RU2811963C1 - Method for conducting geological exploration to identify new oil and gas fields and determine their boundaries in ancient oil and gas basins - Google Patents

Abstract

FIELD: petroleum geological research.

SUBSTANCE: used in the search for oil and gas fields in ancient oil and gas basins, where hydrocarbon deposits were reformed over time. Within the framework of the proposed method, based on the results of a study of core samples taken from wells inside and outside the zone previously identified by comparing the paleostructural and modern plan of the zone and oil and gas accumulation, the boundaries of a new field in the ancient oil and gas basin are established, and further studies of the core and AVO analysis of 3D seismic exploration determine the areas for spot production drilling inside the field, forming a contour of oil and gas potential.

EFFECT: increase in the accuracy of determining zones of distribution of secondary minerals, the presence of which characterizes possible zones of distribution of hydrocarbons in the study area and helps to increase the reliability of identifying new oil and gas fields and determining their boundaries in ancient oil and gas basins.

1 cl, 8 dwg

Description

Technical field

The claimed invention relates to the field of petroleum geological research and can be used in the search for oil and gas fields in ancient oil and gas basins, where hydrocarbon deposits were reformed over time.

State of the art

The difficulty of discovering oil and gas fields in ancient basins lies in the need to search for reformed non-anticlinal deposits, where the fluid seals are secondary transformed reservoir rocks, and not the lithological replacement of reservoir rocks with clayey varieties. In this regard, the classical approach to the discovery of anticlinal and lithologically screened fields, based on lithologic-facies analysis of the distribution of productive deposits, is uninformative or may lead to incorrect results. In addition, at present, the vector for the discovery of new hydrocarbon deposits is shifting to the east (Eastern Siberia, the Far East) and, accordingly, new technologies are needed for discovering deposits, forecasting the distribution of productive strata over the area and selecting the most effective development systems, taking into account new ideas about the geological structure deposits in ancient basins.

The following publications are known from the prior art, revealing various methods for conducting geological exploration, ensuring the identification of new oil and gas fields.

Thus, from the publication “Implementation of the paleoreconstruction method in substantiating geological models” http://www.oilnews.ru/17-17/realizaciya-metoda-paleorekonstrukcij-pri-obosnovanii-geologicheskix-modelej/ a method of conducting geological exploration based on restoration is known paleo-relief using the “benchmark from above” reconstruction method, which is located vertically no higher than 80-100 m from the surface under study. The main objective of this method is to reconstruct the paleo-relief in a local area for the further construction of facies maps and clarification of reserves. In accordance with this method, the paleorelief was restored based on 3D seismic survey and well data, which made it possible to restore elevated zones (paleoislands), within which they were eroded under the influence of alongshore currents and wave activity, which led to the redeposition of predominantly sandy varieties in the submerged parts of the paleorelief (paleodepressions or paleochannels). It is these sandstones that are associated with the productive horizons of the study area. Thus, such an approach disclosed in this source is valid for already discovered and developing fields in order to identify the distribution of productive deposits over an area and drill new production wells, and is difficult to adapt for forecasting and discovery of new fields.

In the publication “Paleotectonic reconstructions and forecast of oil and gas potential prospects of basal deposits of the sedimentary cover of the central part of the Nepa-Botuobisk anteclise” B.A. Lysov, D.D. Popov, 2011, a method for conducting geological exploration works is disclosed, based on a joint analysis of the thickness of promising oil and gas complexes based on paleoanalysis and modern structural plan in order to predict the oil and gas content of basal Vendian terrigenous deposits. The article provides a forecast of the oil and gas potential of various tectonic zones of the Nepa-Botuobisk anteclise. Based on the results of comparing the paleotectonic conditions of the formation of the interval between the top of the Vendian Vanavara Formation (M2) and the roof of the basement, a forecast was made for the distribution and thickness of basal Vendian terrigenous deposits. Thus, by comparing the distribution of basal deposits with the modern structural plan, a forecast was made for the oil and gas potential of Vendian deposits in the central part of the Nepa-Botuoba anteclise.

The known method is characterized by a limited set of studies and does not allow obtaining complete data on the boundaries of the deposit and its volumes.

The RF patent for invention No. 2148166 discloses a “method for developing hydrocarbon deposits”, in which paleotectonic reconstructions are used to identify zones with the greatest fracturing, determine the age of disjunctive faults and establish areas of neotectonic movements in order to optimize well placement and increase the efficiency of field development.

The use of paleotectonic methods is acceptable for the assigned tasks, however, the connection between zones of fracturing and decompression of rocks with the obligatory presence of oil and gas fields has not been reliably established, which reduces the reliability of this method in terms of forecasting. In addition, the method does not provide determination of the boundaries of the deposit, as well as its volumes.

The closest in technical essence to the claimed invention is the method disclosed in the publication “Reformation of deposits in ancient oil and gas basins (using the example of deposits on the eastern slope of the Baikit anteclise of the Siberian Platform)”, A.V. Stupakova, I.I. Khvedchuk, R.S. Sautkin, N.I. Korobova, E.D. Sivkov, Moscow State University named after M.V. Lomonosov, Moscow, Russia, PJSC Krasnoyarskgazprom, Moscow, Russia. This paper examines the stages of formation of modern deposits within the southwestern part of the Leno-Tunguska oil and gas basin. The main oil and gas source strata capable of generating hydrocarbons are described, the structure of a terrigenous reservoir is considered, and a model for the formation of deposits is developed. Secondary transformations of the reservoir were revealed in the zone of development of ancient water-oil contacts (paleo-OWC), completely sealing the void space. The presence of several paleo-OWCs is evidence of multi-stage reformation of deposits. After a change in the structural plan in Paleozoic-Mesozoic times, these zones perform the function (role) of lithological fluid seals. Accordingly, most of the generated hydrocarbons were distributed along the migration path within the slopes of modern large structures and in reformed non-anticlinal traps. Thus, a comparison was made of the paleoplan, within which there could be hydrocarbon (HC) deposits, and the modern structural plan, as a result of which new oil and gas deposits were identified in the most hypsometrically high intervals.

However, this method does not use the analysis of previously identified generation centers, not all promising horizons have been interpreted with subsequent paleostructural constructions, and a comparison of the modern structural plan with the location and size of the ancient trap is not carried out, which does not allow determining the boundaries of the paleo-OWC limiting the distribution of hydrocarbons in the predicted new field.

Thus, the technical problem solved by means of the claimed invention lies in the need to overcome the disadvantages inherent in the above analogues and the prototype by creating a reliable method for conducting geological exploration to identify new oil and gas fields and determine their boundaries in ancient oil and gas basins. The proposed method makes it possible to establish the distribution of hydrocarbons over an area and determine locations for drilling prospecting, appraisal, exploration and production wells.

Brief disclosure of the invention

The technical result achieved by using the claimed invention is to increase the accuracy of determining zones of distribution of secondary minerals, the presence of which characterizes possible zones of distribution of hydrocarbons throughout the study area and helps to increase the reliability of identifying new oil and gas fields and determining their boundaries in ancient oil and gas basins.

The inventive method is based on determining a set of parameters of oil and gas basins, which makes it possible to determine the presence of new oil and gas deposits in ancient oil and gas basins. Based on the results of the research, new patterns were identified in the formation of oil and gas basins, which were used in the development of the claimed method aimed at carrying out geological exploration with the aim of discovering new fields.

The claimed technical result is achieved as a result of the implementation of a method for conducting geological exploration to identify new oil and gas fields and determine their boundaries in ancient oil and gas basins, including the sequential implementation of the following stages:

- formation of a regional composite seismic geological profile passing through previously identified generation centers, migration paths and zones of hydrocarbon (HC) accumulation with reference to reference wells and a geological map of the study area,

- carrying out a structural-kinematic interpretation of seismic horizons along the generated regional composite seismic geological profile with obtaining a modern structural plan of the basin,

- carrying out paleostructural constructions with leveling for each interpreted horizon and identifying changes in the structural plan of the basin over time,

- identification of the oil and gas accumulation zone according to the time of the beginning of the generation of hydrocarbon sources and the type of ancient trap formed by that moment, its size and location on the regional composite seismic geological profile,

- comparison of the modern structural plan with the location and size of the ancient trap to determine the paleo-WOC boundary, limiting the distribution of hydrocarbons and characterized by the absence of effective porosity as a result of secondary transformations at the hydrocarbon-water contact.

- drilling at least one well in a designated oil and gas accumulation zone and one well outside it to establish the paleo-OWC boundary, followed by carrying out a GIS (geophysical survey of wells) complex for each well, including acoustic logging and gamma-gamma density logging,

- sampling of core from drilled wells, followed by lithological studies of the core with identification of secondary minerals, determination of porosity and permeability, while visual examination of thin sections of the core in a petrographic microscope with crossed and parallel nicols determines lithological screens, which are the boundaries of zones of ancient oil-water contacts ( paleo-OWC) in the form of altered impermeable reservoir rocks, characterized by the presence of secondary minerals formed mainly at the boundary of water-oil contacts, as well as low, less than 5%, porosity and low, less than 0.1 mD, permeability,

- determination by GIS, including acoustic logging (AK) and gamma-gamma density logging (GGD-p), and the results of lithological studies

a) fluid-saturated intervals;

b) effective porosity;

c) dense impermeable layers,

d) acoustic properties of rocks in different zones,

- adjustment of acoustic logging (AK) and gamma-gamma density logging (GGD-p) using synthetic curves to eliminate the influence of changes in well diameter associated with the presence of cavities or gas-saturated intervals;

- carrying out AVO analysis to determine fluid saturation over an area with determining the ratio (Vp/Vs) over an area, while recording the change in the Vp/Vs ratio, where when Vp/Vs is less than 1.6, the gas saturation zone is determined, when Vp/Vs is more than 1 .9 determine the zone of water saturation, and with Vp/Vs – from 1.6 to 1.9 determine the transition zone of oil and gas saturation;

- constructing a map of the distribution of productive areas over the field area using AVO analysis and taking into account paleostructural constructions, a modern structural plan, lithological description of the core, GIS interpretation and acoustic properties of the ratio of the interval times of travel of the longitudinal (Vp) and transverse (Vs) waves Vp/Vs.

The Vp/Vs ratio is the inverse ratio of the longitudinal (∆DTP) and transverse (∆DTS) wave travel times. According to the results of core measurements, for gas-saturated reservoir rocks the ratio Vp/Vs is less than 1.6, for water-saturated reservoir rocks Vp/Vs is more than 1.9, and in the transition zone, where reservoir rocks have mixed fluid saturation (water 30-50% , the rest is hydrocarbons) Vp/Vs varies from 1.6 to 1.9, which characterizes the transition zone of oil and gas saturation.

Thus, within the framework of the proposed method, based on the results of studying cores taken from wells inside and outside the previously identified by comparing the paleostructural and modern plan of the zone and oil and gas accumulation, the boundaries of a new field in the ancient oil and gas basin are established, and further studies of the core and AVO analysis are 3D. Seismic surveys identify areas for spot development drilling inside the field, forming a contour of oil and gas potential.

The inventive method characterizes the sequence of actions necessary to achieve a technical result, and if the result is negative at the appropriate stage, further research becomes impractical and unpromising. The use of the invention will allow subsoil users to delineate areas containing hydrocarbons within ancient oil and gas basins and establish zones within them for drilling production wells. In the further description of the invention, the following definitions and terms are used, which are generally accepted in the field of technology of the claimed invention:

Ancient oil and gas basins are sedimentary basins with proven industrial oil and gas potential, having predominantly a Riphean-Paleozoic sedimentary cover and located on an ancient crystalline foundation of Archean-Proterozoic age (Geology of oil and gas: Textbook for universities - M., Nedra, 1990. E.A. Bakirov and etc.).

A modern structural plan is a graphic representation on a certain scale of the distribution over area or section of absolute elevations of any structural surface, presented in the form of a map or profile (Dictionary of Oil and Gas Geology. - L, Nedra, 1988).

Structural-kinematic interpretation is a method that solves a group of problems related to the study of the shape and location in space of various geological bodies by tracing the in-phase axis of reflected waves - coeval reflecting horizons (RG) on seismograms, constructing a depth-velocity model and identifying tectonic faults. (Geophysics in the study of the earth's interior: Textbook. - M., Russian State University of Oil and Gas named after I.M. Gubkin, 2015. Voskresensky Yu.N., Ryzhkov V.I.).

Paleostructural constructions are a group of structural-tectonic methods, including the construction and subsequent analysis of paleostructural maps and profiles associated with the reconstruction of the history of the geological development of the territory and making it possible to accurately establish the time when the formation of the structural plan of the trap began. (Structural geology: Electronic application, M., Nedra, 2014. Miloserdova L.V.).

A trap is a part of a natural reservoir capable of retaining accumulations of hydrocarbons due to favorable geological conditions of the combination of reservoir rock and sealing fluid. (Geology of oil and gas: Textbook for universities - M., Nedra, 1990. E.A. Bakirov and others).

An oil and/or gas deposit is an accumulation of oil and/or gas in a trap, all parts of which are hydrodynamically connected (B.A. Sokolov et al., 2000).

Inversion of the structural plan is a change in the shape of the structural plan over time under the influence of geological events, when swells and uplifts are formed in place of depressions. (according to V.V. Belousov. General geotectonics. Moscow; Leningrad: publishing house and type. Kart. f-ki Gosgeolizdat, 1948, 600 pp.; Inversion tectonics. Edited by Cooper, M.A. and Williams, GD 1989 Special Publication of the Geological Society of London, 44).

Composite seismic geological profile is a profile consisting of interconnected 2D seismic geological profiles, intersecting each other, having different physical and geographical locations and/or created at different times. (Geophysics in the study of the earth's interior: Textbook. - M., Russian State University of Oil and Gas named after I.M. Gubkin, 2015. Voskresensky Yu.N., Ryzhkov V.I.).

Geological time is a period of time during which a sequence of various geological events occurred that led to the formation of layers of rocks (Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978).

Paleo-OWC is an ancient oil-water contact, showing the stabilization of the reservoir at a certain point in geological time and indicating that the reservoir has currently changed (decreased due to destruction or increased due to the arrival of a new volume of hydrocarbons). (according to R.S. Sakhibgareev, Secondary changes in reservoirs in the process of formation and destruction of oil deposits. Leningrad: “Nedra” Leningrad branch, 1989, 260 pp.)

GWC - gas-water contact - a conditionally distinguished surface that separates the zone of complete gas saturation in a gas reservoir from the transition zone, in which gas saturation gradually decreases to zero with depth. (Dictionary of geology of oil and gas. - L, Nedra, 1988).

Brief description of drawings

The claimed invention is illustrated by the following drawings, where

in fig. Figure 1A shows a fragment of a tectonic map of the southern part of the Kurei syneclise (V.S. Staroseltsev et al. 2016) with regional seismic profiles and a composite profile I-I’, laid out in plan.

Fig. 1B shows a regional composite seismic profile I-I’ with reference to wells Nizhnemadashenskaya 138, Pridutskaya 2, Talakanskaya 812,

Fig. 1B shows the results of interpretation of the composite profile I-I’ in the form of a geological section, characterizing the modern structure of the territory of the southern part of the Kurey syneclise and the adjacent territory,

Figure 2 shows paleo-reconstructions of Vendian deposits for the Late Cambrian,

in fig. 3. a photo of a thin section is presented, in which the void space of the reservoir is completely healed with secondary authigenic minerals (quartz) in the paleo-OWC zone (A) and a photo of a thin section with a bitumen film, indicating the existence of an oil deposit (B),

in fig. Figure 4 presents a model of the formation of fields in ancient oil and gas basins that have undergone inversion of the structural plan using the example of the Pridutsky license area with an illustration of the structural rearrangements of the basin,

in fig. Figure 5 presents a diagram of the correlation of productive intervals based on the results of a comprehensive interpretation of geophysical well survey data (GIS) highlighting the filtration-capacitive properties of reservoir rocks within the paleo-trap for wells, both in the zones that gave an influx of hydrocarbons (Pridutskaya 2) and in unproductive zones (Pridutskaya 3),

in fig. Figure 6 shows a component model of sediments and corrections with wave interval time data adjustments made on its basis,

in fig. Figure 7 shows the results of identifying promising areas by combining GIS, petrophysical and seismic methods,

in fig. 8, promising zones associated with paleotraps are highlighted, ranked by geological reserves in billion m ³ .

Positions in the drawings indicate:

1 – roof of the Riphean erosion surface (Ro)

2 – roof of the Vanavara Formation Vendian (M2)

3 – roof of the Teter Formation Cambrian (B)

4 – dolomite layer in the middle part of the Usolskaya Formation of the Cambrian (U)

5 – roof of the Nizhnebelsky formation of the Cambrian (K2)

6 – roof of the Verkhnebelsky formation of the Cambrian (K1)

7 – roof of the Bulay Formation of the Cambrian (H4)

8 – roof of the Evenki Formation of the Cambrian (E3ev)

9 – Ordovician roof (O)

10 – Silurian roof (S)

11 – boundary of angular unconformity

12 – saturation with gas, gas condensate

13 – saturation with oil

14 – oil rim

15 – water

16 – modern VNK

17 – paleo-VNK

18 – dense zone of silicification

19 – impermeable zone, secondary fluid seal

20 – modern gas-water contact (GWC)

21 – gas-saturated reservoir according to GIS ratio Vp/Vs < 1.6

22 – oil and gas saturation (transition zone), ratio 1.6 ≤Vp/Vs ≤ 1.9

23 – zone of water saturation or fluid seals, Vp/Vs ratio > 1.9

24 – geological reserves of 6.57 billion ^m3

25 - geological reserves of 3.94 billion m ³

26 – geological reserves of 3.42 billion ^m3

27 – geological reserves of 5.11 billion ^m3

28 – geological reserves of 1.4 billion ^m3

29 – geological reserves of 22.19 billion m ³

Carrying out the invention

A further description of the invention is given using the example of a specific implementation of the oil and gas potential forecast for the Leno-Tunguska oil and gas basin, located on the ancient Siberian platform, which has a Proterozoic-Phanerozoic sedimentary cover and has undergone numerous structural rearrangements into the Paleozoic, Mesozoic and Cenozoic erathems. The inventive method has been tested in licensed areas and open fields in Eastern Siberia. As an example, the implementation of the claimed method in the south of the Kurei syneclise is presented; the presence of new oil and gas fields in ancient oil and gas basins was established; based on the forecast obtained, new oil and gas fields were discovered in the study area. The main oil and gas content within the study area is associated with terrigenous rocks of Vendian age.

At the first stage, to establish the possibility of the arrival of hydrocarbons within the study area, a seismic geological profile is built, passing through previously identified or suspected sources of generation (the deepest troughs - the Kurey syneclise and the Pre-Patom trough), migration paths (slopes and paleoslopes) and zones of hydrocarbon accumulation (uplifts and anticlines – Baikit anteclise and Nepa-Botuoba anteclise), which are displayed on structural-tectonic maps (Fig. 1.A). To implement this step, use a structural-tectonic map of the research basin from open sources and/or published literature (for example: https://vsegei.ru/ru/activity/intcooperation/tect_map35M/map.php; https://mygeomap.ru/ ). A seismic geological study diagram is applied to the structural-tectonic map (https://rfgf.ru/info-resursy/onlajn-resursy; https://mygeomap.ru/) and seismic geological profiles passing through deep depressions and troughs (foci of hydrocarbon generation) are selected ), slopes (hydrocarbon migration routes), uplifts and anticlines (hydrocarbon accumulation zones) indicated on the structural-tectonic map.

After which, in an interpretive software package (for example, RN-GEOSIM, https://rn.digital/rngeosim/?ysclid=lisntjkb3867364932 or its analogues) from available seismic geological profiles (taken from Rosgeolfond (https://rfgf.ru) or subsoil user archives) create a regional composite 2D seismic geological profile, consisting of interconnected parts of regional and local profiles. The created seismic-geological profile is linked with reference wells (which opened the section and have stratigraphic division) to link reflecting horizons, subsequent structural-kinematic interpretation and paleostructural reconstructions along the entire seismic-geological profile (Fig. 1. B).

Next, a structural-kinematic interpretation of seismic horizons that have a regional distribution and precise age dating is carried out: Ro (1) – the top of the Riphean erosion surface; M2 (2) – roof of the Vanavara Formation Vendian; B(3) – top of the Teter Formation Cambrian; U (4) – dolomite layer in the middle part of the Usolskaya Formation of the Cambrian; K2 (5) – the top of the Nizhnebelskaya Formation of the Cambrian; K1 (6) – the top of the Upper Belsky Formation of the Cambrian; H4 (7) - the top of the Bulay Formation of the Cambrian, E3ev (8) - the top of the Evenki Formation of the Cambrian, O (9) - the top of the Ordovician, S (10) - the top of the Silurian, etc.) according to the regional composite seismic geological profile in order to obtain a modern structural plan pool (Fig. 1. B). As a result, it is ensured -

a) stratigraphic reference of reflections,

b) correlation of reflecting horizons using seismic time sections,

c) selection of target objects for interpretation and associated reflecting horizons,

d) creation of a depth-velocity model,

e) construction of modern structural maps and profiles.

Based on the results of structural-kinematic interpretation, paleostructural constructions are carried out by leveling the overlying reflecting horizons (for example, in interpretation software (RN-GEOSIM (https://rn.digital/rngeosim/?ysclid=lisntjkb3867364932) or using its analogues) or manually) in a horizontal line and determine changes in the structural plan of underlying horizons over time. Analysis of paleostructural constructions allows us to select the time interval in which, at the time of accumulation, for each source of oil and gas formation, a structure capable of capturing hydrocarbons was formed, its geometry and type of trap were determined. Analysis of the geometry and type of trap is carried out visually.

In the described example, paleoreconstructions were carried out for the Vanavara formation (Vendian), since regionally these deposits are oil and gas promising and have good filtration and reservoir properties with a porosity of 13-18% and a permeability of about 20 mD (Melnikov N.V. (1977). Geology and oil and gas potential of the Leno-Tunguska province. M: Nedra, 205 pp.).

Paleoreconstructions of the Vanavara Formation (Vendian) were carried out for Cambrian reflectors. Based on the results of paleostructural analysis, an anticlinal paleostructure with a size of 160x80 km and a height of more than 200 m was determined (Fig. 2).

A fragment of a regional seismic geological profile (Fig. 2) shows the formation of an anticlinal structure in the area of the Pridutskaya 3 – Pridutskaya 2 – Pridutskaya 1 wells from the Vendian to the Cambrian, with an uplift amplitude of up to 200 m, which in the modern structural plan has a slope (Fig. 1B, Fig. .4). Paleoreconstructions for overlying horizons were not carried out due to unclear interpretation or partial absence of these horizons due to erosion, which did not have a fundamental impact on the overall results. Since in neighboring areas the overlying deposits of the Ordovician and Silurian periods have a uniform thickness of sedimentation, the paleostructural plan remained unchanged, and accordingly the shape and size of the Cambrian paleodeposit did not change at that time. Major changes in the structural plan occurred in the Devonian and Mesozoic-Cenozoic times, as evidenced by the sharp angular unconformity between the Carboniferous and underlying sediments (Fig. 1B, position (11)). It was at this time that the modern structural plan was formed.

Next, a comparison of the modern structural plan with the location and size of the ancient trap is carried out to determine the boundary of the zone of ancient oil-water contacts (paleo-OWC zone). Paleo-WOC zones (position 17) limit the distribution of hydrocarbons and are characterized by the absence of effective porosity as a result of secondary transformations at the contact of hydrocarbons with water (Fig. 3, 4). This determines the size of the ancient paleo-deposit, which in the example is about 120 x 50 km, with a height of up to 150 m, the contours of oil and gas potential and the predicted areas of paleo-OWC (Fig. 2, 4).

On the modern structural plan, areas for further drilling of prospecting, appraisal and exploration wells are determined. Such areas are the most hypsometrically high areas of the modern structural plan within the ancient deposit. Areas outside the ancient deposits are unpromising (Fig. 4).

To determine the filtration and reservoir properties of reservoir rocks, at least one prospecting or exploration well is drilled into the paleo-OWC zone and onto the slope of a modern trap in accordance with GOST R 53709-2009.

For drilled wells the following is carried out:

1) GIS complex (AK, GK, PS, SP, BK, GGK-p, IK) (https://studfile.net/preview/4257007/page:4/);

2) continuous core sampling in a prospective interval - (in this case, such an interval is the Vanvar Formation (Vendian)) to determine the filtration and reservoir properties of the formation (every 20 cm according to the recommendations of the State Reserves Committee (Methodological recommendations for calculating oil and gas reserves by the volumetric method. B .I. Petersilye, V.I. Poroskun, G.G Yatsenko - Moscow-Tver: VNIGNI, NPC Tvergeofizika, 2003)), the degree of fluid saturation and the presence of secondary transformations.

A standard set of petrophysical and lithological studies is carried out on the selected core to determine filtration and reservoir properties, secondary processes of mineral formation and leaching of reservoir rocks, highlighting the levels of lack of porosity, characterized by complete healing of the void space with authigenic minerals (Fig. 3).

During lithological examination (visual examination of thin sections in a petrographic microscope with crossed and parallel nicols), special attention is paid to:

- leaching of grains of reservoir rocks down to small caverns, which increases the porosity and permeability of the host strata. Thus, according to lithological studies, intervals with improved reservoir properties and containing hydrocarbons are identified;

- the presence of secondary quartz minerals, which are formed mainly at the boundary of water-oil contacts and completely heal the void space, as a result of which the reservoir rocks become completely impermeable (porosity less than 5%, permeability less than 0.1 mD). In these zones, impermeable lithological screens (secondary lithological fluid seals for oil and gas) are formed, which makes it possible to preserve the hydrocarbon deposit when the structural plan of the trap changes.

In the described example, as a result of petrophysical and lithological studies of the core, it was established that

in the Pridutskaya 2 well, located within the deposit, the values of open porosity from the core as a result of the studies were 15-17% (Fig. 3B). And in the Pridutskaya 3 well, located in the zone of hydrocarbon decomposition and having signs of authigenesis with the development of secondary processes, the open porosity was less than 5% (Fig. 3A). This figure shows that on petrographic thin sections painted with colored resin, pores and voids with open porosity glow blue; in the paleo-OWC zones there is no glow, since the entire space is completely filled with authigenic minerals. Isotopic studies of minerals confirm secondary processes, showing that the age of minerals in the paleo-OWC zone is significantly younger than in the rest of the formation. Thus, according to isotope studies of the core, it was established that lighter isotopes of oxygen (δ ¹⁸ O‰) and carbon (δ ¹³ C‰) are formed in the paleo-OWC zone, which indicates a younger age of authigenic minerals in the paleo-OWC zone compared to with host rocks (N.I. Korobova, N.S. Shevchuk, E.E. Karnyushina, R.S. Sautkin, E.A. Krasnova (2023) Features of the composition and structure of productive deposits of the Vikulovsky formation of the Krasnoleninsky arch and their influence on filtration-capacitive properties. Georesursy 25 (2), pp. 103-119). Based on the results of these studies, it was concluded that secondary impermeable zones are lithological screens for hydrocarbons during the reformation of oil and gas fields.

Using a complex of well logging systems based on petrophysical dependencies and lithological studies, correlation schemes are built with alignment of wells in depth, plotting the results of fluid saturation (Fig. 5 (12,14)) and determined in accordance with the recommendations of the State Reserves Committee (Methodological recommendations for calculating oil and gas reserves by volumetric method. B .I. Petersilye, V.I. Poroskun, G.G Yatsenko - Moscow-Tver: VNIGNI, NPC Tvergeofizika, 2003) the following parameters (numerical values are given for the described example): fluid-saturated intervals - gas, water, boundary porosity - 16% , dense impermeable interlayers - values on the GGC density curve are more than 2.7 g / cm ³ , acoustic properties of rocks in different zones of the paleo-trap - interval time of travel of a longitudinal wave (160-360 μs/m), interval time of travel of a transverse wave (350-600 μs/m).

When working with logging curves and core, the core is linked to the GIS and the logging curves are linked to each other (M.N. Bolshakov, N.A. Skibitskaya, O.O. Marutyan. Linking core data to GIS materials is an important stage in building a digital field models. Current problems of oil and gas. Issue 3(18) 2017. P.1-8). As a rule, gamma ray logging, which has the smallest measurement error and the highest accuracy of depth measurements, is used as a reference curve.

It is important to note that GIS methods that allow determining the acoustic and density properties of rocks - acoustic logging (AK) and gamma-gamma density logging (GD-L) - have a small penetration zone, therefore, changes in the diameter of wells associated with the presence of caverns or gas-saturated intervals has a distorting effect on their readings and, as a consequence, incorrect interpretation and incorrect forecast of oil and gas content.

To eliminate this error, the logging is adjusted using synthetic curves.

To correct AC, a synthetic curve is used that takes into account the travel time of a longitudinal wave in clean clays and in intervals of a gas-saturated reservoir, depending on the depth. To correct acoustic logging in caverns, a synthetic curve (DTCO-COR, Fig. 6) was used, obtained using the Gardner-Castagna equation (Gary Mavko, Tapan Mukerji, Jack Dvorkin. The Rock Physics Handbook: Tools for Seismic Analysis of Porous Media. Cambridge University Press, 2003.) For other specific implementations, other synthetic curves may also be used. The adjustment is achieved as a result of constructing relationships between the values of acoustic logging and the readings of deeper probes, which are not affected by an increase in the diameter of the well in cavern intervals. In the example, resistivity logging data was used for correction - BC (Fig. 6). The adjustment made it possible to interpret the GIS complex and carry out a quantitative comparison of acoustic data when compared with seismic exploration MOGT-3D (https://neftegaz.ru/tech-library/ngk/148126-mogt-metod-obshchey-glubinnoy-tochki/).

The results of well logging interpretation, after correcting the data, showed the presence of hydrocarbons in the Pridutskaya 2 well and their absence in the Pridutskaya 3 well, which is confirmed by the results of core studies and well testing. So, in the example, when testing the Pridutskaya 2 well, a gas influx with a flow rate of 141 thousand m ³ per day was obtained, and in the Pridutskaya 3 well located hypsometrically higher, when testing the Vanavara formation, only a water influx with a flow rate of 0.9-2.8 m ³ was obtained per day. (Fig.5) It was not possible to obtain inflow from the Pridutskaya 1 well, which indicates the absence of reservoir properties in the Vendian formation interval.

Based on the results of a comparison of the ancient deposit, the modern structural plan and the results of a comprehensive GIS interpretation, an estimated modern zone of oil and gas accumulation is established (Fig. 4A), to clarify the boundaries of which CDP-3D seismic exploration is carried out or available field materials are reprocessed in the area associated with the paleo-deposit within license area (in the example - Pridutsky license area).

At the final stage, using AVO-analysis technology (Voskresensky Yu.N., Ryzhkov V.I. Geophysics in the study of the earth’s interior. Moscow: Publishing center (IC) Russian State University of Oil and Gas named after I.M. Gubkin, 2015. 221 p. .) clarify the boundary of the deposit, previously established based on the results of paleostructural reconstructions, modern structural constructions, core studies and interpretation of the GIS complex. In this case, not all available seismic materials are subjected to processing and subsequent dynamic interpretation, but only those falling into the promising zone, which significantly reduces the time and economic costs of geological exploration.

To confirm the identification of gas-saturated reservoir rocks in the section and area, a cross-plot is built (Fig. 7) and the ratio Vp/Vs is calculated - equal to the inverse ratio of the interval times of travel of the longitudinal (∆DTP) and transverse (∆DTS) waves, recorded by acoustic logging . Based on the results of core measurements in the example (Table 1), for gas-saturated intervals the Vp/Vs ratio is less than 1.6, with Vp/Vs more than 1.9 a water saturation zone is predicted, and with Vp/Vs from 1.6 to 1.9 a transition zone is predicted oil and gas saturation zone. This ratio is necessary to compare zones with different saturations and determine the modern boundary of the deposit (Fig. 7, Fig. 8).

Table 1. Interval time measurement results

longitudinal (∆DTP) and transverse (∆DTS) wave paths

∆DTS µs/m ∆DTP µs/m Vp/Vs Reservoir saturation 378 319 1.18 gas condensate 394 301 1.31 gas condensate 418 305 1.37 gas condensate 468 312 1.50 gas condensate 421 333 1.26 gas condensate 419 354 1.18 gas condensate 460 365 1.26 gas condensate 420 256 1.64 transition zone 440 259 1.69 transition zone 447 260 1.72 transition zone 453 258 1.76 transition zone 497 276 1.80 transition zone 517 278 1.89 transition zone 529 272 1.94 water saturation zone 508 255 1.99 water saturation zone 514 243 2.12 water saturation zone 518 190 2.72 water saturation zone 597 225 2.65 water saturation zone 565 264 2.14 water saturation zone

Based on the results of the work carried out, zones that have preserved hydrocarbons inside the ancient deposit are delineated, and reserves are calculated for them in order to determine the need for further geological exploration work (Fig. 8).

Reserves are calculated by the volumetric method for gas and gas condensate in accordance with GOST R 56676-2015. Design of development and development of gas and gas condensate fields. Calculation of gas and gas condensate reserves using the volumetric method. Basic technical requirements; and for oil according to “Methodological recommendations for calculating oil and gas reserves by volumetric method” by V.I. Petersilye, V.I. Poroskun, G.G Yatsenko - Moscow-Tver: VNIGNI, NPC Tvergeofizika, 2003; where the parameters for calculating reserves are determined based on data from a previously studied core, well logging, the results of testing wells for inflow and laboratory analysis of the resulting fluid. The area of oil and gas accumulation is determined based on the results of AVO analysis technology (Fig. 8). Thus, for the Pridutsky license area, the porosity of reservoir rocks is assumed to be 17%, the effective thickness is 8.3 m, the gas saturation coefficient is 0.7, the average initial reservoir pressure in the reservoir is 25.26 MPa, the area of promising areas varies from 5 to 76 km ² (Fig. . 8). Initial geological gas reserves are (Fig. 8): 12–6.57 billion m ³ ; 13 - 3.94 billion m ³ ; 14 – 3.42 billion ^m3 ; 15 –5.11 billion ^m3 ; 16 –1.4 billion m ³ ; 17–22.19 billion ^m3 .

As a result of determining the distribution zones of modern deposits and assessing reserves, it becomes possible to substantiate lithological screens and hydrodynamic connections of various parts of the field, which is a necessary factor for effective production drilling and selection of a field development system.

Thus, the accuracy of determining the intervals of the presence of oil in a well increases due to the identification of lithological screens inside reservoir rocks, limiting paleo-OWC zones, which have extremely low filtration and reservoir properties (porosity less than 5%, permeability less than 0.1 mD) as a result precipitation of secondary authigenic quartz minerals at the boundary of the ancient oil-water contact, as well as determining the boundaries of the deposit taking into account the Vp/Vs ratio over the area.

By applying this approach to geological exploration, it becomes possible to discover new oil and gas fields with the establishment of their boundaries in ancient oil and gas basins.

Claims (16)

A method of conducting geological exploration to identify new oil and gas fields and determine their boundaries in ancient oil and gas basins, including the sequential implementation of the following stages: - formation of a regional composite seismic geological profile passing through previously identified generation centers, migration paths and zones of hydrocarbon (HC) accumulation with reference to reference wells and a geological map of the study area, - carrying out a structural-kinematic interpretation of seismic horizons along the generated regional composite seismic geological profile with obtaining a modern structural plan of the basin, - carrying out paleostructural constructions with leveling for each interpreted horizon and identifying changes in the structural plan of the basin over time, - identification of the oil and gas accumulation zone according to the time of the beginning of the generation of hydrocarbon sources and the type of ancient trap formed by that moment, its size and location on the regional composite seismic geological profile, - comparison of the modern structural plan with the location and size of the ancient trap to determine the paleo-WOC boundary, limiting the distribution of hydrocarbons and characterized by the absence of effective porosity as a result of secondary transformations at the hydrocarbon-water contact. - drilling at least one well in the identified oil and gas accumulation zone and one well outside it to establish the paleo-OWC boundary, followed by carrying out a GIS complex for each well, including acoustic logging and gamma-gamma density logging, - sampling of core from drilled wells, followed by lithological studies of the core with identification of secondary minerals, determination of porosity and permeability, while visual examination of thin sections of the core in a petrographic microscope with crossed and parallel nicols determines lithological screens, which are the boundaries of zones of ancient oil-water contacts ( paleo-OWC) in the form of altered impermeable reservoir rocks, characterized by the presence of secondary minerals formed mainly at the boundary of water-oil contacts, as well as low, less than 5%, porosity and low, less than 0.1 mD, permeability, - determination by GIS, including acoustic logging (AK) and gamma-gamma density logging (GGD-p), and the results of lithological studies a) fluid-saturated intervals; b) effective porosity; c) dense impermeable layers, d) acoustic properties of rocks in different zones, - adjustment of acoustic logging (AK) and gamma-gamma density logging (GGD-p) using synthetic curves to eliminate the influence of changes in well diameter associated with the presence of cavities or gas-saturated intervals; - carrying out AVO analysis to determine fluid saturation over an area with determining the ratio (Vp/Vs) over an area, while recording the change in the Vp/Vs ratio, where when Vp/Vs is less than 1.6, the gas saturation zone is determined, when Vp/Vs is more than 1, 9 determine the water saturation zone, and when Vp/Vs is from 1.6 to 1.9, determine the transition zone of oil and gas saturation; - constructing a map of the distribution of productive areas over the field area using AVO analysis and taking into account paleostructural constructions, a modern structural plan, lithological description of the core, GIS interpretation and acoustic properties of the ratio of the interval times of travel of the longitudinal (Vp) and transverse (Vs) waves Vp/Vs.