petroleum content and mud volcanism in the po (italy) and ......314 petroleum content and mud...

ENERGY EXPLORATION & EXPLOITATION · Volume 25 · Number 5 · 2007 pp. 313–324 313

Petroleum Content and Mud Volcanism in the Po (Italy)and South Caspian (Azerbaijan) Basins:Differences and Their Possible Origins

A.A.Feyzullayev(Geology Institute of Azerbaijan National Academy of Sciences,

H.Cavid av.29A, Baku, AZ1143, Azerbaijan)

INTRODUCTIONThe Po Valley (Italy) and the South Caspian Basin (Azerbaijan), located within oneAlpine-Himalayan tectonic belt are typical intermountain basins (Fig.1). They havevery similar space-time history of formation and development in regional aspects.

Figure1. General view of Po (Italy) and South Caspian (Azerbaijan, Iran andTurkmenistan) Basins

The break-up of Pangea began during the late Triassic; crustal separation between theEurasian and Afro-Arabian continents occurred during the Jurassic leading to thegeneration of the Tethys Ocean (Bernoulli and Jenkyns, 1974; Biju-Duval andMontadert, 1977; Laubsher and Bernoulli, 1977; Dercourt et al., 1986; Ziegler, 1988).Numerous continental and small intervening oceanic domains were generated(Aubouin, 1973; Bernoulli and Jenkins, 1974; Dercourt et al., 1986; Ziegler, 1988)during the stage of separation and fragmentation. Subsequent convergence betweenthe Eurasian and Afro-Arabian continents during Cretaceous and Cenozoic periodscaused subduction of the Tethys Ocean.

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Convergence of the Eurasian and Afro-Arabian continents began in Cretaceous,causing Alpine orogenies (thus the Alps-Apennines and the Caucasus mountain rangeswere produced). This major crustal shortening happened because of a stacking ofcontinental fragments, whereby intervening oceanic domains were overridden,causing the complex configuration of Alpine sheets with extensive flyschsedimentation (Sengor and Yilmaz, 1981). Along the northern margin of Turkey,Pontides were thrust northwards and flysch sedimentation continued in the Black Seaand Caspian Sea Basins (Letouzey et al., 1977).

There was continued opening of the Atlantic Ocean through Early andMiddle Miocene times, but absolute plate motions between the Eurasian and Afro-Arabian continents were small. Afro-Arabian and Eurasian continents continued toconverge throughout Late Miocene times (Haan and Arnott, 1991). The Pliocene is thelast stage in the development of Alpine-Caucasus chains.

The Po and SCB originated in the post orogenic stage of the regional development.The present-day structures of these basins are still controlled by the ongoing collisionof the Arabian and African plates with Eurasia (Philip et al., 1989; Axen et al., 2001;Jackson et al., 2002).

RESULTS OF COMPARATIVE ANALYSISThe collection, systematization and analysis of huge amounts of literature related topetroleum systems of the Po basin and the adjoining Northern Apennines and itscomparison with the data on Petroleum Systems of the South Caspian basin ofAzerbaijan, allow one to conclude that there are many common features of the basinsas:

- sub-oceanic earth crust was generated during the Jurassic (Paleo Tethys ocean);- high recent tectonic activity (high seismicity, horizontal and vertical movements

of Earth crust);- intensive dislocation of sediments (high density of development of fold and fault

systems, thrusting, considerable vertical displacement of sediments);- high sedimentation rate, especially during Pliocene-Quaternary time;- high thickness of sedimentary cover and its young Mesozoic-Quaternary age;- participation of several feed sources that supplied basins with sediments: Russia

craton, Great and Lesser Caucasus, Talysh, Elbourz, Kopetdag mountain systemsin SCB, the Apennines and Alps mountain systems in the Po- basin, very lowtemperature regime, 20-15 oC/km;

- existence of overpressures; - development of mud volcanism and diapirism; - occurrence of industrial accumulations of hydrocarbons (HC) in sedimentary

sections;- existence of several source rock units, which participated in petroleum formation:

in the Po basin Alpine oils were generated from Triassic sources, while the liquidhydrocarbons (HC) of the N. Apennines arose from Tertiary (Miocene) sourcerocks; Upper-Middle Miocene (Diatom suite, Chokrakian horizon) and LowerMiocene-Oligocene (Maikopian suite) deposits took part in the petroleumformation in SCB.

314 Petroleum Content and Mud Volcanism in the Po (Italy) and South Caspian (Azerbaijan) Basins

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At the same time these basins, with the above mentioned, mainly qualitative,common features, have some distinctions in their quantitative values due to localdifferences in geological development and recent structures of these basins: - The SCB and Po basins differ considerably by their total hydrocarbon (HC)resources, 6.47 BTOE (Rachinskii et al, 2006) and about 0.65 BTOE (Linquist, 1999),respectively. - In the Po Valley industrial HC accumulations are predominantly gaseous (up to95% of resources; for comparison, gaseous resources are about 20-25% in the SCB atthe present (Table 1).

Table 1. Quantitative and qualitative characteristics of commercialaccumulations in Po Valley and SCB

Phase type of the HC accumulations Gas (G) Gas- Gas- Oil-Gas Oil (O) Depth

Condensate Condensate-Oil (OG) interval of (GC) (Oil- Gas- productivity,

Condensate) mGCO (OGC)

Po Basin115 (87.7%)* 9 (6.9%) 3 (2.3%) 1 (0.8%) 3 (2.3%) 200- 6200

SCBNone 142 (18.6%)* 52 (6.9 %) 158 (20,7 %) 411 (53.8 %) 8-5240 _ (O),

/6316 (GC)* number of pools (%)

- In contrast to the SCB, gas in the Po basin is mostly biogenic (80% - Mattavelliet al., 1983). (Fig. 2 and 3). The majority of biogenic/diagenetic gases in the Po Basinprovince is sourced and accumulated during Pliocene (with minor Miocene) andPleistocene reservoirs. Some reservoir gases and oils are thermogenic, originatingfrom Mesozoic and Miocene rocks (Mattavelli et al., 1983; Mattavelli and Novelli,1987; Linquist, 1999). Gas in the SCB is mainly catagenic associated with oil and isaccumulated predominantly in Lower Pliocene reservoirs.

Figure 2. Histograms of the methane distribution in the fields of the Po and theSouth Caspian basins

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Figure 3. Isotope composition of carbon in methane of the Po and the South Caspianbasins

- Analysis of the published data has allowed one to establish that there are alsodistinctions in mud volcanism/diapirism.

The development density of the mud volcanoes (MV) in the South Caspian basin ismuch higher than in the Po Valley. About 200 onshore, and more than 120 offshore,MVs were found in SCB and about 60 MVs in the Po Basin (the northern Apenninesside) (Martinelli, 1999).

The MVs of the Po Valley also differ morphologically from MVs of the SCB. Thebiggest mud volcanoes of Italy (Regnano and Nirano), located in the Emilia-Romagnaregion, are characterized by cones with a height up to 2 m. All other mud volcanoesdo not exceed 50 cm (Martinelli, 1999), whereas in the SCB the largest Turagay MV,has a height of about 400 m (Fig. 4)

Figure 4. General view of the largest Italian (left) and Azerbaijan (air photo) MVs

The MVs of Italy are weakly active. Only eruptions of Sasaulo and Regnanovolcanoes were observed and the last paroxysmal eruption took place as long ago as1880 (Martinelli, 1999). In Azerbaijan about 50 volcanoes have erupted almost 200times since 1810; 22 eruptions of the most active, Lokbatan MV, were recorded overthe last century or so.

The HC composition of MV gases of the two basins is very similar. Normalized HCcomposition of MV gases in Po and SCB are CH4: 99.78%, C2H6: 0.22% and CH4:


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99.69%, C2H6: 0.31% respectively. This fact, together with the isotope compositionof carbon (ICC) in methane, ethane, CO2 and IC of H in methane (Table 2) and withgas wetness values (0.0024 in the Po Valley and 0.0031 in the SCB), allow one toconclude that the abundance of biogenic gases in the upper section (Pliocene-Pleistocene) of the Po basin has a slight influence on MV gases; maturity of gases ina majority of MVs in the Po basin (in contrast to the SCB) are much higher.

Table 2. Isotope composition of gases of MVs in the Po Valley and SCB

Region d13 d13 d13 d H 3He/4He 40Ar/36Ar N2/ArCO2, CH4, C2H6,%o %o %o %o (R/Ra)

The Northern 17.9 -48.2 -21.9 -159 0.46 330 60.3Apennines, ItalyThe SE End of 2.03 -46.3 -27.1 -196.5 0.083 455 57.8Caucasus, Azerbaijan

According to IC of helium, MV gases in the SCB originated totally from thesedimentary cover (because average value of R/Ra is less than 0.2), while there is anobvious component of magmatic origin in MV gases in the Po basin.

Comparison of average values of 40Ar/36Ar ratio content of atmospheric argon(36Ar) in argon of MVs in the Po basin is higher than in MVs of SCB (compared withthe 40Ar/36Ar ratio value in atmosphere = 295). This conclusion is well supported bythe values of N2/Ar ratio.

Chemical composition of MV waters is very similar and is represented in bothbasins predominantly by Na and Cl (Table 3).

Table 3. Chemical composition of MV’s waters in the Po Valley and SCB

Vol.,%Region pH

Na Ca Mg HCO3 SO4 Cl

The Northern Apennines, Italy 8.5 35 0.8 1 8.7 0.9 53.6The SE End of Caucasus, Azerbaijan 7.9 36.8 0.7 0.6 12 0.5 49.4

DISCUSSION The data show that the two compared basins, which have very similar regional historyof development differ considerably in their petroleum systems and mud volcanism.What are the reasons for such differences?

Below is a search for a clue to this general question based on some constituentquestions.


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Question 1: Why are hydrocarbon (HC) resources in the Po basin considerably lowerthan in the SCB?

Petroleum resources of any basin depend on several main factors: amount oforganic matter in rocks (TOC, generative index/S1+S2); volume (or at least thickness)of source rocks and their facies composition (because petroleum generative abilitiesmostly have fine grained clayey/silty and carbonate facies), and, finally, the existenceof reservoir rocks that can accumulate all hydrocarbons expelled from the sourcerocks.

According to a great number of investigations of petroleum systems of the SCB itwas determined that most oil resources, which had occurred in the main reservoirs ofthis basin, the Productive series (Lower Pliocene), originated from Oligocene-Miocene source rocks.

According to this premise the following suppositions have been examined:First Supposition: difference in total thickness of sedimentary cover and

Oligocene-Miocene deposits, in particular of the two compared basins.The thickness of sedimentary series in the Po Valley vary from 4-5 km near the

Alpine border to about 12 km near the Apennines (Mattavelli and Novelli, 1987),while in the SCB the thickness varies between 18-25 km (by some estimates 30 km).

The thickness of the Oligocene-Pleistocene rocks in the Po Valley reaches 8 km(Matavelli and Novelli, 1987) and 12-15 km in SCB. There is a lack of Messinian(U.Miocene) sediments in the Tuscany region of the Po basin and available Pliocenesediments are relatively thin.

Second Supposition: difference in facies of potential source rocks.The Paleogene section in the SCB consists mostly (up to 80-90 %) of massive

clayey rocks (especially in the foothills of the Great Caucasus) with some calcareous(dolomite, marls) layers in Middle Eocene and thin sand layers in Miocene sedimentsprecipitated in marine (bay) environments.

According to many publications related to lithofacial analysis of sediments, thePaleogene-Miocene sections in the Po Valley, are comparatively sandier in contrast tothose in the SCB. There are several sand and marl formations in these stratigraphicintervals:

- Monte Piano marls (Marne di Monte Piano) of Eocene-Oligocene age, with thethickness increasing from SE to NE from 80-300 m in Modena and Parma Apennines(Losacco, 1967; Medioli et al., 1967) to 700 m at Costa Merlassino (Segnini, 1967);

- Ranzano Sandstone (Arenarie di Ranzano) (Oligocene), which overlies theMonte Piano marls. This formation is 2000 m thick at Costa Merlassino, 770 m atMonte Barigazzo and about 100-200 m in the lower Piacenza, Parma and ModenaApennines (Sestini, 1970a). The thickest turbidites, Ranzano sandstones of theLigurian-Emilian Apennines, have, on the whole, a proximal flysch facies withabundant conglomerates and olistostromes.

- Loiano Sandstone Formation (Oligocene) with very high thickness (1400-1500m), which consist of massive beds of coarse-grained, poorly cemented feldspathicsandstone and irregular conglomerate beds at various horizons. According to Merla(1951) and Gazzi (1963) this deposit formed in a shallow-water, deltaic environment.


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- Antognola marls (Marne di Antognola) (Upper Oligocene-Aquitanian) occurdiscontinuously above the Ranzano sandstone or the Monte Piano marls. Thethickness of the Antognola marls varies from 370 m (near Vetto, Parma) to 700-800m in the Secchia Valley and Modena foothills.

The Lower-Middle Miocene section is predominantly marly-sandy, which wasformed in a shallow marine environment (Sestini, 1970a):

- in Lower Miocene: Manciano Sandstone (Burdigalian), probably deposited fromlittoral to very shallow marine environment.

- in Middle Miocene: Termina marls (Tortonian) with 150-580 m thickness,composed of Sgrilla Sandstone and Ponsano Sandstone (Helvetian or Tortonian); SanMarino limestone formation; transitional sandy unit (Helvetian); Monte FumaioloSandstone (Helvetian) with up to 200 m thickness.

The distribution of sedimentary facies during Upper Miocene-Pleistocene wasconsiderably affected by continued tectonic movements. A general “swallowing” ofcontinental shelves in the Messinian (U.Miocene), causing brackish and evaporateenvironments, was followed by extensive subsidence and marine transgression in thePliocene. The Messinian - Pleistocene deposits are predominantly clastic, formed fromthe erosion of folded and uplifted rocks (Sestini, 1970b).

As a whole, the Po basin is characterized by predominantly turbididic sedimentsand the SCB - by lacustrine and deltaic sediments.

Third Supposition: difference in organic matter content.The content of organic matter ranges from 0.1 to 0,7% (on average 0.4%) in

Miocene sediments of the Po basin (Mattavelli and Novelli, 1987). The content of organic matter is comparably higher in Oligocene-Miocene rocks of

the SCB basin, varying within 0.10-2.44 % (on average 0.68%) in Middle-UpperMiocene (Diatom and Chokrakian suits), and 0.10-15.1 (on average 1.86%) inOligocene-Lower Miocene (Maikopian series).

Fourth Supposition: difference in rate of transformation of OM to HCs (OMmaturation).

According to Chiaramonte and Novelli (1986), Mattavelli and Novelli (1987) andAnnelli, Mattavelli and Pieri (1996) the “oil-generative window” in the Po Valleyoccurred during the last 3 Ma at great depth (5500-7000 m). Miocene source rocksreached the hydrocarbon generating phase only in Apenninic areas, where sedimentarynappes piled up and created temperature conditions suitable for OM maturation.

The onset of oil generation in the SCB is located at a depth of about 3,5-4 km(where the South Caspian depression joins the Great Caucasus mountain) and about 5km (in the submerged part of the SCB). Almost all Oligocene, the majority ofMiocene, and even the lower part of Pliocene (in the central deep buried part of abasin) deposits occur below oil onset. This fact, and differences in the areas of basins(the Po basin about 115, 000 sq.km; the SCB 160, 000 sq.km) allows one to concludethat the volume of rocks participating in the petroleum generation in the SCB is morethan in the Po basin.

Fifth Supposition: difference in existence and quality of the reservoir rocks.Unfortunately, the published data are not enough to estimate the thicknesses and

reservoir quality of rocks (grain size composition, porosity, permeability etc.) for the


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Po basin. Nevertheless, according to the lithofacies analysis described here, and somedesultory information about reservoir quality, there are no such problems withreservoir in Po Valley.

Thus, one concludes that according to four main parameters (thickness of potentialsource rocks, their facies composition, abundance of OM, and its transformation rate)the Po Basin have generated considerably less volume of petroleum compared to theSCB.

In other words, there were unfavorable conditions for the generation of hugeamount of hydrocarbons in Po basin in contrast to SCB.

Question 2: Why are the petroleum resources in the Po basin predominantly gaseousin contrast to resources in the SCB?

A possible answer to this question is the difference of the OM type in the twobasins. Analysis confirms this supposition (Fig. 5).

Figure 5. Average composition of organic matter in Pliocene-Oligocene rocks in thePo and South Caspian basins: 1-herbaceous OM; 2-woody OM: 3-amorphous/marine

OM.

OM in Miocene rocks (the Marnoso-Arenacea formation, as well as the overthrustedunits) in the Po Valley has mainly a terrigenous nature (Mattavelli and Novelli, 1987;Annelli, Mattavelli and Pieri, 1996) in contrast to OM in the SCB. Marinephytoplankton and amorphous matter content in OM are about 14% and there is aslight increase in Lower-Middle Miocene (about 19%) (Mattavelli and Novelli, 1987).


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In the SCB, amorphous OM in Oligocene-L. Miocene (Maikop suite) rockspredominates (> 80 %) (Saint-Germès et al, 2002), in the Diatom series this indexvaries within 60-100%.

Thus, OM in the Po basin is predominantly terrestrial (mostly gas prone) andpredominantly marine in the SCB (mostly oil-gas prone).

This phenomenon explains why the quantity of gas in the Cortemaggiore field (thePo basin, Miocene reservoir) (calculated in equivalent tons of oil) is twelve timeshigher than that of the oil (Mattavelli and Novelli, 1987).

The reason for a high gas content of the Po basin was also the presence offavorable conditions for the formation and preservation of biogenic (diagenetic) gasesassociated with broad development of consedimentary, lithologically andstratigraphically screened traps during Pliocene-Pleistocene (Sestini, 1970). Theturbidity character of sedimentation was favored formation of such traps.

Question 3: Why is the density of development of MV and their activity in the Po basinlower than in the SCB?

The formation of MV occurs usually in young basins with high sedimentation rates,where thickness of sedimentary cover is more than 10 km and organic-rich massiveclayey rocks predominate in the sections.

Maximum sedimentation rate reaches 1000 m/My. (by some estimation it is 500m/My.) in the Po basin is considerably less than in the SCB (on average 2000 m/My).In addition, the Po basin differs in the thickness of sediments and their facies.

CONCLUSIONThe results of the above analysis provide answers to all questions and, finally, allowone to obtain an idea about common and different characteristics of petroleum systemsof the Po and SCB and also their possible origin.

The poor hydrocarbon resources in the Po basin, in comparison to those of the SCBare conditioned by lower thickness of hydrocarbon generating Paleogene-Neogenecomplexes and their more sandy, sandy-carbonate lithofacies composition, togetherwith relatively lower organic matter content.

The predominance of the gas component in hydrocarbon resources of the Po basincan be explained by two factors: first, OM in Palaeogene-Neogene sediments ispredominantly gas prone; second, favorable conditions for the preservation ofbiogenic gases existed in Pliocene-Pleistocene time.

The formation of biogenic gases occurs at the first (diagenetic) stage oftransformation of OM into hydrocarbons due to activity of methane generatingbacteria in a relatively low temperature environment (below 60-70 0C). Despite thefact that formation of such gases is characteristic to all gas-oil bearing basins, theircommercial accumulations are relatively rare. This rarity is related to the gasformation at the stage of sedimentogenesis and diagenesis of rocks, when they arepoorly lithified, structural traps and sills necessary for accumulation and preservationof biogenic gases are not yet formed. Thus, these gases are not usually preserved andthey dissipate into the atmosphere. Their preservation is possible in basins that have,due to inclement climate, a layer of permafrost (Western Siberian basin) or where


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ithostratigraphically-limited consedimentary traps are widely developed due tospecific paleogeographic conditions of sedimentation (the Po basin).

The known differences in development density, morphology and activity of mudvolcanoes in the Po and the South Caspian basins are conditioned by the difference ofsedimentation rate, thickness and facies composition of sediments, as well as by thetotal content of OM in two basins.

This article was prepared with a support of Cariplo Foundation at frame ofFellowship-2007, organized by Landau Network-Centro Volta, Italy.

ACNOWLEGEMENTSI am grateful to all colleagues from the Geology Department of the University ofPavia, Prof. Cesare Perotti and Andrea Di Giulio for their support of myinvestigations, and as well for the library assistance provided by Claudia Olivati ingathering literature sources.

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