GEOL. CROAT. 49/2 1333 . 334 1 ZAGREB 1996 Ahslrac! - Nole Justifiability of the Biostratigraphy Explorations in Petroleum Geology Kresimir KRIZMANIC and Vlasta PREMEC FUCEK 1. INTRODUCTION Tn consequence of the ever demanding tasks which have been addressed to the oil industry concerning both efficiency or the exploration process and the empha­ s ised economy, the trend of neglecting fundamental geo logical disciplines, In which biostratigraphy belongs, has become obvious. Petrolcllm geology discriminates against "classic" geological exp lorations in favour of seismic and well­ log methods because of the swiftness, lTIalhcmatical exactness and economic saving. However, together with the undisputed qualities or geophysical techniques, it is ve ry often forgotten that such data, which are to be used in thc correlation ancimocielling of a given geolog­ ical object (depression, fi e ld), must be calibratcd with the data ga ined by petrographical, sed imentological, and also, biostratigraphical analyses and interpretations (LUCIC & KRIZMANIC. 1993). Therefore, sllch a short -term economic bcnefit, that has been achieved by not using biostratigraphy in the exploration process, is often later negated . 2. METHODS AND GOALS OF mOSTRATIGRAPHY RESEARCH Biostratigraphy as a science bloomed wi th the development of the oil industry. Through deep drilling explorations, palacontologists obtain multiwdinous data on microfossil assemblages in the rock samples (cut ­ tings or cores). Although such exploration in drill-holes is accompanied by many problems, there arc also some advantages. Biostratigraphy is based on the recogn ition or species, definition or their geological ranges, and there­ fore determination of age. After estab li shing the strati ­ graphical borders and resolving the stra tigraphy in a few wells, the lateral extent of the subs urrace layers becomes known through well -to -well correlation. How­ ever, biostratigraphical data are more valid when utilised in conjunction with other related sciences (biol- INA -Naftaplin. Laboratory Dept., Lovinciceva I, HR-IOOOO Zagreb, Croalia. PROCEEDINGS ogy, ecology, radiometlY, eustacy, magnetometry, etc.), to reconstruct a mosaic or gcological evenls. Only three palaeontologica l disciplines utilised the most in the biostratigraphical research of the INA-Na r­ taplin Oil Industry are mentioned herc. 2.1. MACROPALAEONTOLOGY Dispite the often very rare macrofossil occurence in boreho le materials particular mollusc genera and species which were considered to be outstandingly use­ ful in the correlation of sedi ments and oil and gas sa tu­ rated beds in Paratethys can be traced. Some well­ known index fo ssi ls (e.g. Radix croat/ca, Congeria banaficG, Paradael/a abichi, Conge ria r/IOJIlhoidea) were generally accepted as age indicators ror many years, with little additional consideration of their va li­ dity . 2.2. MICROPALAEONTOLOG Y Microfossil assemblages of fo raminifera, oSl racoda and silicoplacentina were analyzed from borehole rock samples. The appearance or species, slrat igraphic range, abundance, as well as the composition of whole bio­ coenoses have been closely examined. Besides conven­ tional palaeontological literature, contributions rrom INA-Naftaplin's bios tratigraphers arc also utilised as in a successive series of sediments phylogenetic lineages arc easily noted. A good example of these arc the clastic Middle to Upper Eocene sediments in the northe rn Adriatic, which contain fossils representing the com­ plete evolutionary lineage of the TurborOfa/ia ccrroaZl/ ­ lensis group, as well as the species sLlccession or the genus Globigerillatheka (PREMEC FUCEK. 1995; PREMEC FUCEK et al.. in press). Consequently, the lack of other index fossils in our latitude, proved thi s lineage to be advantageous in resolving biostratigraphic problems. 2.3. PALYNOLOGY Due to the costs of maceration, s lide preparation and analys is, palynology is used mainly for sed im ents where there are no olher palaeontological data, e.g. sometimes the sediments are micropalaeontologicJly sterile , but palynologiealy ve ry rich. This is commonly :13-1 the case with Uppcr Tertiary sediments which are most often the most interesting for petroleum geology . In these homogenous sandstones it becomes feasible to determine stratigraphical and palaeoecological charac­ te ristics by using palynological biozonation (SUTO­ SZENTAI, 1988). Palaeontologists obtain information on palaeoenvi­ ronment through collecting data about the habitat (marine, brackish, freshwaler. .. ), mode of life (plank­ tonic, benthic ... ), climatic conditions (tropic, moder­ ate ... ), etc. Morphology of the taxa, chemism, preserva­ tion and many other parameters are also ex tremely important. All observations arc compared with relevant lit e rature and if some new inrormation is obtained it is published . Biostratigraphical and palaeoecological data are cnhanced with petrographical and sedimento logical rock characteristics, chemical analyses etc., so that all tile information on age, type or rock and depositional environment becomes known. Sometimes the structural and tectoni cal rramcwork may also be defined. Such a multidisc iplinary approach based on the results of the scientific investigation of many spccialists, is highly reliable and can be checked and compared with other scientific investigations. 3. IlIOSTRATlGRAPHICAL APPLICATION IN THE OIL INDUSTRY In the contexl of industrial appl icability, biostratig­ raphy plays a unique role in resolving the multiplicity or geological problems which arise during the explo­ ration and exploitation activities of petroleum compa­ nies. 13y using chronostratigraphical and palaeoecologi­ cal data, accurate and detailed well -to-well correlation is made in order to facilitate the interpretation and pre­ diction or reservoirs. Biostratigraphy is establi shed as one of th e main techniques used for this purpose because easily distingu ishable changes in the fossil record commonly occur with in sequences which appear uniform on lithologic and electric logs. Structural traps for oil and gas ca n sometimes be si mply recognised llsing geophys ical techniques. However, a la rger prob­ lem is involved in the regional mapping of palaeogeo­ graph ic and env ironmental parameters which influence the st ra tigraphic control s of hydrocarbon distribution. The accuracy or these maps depends on the precision with which isochro nous surfaces can be identified (STAINFORTH ct aI., 1975). Exclusive usc or seismic, well log or lithostratigraphic data is a mistake, because rocks of physically similar composition at many points, commonly prove to be different in age. Furthermore, in a sense or a new cconomy, the exchange of knowledge and information through con­ versation becomes more and more important in busi ­ ness. As each individual contributes personal ideas and Geologia Croatica 49(2 experience in group discussion amongst specialists with multifarious knowledge and a common aim, such ideas become refined by the group. They make a team, with­ out the coercion or initiative of superiors. The result or such teamwork is, because of enthusiasTll, healthy COTll ­ petition, rivalry and a desire for new understanding, very often remarkabl e by its originality , lucidity , applicability and rapidity. Direct saving and profit is shown, along with better efficiency, in improving hori ­ zontal and vertical communication as well as st imulat­ ing dynamism within the finn. The competitiveness of the company is also improved because on ly such agile, prompt, and dynamic systems are competitive. 4. INSTEAD CONCLUSION 1 beg of VIP cord possessors iJ/ oil cO lI/poJ/ies J/ot to lIegIeet tile II/lldall/ellfal discipIil/ cs such ns biostrnLig­ rnplly and sedilllellfoIogy, Cl7l1se withollt thelll there is J/G prosperous geology. Dr. Jean DERCOURT, President of the Commission for the Geological Map of the World, excerpt of the oral presentation on 81b MAEGS , Budapest, 1993. 5. REFERENCES LUCIC, D. & KRIZMANIC, K. (1993): Laboralorijske tehnikc i metodika primjene rezultata tijekolll geo­ [oskog istrazivanja i razrade podzemlja. - Naftap lin, 1, 15-19, Zagreb. PREMEC FUCEK, V. (1995): Biostratigraphy and Planktonic Foraminiferal Lineages of the Middle to Late Eocene Succession in the North Adriatic Sea.­ First Croat. Geol. Congr., Opatija 18-21.10.1995., Abstracts, 77, Zagreb. PREMEC FUCEK, V., BAI3IC, Lj., BAJRAKTARE­ VIC, Z. & GUSIC, I. (in pres s): Planktonic Foraminiferal Biostratigraphy and Paleoecology of the Mjddle to Upper Eocene Succession in the North Adriatic Sea. - IGCP-28 6, Early Pal eogene Bentos, LjUbljana. STAIN FORTH, R.M., LAMB, J.L., LUTERBACHER, H. , BEARD, J.H. & JEFORDS , R.M. (1975): Ceno­ zoic planktonic foraminiferal zonation and chanlC­ teristic of index forms.- Paleontological Contribu­ tions , 62, The University of Kansas, Lawrence, 425p. SUTO-SZENTAI, M. (1988): Microplankton Zones of Organic Skeleton in the Pannonian s .1. Stratum Complex and in the Upper Par! of the Sarmatian Strata.- Acta Bot. Hungarica, 34/3-4, 339-356, Budapes t.