Uticaj paleosredine na raspodelu i obilnost zasićenih i aromatičnih ugljovodonika u sedimentima topličkog basena

Abstract

Osnovni cilj ove doktorske disertacije bio je proučavanje uticaja paleosredine taloženja na raspodelu i relativnu obilnost zasićenih i aromatičnih ugljovodonika u miocenskim jezerskim sedimentima Topličkog basena. Rezultati ovog istraživanja omogućili su procenu porekla, naftno- gasnog potencijala i zrelosti organske supstance u ispitivanim sedimentima. Takođe, ova disertacija je razmatrala uticaje sedimentoloških procesa i tektonike na razvoj Topličkog basena i sastav sedimenata. Toplički basen do sada nije bio predmet ovakve vrste naučnih istraživanja. Ovo istraživanje je obuhvatalo interdisciplinarnu analizu i interpretaciju organskog i neorganskog dela odabranih sedimenata iz četiri istražne bušotine: BL3, BL4, BL5 i BL7. Neorganski deo je ispitivan primenom optičkih ispitivanja, rendgenske difraktometrije, rendgenske fluorescentne spektroskopije, indukovane kuplovane plazme sa masenom spektrometrijom, skenirajuće elektronske mikroskopije, kompleksometrijske analize sadržaja kalcijuma i magnezijuma. Organski deo sedimenata je analiziran upotrebom petrografskih ispitivanja, Rok-Eval pirolize i gasnohormatografsko-masenospektrometrijske metode. Na osnovu sedimentoloških, litoloških i mineraloških rezultata, Toplički basen je podeljen na zapadnu i istočnu subbasensku jedinicu. U dubljoj, zapadnoj subbasenskoj jedinici, ispitivani su sedimenti bušotine BL7, dok su u plićoj, istočnoj subbasenskoj jedinici, ispitivani sedimenti bušotine BL3 i BL5. Bušotina BL4 nalazi se blizu prelaza između dve subbasenske jedinice, preko kojeg je omogućena potencijalna komunikacija zapadnog i istočnog dela basena. Postojanje dve nezavisne subbasenske jedinice omogućila je proučavanje uticaja različitih procesa na taloženje sedimenata i karakteristike organske supstance. Pored navedenog, na osnovu litoloških i mineraloških podataka, miocenski sedimenti su raščlanjeni na nekoliko litofacijalnih jedinica koje su se međusobno razlikovale po strukturnim i teksturnim osobenostima. U okviru ove disertacije detaljno su ispitivani tufozni, analcimizirani i bituminozni finozrni klastiti Čučala i finozrni klastiti Prebreze, taloženi u jezerskoj ili močvarnoj faciji. Sedimenti močvarne facije odlikuju samo bušotinu BL5. Vulkanizam je pratio taloženje sedimenata u Topličkom basenu, što se odražavalo kroz prisustvo vulkanoklastičnog materijala duž vertikalnih profila ispitivanih bušotina. Vulkanski procesi su bili izraženiji u zapadnoj subbasenskoj jedinici, na šta je upućivao viši sadržaj analcima u sedimentima bušotine BL7, kao i pojava tufogenog paketa, debljine 7 m. Značajna količina vulkanoklastičnog materijala koja je transportovana sa zapadnog dela u basen, podstakla je proliferaciju algalnih i planktonskih zajednica, istovremeno pospešujući primarnu bioproduktivnost, što je uticalo na sadržaj organske supstance. Razvoj Topličkog basena praćen je tektonskim istanjivanjem kore, što je dovelo do pojačanog termalnog fluksa iz litosfere, čime je prouzrokovano pregrevanje organske supstance, koje je dalje podstaklo proizvodnju ugljovodonika i povećanje njihovog ukupnog sadržaja. Organska supstanca sedimenata bušotine BL7 je nematurisana do rano maturisana i izgrađena od kerogena tipa I ili II, koji imaju tendenciju ga generisanju tečnih ugljovodonika. Akumulacija i očuvanje organske supstance u sedimentima BL7 je bilo omogućeno zahvaljujući anoksičnim i mezo-slanim ili hiperslanim paleouslovima sredine, o kojima svedoči predominacija fitana, prisustvo i-C30, β-karotana, perilena, gamacerana, δ-metiltrimetildecilhromana, 1,5- i 1,4-dimetilkarbazola. Taloženje sedimenata u istočnom delu Topličkog basena praćeno je intenzivnim aluvijalno- jezerskim procesima, što se odrazilo kroz izraženo prisustvo peskovite i šljunkovite komponente duž vertikalnog profila bušotine BL3, a u manjem obimu i u bušotini BL5. Peskovi i šljunkovi su značajnije razblažili organsku supstancu ispitivanih sedimenata, koja je nematurisana i izgrađena od kerogena tipa III ili mešovitog kerogena tipa II/III. Predominacija viših neparnih n-alkana (n-C27, n- C29 i n-C31) i hopana u sedimentima bušotine BL3 i BL5, upućivali su na pojačano učešće viših kopnenih biljaka i mikrobiološki prerađene organske supstance. Sredina sedimentacije je bila generalno anoksična i veoma dinamična. Na dinamiku sedimentacije uticala je energija jezera i Doktorska disertacija Nikola M. Burazer aluvijalni sistem putem kojeg se transportovala velika količina klastičnog materijala različite veličine. Klastični materijal je sa sobom dopremao organsku supstancu različitog porekla i stratigrafske pripadnosti, time ukazujući na prisustvo resuspendovane organske supstance u istočnoj subbasenskoj jedinici. Organska supstanca, istaložena pod ovakvim okolnostima, je odražavala kompleksne informacije o paleosredini taloženja. S obzirom da su više kopnene biljke dominantan prekursor u organskoj susptanci bušotina BL3 i BL5, postojala je mogućnost razmatranja uticaja paleoklimatskih uslova na raspodelu i obilnost ugljovodonika. Paleoklimatski uslovi su odražavali pojavu određenih biljnih taksona u organskoj supstanci ispitivanih sedimenata. Naime, visoke C-value i Rb/Sr, odnosno niske vrednosti Sr/Cu odnosa, praćene obilnijim učešćem zasićenih i aromatičnih triterpenoida koji vode poreklo od skrivenosemenica, odlikovali su veliku grupu ispitivanih sedimenata. Rezultati neorganskih geohemijskih parametara i prisustvo skrivenosemenica odražavali su tople i semi-humidne/semi- aridne do semi-humidne paleoklimatske uslove sredine. Dobijeni podaci su u skladu sa dosadašnjim paleoklimatskim istraživanjima neogenih basena jugoistočnog dela Evrope. S druge strane, tektonski procesi rastezanja kore i izdizanje (ekshumacija) Jastrebačkog kompleksa su pratili razvoj Topličkog basena tokom neogena. Razlike u sedimentološkim, litološkim, mineraloškim i organsko-geohemijskim osobenostima duž vertikalnog profila bušotine BL4, ukazale su na pojavu cikličnosti u taloženju i četiri litočlana, A, B, C i D. U ovom slučaju cikličnost je povezana sa tektonskim procesima. Naime, stvaranje prirodne „barijere“, u vidu ekshumiranog Jastrebačkog kompleksa, kontrolisalo je prinos klastičnog materijala sa margina u zapadni i istočni deo ispitivanog basena. Tokom inicijalne faze razvoja Topličkog basena, prirodna „barijera“ je sprečila transport erodovanog materijala koji je odlikovao sedimente istočne subbasenske jedinice. Litočlanovi A i B odražavali su tip organske supstance i paleouslove slične kao u sedimentima zapadnog dela Topličkog basena (bušotina BL7). Mirna, anoksična i mezoslana sredina sedimentacije, pohranjena makro- i mikro- nutrijentima oslobođenih tokom vulkanskih procesa, odlikovala je organsku supstancu litočlanova A i B. O ovome svedoče znatne količine analcima, predominacija fitana, značajnije koncentracije i-C25 i i-C30, kao i izraženo prisustvo β-karotana. Organska supstanca bila je pretežno obogaćena algalnim prekursorima. Sedimentacija se nastavila formiranjem litočlanova C i D. U tom trenutku, prirodna „barijera“ je sprečavala transport erodovanog materijala sa zapadne subbasenske jedinice. U ovom slučaju, organska supstanca odražavala je tip i paleouslove slične kao u sedimentima istočnog dela Topličkog basena (bušotine BL3 i BL5). Organska susptanca bila je mešovitog porekla, istaložena u anoskično- disoksičnim i slatkovodnim uslovima sredine. U prilog iznešenom, potvrdu je predstavljalo i izraženije prisustvo peskovite komponente dopremljene aluvijalnim sistemom, promenljive vrednosti Pr/Ph odnosa, smanjene relativne koncentracije i-C25 i i-C30, kao i odsustvo β-karotana. Sedimentacija u Topličkom basenu se okončala prinošenjem grubozrnih i srednjozrnih klastita sa svih margina, čime se polagano završila sedimentacija i nagovestilo gašenje neogenog jezerskog basena.

The main goal of this doctoral dissertation was to study the influence of the paleoenvironmental conditions on the distribution and relative abundance of saturated and aromatic hydrocarbons in the Miocene lake sediments of the Toplica basin. The results of this research enabled the assessment of the origin, oil and gas potential and maturity of the organic matter in the investigated sediments. Also, this dissertation considered the influence of sedimentological processes and tectonics on the development of the Toplica basin and composition of sediments. The Toplica basin has not been the subject of previous scientific investigations. The research included an interdisciplinary analysis and interpretation of the organic and inorganic parts of selected sediments from four boreholes: BL3, BL4, BL5 and BL7. Examination of the inorganic part of the sediments was made possible by applying optical examinations, X-ray diffractometry, X-ray fluorescence spectroscopy, Inductively coupled plasma mass spectrometry, scanning electron microscopy, and complexometric analysis of calcium and magnesium content. The organic part of the sediments was analyzed using petrographic tests, Rock-Eval pyrolysis and gas chromatography-mass spectrometry methods. Based on sedimentological, lithological and mineralogical results, the Toplica basin was divided into western and eastern sub-basin units. In the deeper, western subbasin unit, the sediments of borehole BL7 were investigated, while in the shallower, eastern subbasin unit, the sediments of boreholes BL3 and BL5 were examined. The BL4 borehole was located near the transition between these sub-basin units, through which the potential communication of the western and eastern parts of the basin was probable. The existence of two independent sub-basin units opened the possibility of studying the influence of different processes on the deposition of sediments and the characteristics of organic matter. In addition to the above, lithological and mineralogical data allowed the categorization of Miocene sediments into several lithofacies, which were different in terms of structural and textural properties. Within the framework of this dissertation, the tuffaceous, analcimized and bituminous fine-grained clastites of Čučale and fine-grained clastites of Prebreza deposited in lake or swamp lithofacies were examined. Swamp sediments were only detected within the borehole BL5. Volcanism characterized the deposition of sediments in the Toplica basin, which was reflected in the presence of volcaniclastic material along the vertical profiles of the examined boreholes. Volcanic processes were more pronounced in the western subbasin unit, which was indicated by the higher content of analcime in the sediments of borehole BL7, as well as the appearance of a 7 m thick package of tuffs. Significant proportions of volcaniclastic material transported from the western part into the basin enhanced the proliferation of algal and planktonic communities, simultaneously increasing primary productivity, which affected organic matter content. The development of the Toplica Basin was accompanied by crustal thinning that led to an increased thermal flux from the lithosphere, which caused overheating of the organic matter and further stimulated the production of hydrocarbons and an increase in their total content. The organic matter in sediments of the borehole BL7 was immature to early mature and composed of type I or II kerogens, which tend to generate liquid hydrocarbons. The accumulation and preservation of organic matter in BL7 sediments were attributed to the anoxic and meso-saline or hypersaline paleoenvironmental conditions, which is evidenced by the predominance of phytane, presence of i-C30, β-carotane, perylene, gammacerane, δ-methyltrimethyltridecyl chroman, 1,5- and 1,4-dimethyl carbazoles. The deposition of sediments in the eastern part of the Toplica basin was characterized by intense alluvial-lacustrine processes, which were reflected in the pronounced presence of sand and gravel components throughout the borehole BL3 and, to a lesser extent, in well BL5. Sand and gravel components significantly diluted the organic matter of the investigated sediments, which was immature and consisted of type III kerogen or mixed type II/III kerogen. The predominance of odd Doktorska disertacija Nikola M. Burazer long-chain n-alkanes (n-C27, n-C29 and n-C31) and hopane in investigated sediments indicated an increased participation of higher plants or microbiologically reworked organic matter. The paleoenvironmental conditions were generally anoxic and very dynamic. The sedimentation dynamics were influenced by the energy of the lake and the alluvial system through which a large amount of clastic material of different sizes was transported. The clastic material transported organic matter of mixed origins and stratigraphy, indicating resuspended organic matter in the eastern subbasin unit. The organic matter deposited under these circumstances reflected complex information about paleoenvironmental conditions. Given that higher terrestrial plants were the dominant precursor in the organic matter of boreholes BL3 and BL5, the possibility of considering the influence of paleoclimatic conditions on the distribution and relative abundance of hydrocarbons opened up. Paleoclimatic conditions reflected the occurrence of specific plant taxa in the organic matter of the investigated sediments. Namely, high C-value and Rb/Sr ratios, low Sr/Cu ratio values, followed by greater participation of saturated and aromatic triterpenoids originating from angiosperms, characterized a large group of investigated sediments. The results of inorganic geochemical parameters and the presence of angiosperms suggested warm and semi-humid/semi-arid to semi-humid paleoclimate conditions. The obtained data were in accordance with the previous paleoclimatic research of Neogene basins of the southeastern part of Europe. On the other hand, tectonic processes of crustal extension and uplift (exhumation) of the Jastrebac complex followed the development of the Toplica basin during the Neogene. Differences in sedimentological, lithological, mineralogical and organic-geochemical characteristics along the vertical profile of the BL4 borehole indicated the occurrence of cyclicity in deposition and lithomembers A, B, C and D. In this case, cyclicity is associated with tectonic processes. The formation of a natural "barrier" (exhumed Jastrebac complex) controlled the flow of clastic material from the margins to the western and eastern parts of the studied basin. During the initial phase of the development of the Toplica basin, a natural "barrier" prevented the transport of eroded material that characterized the sediments of the eastern sub-basin unit. Therefore, lithomembers A and B reflected the type of organic matter and paleoenvironmental conditions similar to those in the sediments of the western part of the Toplica basin (borehole BL7). A calm, anoxic and mesosaline paleoenvironment, stored with macro- and micro-nutrients released during volcanic processes, characterized the formation of lithomembers A and B. This was evidenced by significant amounts of analcime, the predominance of phytane, significant concentrations of i-C25 and i-C30, and the pronounced presence of β-carotene. The organic matter was predominantly enriched in algal precursors. Sedimentation continued with the formation of lithomembers C and D. At that point, a natural "barrier" prevented the transport of eroded material from the western subbasin unit. Therefore, the organic matter reflected the type and paleoenvironmental conditions similar to those in the sediments of the eastern part of the Toplica basin (boreholes BL3 and BL5). The organic matter was of mixed origin, deposited under anoxic-dysoxic and freshwater environmental conditions. In support of this, a more pronounced presence of the sandy fraction supplied by the alluvial system, fluctuating values of the Pr/Ph ratio, reduced concentrations of i-C25 and i-C30, as well as the absence of β-carotene characterized these lithomembers. Sedimentation in the Toplička basin ended with the deposition of coarse-grained and medium- grained clastites from all margins, which slowly ended the sedimentation and indicated the extinction of the Neogene lake basin.