Pyroclastic flow sediment (ignimbrite) in Miocene Tar

Pyroclastic flow sediment (ignimbrite) inMiocene Tar Formation, Drava DepressionD. Bigunac, K. Krizmaniæ, B. Sokoloviæ and Z. Ivanièek

PRELIMINARY COMMUNICATION

In the 2009 one of the exploration wells near Croatian-Hungarian borderline instead of prognosed primarytarget (Miocene barrier bar composed of sandstones, biogenic limestones and clayey limestones) 72 mshallower drilled through massive, 88 m thick pyroclastic flow sediment. Petrological analyses had beendone on core as well as on drilling cuttings and the sediment is described as devitrified vitroclastic weldedtuff, i.e. ignimbrite. As a result of micropaleontological and palynological analyses from overlying andunderlying deposits, the age of the pyroclastic flow sediment was determined as the most probablyBadenian. Lithostratigraphically that ignimbrite belongs to the Tar Formation.

Key words: pyroclastic flow, ignimbrite, Tar Formation, Miocene, Drava Depression

1. INTRODUCTIONIn order to explore hydrocarbon potential in the DravaDepression, near Croatian-Hungarian border, INA andMOL during 2007 and 2008 acquired 189 km2 of 3Dseismic, both on the Hungarian and Croatian side. As-signing of well location was based on interpretation of 3Dseismic data. Geochemical analyses were performed oncuttings and cores of nearby wells. In the Early-MiddleMiocene rocks of those wells gas-prone source rockswith low generating potential were found. However, hy-drocarbon shows are registered in the area, what provedthe lateral secondary migration. Based on previous re-sults the new exploration well, here named AA-1(Figure1), was drilled in the 2009 on Hungarian side.

The main objective was to verify hydrocarbon presencein two predicted reservoirs of Miocene. Those are barrier

bar and coarse clastic series. According to seismic im-age, lenticular depositional body near AA-1 well was in-terpreted as barrier bar. However, that well drilledmassive, not-bedded pyroclastic flow sediment, i.e.ignimbrite, which belongs to Miocene Tar Formation (Fig-ure 2). This formation belongs to Hungarianlithostratigraphic nomenclature and is dated inKarpatian–Badenian period, i.e. time span 17.5-14.79Ma (e.g. references4, 5, 15). Tar Formation comprises vari-able pyroclastic facies, it can occur as stratified and asunstratified, ignimbritic as described in north, middleand south-west Pannonian Basin (e.g. references3,5, 15,17).

Pyroclastic rock drilled by AA-1 well is interpreted asthe deposit of volcanically produced hot, gaseous densitycurrent, i.e. pyroclastic flow, that frequently developfrom explosive volcanic eruptions, when hot masses

composed of pyroclastic frag-ments and ash with high gascontent flow, roll and traveldownhill driven by mecha-nisms similar to gravity massflows (e.g., references2,18). Thespeed of those fast movingfluidized bodies of pyroclastsdepends on gradient of theslope and the size of the flow.High velocities and ability offlows to move over andaround obstacles testifies totheir great mobility, but mostimpressive are the distances(more than 100 km) overwhich ignimbrites have beentraced and heights (over 600m) that they can surpass.Pyroclastic flows originatefrom different tectonic set-tings and eruptions producingthem may vary in volumefrom less than 1 km3 to morethan 1000 km3 . Genesis2 of

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Fig. 1. Location mapSl. 1. Polo�ajna karta

such flow is most frequently connected to collapsing oferuptional column, which initiates processes of formingpyroclastic flow that finally form pyroclastic flow depos-its (e.g. ignimbrites). Such volcanic products could beformed during hours and weeks, i.e. during instanta-neous geologic events.3

The term ignimbrite is used for deposits of pumice-richpyroclastic flow deposits whether welded or unweldedand because of many transitional varieties it is recom-mended to be used for all deposits formed by the em-placement of pyroclastic flows.2 Welding of glass shardsis one of the most characteristic features of ignimbritesdeposited at high temperatures. The degree of weldingdepends on number of factors e.g. temperature, viscositythat develop during movement of pyroclastic flow andgas content.2

The range of silica content (Table-1) exhibited by therock from AA-1 core is from 71-73%. In the base of ana-lysed ignimbrite there is “transitional” zone consisting ofpyroclastics in alteration with volcanic greywackes orconglomerates. That interval also probably belongs toMiddle Miocene as the analysed part of the Tar Forma-

tion, i.e. is part of the Budafa Formation (Figure 2). Thedeepest drilled rocks are gneiss, chronostratigraphicallyprobably of Palaeozoic age, i.e. lithostratigraphically ofthe Babócsa Complex Group.

Two cores (18 m each) were planned in prognosed res-ervoirs, but eventually one (6 m planned, 5.6 m obtained)was taken in welded tuff (Tar Formation). Gas shows areregistered. Also cutting samples had been collected fromthis interval and from overlying and underlying rocks.

2. GEOLOGY OF THE ANALYSEDAREA

In this chapter the complete review of regional geologicalsettings is given. It comprises the short stratigraphicaland structural evolution.

2.1. Stratigraphy

The oldest rocks belong to crystalline basement. Thoseare low to middle grade gneiss, mica, i.e. mica-quartzschist and amphibolite. Chronostratigraphic age is notdetermined.

Depending on paleorelief, Miocene sediments were de-posited on Mesozoic carbonates or Palaeozoic magmaticor metamorphic rocks. Sediments are heterogeneous:conglomerates, breccias, sandstones, marls and of volca-nic origin. Volcanic activity that started in early Mioceneoccurred throughout Pannonian Basin (e.g., references 1,

12, 14, 16).

Pannonian and Pontian are characterized by alterationof sandstones and marls due to successive turbiditeevents in Croatian part9 that were periodically inter-rupted by hemipelagic sedimentation.

Late Pliocene and Quaternary sediments are repre-sented by terrestrial lithofacies.

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D. BIGUNAC, K. KRIZMANIÆ, B. SOKOLOVIÆ AND Z. IVANIÈEK PYROCLASTIC FLOW SEDIMENT (IGNIMBRITE) IN...

Fig. 2. The AA-1 well geological column (from LowerPannonian to well depth - actual (right) andprognosed (left). Hungarian lithostratigraphicalnomenclature is applied.

Sl. 2. AA-1 geološki stup (od donjeg panona do konaènedubine) - ostvareni (desno) i prognozirani (lijevo).Prikazana je maðarska litostratigrafska nomenklatura.

Fig. 3. Ignimbrite thickness map with lines of sectionsshown on Figures 4, 5 and 6

Sl. 3. Karta debljina ignimbrita s oznaèenim geološkimprofilima prikazanim na slikama 4,5 i 6

2.2. Structural settings

The main geotectonic character-istic of the area of interest is nor-mal faulting system related tostrong tectonic activities duringAlpine orogenesis that began dur-ing the late Eocene and Oligoceneand continued throughout muchof the Neogene accompanied byintensive volcanic activity. Ex-change of deep depressions andelevated parts characterizedpalaeorelief. The deepest part issituated at the south-eastern partoriented NW-SE, while towardsnorth studied area is upliftedalong the numerous north-west-southeast trending normalfaults that represent Early-Mid-dle Miocene synrift extensionalphase of Pannonian Basin.9 Dueto Late Badenian (postrift)compressional, or transpres-sional, tectonic event NE-SW di-rected reverse faults took place.Late Miocene slight compres-sional (uplifting) deformation isreflected by tilted blocks andNW-SE oriented small reversefaults. E-W directed steep normalfaults situated in the northernarea indicate Pliocene (– Quater-nary?) transtensional – extensio-nal tectonic phase that occurredlocally during regional transpres-sional9 tectonic phase.

Based on seismic interpretationand the corresponding well data,structural maps of Miocene bar-rier bar and coarse clastic serieswere interpolated and were basefor location of the AA-1 well (e.g.,references6,7). The two prognosedreservoirs were target. Explor-atory well was drilled and thewhole area was reinterpreted us-ing new obtained data. Previouslyinterpreted lenticular depositio-nal body, after reinterpretation,maintained its shape and direc-tion, but was located 72 m shal-lower, with different lithofacies(Figures 3, 4, 5 and 6).

The ignimbrite (Figure 3) covers14 km2. It is approximately 6 kmlong and 3 km wide body with itslonger axis positioned in SW-NEdirection. Maximum thickness is190 m in the central part and it isthinning gradually toward mar-gins with several smaller maxi-mums.

PYROCLASTIC FLOW SEDIMENT (IGNIMBRITE) IN... D. BIGUNAC, K. KRIZMANIÆ, B. SOKOLOVIÆ AND Z. IVANIÈEK

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Fig. 4. Cross-section A-A’ (strike is given on Figure 3)Sl. 4.Geološki profil A-A' (polo�aj je prikazan na slici 3)

Fig. 5. Cross-section B-B’ (strike is given on Figure 3)Sl. 5. Geološki profil B-B' (polo�aj je prikazan na slici 3)

Fig. 6. Cross-section C-C’ (strike is given on Fig. 3)Sl. 6. Geološki profil C-C' (polo�aj je prikazan na slici 3)

3. PETROGRAPHICANDBIOSTRATIGRAPHICANALYSES ANDINTERPRETATION

Analyses of the core and drill cut-tings from ignimbrite and its topand bottom included the follow-ing procedures: detailed macro-scopic description, inspectionunder ultraviolet light (Figure 6),SEM (Scanning Electronic Micro-scope) analyses, various petro-physical measurements and geo-chemical analyses. Pyroclastic de-posits were classified accordingto the size of component particlesafter.8 Chemical analyses weredone according to.11 Biostra-tigraphic analyses (palynologicaland micropaleontological) of drillcuttings from overlying and un-derlying deposits of thepyroclastic interval were carriedout in effort to define age anddepositional context of ignim-brite. Dinoflagellatae cysts weredetermined according to19 andfindings of pollen according to.13

3.1. Petrographic analysesand interpretation

Macroscopic appearance of coresamples (Figure 7, Figure 10 a) isvery light gray to medium lightgray and structure is massive.Core samples show zones ofstrong tectonic (fracturing) andchemical compaction (stylolites,Figure 10 e). Fracturing and vuggy(secondary) porosity is result oftectonics and dissolving. Treatedwith chloroform, core sample em-anate pale yellow colour long frac-tures and vugs under UV light(Figure 7).

Geochemical analyses showedthat core contains mature bitu-men, dominantly from algal pre-cursor with some indices ofterrestrial material. Petrophysical

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Fig. 7. Photos of sample exposed to normal (left) and UV (right) lightSl. 7. Fotografija uzorka izo�enog normalnom (lijevo) i UV (desno) svjetlu

Fig. 8. SEM image of spherulites (left) and of inter-crystal pores (right)Sl. 8. SEM fotografija sferulita (lijevo) te interkristalnih pora (desno)

Fig. 9. SEM image (left) of open fracture and of lithophysae (red arrow), central vug inspherulites (right)

Sl. 9. SEM fotografija (lijevo) frakture i litofize (crvena strelica), centralne šupljine u sferulitu(desno)

Samples SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) Na2O (%) K2O (%)

AA-1Core – 1, IV m

71.55 13.30 0.11 0.28 0.02 2.88 6.55

AA-1Core – 1. IV m

71.65 13.34 0.25 0.15 0.07 3.15 5.28

AA-1Core – 1. IV m

73.28 13.03 1.71 0.07 0.13 4.16 4.67

Table 1. Chemical analyses of core sample

measurements showed high val-ues of fracture porosity, but lowpermeability values.

Core samples represent pyro-clastic sediment flow deposit -devitrified vitroclastic welded tuffwhich tends to develop from rela-tively high viscosity silicic(rhyolitic) magma. The whole-rock chemical analysis for de-scribed ignimbrite is given in Ta-ble 1.

Analyzed thin sections fromAA-1 core consist of glass shards,pumice, crystals and lithic frag-ments. Devitrification, secondaryprocess during cooling, is charac-terised by the formation ofaxiolithic and spherulitic (Figure8, Figure 10 b,g,h) forms of radi-ating quartz and alkali feldsparcrystal fibres with spherical, fan(fibres radiate from a point),axiolitic (fibres radiate from aline) and mosaic texture. Spheru-lites with various morphologiesrepresent common product ofhigh-temperature devitrification10

of volcanic glass that consists ofradiating arrays of crystal fibreswhere single crystals have slightlydifferent crystallographic orienta-tion form adjacent crystals. Lithicfragments as pyroclasts and crys-tal fragments are also present(Figure 10 c,d) as well as brokencrystals which are common char-acteristics for ignimbrites.Chalcedonic aggregates (Figure10 g) and pyrite crystals are alsonoticed in thin sections.

Porosity in thin sections is de-rived from micro fractures andvuggies (Figure 10 f). In SEM im-ages such inter-crystalline (5-10mm) pores and open fractures(Figure 9 left) are visible. Somespherulites (Figure 9 right) have acentral vug, called lithophysae,while others are filled. It is be-cause the internal crystal fibrestructure can be re-crystallized toa quartz-feldspar mosaic as a re-sult of later alteration. That can destroy or modify theoriginal devitrification textures.

3.2. Biostratigraphical analyses andinterpretation

Pyroclastics from core could not be biostratigraphicallyanalysed. So, drill cutting samples from top (Figure 11)and base (Figure 12) of ignimbrite were palynologicallyand micropaleontologically analysed. Younger sediments

are represented by fossiliferous, sandy and argillaceousmicrite.

3.2.1. The ignimbrite top

Drill cuttings were treated by conventional method ofmaceration. In sediments overlying pyroclastic (top ofignimbrite) deposit organic matter is relatively well rep-resented. In most of samples terrestrial material, whichvaries in size and type, prevail over amorphous organic


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Fig. 10. AA-1 core samplesSl. 10. Uzorci jezgre iz AA-1

matter. Fossils are rare and generally represented bybisaccate, porate, colpate and other kinds of pollengrains together with some green algae remnants(Botryococcus). Macerals are biodegraded and mechani-

cally re-worked. Sedimentation took place probably inmarine environment, shallow (absence of dinoflagellates)and coastal environment (terrestrial organic matter andpollen grains). Micropaleontological analyses showedfindings of planktonic foraminifera (Orbulina sp.,Globigerinoides sp., Globigerina sp.) and remnants ofalgae Lithothamnium. All those facts indicated on ma-rine depositional environment of Badenian.

3.2.2 The bottom of ignimbrite

However, the chronostratigraphic classification of the in-terval underlying the ignimbrite is uncertain regardingparticular stage because of lack of index species. It is as-signed to Budafa Formation with Middle Miocene age,probably Badenian. Lithologically, this interval is repre-sented by pyroclastics, conglomerates (according to mudlogging reports) and lithic greywacke (Figure 12) consist-ing of poorly sorted, sub-angular, sandy detritus infine-grained matrix (silty to sandy, in places hematitic).The detritus consist of quartz, feldspar and rock frag-ments. Thin sections were micropaleontologically sterile,but palynological analyses have the following results:

Sporomorphes of the palynofacies is lacking in sporesbut rich in diverse pollen grains. Very often Momipitessp., Myricipites sp. (Figure 13) and Engelhardtioiditessp. (Figure 14) are identified. Bisaccate grainsCathayapollenites sp. (Figure 15), Pinuspollenitesmioceanicus and many other are abundant. Besidessome undefined pollen grains, following taxa have beendetermined: Caryapollenites simplex, Tricolporopol-lenites sp., Monocolpopollenites sp., Alnipollenites sp.and very rarely Eucalyptus sp.

Green algae of the genus Botryococcus sp. and follow-ing dinoflagellate cysts were determined: Spiniferitesmirabilis (Figure 16), Spiniferites sp. (Figure 17),Impagidinium sp., Impagidinium cf. patulum,Batiacasphaera sphaerica (Figure 18), Achomosphaerasp.

Ratio between sporomorphs and phytoplankton as wellas amount of terrestrial material varies from sample tosample, depending on palaeo-depth and distance frompalaeo-land. Generally, it was high-energy environment(mechanically damaged fossils), marine, near-shore, rel-atively shallow, but deeper than environment of sedi-ments deposited in the top of ignimbrite (dinoflagellatesare observed). According to palynobiocenosis this sedi-ment is of Badenian age, and belongs to the BudafaFormation.

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Fig. 12. Lithic greywacke with hematitic matrixSl. 12. Liièna grauvaka s hematitiziranim matriksom

Fig. 11. Fossiliferous, sandy and argillaceos micriteSl. 11. Fosiliferni, pjeskoviti i glinoviti mikrit

Fig. 13. Myricipites sp.;transmited light

Fig. 14. Engelhardtioidites sp.;transmited light

Fig. 15. Cathayapollenites sp.;transmited light

4. CONCLUSIONSDescribed type of pyroclastic sediment flow deposit hasnot yet been observed on the outcrops or wells in thiszone of the Drava Depression. More than 10 kilometresto the south-west, towards Virovitica (Figure 1) and to thesouth-east, evidences of Miocene volcanism in Croatiaexist in the form of thick series of basic to intermediateeffusive rocks commonly represented by basalts andandesites intercalated with marls or in form of thinpyroclastic fall deposits which are tuff layers within thickclastic sediments, drilled by several exploration wells,but pyroclastic flow deposits formed after explosive vol-canic eruptions were not determined.

Moreover, pyroclastic flow deposits of the Tar Forma-tion are described in numerous locations in Hungary. Inanalysed part of the Drava Depression (Figure 1) 88 mfrom pyroclastic interval had been drilled by the AA-1well. That is lithologically defined as devitrifiedvitroclastic welded tuff, i.e. ignimbrite. Lithostratigraphi-cally it belongs to the Tar Formation, and chrono-stratigraphically to Badenian.

Analysed ignimbrite did not show internal bedding. It isrich in glass shards, pumice, crystals and lithic frag-ments. The thickness of the mapped welded tuff is up to190 m. The dominant component is ash-sized (<2mm),vitric, lithic and crystal fragments being set in veryfine-grained matrix. Vitric particles, glass shards andpumice are abundant. Texture is granophyric andspherulitic. Lithic fragments as lithic pyroclasts andcrystal fragments (commonly broken feldspar andquartz) are also present, as well as chalcedonic aggre-gates and pyrite crystals. Other characteristic features ofthis deposit are light colour and rhyolitic chemical com-position. Basic structural features in the core arefractures, vugs and stylolites.

Micropaleontologically based ages of the top of theignimbrite were concluded from foraminifera and algaefindings and palynologically based ages fromdinoflagellate cysts and findings of pollen. The top of ana-lysed ignimbrite contained the Badenian marinemicrofossils. Samples from bottom aremicropaleontologically sterile but palynobiocenosis de-termined from palynological analyses, also indicated onBadenian marine environment. Consequently, it is as-sumed that the ignimbrite had been created frompyroclastic flow in Badenian and into shallow, marineenvironment.

5. ACKNOWLEDGEMENTS

This paper resulted from multidisciplinary geological in-vestigation done in INA Exploration and Production BUin 2010, during 3D seismic data reinterpretation andpost-well appraisal of AA-1 well. The authors thank toNevenka Romaniæ-Kristensen for her 3D interpretationand mapping and Dr. Tomislav Malviæ, Assoc. Prof., forreviewing of the manuscript.

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Authors:

Dijana Bigunac, INA-Industry of Oil Plc., Exploration and Production, Sec-tor for Geology and Reservoir Management, Department for Geomodelling,Šubiæeva 29, Zagreb, Croatia

Krešimir Krizmaniæ, INA Industry of Oil Plc., Exploration & Production BD,Field Engineering & Operations Sector, E&P Research Laboratory Depart-ment, Geology and Geochemistry BU, Lovinèiæeva bb, Zagreb

Branko Sokoloviæ, INA Industry of Oil Plc., Exploration & Production BD,Field Engineering & Operations Sector, E&P Research Laboratory Depart-ment, Geology and Geochemistry BU, Lovinèiæeva bb, Zagreb

Zlata Ivanièek, INA Industry of Oil Plc., Exploration & Production BD, FieldEngineering & Operations Sector, E&P Research Laboratory Department, Ge-ology and Geochemistry BU, Lovinèiæeva bb, Zagreb

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Pyroclastic flow sediment (ignimbrite) in Miocene Tar