Geological and Physical-Chemical Characteristics of Lower Paleozoic Deposits of Volhyno-Podillya, Western Ukraine
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BIULETYN PAŃSTWOWEGO INSTYTUTU GEOLOGICZNEGO 449: 119–130, 2012 R. GEOLOGICAL AND PHYSICAL-CHEMICAL CHARACTERISTICS OF LOWER PALEOZOIC DEPOSITS OF VOLHYNO-PODILLYA, WESTERN UKRAINE CHARAKTERYSTYKA GEOLOGICZNA I FIZYKOCHEMICZNA DOLNOPALEOZOICZNYCH OSADÓW WOŁYNIA I PODOLA (ZACHODNIA UKRAINA) IHOR KUROVETS1, DANYLO DRYGANT2, IHOR NAUMKO1, SERHIY KUROVETS3, YURIY KOLTUN1 Abstract. The 340 to 1102 m thick multi-facies sequence was deposited during the Silurian at the Volhyno-Podillya margin of the East European Platform. The open shelf facies bordered from the west by a barrier formed by reefs, bioherms, crinoids, or by banks of detrital sediments (at the boundary of the lagoon and the open sea). Lagoonal deposits are represented by thin intercalations of sedimentary dolo- mites, sometimes with intercalations of organogenic limestones, marls, argillites, gypsum and anhydrite. Within the open shelf and slope facies, marls, lumpy clayey limestones and argillites are prevailing. A zone of reefal buildups has a rather complicated and insufficiently investigated structure. The exclusively terrigenous deposits of the submerged part of the continental slope form the entire Silurian sequence in the central part of the L’viv Depression and in the Carpathian Foredeep. Open porosity of the Silurian argillites vary from 0.6 to 2.4% and they are almost impermeable (less than 0.001 μm2·10–3). Volume weight ranges between 2.64 and 2.8 g/cm3. Natural radioactivity of rocks varies from 9 to 18 mcR/hr, apparent electric resistance is 20–154 Ohmm, interval time of longitudinal waves spreading is 126–365 μcs/m. TOC content, determined in 21 samples using the thermal analy- sis, varies from 0.75 to 2.38%. The composition of volatiles of fluid inclusions and closed pores of argillites shows the predominance of methane (up to 100 vol. %). In deeper horizons, carbon dioxide, nitrogen and ethane (a few percent) appear. Key words: geology, petrophysics, fluid inclusions, Lower Paleozoic, shale gas, Volhyno-Podillya. Abstrakt. Zróżnicowane facjalnie utwory syluru, miąższości od 340 do 1102 m, były deponowane na wołyńsko-podolskiej krawędzi platformy wschodnioeuropejskiej. Bariera budowana przez rafy, biohermy i ławice materiału detrytycznego graniczyła na zachodzie z facjami otwartego szelfu. Na wschód od tej bariery, w lagunie, tworzyły się osady dolomitowe z przeławiceniami organogenicznych wapieni, margli, iłowców, gipsu i anhydrytu. Osady szelfu i skłonu są reprezentowane głównie przez margle i ilaste wapienie gruzełkowe oraz argility. Strefa bariery rafowej ma dość skomplikowaną i jeszcze niedostatecznie rozpoznaną budowę. Osady terygeniczne występują tylko w zanurzonym odcinku skłonu kontynentalnego w centralnej części depresji lwowskiej oraz w podłożu zapadliska przedkarpackiego. .(Efektywna porowatość argilitów sylurskich wynosi od 0,6 do 2,4% i są one prawie nieprzepuszczalne (mniej niż 0,001 μm2ּ10–3 Gęstość objętościowa waha się pomiędzy 2,64 i 2,8 g/cm3. Promieniotwórczość naturalna skał sięga od 9 do 18 mcR/h, a pozorna oporność właściwa wynosi 20–154 Ohmm, czas interwałowy rozchodzenia się fal podłużnych – 126–365 μcs/m. Zawartość TOC, określona w 21 próbkach z wykorzystaniem analizy termicznej, waha się od 0,75 do 2,38%. Skład substancji lotnych z inkluzji fluidalnych i zamkniętych porów w argilitach wykazuje dominację metanu (do 100% obj.). W głębszych horyzontach pojawiają się dwutlenek węgla, azot i etan (kilka procent). Słowa kluczowe: geologia, petrofizyka, inkluzje fluidalne, dolny paleozoik, gaz łupkowy, Wołyń, Podole. 1 Іnstitute of Geology and Geochemistry of Combustible Minerals of NAS of Ukraine, Naukova 3a, 79060 Lviv, Ukraine 2 State Natural Museum of NAS of Ukraine, Teatralna 8, 79000 L’viv, Ukraine 3 Ivano-Frankivs’k National Technical University of Oil and Gas, Ukraine, Karpatska 15, 76000 Ivano-Frankivs’k, Ukraine 120 Ihor Kurovets et al. INTRODUCTION Early Paleozoic deposits of the south-western margin of et al., 2011), their structure, facies and sedimentological pe- the East European Platform are considered presently as one culiarities are known only in general terms. The objective of of the most prospective targets for shale gas exploration (Po- this study is to specify the depositional and facies environ- prawa, 2010). In Ukraine, these are Silurian and Ordovician ments of these strata, which would be useful for understand- deep-marine sediments, which extend towards the SE from ing the possible shale gas occurrence in this territory, and to the Polish border to Romania (Drygant, 2010). Although investigate the geochemical and petrophysical characteris- these deposits are prospective for hydrocarbons (Kurovets tics of black shales. EXPERIMENTAL Petrophysical investigations. Methods of petrophysical Mass-spectrometry. Taking into account the specificity laboratory investigations of core samples includes a range of shale gas, the methods, developed in the Department of of study methods of the rock matrix, diagnostics and esti- Deep Fluids of IGGCM NAS of Ukraine (Naumko, 2006; mates of physical-mechanical parameters of reservoir rocks, Naumko et al., 2009), shall be involved in the determination independently from the previous conclusions on the type and of its concentrations and features of spatial distribution with- characteristics of the pore space of the rocks. Errors of mea- in the productive sequences of black shale formations. Their surements of the parameters are characterized by the follow- peculiarity is that the release of volatiles from fluid inclu- ing values: for integral time, the mean square error is 1.5– sions in minerals and from closed pores in rocks is made by 2%, for relative resistivity 2–2.5%, for saturation parameter mechanical crushing of rocks, which juxtaposes this process 3–5%, for open porosity 5–7%, for permeability 10–15%. with the conditions of breakdown of rocks within the zone of Differential Thermal Analysis. TOC measurements in artificial hydraulic fracturing. this study were made using the Differential Thermal Analy- Mineral-fluidological indexes: volatile content, relative sis. Thermogravimetric investigations were performed using gas saturation ∆Р, Pa (pressure increase in the filling system the NETZSCH STA 449 F3 Jupiter instrument within the of mass-spectrometer compared to the residual pressure in temperature range of 25–800°С, heating rate was 20°С/min the order of 1·10–3 Pa resulting from the release of volatiles) in argon atmosphere. Sample weight ~300 mg. Precision of and relative water saturation СН О (water vapour content in –2 2 temperature measurements 1°С, mass change 1·10 mg. At the total volume of the released volatiles; absorber – Р2О5) the temperature of up to 120°С, water is released from the were determined using the mass-spectrometric chemical sample, at 120–300°С – free hydrocarbons, at 300–390°С – method on time-of-flight mass-spectrometer МСХ-3А (1– bound hydrocarbons, at 390–550°С – СО2 is formed during 200 а.u.m.). burning of kerogen (С >40). SEDIMENTARY ENVIRONMENTS BASIN eVOLUTION a significant increaseof the thickness in the western direc- tion (Fig. 1) (Drygant, 2000). It rests on the surface eroded Nature and intensity of the processes, which had an during a long regression (caused by the Taconic orogeny). impact on the formation of Silurian strata of the south- The surface is made up of Ordovician, Cambrian and Ven- western margin of the East European Platform have been dian rocks (Fig. 2). determined by the location of the region, which tectonical- The 340 to 1102 m thick multi-facies sequence was de- ly belonged to the zone of peri-cratonic subsidence, and posited during the Silurian at the Volhyno-Podillya margin was the floor of an epicontinental sea. The initial simple of the East European Platform. The minimum thicknesses structure of this sequence was significantly complicated (340–385 m) are found in the eastern part of the region, where by the contrast and multi-directional tectonic movements the shallow-water and lagoon facies prevail in the sequence. during the long intervals of the continental evolution of The maximum thicknesses (989–1102 m) are observed in the the region at the end of Early Devonian and in the Late Carpathian Foredeep and central part of the L’viv Depres- Carboniferous (Drygant, 2010). The Wenlock–Ludlow sion, where the slope and depressional facies (mainly grap- deposition was compensated by the more or less uniform tolitic argillites) occur (Einasto et al., 1980). subsidence, while in the Pridoli–Early Devonian, it was Analysis of maps of isopachs distribution and facies oc- obviously excessive in its western part (Teisseyre-Torn- currence shows that the open-shelf facies bordered from the quist Zone – TTZ). The continuous Silurian–Lower Devo- west on the bottom of a barrier, formed by reefs, bioherms, nian sequence (structural complex) shows almost a sub- crinoids or the banks of detrital sediments at the boundary of meridional strike of the clearly defined facies zones and the lagoon and the open sea. In spite of the lateral migration Geological and physical-chemical characteristics of Lower Paleozoic deposits of Volhyno-Podillya, Western Ukraine 121 BELARUS POLAND 0 25 50 km ROMANIA , Eastern extent of the Silurian deposits L viv Depression limit Eastern extent of the Lower Devonian deposits Borehole and thickness Tyver Superhorizon Extent of biohermal barriers Dnister Series (A – Bahovytsya, B–Konivka, C–Isakivtsi suite) Western extent of the Silurian deposits Teisseyre–Tornquist Zone Fault Fig. 1. Isopach map