Secondary dolomitization of the Domanik black shales as an indicator of gas-water hydrocarbon-containing fluids upward migration in the Tatarstan territory

The share of unconventional oil reserves is high in Russia and Volga-Ural region, including shale oil reservoirs. It contains around 65% of the total volume of hydrocarbon reserves in region, according to various data. Shale hydrocarbons are widely used in Russia. The main part of the Domaniki oil black shale is confined to carbonate-siliceous carbonaceous complexes of the Domanik horizon of the Francian stage of the Upper Devonian system. According to previous studies in the Domanik formation sedimentary complexes of the Volga-Ural oil and gas province, two types of rocks are distinguished - the Domanikites themselves and Domanikoids. Domanikites contain from 5 to 20% organic matter. Both types of carbonaceous rocks have a practically similar lithological composition. The difference between them lies in variations in siliceous component content and number of authigenic minerals. If content of very fine-grained silica was determined by primary sedimentation factors, then authigenic minerals are indicators of secondary lithogenesis processes. In this work, an attempt is made to compare organic matter content with dolomite mineralization, which is an active agent in mineral formation.


Introduction
In recent years, in connection with commercial shale oil production, there has been an increase in interest to high-carbon formations sediments. On the Russia territory, one of these objects is the Domanik black shale (also call Domanikites) deposits of the Russian platform [1,2,3]. From one side such deposits are considered as oil-saturated rocks; from another side are unconventional reservoirs [4,5,6,7]. The main part of the Domanikites is confined to carbonate-siliceous carbonaceous complexes of the Domanik horizon of the Francian stage of the Upper Devonian system. Although their manifestations can be traced even higher in the section, up to the Tournaisian stage of the Lower Carboniferous system. According to previous studies in the Domanik formation sedimentary complexes of the Volga-Ural oil and gas province, two types of rocks are distinguished -the Domanikites themselves and Domanikoids. Domanikites contain from 5 to 20% organic matter. This is indicated in a number of papers [8,9]. Hydrothermal activity in addition to silica served as a source of biophilic elements, which caused a surge in biota vital activity [10]. Mineralized water of the hydrothermal solutions rose upwards and carried various chemical elements, including elements IOP Conf. Series: Earth and Environmental Science 516 (2020) 012002 IOP Publishing doi:10.1088/1755-1315/516/1/012002 2 important for the life of the plankton. This stimulated rapid life activity in the seas and accompanied (after the death of organisms) by the accumulation of large quantities of organic matter in the sediments. At the same time, there was a periodic change in the oxidation-reduction potential. Periodic reducing conditions contributed to the preservation of organic matter. Therefore, the organic matter is preserved in the carbonate-siliceous layers [11,12,13]. As a rule, they occupy territory of uncompensated depressions in the Famennian stage rock formation basin. Domanikoids contain less than 5% organic matter. They are developed both in axial and onboard zones of the Kama-Kinelsky system of depression (KKSD) in the Mendym horizon deposits. Both types of carbonaceous rocks have a practically similar lithological composition. The difference between them lies in variations in siliceous component content and number of authigenic minerals. If content of very fine-grained silica was determined by primary sedimentation factors, then authigenic minerals are indicators of secondary lithogenesis processes. In this work, an attempt is made to compare organic matter content with dolomite mineralization, which is an active agent in mineral formation.

Object
The object of study was core material of several deposits, selected from the Semiluk and Mendym horizons of the Upper Devonian formations from the Volga-Ural region, Russian Federation (figure 1). Tectonically the studied wells are confined to the eastern side of the South Tatar and North Tatar Arches and to the western slope of the Melekess Depression. A feature of the studied object is high facies variability of sediments; a high content of organic substances, including hydrocarbons; high porosity and very low permeability. These rocks are distributed along the eastern part of the Russian platform. Depth of occurrence of such deposits varies from 1630 to 1900 m. The thickness of the sediments usually does not exceed 20-30 m. Current temperatures of rocks do not exceed 30°C. In sections of all the studied wells, an alternation of three main lithological rocks is observed: 1) carbonate-siliceous rocks enriched with organic matter, 2) limestones (mudstone, wackstone) and 3) clastic (lithoclast or rudstone) limestones composed of carbonate fragments cemented by carbonatesiliceous material, enriched with organic matter.
Carbonate-siliceous rocks enriched with organic matter are dense, cryptomerous domanikites and domanikoids of dark gray and black color. Color variations are determined by organic matter content. Limestones macroscopically have cryptomerous structure. Under the microscope rock structure is usually pelitomorphic (mudstone). Less often, a small amount of organic residues, bioclasts (wackstones), is found in limestones. Usually found in the form of small in thickness (from a few millimeters to the first tens of centimeters) layers alternating with carbonate-siliceous rocks. The rocks are light gray, dense, usually with massive texture. Virtually do not contain of organic matter.
Carbonate breccia (lithoclast limestones). A diagnostic sign is their fragmental structure. They can be found in various areas of sections. The power of such formations is up to tens of centimeters. They are rocks composed of weakly rounded fragments of light gray limestone, cemented with dark gray carbonate-silicon-carbonaceous material.

Methodology
Considering the cryptomerous structure of rocks, the main research methods were optical-microscopic, X-ray and thermal analysis methods. Optical microscopic analysis was carried out on a CarlZeiss Axio Imeger A2 polarizing microscope equipped with high-resolution camera. X-ray analysis was carried out on a Bruker D2 Phaser diffractometer. Thermal analysis of the rocks was carried out on a NETZSCH STA 449 F3 Jupiter instrument, which made it possible to record both loss mass of rock and manifestation of exo-and endo effects in thermal transformations process of organo-mineral components. The main research methods also include SEM analysis performed on an XL-30ESEM electron microscope (FEI, Philips, Netherlands) equipped with an EDAX energy dispersive spectrometer (USA). As additional methods, X-ray fluorescence analysis, analys of fluid inclusions in dolomite and homogenization temperatures, petrophysical properties of rocks, study of organic matter by Rock-Eval and geochemical methods were used.

Optical microscopy data
According to optical microscopy, authigenic dolomites are found both in the form of isolated, nontouching crystals, and in the form of dense aggregate clusters. In intergrowths, rhombohedral crystals of dolomites come into contact mainly with faces. Dark brown clumps of condensed organic matter are observed between the grains. At some sites very fine-grained dolomite completely replaced calcite cement of the rock and performing the function of cementing material. Most authigenic dolomite is associated with black puffs and lenses enriched by organic matter. Figure 2a shows a fragment of a stylolite seam, the cavity of which is intensively pigmented with organic matter. Secondary dolomitization in form of rhombohedral crystals forming fine-grained aggregates is observed in the stylolite seam cavity.
The dolomite grains size in the studied sediments varies widely from 0.01 to 0.4 mm. Grains usually have an isometric, less often irregular shape. Among them, pronounced rhombohedra stand out, forming isolated individuals or small clusters. Cleavage cracks, relicts of primary pelitomorphic calcite, claybituminous matter are noted in the largest dolomite grains. In association with dolomites, pyrite framboids up to 0.15 mm in size and small grains of authigenic calcite are noted. Quartz, feldspar, and rare muscovite flakes act as accessory minerals. Both in polarized light.

Electron microscopic data
Electron microscopic studies confirmed the relationship between authigenic dolomites with areas enriched by organic matter (figure 3a). Dolomite is found in the form of hipidiomorphic crystals of rhombohedral habitus ranging in size from 0.05 to 0.15 mm. The crystals contacts with the host rock are sharp, even, often underlined by kerogen rim. In domanikites, dolomitic newly formed structure are at different stages of formation. Together there are perfect crystals with well-developed edges and faces, and partially crystallinity types. In individual samples of domanikites, dolomite crystals form irrational intergrowths. Relatively perfect (even) faces of dolomite crystals are complicated by growth microdefects in the form of unfilled holes. Apparently, holes on the authigenic dolomites faces formed due to local blocking of crystal growth by kerogen films. The spaces between crystals are filled with organic matter. According to microprobe analysis dolomites are nonstoichiometric. In crystal lattice of the mineral, calcium ions prevail over magnesium ions. The constant presence of iron lines in the spectra indicates that lack of magnesium ions in dolomite is partially compensated by Fe2 + ions. Dolomite grains which metasomatic replacing limestones are more stoichiometric and do not contain iron ions in the crystal structure.

Thermal research methods data
Some of the domanikites samples folded by dolomites in association with coarse-grained calcite were investigated by the thermobaric geochemistry method to determine homogenization temperature of gasliquid inclusions. The data obtained show that the average temperature of inclusions homogenization which found in secondary calcites is 117°C, the minimum is 97°C, and the maximum is 137°C. Given location proximity of syngenetic dolomites with them, it can be assumed that the dolomitization process occurred at similar temperatures.
The organic component of the samples was identified by thermal analysis of exothermic effects in temperature range up to 600°C (DSC curve), and its quantitative content was estimated by the TG curve. The first exothermic effect and associated mass loss in temperature range 110-375°С (figure 3b) corresponds to thermal decomposition of light hydrocarbon fraction. A similar effect in range of 375-490°C corresponds to the destruction of the heavy hydrocarbons fraction.
Thermal decomposition data of organic matter, it was found that the maximum amount of Corg is associated with carbonate-siliceous rocks (5-15%). Moreover, the more silica in them -the more Corg. According to the content of Corg, these rocks correspond to the definition of domanikites. In carbonate rocks, the Corg content does not exceed 5%, i.e. these are typical domanikoids. The lowest content of Corg is noted in carbonate breccias (less than 1%). Here, organic matter is concentrated in carbonate-clay cement of rocks, which binding lime fragments. An analysis of organic matter fractions ratio in the Domanik horizon rocks of showed that, despite their lithotype, heavy hydrocarbons prevail in association with medium hydrocarbons. Light fractions hydrocarbons are either absent or in small quantities. Moreover, in all cases, there is a tendency to an increase of light hydrocarbons in dolomitization areas. In some wells, where secondary dolomites form small interlayers, dark brown oil stainings are noted in cavities and pores spaces of dolomite metasomatites. Thus, there is a clear relationship between light hydrocarbons of oil series and dolomitization processes ( figure 4 a, b).

Secondary dolomitization characteristic
Taking into account structural features of secondary dolomites, the homogenization temperature of gasliquid inclusions in calcites accompanying them, and paragenesis with light hydrocarbons, dolomitization processes can be considered as indicators of oil-water fluids migration. During fluid