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 V. Vladyka comprehensive research Trends of changing reservoir properties center of UkrNDIgaz of Sarmatian stage rocks at the A. Push Expert of DKZ of northwestern part of Bilche-Volytsia for Industrial Geophysics M. Nesterenko zone in different facial terms Dr. of geol. science R. Balatskyi of sedimentation Lviv comprehensive research center of UkrNDIgaz

UDK 553.82

За даними геофізичних досліджень свердловин охарактеризовано фізичні властивості порід-колекторів, сфор- мованих у різних фаціальних умовах (глибоководні, перехідні, мілководні), залежно від виділених типів розрізів у сарматських відкладах. Ключові слова: порода-колектор, відкрита пористість, газопроникність, коефіцієнти шаруватості, піскуватості, трансгресія і регресія моря. По данным геофизических исследований скважин охарактеризованы физические свойства пород-коллекторов, сложившихся в различных фациальных условиях (глубоководные, переходные, мелководные), в зависимости от выделенных типов разрезов в сарматских отложениях. Ключевые слова: порода-коллектор, открытая пористость, газопроницаемость, коэффициенты слоистости, пещанистости, трансгрессия и регрессия моря. On the basis of geophysical well logging data the physical properties of reservoir rocks formed in different facial conditions (deep-water, transitional, shallow-water) were characterized depending on selected types of cross-sections in Sarmatian deposits. Key words: reservoir rock, porosity, permeability, factors of layering, sandstone, transgression and regression of the sea.

A complex of well logging actions (WL) allows describing high-ground vaults, elevations and slopes, underwater shafts, spits, various properties of natural bodies, i.e. the electrical resistivity of bars, and this is due to the unevenness of their distribution in the area various parts of the reservoir, the natural and given radioactivity, [2, 3]. natural polarization and rate of passage of elastic waves, in a flexible The first type of cut is the deepest part of the shelf. This type manner. All parameters of these methods are geological indicators of cut includes the Sarmatian sediments of Vyzhomlia and Vyshnia which allow studying the conditions of sediment accumulation in fields, as well as and Oselia areas (see the figure). rocks. The second type of cut is a shallow shelf. This type cut was open The development of a sedimentic logging analysis technique in Sarmatian deposits in Letniany, Kniahynychi, and Pyniany fields based on well logging methods has been the task of the Dr. of (see the figure). geological-mineral sciences, T.S. Izotova for many years. However, The third type of cut is the coastal part, the transition to these works have not been completed for objective reasons. In the Verkhnia Dashava and Nyzhnia Dashava deposits. Because of the northwestern part of Bilche-Volytsia zone (BVZ) T.S. Izotova small amount of information, this type of cuts is little studied. It was identified four types of cuts. [1] open at Rubanivka and fields (see the figure). The authors of this article, based on joint research of WL The formation of microlayered Sarmatian cuts occurred in techniques and detailed descriptions of microfaunal research, certain sedimentation conditions affecting the texture, structure and conducted the further in-depth study of the conditions of sediment lithological composition of layers. These genetic features of the cut accumulation in the northwestern part of BVZ. For this purpose, are determined using the WL methods based on parameters of the the most representative coring intervals selected by core sample and stratification, coefficient of frequency change of lithological varieties microfauna were selected in wells, which revealed the Sarmatian and sandy factor. deposits. On the basis of these studies, three types of cuts were The stratification of Sarmatian section is the result of changes identified. in facies and hydrodynamic regimes of sedimentary flows. Each A characteristic feature of the Sarmatian deposits is variation layer in the microlayer cut allocated according to the WL data meets of capacity of individual packs and layers caused by the sediment certain deposit sedimentation conditions. accumulation conditions and intensity of terrigenous material arrival The texture of a certain pack of the microlayered cut can be to the pool. Depending on the base level of erosion, the sandstones determined by studying the individual strata of different lithology were deposited not only in the humble areas of relief, but also in the and thickness. The cut stratification coefficienth ( L) is determined

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through the total thickness of all layers Σ hi and the number of all KL<0.8, indicates the continuous change in lithological varieties of layers (n): hL= Σhі /n. rocks and a sharp change in deposit sedimentation conditions. As regards the value of the stratification coefficienth ( L), the cuts The sediments of Sarmatian rocks are the examples of such are divided into thick-layered hL >4, medium-layered hL from 4 to 2, changes. The stratification of sandstone, mudstone and siltstone is thin-layered hL – from 2 to 0.8 and microlayered hL <0.8 m. changing continuously. For example, in Shehyni well 2 (interval According to the stratification coefficient, the cuts of Sarmatian 960–1100 m) KL=0.3, in Oselia well 1 (interval 1100–1300 m) KL = rocks in wells of Vyshnia, Oselia and Shehyni areas can be attributed 0.12; and at Vyshnia well 5 (interval 940–1040 m) KL=0.1. to thin-layered. The cuts of wells in Zaluzhany, Mainytchi and The distribution of sandy rocks laterally, according to the Letniany areas (see the figure) belong to the medium-layered. WL data, in microlayered cuts is investigated using the sandiness An important texture feature of the layered deposits is the coefficient (KSands). This figure is one of the major ones for zonal and coefficient of the frequency of change of individual lithological local forecasts in productive saturated cuts. varieties of rocks (KL): In terms of sandiness, the microlayered rocks, based on the WL data, are subdivided into three groups: with mostly alevritic rocks KSands=ΣhSands/Σh>0.5, with an average sandiness level KSands=ΣhSands/ Σh from 0.5 to 0.3; and with mostly clay rocks KSands=ΣhSands/Σh<0.3. where hi∙ η is the product of the average thickness of layers of In sections of Sarmatian sediments the clay rocks are individual lithological units and their percentage content (η) in the predominating, e.g. Shehyni well 2, KSands= 0.06; Oselia well 1, KSands= whole pack. 0.04, Vyshnia well 5, KSands=0.08. The most arenose cut (according to To determine the frequency of changes in lithological varieties WL) is located in the interval of 1250 to 1350 m, Vyzhomlia well 2. of rocks, it is required to know the η value. This indicator is defined The sandstone layers are clearly distinguished by GC method. The as a percentage number of an individual lithological size, and its cut sandiness coefficient is 0.7, confirming the inflow of gas into the 3 percentage content in relation to the total number of all lithological well (Qg= 12.2 thousand m /day). varieties in the pack. In the context of the shallow shelf part, the sandiness coefficient In terms of lithological changes, the rocks are monofacial and (Csand) according to Zaluzhany well 30 is 0.55, at Mainytchi well 7 is polyfacial. If the coefficient K ( L)≥0,8, the deposit sedimentation 0.72, and at Letniany well 10 – 0.68. The amount of sandy material in conditions are stable in a certain period of geological time span. If these cuts is much higher.

Fig. Typical cuts of the Sarmatian deposits formed in various deposit sedimentation conditions

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The sediment logging analysis involves identification of The second type of the cut of Sarmatian rocks was formed the structure of sedimentary rocks. As regards the structural in shallow shelf. These cuts are significantly different from the features, the logging methods determine the particle size cuts, which were formed in the most recessed shelf (see the distribution of sandstones, siltstones and clay dispersion degree. figure). The indicators of well logging methods, i.e. a lateral The first difference observed using the well logging methods is micrologging (LML), lateral logging (LL) and lateral the nature of rock layers. While the cuts of Vyzhomlia-Shehyni area logging sensing with small probes (LLS), are formed in the are composed of alternating sandstones, siltstones and mudstones, permeability area. Based on their values, we can determine the in the cuts of Letniany-Zaluzhany type the sandstone and siltstone area permeability depth, the porosity rate and other elements is more prevalent, but there is almost no mudstone layer. The second characterizing the permeability area. After determination of difference of cuts is in the thickness of layers to their inherent micro- the porosity parameter according to WL, the changes in the or thin-layered nature. The cuts of Letniany-Zaluzhany type are mean diameter of the grains of sand and alevritic rocks can be characterized by layers of greater thickness, which varies from 0.6 described. to 2 m. Using the LML methods and micrologging (ML), together The WL methods in these cuts are better differentiated. with an average diameter of sandstone grain, the conclusions The «own polarization» (OP) method allocates well the can be drawn about the signs of the basin regression or reservoir rocks in the cuts (Mainychi well 7, the interval of transgression. Dev reduction suggests the transgressive events 2250 to 2350 m). The Kniahynychi-Pyniany cuts are somewhat in the accumulation of rocks. Dsir increase or change ( ρmbk) different from Letniany-Zaluzhany type by their cycles. vertically indicates the signs of the sea regression. The cycles observed in Kniahynychi-Pyniany cuts are less The irregular distribution of clayness in rocks and irregular powerful than Letniany-Zaluzhany cuts (Kniahynychi well 1, grain size of the sandstones indicate the rapid streams. The the interval of 2150 to 2050 m, Mainychi well 7, the interval complex of the abve mentioned indexes gives a posibility of 2240 to 2460 m). to determine genetic nature of different types of cuts in the The clay packs delineating the sand and silt are clearly northwestern part BVZ with a sufficient probability. distinguished by means of WL and retain the thickness of 10 The first type of cut (Zaluzhany well 30, the interval of 2645 to 2660 m) to 20–30m The figure shows a typical cut of Sarmatian rocks at (Kniahynychi well 1, the interval of 2140 to 2165 m). Vyshnia area. It is the deepest part of the shelf. The cuts are The cuts of Letniany-Zaluzhany- Kniahynychi types are characterized by thin-layered texture of rocks. The layers 1 combined by the low clay content in rocks. In the sandstone of the to 0.4 m, with high value of the natural radioactivity of rocks second type of cut the clay content reaches 10–15%. They have a 8-12 γ, and layers of both regressive (Vyshnia well 5, interval of high content of sand component (KSands>0.5). Their thicknesses 1300 to 1250 m) and transgressive (Vyzhomlia well 13, interval in stratification represent 0.6 meters or more. The sandstones in of 1340 to 1360 m) types are predominating. these sections (according to the logging data) have larger grains The electrometer recorded the cycled nature of deposit (Dev to 109 microns) and a large filtration area (up tod 8) affecting sedimentation in wells; there are significant megacycles (Vyzhomlia the interpretation of well logging techniques. Due to this effect, well 13, interval of 1370 to 1270 m). The pack of argillaceous rocks the apparent electrical resistance on PivotChart varies from 5 (4 m) is varied, alternating with sandstones and siltstones in the to 9 Ohm∙m; in natural radioactivity is 4 to 8 u, the interval time interval of 1270 to 1210 m. The megacycles are ending with a large of elastic wave passage in reservoirs (AT) varies from 204 to stack of argillaceous rocks. The cuts sandiness coefficient is from 240 mcs/m. 0.04 to 0.08. The sandiness is extremely variable. The most sandy cut As regards the sandiness coefficient, the most sandy horizons is located in Vyzhomlia well 3 (interval of 1250 to 1350 m), where are ND-1-10 at Letniany and Pyniany fields. ND-10-15 horizons in KSands=0.7. The apparent resistance of rocks is 2 to 5 ohm∙m. Judging these fields are represented by cuts dominated by siltstones, with a by SP self-polarization curves, the cut is clearly differentiated small number of highly porous sand strata. (Vyzhomlia well 13, interval of 1200 to 1350 m). In Letniany field the porosity ratio varies from 14.9 to 25.2%; The cut clay content is very high. The collector rocks have the the most common open porosity factor is 15.8 to 16.8% and the 15 2 clay content up to 30%. The mean clay content coefficientC c at average gas permeability is about 0.2∙10 m . The deposit in the Vyzhomlia deposit is equal to 21%. Nyzhnia Dashava deposits on Letniany field lies in the interval of As regards their granulometric composition, the sandstones 800 to 1200 m. in the recessed shelf are medium and fine-grained. The statistical In Krukenytsia depression, in Kniahynychi well 1, in the analysis of the data shows that the medium-grained component (from interval of 1540 to 2414 m the lower Sarmatian deposits have the 0.5 to 0.1 mm) holds a 29.6% share, and a fine-grained component porosity ratio of 7 to 17%, and the permeability coefficient of 0.1∙1015 (from 0.2 to 0.01 mm) holds a 30% share. The content of clay shares to 1∙1015 m2. is up 38%, and the carbonate shares are 31%. The carbonate cement occurs in the amount of 11–22%. Its mean value is 18.5%. These are The third type of cut the sandstones of alevritic, medium-clay lithotypes with carbonate An example of this type of cut can be the cut of Sarmatian cement. sediments at Rudky and Rubanivka deposits (see the figure), which The rocks with open porosity ratio of 7 to 15 are the most has its special characteristics. common. There are very little sandstones with porosity of less than The deposit sedimentation conditions of the lower Sarmatian 7 and more than 20%. sediments in this section are different from the deposit sedimentation

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