At present, many oil and gas deposits of small (up to 2.0 km) and medium (up to 4.0 km) depths have already been worked out or have been largely depleted. At the same time, demand for liquid and gaseous hydrocarbons (HCs) continues to grow steadily. This trend has developed in almost all oil- and gas-producing countries of the world, including Russia. Therefore, many oil and gas organizations have already turned to the search, exploration, and commercial development of superdeep (5.0–10.0 km) oil and gas deposits. The difficulty in the study and development of such deposits includes the fact that they are located at great depths inaccessible to direct observation, as well as in the complex composition and behavior of the fluids themselves, which are mixtures of various hydrocarbons, aqueous solutions, and their vapors with broadly variable bulk proportions. We used these components as the basis for experiments, which, according to our ideas, can provide significant assistance in understanding, whether or not heterogeneous multiphase water–hydrocarbon (i.e., oil and gas) fluids of small and medium depths are transformed into a homogeneous water–hydrocarbon liquid, similar to deep natural oil in the phase composition and state; if yes, it is necessary to estimate the specific conditions of this process.

The discovery of large gas condensate fields at depths of 8.8–11.5 km in Mexico, China, and other countries in recent decades [1–3] has contributed (and continues to contribute) to a lively discussion about the maximum likely depths (or, in other words, temperature stability) of oil in the Earth’s interior. Moreover, it is especially important to study the conditions for the heterogeneous and homogeneous states of deep water–hydrocarbon systems.

We tried to obtain unambiguous experimental data on the phase state of deep water–hydrocarbon fluids, using, among other things, a new approach previously developed by the authors of this paper [4–7]. Its essence is to conduct experiments on the synthesis of quartz crystals with trapped water–hydrocarbon inclusions obtained simultaneously with the interaction of bituminous or carbonaceous rocks, as well as crude oil with hydrothermal solutions [4, 7]. Quartz was selected for inclusion trapping due to its high mechanical strength and chemical resistance; these are the properties that are essential both to prevent crystal cracking upon the formation of a fluid inclusion and for subsequent thermometric studies. In addition, quartz is grown in autoclaves of a simple design, using a reliable hydrothermal temperature gradient method in a wide temperature range of 240–700°C and pressures of 7–150 MPa [4]. We applied aqueous solutions of sodium bicarbonate (7 wt % NaHCO3) and sodium carbonate (5 wt % Na2CO3) with a high solubility of quartz as solvents in the experiments [8].

After the experiments, polished plates with a thickness of 0.5–2.0 mm were prepared from synthetic quartz for the study of fluid and solid inclusions. The behavior, phase composition, and states of the inclusions were studied in situ upon their heating and cooling in a measuring microthermometric complex based on a THMSG-600 microthermal camera from Linkam (England) and an Amplival microscope (Germany) equipped with an additional UV light source, a set of long-focus lenses, a video camera, and a computer control [9, 10]. The complex allows real-time monitoring for the behavior and phase states of fluids in inclusions in the temperature range from –196 to +600°С and recording videos with continuous automatic recording of the temperature and the rate of its increase and decrease. Actually, thermometric measurements were stopped at temperatures of 405–410°C, since at higher temperatures fluid inclusions lost their tightness due to fracturing, up to complete destruction with an explosion.

Liquid and gaseous hydrocarbons in fluid inclusions were identified by the basic absorption bands of IR spectra in the range of 6000–2600 cm–1, recorded using a Continuum IR microscope and a Nicolet Nexus single-beam FTIR spectrometer with a minimum aperture size of 5 μm (a resolution of 4 cm–1). The distribution of hydrocarbons in the inclusions was monitored using a CRAIC QDI 302 microspectrophotometer based on a LEICA DM 2500 P microscope and a ZEISS AXIO Imager microscope (Germany) equipped with an additional UV light source.

In total, more than 50 experiments lasting from a few to 40 days were carried out to solve the problems posed in our study regarding the phase composition and states of water–hydrocarbon fluids in the Earth’s interior. As a result, we synthesized wedge-shaped quartz crystals (Appendix, Fig. 1) containing water–hydrocarbon inclusions ranging in size from 0.n to 1 mm with various morphologies: tubular, needle-shaped, oval, and irregular. Inclusions formed in the overgrown layer directly during crystal growth fully reflect the phase state of the crystallization medium. The mechanism and conditions for the origin of inclusions controlled the formation of fluid inclusions with different proportions of water (L1), gaseous (G), and liquid hydrocarbon (L2 and L3) phases [7] (Fig. 1, Appendix Figs. 2–7). There are often precipitates of solid bitumen (SB) of spherical or irregular shape in streaks and drops of liquid hydrocarbons. The presence of hydrocarbons was detected under UV light in all fluid inclusions in quartz, even from experiments containing ≤0.01–0.02 vol % of oil in the initial solutions (Appendix, Figs. 3, 4). Inclusions of three different types were selected for study and comparison: essentially aqueous with the bulk phase proportions of L1 > G, L1 > G > L2, L1 > G > L2 \( \gg \) SB; essentially gaseous with G > L1 > L2, G > L1 > L2 \( \gg \) SB; and essentially hydrocarbonic with L2 > G, L2 > G > L1, L2 > G > L1 > L3. The behavior and phase state of water–hydrocarbon fluids in inclusions of three different types were studied in situ upon heating and cooling in the range of 25–410–25°C. Such microthermometric studies allowed us to detect changes in the phase composition and state of water–hydrocarbon fluids upon subsidence (i.e., temperature increase) and uplift (i.e., temperature decrease) of the host rocks.

Fig. 1.
figure 1

Synthetic fluid water–hydrocarbon inclusions in quartz: essentially aqueous with the bulk phase proportions of (a) L1 > G, (b) L1 > G > L2, (c, d) L1 > G > L2 \( \gg \) SB; essentially gaseous with (e) G > L1 > L2, and essentially hydrocarbonic with (f, h) L2 > G > L1, (g) L2 > G > L1 > L3, and (i) L2 > G.

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Fig. 2.
figure 2

Microthermograms of essentially aqueous hydrocarbon inclusions with the bulk phase proportions of (a) L1 > G, (b) L1 > G > L2, and (c) L1 > G > L2 \( \gg \) SB.

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Fig. 3.
figure 3

Microthermogram of an essentially gaseous hydrocarbon inclusion with a bulk phase proportion of G > L1 > L2.

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Fig. 4.
figure 4

Microthermogram of an essentially gaseous hydrocarbon inclusion with a bulk phase proportion of L2 > L1 > G.

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Let us consider the results of a microthermometric study of essentially aqueous inclusions. Fluid homogenization in inclusions with a bulk phase proportion of L1 > G (Fig. 2a) occurs at a temperature of 350°C due to a gradual decrease in the gas phase (G) until it is completely dissolved in an aqueous solution (L1). The inclusion seems to be two-phase; however, under UV light, a rim of liquid hydrocarbons is observed at the boundary of the aqueous solution and the gaseous phase, which is not visible under ordinary and polarized light, but manifests itself by a characteristic fluorescent glow under UV light as a phase of liquid hydrocarbons (Appendix, Figs. 3, 4).

In the case of the bulk phase proportions of L1 > G  \( \gg \) L2, water–hydrocarbon fluids are in a three-phase state up to a temperature of 220–250°C. With increasing temperature, the liquid hydrocarbon phase dissolves in an aqueous solution, and fluids transform into a two-phase (L1 > G) state (Fig. 2b). Complete homogenization of inclusions occurs at a temperature of 380–390°C. It should be noted that essentially water–hydrocarbon inclusions, in which the bulk fraction of the hydrocarbon phase is >10 vol %, or approximately equal to the fraction of the gaseous phase (L1 > G ≥ L2), undergo complete or partial depressurization without reaching homogenization. The stable existence of two-phase fluids (L1 > L2) without a free gas phase was observed up to temperatures of 385–405°С, which was followed by depressurization of inclusions (Appendix, Fig. 8).

In the cases when a solid bitumen phase with bulk phase proportions of L1 > G \( \gg \) L2 > SB occur in essentially aqueous inclusions, at first, when the temperature increases to 280–300°С, the liquid hydrocarbon streak is completely dissolved in the gas with the formation of a two-phase (L1 ≥ G) fluid, and then at 390–400°C, the gas phase disappears with the transition of the fluid to a homogeneous liquid state (Fig. 2c). The spherical precipitates of solid bitumen SB located in the rim L2, do not undergo any changes.

In rare cases, essentially gaseous hydrocarbon inclusions with the phase proportions of G > L1 > L2 and G > L1 > L2 \( \gg \) SB are formed in quartz crystals. They are secondary and were formed due to an increase in the volume of essentially liquid inclusions when blind fractures appeared in the walls of vacuoles upon heating–cooling of the studied samples (Fig. 1e). The microthermograms of such inclusions initially show the dissolution of the liquid hydrocarbon phase in the gas with the formation of a two-phase fluid (G > L1) as well at a relatively higher temperature (300–320°C) compared to the previous case. A further increase in temperature to 360–400°С results in complete disappearance of the aqueous phase (L1) with the formation of a homogeneous gaseous fluid (Fig. 3).

The gas phase first disappears and the fluid becomes two-phase hydrocarbon–water (L2 > L1) in essentially hydrocarbon inclusions with bulk phase proportions of L2 > L1 > G at 260–280°С in the liquid hydrocarbon phase. Then, as the temperature increases to 350–360°C, the aqueous phase L1 is completely dissolved in the L2 phase with the formation of a homogeneous hydrocarbon fluid (Fig. 4). Dissolution of drops of the hydrocarbon phase L3 is first (up to 200°C) detected in the inclusion of the composition L2 > L1 > G > L3 ≥ SB; then gas (G) and aqueous phases (L1) are dissolved in the main volume of liquid hydrocarbons L2 (Appendix, Fig. 17).

In general, water–hydrocarbon inclusions in other quartz crystals synthesized under the similar T–P conditions behave similarly in general terms upon heating and cooling (Appendix, Figs. 8–18). The temperatures of disappearance and appearance of the phase of liquid (L2) and gaseous (G) hydrocarbons upon fluid homogenization and heterogenization usually differ in the range from 10 to 50°C. This is due to the different filling of autoclaves and unequal proportions of petroleum hydrocarbons in the initial solutions. The phase of solid hydrocarbons (SBs) in the inclusions almost does not change during microthermometric studies. Repeated heating and cooling of inclusions reproduce their behavior, phase composition, and states, which indicates the reversibility of the processes studied.

Thus, experiments on the formation of liquid and gaseous hydrocarbons upon interaction of bituminous rocks and crude oil with hydrothermal solutions provided unambiguous evidence for the heterogeneous state of water–hydrocarbon fluids at low, medium, and high depths. This is especially clear on microthermograms obtained by simultaneous study of fluid inclusions with trapped liquid and gaseous hydrocarbons. At the same time, experiments show that at temperatures of ~380–400°С and an oil content up to 10 vol % or higher, such inclusions reach a homogeneous state. However, under natural conditions, such temperatures and the corresponding homogeneous states of water–hydrocarbon fluids have not yet been established in any oil and gas basin of the world. The experimental data obtained confirm that, within the temperature stability of oil (450–550°С), there may be natural conditions that correspond to a homogeneous and even supercritical state of fluids. This should contribute to the development of the theory of the origin of liquid and gaseous hydrocarbons, as well as the successful search for and exploration of deep oil.