedimentation rates together with rhythmic and cyc1ic deposition of Miocene c1ays and sands as well as the vigorous generation of microbial methane caused that the gas produced in c1aystone layers readily migrated to the sandstone beds capped, in tum, by succeeding c1aystones. Such generation and accumulation system of microbial gases gave rise to the formation of multi-horizontal gas fields (Fig. 8).
Conclusions
The TTI modelling proved that the autochthonous Mio-cene strata of the outer basin of Carpathian Foredeep loca-ted north of the orogen edge have generaloca-ted only the microbial methane at depths less than about 2,400 meters below the Miocene seafloor. In the part of the Miocene outer basin covered by the Flysch Carpathians the low-tempera-ture thermogenic gases have be en generated down to about 7,500 meters of depth. The Lower Miocene molasse of the inner basin located at depths interval 7,000-11,000 meters might have generated only the high-temperature thermoge-nic gases. The site of hypothetical Lower Miocene strata of the inner basin may well be occupied by a depression filled with the Upper Carboniferous coal-bearing formation (Ko-zikowski, 1966) which seismic response is similar to that of the Miocene molasse. Nevertheless, the gas accumulations can be expected beneath 7,500 meters of depth, i.e. under the cover of the Carpathian overthrust.
Methane which is the main component of naturaI gas deposits accumulated within the autochthonous Miocene strata of the Carpathian Foredeep was formed during the microbial processes. Higher gaseous hydrocarbons were generated by low-temperature thermogenic reactions from
the Miocene source rocks buried under the Carpathian over-thrust. Small amounts ofthese components occur exc1usive-ly in the deepest horizons of the Miocene gas deposits (e.g.,
Przemyśl and Husów). Maximum yield of the microbial
methane generation ca1culated for depth interval 900 to 1,500 meters is about 5 cubic meters per cubic meters of source rocks. It is probabie that the generation process of microbial methane still continues recently. Accumulation of microbial methane within the autochthonous Miocene strata was facilitated by high sedimentation rate and rhythmic and cyc1ic deposition of c1ays and sands.
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A study of organie matter and habitat of gaseous hydroearbons
in the Mioeene strata of the Polis h part of the Carpathian Foredeep
Maciej
J.
Kotarba*, Tadeusz Wilczek**, Paweł
Kosakowski*, Adam Kowalski*,
Dariusz Więcław*
Geoehemieal studies on the organie matter jrom the autoehthonous Miocene strata oj the Carpathian Foredeep demonstrated the presenee oj gas-prone type 1/1 kerogen in both the Upper Badenian and Lower Sarmatian sediments with rare admixtures oj algal type
/1 kerogen. The TOC eontents vary jrom 0.02 to 3.22 wt. %. Down to the depth 3,200 meters the organie matter is immature. 1ts transjormation degree eorresponds to the vitrinite refleetance Rojrom 0.25 to 0.6% and to the Rock Eval Tmax temperatures jrom 415 to
4380
C. Insignifieant variability in spatial and depth distribution oj geoehemieal parameters and indiees (TOC, TEITOC, Tmax, HI, Ro,
CPI, etc.) suggest that deposition eonditions oj the organie matter were general!y homogenous within the jul! thiekness oj Mioeene
sequenee down to the reeent depth about 3,200 meters. Sueh eonditions resulted in the laek oj diversity oj the TOC, the genetie type and
the transjormation degree ojthe studied kerogen. The Miocene terrestrial OM is immature and generated almost exclusively the mierobial methane. The low-temperature thermogenie proeesses have been aetive beneath the depth about 3,200 meters i.e., mainly under the Carpathian overthrust.
Key words: petroleum exploration, geoehemieal methods, stable iso top es, organie materials, kerogen, natural gas, genesis, Miocene,
Carpathian Foredeep, Poland
Introduction
The studies reported below aimed to measure the
geo-chemical parameters and indices
ot
the potential sourcerocks (c1aystones and mudstones) within the autochthonous
Miocene sequence
ot
the Carpathian Foredeep in order toevaluate their hydrocarbon generation potential during
transtormation
ot
the organie matter (OM).Genetic constrains
ot
depositional environmentot
theOM can be characterized by hydrocarbon geochemical
pa-*University of Mining and Metallurgy, Department of Fossil Fuels, al. Mickiewicza 30,30-059 Kraków, Poland,
email: kotarba@uci.agh.edu.pl
**Polish Oil and Gas Company, Geological Office Geonafta, ul. Jagiellońska 76,03-301 Warszawa, Poland
rameters: Rock Eval hydro gen (HI) and oxygen (Ol) indiees,
n-alkanes and isoprenoids distribution, stable carbon isotope composition in bitumens, their individual tractions (satura-ted hydrocarbons, aromatic hydrocarbons, resins and as-phaltenes) and kerogen as well as maceral compositions.
Transtormation degree of the OM was evaluated
trom
thevitrinite reflectance Ro and the Rock Eval T max temperature.
The selection
ot
core material collected from wellstor
the studies on dispersed OM takes into consideration the
spatial lithotacial development
ot
the autochthonousMio-cene molasse tormation in the Polish part
ot
the CarpathianForedeep. Thus, the
tour
representative test areas werecho-sen in the Carpathian Foredeep for comparison
ot
thepre-sumed spatial variability of geochemie al parameters and
indices
ot
the source rocks which practically reflect thevariability of hydrocarbon generation conditions (Fig. 1).