Chemical geothermometry: application to mud volcanic waters of the Caucasus region

Abstract

The generation temperatures of gas-water fluids released from mud volcanoes in different provinces of the Caucasian region have been constrained using Mg/Li (Т_Mg/Li) chemical geothermometry. Mud volcanic fluids in the Taman Peninsula (Kerch-Taman mud volcanic province) were generated at temperatures (Т_Mg/Li) from 41 to 137°С. The depths of the respective mud reservoirs estimated from Т_Mg/Li values and local geothermal gradient are in a range of 1.0 to 3.4 km which spans the Maykop Formation of marine shale. For the South Caspian province, the Т_Mg/Li values of waters vary from 18 to 137°C and the respective root depths Н_Mg/Li of mud volcanoes range from ~ 0.85 to 6.5 km. The obtained T_Mg/Li values for the analyzed mud volcanic waters from Caucasian provinces are in positive correlation with НСО₃⁻ contents and water oxygen isotope compositions ({\delta }_{18}{\mathrm{O}}_{{\mathrm{H}}_2\mathrm{O}} and Δ{\delta }_{18}{\mathrm{O}}_{{\mathrm{H}}_2\mathrm{O}}) and in high negative correlation with Cl⁻. The increase of Т_Mg/Li toward the Greater Caucasus Range, as well as the lateral T_Mg/Li patterns in the Taman and South Caspian mud volcanic provinces, support the idea that mud volcanic fluids generate at temperatures increasing progressively toward the Alpine orogenic belt.

Keywords

mud volcano / fluid / chemical geothermometry / stable isotopes / Caucasus region

Introduction

Mud volcanoes (MVs) are specific landforms created by extrusion and eruption of liquefied mud, saline water and gases (mainly methane and CO₂). They are common to many sedimentary basins in compressional settings, including intermontane basins within young orogenic belts, like the Caucasus, South Caspian Basin, and Mediterranean regions (^{Jakubov et al., 1971}; ^{Dimitrov, 2002}; ^{Kopf, 2002}; ^{Mazzini et al., 2009}; ^{Oppo et al., 2014}; ^{Mazzini and Etiope, 2017}). More than 1500 onshore and offshore mud volcanoes worldwide cluster in mud volcanic provinces and are normally dormant but discharge mud volcanic fluids (MVF) (^{Milkov, 2000}; ^{Dimitrov, 2002}; ^{Kopf et al., 2003}; ^{Kholodov, 2013}; ^{Mazzini and Etiope, 2017}). The sources of the fluids and their generation temperatures, as well as the depths of the feeding mud chambers and roots, remain poorly understood (^{Kholodov, 2002} and ²⁰¹³; ^{Kopf, 2002}; ^{Kopf et al., 2003}; ^{Mazzini et al., 2009}; ^{Etiope, 2015}; ^{Kokh et al., 2017}; ^{Mazzini and Etiope, 2017}).

Mud volcanoes are abundant in the Caucasus segment of the Alpine-Himalayan belt, with at least 400 edifices in the Indol-Kuban Trough (Russia) and the Kura intermontane basin that comprises the Middle (East Georgia) and Lower (Azerbaijan) Kura subbasins. The Lower Kura basin, known for offshore mud volcanism, borders the Caspian Sea. The total sediment thickness in the South Caspian basin exceeds 20–25 km, while the MV roots exceed 8– 9 km (^{Jakubov et al., 1980}; ^{Rakhmanov, 1987}; ^{Guliev et al., 1988}; ^{Nadirov et al., 1997}; ^{Ali-Zade, 2008}; ^{Sobissevitch et al., 2008}). Fluid systems at these depths are hardly accessible for drilling and remain most often uninvestigated. The chemistry of MV fluids has implications for the compositions of sediments and their maturation conditions, as well as for the origin depths and generation mechanisms of fluids. The respective reconstructions can bridge the gaps of knowledge on diagenesis and maturation processes.

The depths of mud reservoirs can be inferred from constrained MV fluid generation temperatures and the known local temperature gradients. Waters released from dormant mud volcanoes have permanently low temperatures of 10°C to 30°C. However, the measured in situ temperatures of emergent MV waters in salsas and gryphones are not the true subsurface water temperatures: the ascending waters cool down under an adiabatic effect of gas expansion or as a result of seasonal and diurnal air temperature variations.

Although the temperatures of emergent mud volcanic waters provide no idea of their initial values at origin depths, these values can be constrained by chemical geothermometry. Calibrated water geothermometers were developed in the 1960-1980-s and used for temperature estimation in geothermal systems and diagenetically altered sediments (^{Bödvarsson, 1961}; ^{Bödvarsson and Pálmason, 1961}; ^{Fournier and Truesdell, 1973}; ^{Fournier, 1977}; ^{Fouillac and Michard, 1981}; ^{Kharaka and Mariner, 1989}). Geothermometry stems from empirical relationships between element concentrations or their ratios in waters and temperatures measured directly in drilled aquifers or reservoirs. Analysis of temperatures measured in subsurface fluids revealed that their warming is coupled with increasing salinity (concentrations of total dissolved solids, TDS). Iceland researchers (^{Bödvarsson, 1961}; ^{Bödvarsson and Pálmason, 1961}) proposed the terms base temperature for water temperature at the respective TDS formation level and base depth for the position of the respective isotherm.

Different chemical geothermometers have been proposed to estimate temperatures in both geothermal systems and more or less diagenetically altered rock reservoirs. The SiO₂, Mg/Li, Na/Li, and Na/K geothermometers are most important for reconstructing sediment maturation temperatures, as well as the temperatures of oil and gas reservoirs at the time of fluid generation (^{Bödvarsson, 1961}; ^{Bödvarsson and Pálmason, 1961}; ^{Fournier and Truesdell, 1973}; ^{Fournier, 1977}; ^{Fouillac and Michard, 1981}; ^{Kharaka and Mariner, 1989}; ^{D’Amore and Arnórsson, 2000}). Soon after coming into use, the geothermometers were found out to frequently yield appreciably different reservoir temperatures owing to the lack of equilibrium between the solution and the dissolved/precipitated minerals, or as a result of reactions, mixing or degassing during fluid up-flow. The Na/Li and Mg/Li thermometers yield more reliable estimates than the SiO₂, K/Na, and K/Na/Ca ones because they are less sensitive to host lithology and better suitable for basinal waters (^{Kharaka and Mariner, 1989}; ^{Lavrushin et al., 2003}). Qualitative interpretations and constraints on fluid-forming processes and source depths can be obtained using isotopic tools, besides other geochemical fingerprints (^{D’Amore and Arnórsson, 2000}). Water isotopic patterns have implications for the depths of diagenetic alteration in the case of smectite–illite transformation accompanied by release of abundant freshened water into the pore space (^{Kopf, 2002}; ^{Lavrushin et al., 2003}; ^{Chelnokov et al., 2018}; ^{Sokol et al., 2019}).

The geochemical-based approaches to MV fluid generation conditions are especially valuable for the areas not covered yet by petroleum exploration drilling. On the other hand, most of the available studies are limited to restricted MV areas, single edifices, and even single discharges. In this respect, the regional-scale patterns which would reveal general trends with a perspective on mud volcanism over vast territories remain unknown. This paper provides the first comparative analysis of geochemistry and reconstruction of MV water temperature patterns in different parts of the Caucasus region extending over a 1500 km long zone from the Kerch Peninsula in the west to the Caspian Sea in the east. The reconstructions of fluid generation temperatures and respective depths are based on a vast collection of monitoring data from 2009 through 2013. The results were partly published earlier (^{Kikvadze et al., 2014}; ^{Lavrushin et al., 2015)}, but the greatest portion of data has never been reported before.

Geological setting

The present tectonic framework of the Caucasian region results from Late Cenozoic folding and thrusting associated with the continental collision of Eurasia with Africa and Arabia (^{Khain, 1982}; ^{Zonenshain and Le Pichon, 1986}; ^{Gamkrelidze and Giorgobiani, 1989}; ^{Philip et al., 1989}; ^{Leonov, 2007}; ^{Mosar et al., 2010}; ^{Kadirov et al., 2015}). The N–S cross section of the region comprises several major units (Fig. 1): the Indol-Kuban and Terek-Caspian foredeeps, the Great Caucasus Range, and the Kura and Rioni intermontane basins between the Great and Lesser Caucasus systems (^{Khain, 1982}; ^{Philip et al., 1989}).

The collisional processes in the southern slope of the Great Caucasus are still active at present. According to GPS data, the area is moving northward at 3–4 mm/yr but the velocity of the Lesser Caucasus and Kura continental blocks is as high as 13 mm/yr (^{Mosar et al., 2010}; ^{Kadirov et al., 2015}). The plate motion induces high seismicity, thrusting, and formation of shear zones in the Caucasus southern slope and along the basin borders.

The sedimentary section of the intermontane and foreland basins begins with Early Jurassic strata and encompasses almost all Mesozoic and Cenozoic stratigraphic units. This section includes the Maykop Formation of Oligocene–Early Miocene shale which often constitute almost half of the sediment thickness. For instance, it occupies 5–6 km out of a 13–14 km section in the central Kura basin (^{Krasnopevtseva et al., 1977}; ^{Chelidze, 1983; Iosseliani and Diasamidze, 1983}; ^{Adamia, 1985}; ^{Radzhabov et al., 1985}; ^{Ali-Zade, 2008}) or 3–5 km out of 8 to 12 km thick sediments in some parts of the Indol-Kuban Trough (^{Jakubov et al., 1980}; ^{Nadirov et al., 1997}; ^{Shnyukov et al., 1986} and ²⁰⁰⁵; ^{Rakhmanov, 1987}). All sedimentary basins of the Caucasus region (except for Rioni) are rich reservoirs and developed pays of oil and gas.

The Caucasus region hosts a great number of active mud volcanoes in mud volcanic provinces that belong to different tectonic units (Fig. 1). The South Caspian province is the world largest areas of mud volcanism with more than 180 onshore edifices (^{Jakubov et al., 1980}; ^{Aliyev et al., 2009}; ^{Feyzullayev, 2012}), and as many offshore volcanoes within the southern Caspian Sea. Most of the edifices belong to the South Caspian sedimentary basin, with the Kura basin as its onshore extension. Mud volcanism in the Kura basin occurs in its Middle and Lower subbasins. The latter accommodates Jurassic and Cretaceous sediments overlain by Cenozoic molasse, 10–12 km thick in total (^{Guliev et al., 2013}), and includes the Fore-Caspian, Apsheron, Shemakha-Gobustan, and Fore-Kura petroleum areas in Azerbaijan (^{Jakubov et al., 1971}). Offshore seismic surveys in the South Caspian Sea allowed tracking feeder channels of mud volcanoes to 8–10 km below the surface, but no onshore data are available (^{Ali-Zade, 2008}; ^{Feyzullayev, 2012}).

The Kakhety province has six MVs in its territory and it is located in Eastern Georgia at the transition from the Kura intermontane basin to the Great Caucasus southern slope, where 13–14 km thick consolidated crust is overlain by sediments, including a 4–6 km thick Mesozoic section (^{Krasnopevtseva et al., 1977}; ^{Jakubov et al., 1980}; ^{Iosseliani and Diasamidze, 1983}; ^{Adamia, 1985}; ^{Adamia et al., 2008}; ^{Aliyev et al., 2015}). The thickest Cenozoic sediments correspond to the Oligocene-Early Miocene Maykop Formation (5–6 km) and the Late Miocene Shirak Formation (2–2.5 km). The large sediment thickness in the Middle Kura basin may be as a result of the presence of multiple thrust complexes (^{Mosar et al., 2010}; ^{Adamia et al., 2011}).

Another large MV province, that of Kerch-Taman, lies in a zone of thick Pliocene–Quaternary sediments of the Indol-Kuban Trough which borders the Greater Caucasus Range in the north (Fig. 1). Mud volcanism in the region culminated during the Middle Miocene Chokrak and Middle-Late Sarmat deposition events but has decayed lately. Forty out of 80 mud volcanoes in the province are currently active (^{Shnyukov et al., 1986} and ²⁰⁰⁵; ^{Kopf et al., 2003}; ^{Lavrushin et al., 2003} and ²⁰⁰⁵; ^{Olenchenko et al., 2015}; ^{Sokol et al., 2018} and ²⁰¹⁹). The zones of mud volcanism are located in basin borders deformed by folds and thrusts as in the Kura basin.

Thus, there are two main factors favorable for mud volcanism in the Caucasus region: 1) the presence of thick Cenozoic shales of which some are producing oil and gas reservoirs; 2) high activity of Cenozoic and recent geodynamic processes. This combination of factors is typical of many MV provinces worldwide (^{Kopf, 2002}).

Materials and Methods

Sampling

Mud volcanic fluids in the Taman Peninsula and Azerbaijan were monitored during several successive field campaigns of 2009, 2010, 2012, and 2013. Waters were sampled from 14 MVs in the Taman Peninsula in 2009 and from 56 edifices in Azerbaijan (South Caspian province) during the trips of 2010, 2012 and 2013 (Figs. 2 and 3; Tables 1, 2 and S1). Additionally, a flowing abandoned borehole (13-1/10) was sampled at the foot of Neftechala volcano. Our results have been compared with earlier data collected in the September of 1997 from MVs in the Kakhety province (^{Lavrushin et al., 1996}, ²⁰⁰³ and ²⁰⁰⁵).

The sampled sites were located using a Garmin GPS, in a WGC-84 system (accuracy of 3–5 m). The temperature and pH of every sampled water issue were measured in situ with a manual Hanna Instruments PH ORP Combo Meter & Temperature Gauges (HI98121) to a precision of ±0.1°C and ±0.1 pH. Eh measurements were performed in situ using a Pt electrode, with an EXPERT 001 manual pH-meter, to a precision of ±0.1 mV Eh.

Sampling of water and free emanating gases for laboratory studies were from the most active salsas in the central part of the MV field; at large volcanoes, a central and several side salsas were sampled. The sampling techniques were detailed previously (^{Kikvadze et al., 2014;Lavrushin et al., 2015}).

MV waters were sampled simultaneously into plastic vials and bottles. Water was filtered through a 0.45-μm Nucleopore filter a few hours after sampling, in order to avoid initial salt precipitation and colloid coagulation. Major cations and trace elements were analyzed in 15 water aliquots which were stored separately and then acidified with 0.5 mL of 15N distilled nitric acid; anions were determined in another 50 mL aliquot of unacidified water; other 15 mL water aliquots stored in glass bottles were used for analysis of H and O isotopes. Gas was sampled by pumping using a small water siphon, into 100 cm³ glass bottles.

Analytical procedures

Major-element concentrations in water samples were measured at the chemical analytical laboratory of the Geological Institute (GIN, Moscow, Russia) and at the Analytical Center of the Institute of Microelectronics, Technology and High-Purity Materials (IMT, Chernogolovka, Russia), by ICP-AES, ICP-MS, and acid and AgNO₃ titration. The analytical details were summarized previously (^{Lavrushin, 2012}; ^{Lavrushin et al., 2015}). Trace element compositions were analyzed at IMT (Chernogolovka) on a Thermo Jarrel (USA) and a Thermo Elemental Х-7 ICP-MS (USA) analyzers following the procedure from (^{Karandashev et al., 2016}). The accuracy and precision were monitored by measuring the IAPSO salinity standard on a regular basis (Gieskes et al., 1992). The concentrations measured for IAPSO were generally within 5% of seawater values at the salinity on the IAPSO standard bottle. The precision and accuracy were 10– 15 rel% for all analyzed elements.

Water salinity (Table 2) was estimated by summation of main dissolved salts: Na, K, Ca, Mg, Cl, HCO₃ and SO₄.

Oxygen and hydrogen isotope compositions of H₂O were studied at the Laboratory of Isotope Geochemistry and Geochronology of the Geological Institute (Moscow) using Thermoelectron instruments, including a Delta-V-Advаntage mass spectrometer, and a Finnigan-TC/EA thermochemical element analyzer (for δD in H₂O). All δD and δ¹⁸O values are quoted in‰ relative to the V-SMOW standard. The δ¹⁸O and δD estimates are accurate (reproducible) to no worse than ±0.2‰ and 3‰, respectively. More analytical details were reported earlier (^{Lavrushin et al., 2015}; ^{Pokrovsky et al., 2017}). All the results are summarized in Tables 1–2.

The bulk chemistry of MV gases was analyzed on a Crystal-2000 M gas chromatography analyzer at the Geological Institute (GIN, Moscow, Russia), by absolute calibration of each component against standard gas mixtures. Carbon dioxide was determined on a flame-ionization detector with a built-in methanizer. The error was within 0.5 vol.% for each component.

Fluid generation temperatures were estimated using an Mg–Li geothermometer designed originally (^{Kharaka and Mariner, 1989}) for low to high saline pore and edge waters in oil and gas basins, with temperatures up to 300°C:

T_Mg/Li (°C) = 2200/(lg((√Mg)/Li) + 5.47) – 273.15.

The Mg-Li thermometer is less sensitive to water salinity and aquifer lithology and the most suitable for basinal waters (^{Kharaka and Mariner, 1989}); it provides more reliable estimates than its SiO₂, K-Na, Na-Li and K-Na-Ca counterparts (^{Lavrushin et al., 2003}).

Results

Chemistry of MV free gas

The gas phase of MVFs in the Caucasus region contains ~70 vol.% to 99 vol.% CH₄, a few percent to 30 vol.% CO₂, and vanishing amounts of heavy CH gases (<<0.5 vol.% ethane,<<0.005 vol.% propane, etc.) (^{Kikvadze et al., 2014}; ^{Lavrushin et al., 2015}). Average carbon dioxide contents are 7.2 vol.%, 5.3 vol.%, and 4.3 vol.% in the Taman Peninsula, in the South Caspian province, and in the Kakhety area, respectively, but is often much lower or even below detection limit. The highest CO₂ contents reaching ~30 vol.% were measured in gas released from Kuchugur volcano in the Taman Peninsula, where CO₂ content in MVFs generally increases toward the Kerch strait. The gas phase composition correlates with the mud volcanic activity in both the Taman and Azerbaijan provinces. The concentration of CO₂ in MV gases is commonly high in central most active high-rate issues but lower on the periphery. Nitrogen is most often<1 vol.% and rarely reaches 5.4 vol.% in the MV gases, whereas inert gases and hydrogen occur as impurities (^{Kikvadze et al., 2014}, ^{Lavrushin et al., 2015}).

Chemistry of MV water

MV waters in both Taman and South Caspian provinces are Na⁻, Cl⁻ and HCO₃⁻ dominated with lesser amounts of Mg⁺ and Ca²⁺ (Fig. 4). The Taman MV waters have HCO₃⁻-Cl/Na and Cl-HCO₃/Na compositions with high contents of HCO₃⁻ while the South Caspian waters are mainly of Cl/Na and Cl-HCO₃/Na types. Commonly the waters that emanate CO₂-rich gases are also enriched in HCO₃⁻ (Fig. 5), i.e., HCO₃⁻ concentrations correlate with the gas phase composition. Generally the HCO₃⁻ concentrations in the Taman MVFs increase progressively west- and south-westward, along with CO₂ increase in the gas phase.

Waters released from the Taman MVs differ from those in the South Caspian province in TDS ranges: 9.6 to 20.1 g/L and 8.6 to 63.8 g/L, respectively (Table 2). This may reflect the original difference between the compositions of pore waters in the Indol-Kuban Trough and the Lower Kura basin. On the other hand, salinity of the greatest part of pore water samples in both MV provinces may be as low as 20 g/L (Table 2), possibly, due to freshening by waters from hydrocarbon reservoirs or due to smectite dehydration at depths.

Oxygen and hydrogen isotope compositions of H₂O

The oxygen and hydrogen isotope compositions of MV waters are largely similar in MVFs from different Caucasian provinces (Fig. 6) and different from those of local surface (stream), marine, and pore waters, and especially, CO₂-rich springs in Northern Caucasus (^{Lavrushin, 2012}; ^{Dubinina, 2013}). The Northern Caucasus CO₂-rich mineral springs also differ from MVFs in low salinity (Table 2), which is below 2.5 g/L in 94 (out of 172) samples and below 5.0 g/L in 43 other samples (^{Lavrushin, 2012}) while MVFs in different Caucasian provinces contain 13.3 to 22.4 g/L TDS on average.

The δ¹⁸O–δD compositions of MVFs from all Caucasian provinces (Fig. 6) fall to the right of the Craig line for meteroric waters (^{Craig, 1961} and ¹⁹⁶³). The maximum δ¹⁸O_H2O values for MV waters from each province are much higher than in the meteoric endmember which is apparently about −5‰ in the region (^{Lavrushin et al., 2005}; ^{Dubinina et al., 2005}; ^{Lavrushin, 2012}).

Thus, the δ¹⁸O and δD values in all studied water samples suggests the same formation mechanism for MV waters all over the Caucasian region. In the δD–δ¹⁸O diagram, the modern oceanic water (SMOW), with δ¹⁸O_H2O = 0±1‰ (at 19.35 g/L Cl⁻ and 0.14 g/L HCO₃⁻) (^{Horn, 1969}), plots much above the composition field of the Taman MV waters. The intersection of the Taman MVF trend with the Craig line, in its turn, lies high above the trends for both meteoric surface waters and pore waters in the Azov-Kuban and Kura oil and gas basins, which have similar δD and δ¹⁸O compositions (Table 3). Many data points of CO₂-rich springs from the central (Elbrus-Kazbek) and western segments of the Greater Caucasus, as well as waters from boreholes, lie farther below the δD and δ¹⁸O fields of pore waters from hydrocarbon reservoirs (^{Lavrushin, 2012}; ^{Dubinina, 2013)}.

The oxygen isotopic shift (Dd¹⁸O_H2O) is a “stagnation” criterion for pore waters or, in a more general case, an indicator of the formation depth of groundwater (^{Ferronsky and Polyakov, 2012}). Higher δ¹⁸O_H2O in the Caucasian MV fluids hardly can result from greater inputs of evaporitic brines, as we obtained the lowest δ¹⁸O_H2O for the most saline samples (Tables 1 and 2). Note that δ¹⁸O_H2O is in negative correlation with Cl⁻ (Fig. 6). The δ¹⁸O and δD of MVFs bear strong impacts of dehydration waters released during illitization of smectite (^{Revil, 2002}; ^{Dählmann and de Lange, 2003}; ^{Lavrushin et al., 2005}; ^{Chelnokov et al., 2018}; ^{Sokol et al., 2019)}.

Discussion

Mud volcanic waters: major-ion chemistry and stable isotope composition

In spite of considerable geographic variations, the chemistry of MV waters worldwide shows distinct correlations with tectonic settings (^{Dimitrov, 2002}; ^{Kopf, 2002}; ^{Oppo et al., 2014}; ^{Kokh et al., 2015}; ^{Ershov and Levin, 2016}; ^{Mazzini and Etiope, 2017}; ^{Sokol et al., 2019}). Mud volcanoes within the tectonically active circum-Pacific belt release several types of waters: HCO₃⁻-Cl/Na, Cl-HCO₃/Na, and Cl/Na, with high concentrations of HCO₃⁻, especially, in Sakhalin Island where average salinity reaches 15–16 g/L (^{Lagunova and Gemp, 1978}; ^{Ershov et al., 2016}; ^{Chelnokov et al., 2018}), and in Taiwan Island (^{Gieskes et al., 1992}; ^{You et al., 2004}). Generally, the HCO₃-Cl/Na and Cl⁻HCO₃/Na-type waters become more saline as HCO₃⁻ increases. The MV waters of Cl/Na type show large salinity ranges and most often occur in the Alpine-Himalayan belt. Their salinity reaches almost the highest brine values (25–50 g/L) within the influence zone of the Great Caucasus orogeny (Georgia and Azerbaijan), as well as on the eastern extension of the South Caspian MV province (Turkmenistan). Unlike these, the waters of the Kerch-Taman MV province in the area of decaying orogeny mainly have Cl^-HCO₃/Na chemistry and a lower salinity of ~14 g/L (^{Lagunova and Gemp, 1971}, ¹⁹⁷⁸; ^{Dimitrov, 2002}; ^{Kopf, 2002} and ²⁰⁰³; ^{Lavrushin et al., 2003}; ^{Kokh et al., 2015}; ^{Olenchenko et al., 2015}; ^{Ershov and Levin, 2016}; ^{Sokol et al., 2019}).

The new data set from the Great Caucasus orogeny zone fits well this general pattern and provides a more detailed picture. The distribution of main MV water types (Cl/Na, Cl^-HCO₃/Na, and HCO₃-Cl/Na) is random in the Taman and Kakhety areas but more regular in the South Caspian province. Specifically, waters are mostly of Cl/Na type in the Fore-Kura petroleum province but mainly have HCO₃-Cl/Na or Cl^-HCO₃/Na compositions in the Shemakha-Gobustan and Apsheron areas. The HCO₃/Na-type MV waters tend to the tectonically active Greater Caucasus frontal zone of continental collision (^{Lavrushin et al., 2015}). They are mainly restricted to the southern slope of the Greater Caucasus, while the Cl-type MV waters are more common to the Kura and Caspian basins which are filled with thick sediments (Fig. 7) and less stressed tectonically (^{Guliev et al., 2002}; ^{Reilinger et al., 2006}; ^{Aliev and Bairamov, 2007}; ^{Lavrushin et al., 2015}).

Note that the chemically different waters differ also in δ¹⁸O and T_Mg/Li: these values are generally higher in HCO₃⁻-rich waters than in those of the Cl/Na type (Table 2). The Azerbaijan MVs discharging mainly HCO₃-Cl/Na waters are located in active tectonic and seismic zones around the southern slope of the Great Caucasus Range (Fig. 7), where plate velocities decrease dramatically to 4 mm/yr from ~13 mm/yr in the Kura basin (^{Mosar et al., 2010}; ^{Kadirov et al., 2015}).

Judging by T_Mg/Li values (Table 2), HCO₃⁻-rich MV waters form at higher temperatures than those of Cl/Na-type, either due to higher temperature gradients in active areas or because hotter waters from deeper aquifers can rise to the surface through faulted crust.

The Cl/Na MV waters of the Fore-Kura area and the Cl-HCO₃/Na and HCO₃-Cl/Na MV waters of the Shemakha-Gobustan and Apsheron areas differ markedly in oxygen isotope composition. The former (Table 1) have high salinity (25 to 84 g/L) and relatively low δ¹⁸O_H2O (≤ +4.4‰) while the latter (Tables 1 and 2) are less saline (10–20 g/L) but isotopically heavier, with up to+ 10.4‰ δ¹⁸O(H₂O). The heavier MV waters with high ¹⁸O and HCO₃⁻ contents may result partly from dehydration of marine shale during sediment maturation and compaction at temperatures up to 200°C (^{Seletskii, 1991}; ^{Giggenbach, 1995}; ^{Lavrushin et al., 1996}; ^{Dählmann, and de Lange, 2003}; ^{Chelnokov et al., 2018}). The percentages of dehydration water in MVFs from different Caucasian provinces, estimated after ^{Chelnokov et al. (2018)}, are approximately 20% to 80% in Kerch (^{Sokol et al., 2019}), 20%–60% in Taman, 20% to 55% in Shemakha-Gobustan, 25% to 50% in Apsheron, and as low as 10% to 35% in the Fore-Kura area (Table 3). A zone of rapid smectite illitization and dehydration was detected in the 7–13 km depth interval by seismic surveys in the South Caspian region (^{Guliev et al., 1988}).

The Cl⁻ and HCO₃⁻ contents, as well as δ¹⁸O_H2O and δD_H2O values, of the Caucasian MVFs correlate well with the constrained temperatures T_Mg/Li (Fig. 8). The obtained T_Mg/Li values for all MVFs from the Caucasian provinces are in positive correlation with HCO₃⁻ contents and oxygen isotope compositions (both δ¹⁸O_H2O and Dd¹⁸O_H2O) and in high negative correlation with Cl⁻ (Fig. 8): the Cl⁻ contents decrease with increasing T_Mg/Li.

Constrains of fluid generation depth

The Greater Caucasus backarc basin underwent inversion in the Early Cenozoic induced by the Arabia-Eurasia collision and the Caucasian orogeny. Since that time on, the orogeny has controlled all geological structures and events in the region and its surroundings (^{Mosar et al., 2010}), including the ongoing mud volcanism in south-eastern Europe. It can be illustrated that this control is especially prominent in the eastern part of the region by the case of the Azerbaijan MVs. The Greater Caucasus is a doubly verging fold-thrust belt, with a pro- and a retro wedge actively propagating into the foreland sedimentary basins of Kura in the south and Terek in the north (Fig. 7). The orogenic front of the Greater Caucasus in Azerbaijan lies at the foothills of the Lesser Caucasus, south of the Kura foreland basin (^{Mosar et al., 2010}).

Thus, MVs of the South Caspian province fall within four tectonic units (Fig. 7) with different recent convergence rates and earthquake frequency and magnitudes (^{Mosar et al., 2010}). The Kura basin has the lowest (or locally absent) seismic activity and the thickest crust and Mesozoic-Cenozoic sedimentary fill, whereas the Shemakha-Gobustan and Apsheron areas of Azerbaijan adjacent to the orogenic front are highly seismic and heavily faulted (^{Aliev, 1985}; ^{Guliev et al., 2004}). As a consequence, the background conductive heat flux within the South Caspian province is notably higher in the Shemakha-Gobustan and Apsheron areas than in the Kura basin: 50–90 against 20–30mW/m² (^{Aliev, 1985}). The respective geothermal gradient ranges from 13 to 17°C/km (mean value 15°C/km) in the Fore-Kura area and the Baku archipelago to 20–22°C/km (mean value 21°C/km) in the Shemakha-Gobustan and Apsheron areas of Azerbaijan (^{Aliev, 1985}; ^{Guliev et al., 2004}).

The large difference in background conductive heat flux leads to differences in fluid generation temperatures (T_Mg/Li) of MV waters sampled from adjacent areas. The highest values (75°C and 137°C) were inferred for two MVs located in the Fore-Caspian area nearest to the Greater Caucasus Range, which corresponds to a fluid generation depth (H_Mg/Li) of 3.6 to 6.5 km. Slightly lower average fluid generation temperatures (T_Mg/Li) were obtained for the Apsheron and Shemakha-Gobustan areas: 63.0±8.6°C and 66.0±7.0°C, respectively (Table 4; Fig. 7), which correspond to an average fluid generation depth of 3.1 km. The T_Mg/Li value is much lower (37.5±7.0°C) for MVFs of the Fore-Kura area, the most distant from the Greater Caucasus. However, the average H_Mg/Li depth (~2.5 km) is not much shallower than in adjacent Azerbaijan, given the considerably lower regional geothermal gradient. The respective depth intervals in all these areas are occupied by the Maykop shale and younger sediments.

The MV waters sampled in the Taman Peninsula fall into three groups according to the estimated fluid generation temperatures (T_Mg/Li) (Fig. 9). The T_Mg/Li values are relatively low (41°C to 66°C) in the northern part of the peninsula near the Azov coast, higher (75°C–89°C) in the south, around the Taman Gulf and on the Black Sea coast, and the highest (86–137°C) in the extreme east of the peninsula (east of the N–S Djiga fault impacted by the Greater Caucasus orogeny). The temperature zoning is generally oriented in the W–E direction and controlled by the folding pattern in sediments (^{Shnyukov et al., 1986}). Unlike other areas of the Caucasian collisional zone with generally NW-striking folds in Mesozoic-Cenozoic sediments, the folds in the Taman Peninsula are mostly narrow and W–E trending. The MV waters within anticlinal folds in the north of the peninsula, where the Maykop Formation spans the depths from 0.6 to 1.6 km to 2–4 km (^{Rostovtsev, 2000}), originate at H_Mg/Li ~1.0–1.5 km, as calculated with reference to a geothermal gradient of ~40°C/km (Lagunova, 1975). The respective fluid generation depths in the southern peninsula part, where the Maykop shale is thicker and lies between ~1.0–2.6 km and 5–7 km (^{Rostovtsev, 2000}), are 1.9–2.2 km. These estimates are consistent with the inference (^{Shnyukov et al., 1986} and ²⁰⁰⁵; ^{Kokh et al., 2015}; ^{Sokol et al., 2018} and ²⁰¹⁹) that MVs in the Kerch and Taman peninsulas feed mainly from the Middle Maykop strata. The group of large MVs east of the Djiga fault is associated with NW folds, especially with the anticlines that have Cretaceous cores (^{Shnyukov et al., 1986} and ²⁰⁰⁵). These mud volcanoes have the deepest roots in the peninsula: H_Mg/Li ~ 2.2–3.4 km.

Conclusions

The Caucasian collision zone, a classical area of intracontinental mud volcanism, comprises two large onshore MV provinces (Kerch-Taman and the Caspian) and a smaller province (Kakhety) between them. The two large units are the Kerch-Taman province in the west and the Caspian one in the east, which are exposed to the influence of the Caucasian orogeny decaying westward and northward. The two provinces differ markedly in total thickness and petroleum potential of sediments correlated with trends in the generation depths of fluids that feed MVs at different sites of the vast territory.

The reported study on the formation conditions of fluids in the Caucasian mud volcanic provinces shows that the MV waters formed within a temperature range of 18°C to 137°C. They are i) hot HCO₃-rich waters with high δ¹⁸O_H2O values and ii) relatively cold chloride waters with lower δ¹⁸O_H2O. According to Mg/Li geothermometry, less saline HCO₃/Na-type MV waters generate at greater depths than more saline Cl/Na MV waters. The lateral MVF temperature distribution agrees with the patterns of major-ion chemistry and δ¹⁸O_H2O and δD_H2O values. The correlation is especially prominent in the South Caspian MV province. Namely, the HCO₃-Cl/Na-type waters of relatively low salinity formed at higher temperatures than the Cl/Na-type brines. The MVF base temperatures in different Caucasian mud volcanic provinces increase concordantly toward the Great Caucasus Range, this being the evidence of relation between fluid generation conditions and regional tectonic activity (and heat flux). The HCO₃-rich MV waters in the region occur mainly within active tectonic zones, which agrees with the previously revealed global tendency. Taking into account local variations in geothermal gradient, the mud reservoirs can be expected to lie in the depths interval from ~ 0.85 to 6.5 km within Cenozoic sediments, mostly corresponding to the Maykop Formation shales. The temperature and chemistry patterns in MV fluids of the Caucasus region are largely due to sediment maturation at different temperatures and depths.

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