Hybrid carbonate reservoirs in saline lacustrine basins are widely developed in petroliferous basins of China. Significant explorational discoveries have been made in hybrid carbonate reservoirs from many basins, implying great resources potential and prospects^{[1,2,3,4,5,6,7,8,9]}. However, exploration activities have also shown that hybrid carbonate reservoirs are mostly characterized by complex fabric, strong heterogeneity, and rapid changes in the scale and distribution of sweet reservoirs spatially and temporally, thus restricting their exploration scale and efficient development. Moreover, formation mechanism and distribution of reservoirs need to be further clarified.

The deep layers in the Yingxi area of the Qaidam Basin have been one of the hotspots and difficult areas in the exploration of hybrid carbonate rock tight/shale oil in this saline lacustrine basin in recent years. The rich drilling data accumulated in the area laid the data foundation for deeply understanding hybrid carbonate reservoirs. The burial depth of primary subsalt oil zone in the upper part of the Lower Ganchaigou Formation (E₃²) of the Paleogene is generally greater than 4 km. Although reserves at 100-million-ton level have been discovered, the daily single-well oil and gas production varies greatly (from non-industrial to 1000-ton level) depending on reservoir types and physical properties. The genetic type and distribution of high-quality reservoirs remain unclear. These factors have severely restricted the beneficial development of the area and the outstep expansion. To solve this problem, previous studies discussed the characteristics of reservoir rocks and minerals, pore types and genesis to highlight the control of dolomitization intercrystalline pores, abnormal high pressure microfracture and tectonic breccia fracture-vug on high and stable production of oil and gas from different perspectives^{[3-4, 10-18]}. In respect of hybrid carbonate reservoirs in saline lacustrine basins, it is generally accepted that there is an essential relationship between lithofacies and the genesis and distribution of the reservoir^{[2, 19-23]}. However, in the Yingxi area, the studies on hybrid carbonate reservoirs in the saline lacustrine basin are relatively scarce, so that the preservation and distribution of high- and stable-yield reservoirs have not been well understood. Based on a large dataset of core, thin section and geochemical analysis, we systematically characterize the lithofacies of the hybrid carbonate reservoirs in the study area and explore the formation mechanism of these reservoirs. On this basis, different pore types have been established. The development and distribution pattern of layers is expected to shed light on the theory of the formation of hybrid carbonate rocks in the saline lacustrine basin, thus providing a case study for clarifying the distribution of high-quality reservoirs to support the efficient development and the exploration expansion in the Yingxi area and elsewhere.

The Yingxi area is located at the western margin of the Yingxiongling structure in the Qaidam Basin, adjacent to Ganchaigou in the north, Youshashan in the south, and Southwest Qaidam area in the west. According to the structural setting in the deposition-accumulation period, the Yingxi area is divided into a slope zone, an inherited depression zone, a depression inversion structural zone, and an intra-depression paleo-uplift zone (Fig. 1a). Upwardly, the Cenozoic includes the Paleogene Lulehe Formation (E₁₊₂), Lower Ganchaigou Formation (lower part: E₃¹ and upper part: E₃²), the Neogene Upper Ganchaigou Formation (N₁), Lower Youshashan Formation (N₂¹), Upper Youshashan Formation (N₂²), and Shizigou Formation (N₂³), and the Quaternary Qigequan Formation (Q₁₊₂) (Fig. 1b). The target interval - the upper part of the Lower Ganchaigou Formation (E₃²) was deposited and evolved under the background of arid paleo-climate in early, middle and late stages, i.e. initial salinization, salinization, and saline lake^[24,25] (Fig. 1c). In the early stage (the deposition period of pay zone VI), the main part of the lacustrine basin was in a semi-deep to deep lacustrine environment, dominated by dark-colored mudstone deposits, forming the major source rocks in the area. In the middle stage (the depositional period of pay zones V-IV), under a saline lacustrine sedimentary environment, due to the intermittent source supply and intensive evaporation, the salinity of the lake water was generally high, and the deposits of shore-shallow to semi-deep lacustrine facies evolved at high-frequency oscillation^[26,27], resulting in concentrated development of hybrid carbonate reservoirs. In the late stage (the depositional period of pay zones III-I), featured by lowest source supply and intensive evaporation, multiple sedimentary cycles from evaporation and concentration of lacustrine basin to saline basin were developed, forming thick evaporite deposits that serve as caprocks and carbonate successions with potential reservoirs.

In the Yingxi area, the E₃² carbonate rocks in saline lacustrine basin show distinct hybrid characteristics. Three components (clastic, clay and carbonate) were used to classify rocks and describe reservoir characteristics^{[11, 15]}. Based on the observation of cores and thin sections, we have found various types of rock fabrics in the E₃² reservoirs. Accordingly, the reservoirs are divided into five types of lithofacies, i.e. mixed granular facies, massive calcareous dolostone facies, plaque gypsum-bearing to gypso calcareous dolostone facies, laminated dolomitic limestone facies, and tectonic breccia calcareous dolostone facies (Table 1 and Fig. 2).

Cross bedding can be observed in the core (Fig. 2a). Under the optical microscope, the facies shows a sparry mixed granular structure. The grains are mainly composed of well-sorted and highly rounded argilliferous to argillaceous carbonate sand fragments and terrigenous clastic. The proportion of clastic particles can reach up to 80%. Quartz, feldspar and rock debris can be observed, and the epidermal ooids with clastic particles as the core can also be seen (Fig. 2b). In terms of rock and mineral composition (61 samples), the carbonate minerals of the sand particles are dominated by ankerite, accounting for 40%-100% of the total carbonate, with an average of about 70%; the clay content is 0-40%, with an average of about 15%, but it does not affect the pore structure of the rock. The inter-granular sparry cement is mainly anhydrite (0-25%, averaging about 8%).

The storage space of this lithofacies is mainly composed of dissolved pores of anhydrite cement, and anhydrite dissolution remnants can be observed under optical microscope (Fig. 2c, d). The pore size is micrometer level and the reservoir had a porosity of 1%-10% with the peak value of 4%-8% and the permeability of (0.01-0.10)×10^-3 μm².

The lithofacies is observed with relatively homogeneous massive hybrid structure in the core, without bedding (Fig. 2e). Under the observation of thin sections of rock through electron microscope, the carbonate, terrigenous grain and clay are hybrid to form a micritic structure (Fig. 2f). In view of the mineral composition (517 samples), the carbonate accounts for 30%-92% with an average of about 58%, in which ankerite dominates, accounting for 40%-100%, on average 79.5%; the terrigenous felsic components are mainly mud level, followed by silt-powder level, with a content of 3%-47%, on average 18%; the terrigenous clay content is 0-41%, with an average of about 15%; the evaporite mineral is mainly anhydrite, with a content of 0-15%, on average 3.4%.

The storage space of this lithofacies is dominated by dolomitized intercrystalline pores, with multi-stage structural microfractures which are mostly filled (Fig. 2g). Under the scanning electron microscope, mud to micrite-level euhedral to semi-euhedral dolomite intercrystalline pores are observed, and the intercrystalline pores without the communication by fractures are mostly distributed in a triangular shape, with poor pore connectivity (Fig. 2h). The pore size is mainly in nanometer, and the reservoir has a porosity of 1%-13% with peak value of 4%-6% and a matrix permeability of (0.001-0.100)×10^-3 μm².

This lithofacies is characterized by the development of a large number of evaporative salt minerals, and is distinguished from the aforementioned massive argillaceous calcareous dolostone facies. The lithofacies shows massive bedding as a whole in core, and a large number of evaporative minerals such as alabaster anhydrite are distributed in a porphyritic manner (Fig. 2i). Under the microscopic observation, the carbonate and fine-grained terrigenous grain and clay are hybrid to form a micrite structure, and crystals of evaporite minerals such as anhydrite are developed (Fig. 2j). In terms of mineral composition (146 samples), the carbonate content is 17%-70%, with an average of about 40%, in which ankerite accounts for 50%-100%, with an average of 88%. The total carbonate content is lower than that of massive argillaceous calcareous dolostone facies. The particles of terrigenous felsic components are mainly mud-sized, as well as silt- sized, with a content of 2%-47%, on average 16%. The content of terrigenous clay is 0-25%, with an average of about 8.2%. The evaporite minerals are mainly anhydrite and glauberite, with a content of 16%-60%, on average 30%.

The storage space of this lithofacies is dominated by two types of pores: dolomitized intercrystalline pores, and phenocryst dissolved pores of evaporative salt minerals such as anhydrite and glauberite, with structural micro-fractures locally (Fig. 2k, l). Because this lithofacies is highly dolomitized, the intercrystalline pores in dolomite are more developed than that of massive calcareous dolostone facies. The pore size is nanometer-micrometer, and the reservoir has a porosity of 1%-14% with a peak value of 6%-8% and a matrix permeability of (0.001-0.500)×10^-3 μm².

The lithofacies shows the characteristics of shale structure in core (Fig. 2m). Under the microscopic observation of thin sections, it is typically laminar with the superposition of dark-colored lime-bearing muddy-silty terrigenous grain laminae and pure carbonate laminae. The pure carbonate laminae are composed of xenomorphic calcite grains, with the grain size of 0.03-0.06 mm generally (Fig. 2n). Due to the difference in the component content of the two types of lamina, the laminae are correspondingly different in thickness, generally in the range of 0.1-0.5 mm. In terms of mineral composition (272 samples), total terrigenous grains are roughly equivalent to total carbonate. The latter is 30%-67%, with an average of about 43%; it is dominated by calcite and contains ankerite of 0-65%, with an average of 35%. The grains of terrigenous felsic components are mainly mud-sized, as well as silt-sized, with a content of 12%-54%, on average 25%. The content of terrigenous clay is 5%-43%, with an average of about 24%. The evaporite minerals are mainly anhydrite, with a content of 0-15%, on average 3.7%, and it mainly appears in the form of cemented calcite inter-granular pores.

The storage space of the lithofacies is dominated by dissolved pores and laminar fractures of anhydrite cement between the calcite grains within the calcite lamina, and the dissolved pores and laminar fractures are saturated with oil (Fig. 2o-p). The dissolved pores are mainly in micron scale, and the reservoir has a porosity of 1%-13% with the peak value of 3%-7% and a matrix permeability of (0.01-5.00)×10^-3 μm². The development of laminar fractures can significantly improve the seepage capacity of the reservoir.

Wang et al.^[18] identified breccia of sedimentary, diagenetic and tectonic origins through observation of cores and rock thin sections in the study area. The one that can become reservoir is primarily the breccia of tectonic origin, namely, tectonic breccia calcareous dolostone facies. According to core observation, it is mainly manifested as a breccia formed by the abovementioned four facies types of sedimentary origin after tectonic reformation. With rock thin section, the breccia can be seen with the phenomena such as compression, fracture, and distortion. The breccia is angular, with flat edge, and without grounding and directional arrangement. However, all breccia retain the sedimentary structure and mineral composition characteristics of original rock inside (Fig. 2q-t). The mudstone layers above and below the breccia interval are flat (Fig. 2q).

The storage space of this lithofacies includes the dissolved pores and dolomite intercrystalline pores that the abovementioned four lithofacies types have, and also a large number of tectonic brecciated pores and reticular fractures. Pores and vugs with regular edges and filling of late cements (e.g. anhydrite) with different contents can be seen under the microscope (Fig. 2r-t). The pore size of the reservoir interval with low cement filling is from micrometers to centimeters. The reservoir generally has a porosity of greater than 8% and a permeability of greater than 1×10^-3 μm².

In summary, in addition to the breccia facies of tectonic origin, the other four types of lithofacies of sedimentary origin are evidently different in mineral composition characteristics. Laminated dolomitic limestone facies is typically characterized by high content of terrigenous grains and low content of dolomite. Mixed granular facies and massive calcareous dolostone facies are similar in the content and variation of terrigenous grains and dolomite. Plaque gypsum-bearing to gypso calcareous dolostone facies is characterized by high evaporative salt mineral content, low terrigenous grain content and the highest proportion of dolomite (Figs. 3 and 4). Moreover, due to differences in rock fabric and mineral composition, these lithofacies are variable in the types and sizes of matrix pores, and show highly heterogeneous physical properties. Clearly, the subsalt hybrid carbonate rock in the Yingxi area has typical characteristics of lithofacies-controlled reservoir, except for the breccia facies of tectonic origin. Therefore, the analysis of the sedimentary sequence combination of the reservoir lithofacies is necessary to understand the mechanism of lithofacies-controlled reservoir and the temporal and spatial distribution of reservoirs.

Through core observation and comprehensive analysis of rock and mineral geochemistry, under the background of arid paleo-climate and paleo-topography of alternate occurrence of uplift and depression, the E₃² subsalt strata in the Yingxi area were mainly composed of terrigenous grain deposits during the rise of lake level. Various carbonates were developed in the saline deposition stage during the decline of lake level. Generally, there are two types of sedimentary sequence combinations: low-energy saline sedimentary sequences, and low-energy to high-energy saline sedimentary sequences (Figs. 5 and 6).

As shown in Fig. 5, the C1 sedimentary sequences include (from early to late) massive mudstone facies, laminar dolo-limestone facies, massive calcareous dolostone facies and plaque gypsum-bearing to gypso calcareous dolostone facies. It is speculated that evaporite facies appeared in the late saline lacustrine basin center, and it represents the complete sedimentary sequence of C1. It is mainly distributed in the low-energy setting of the stable transition from semi-deep lacustrine to shallow lacustrine facies in the depression area of the lacustrine basin (Fig. 7a). During the initial decline of lake level, the terrigenous muddy-silty suspended components in the lake water took a high content, and were quickly mixed with the calcite precipitated by evaporation to form massive limestone. Afterwards, the content of terrigenous grains decreased, and the calcite and mud formed massive lithofacies in a quiet and semi-deep lake water environment. Affected by frequent replenishment of sources and desalination of water bodies, the abovementioned complete sedimentation process was mostly interrupted and a new cycle was started. Therefore, a complete C1 sedimentary sequence combination is relatively rare, but mostly missing to reflect seasonal laminar rhythms. As the lake level further dropped, evaporation and concentration were strengthened, the salinity of lake water continued to rise, and the depression area tended to be shallow lacustrine. In the sedimentary environment, carbonates and sulfates minerals successively entered a supersaturated state and precipitated in large quantities, and then massive and plaque facies were successively deposited above the laminar facies. The number and type of missing lithofacies depend on the sedimentary process during the source replenishment period. If source replenishment occurs after the deposition of laminar facies, the sequence combination of the lithofacies is only composed of massive mudstone-laminated dolomitic limestone.

As shown in Fig. 6, the C2 sedimentary sequences include (from early to late) massive mudstone facies, massive calcareous dolostone facies, plaque gypsum-bearing to gypso calcareous dolostone facies, and mixed granular facies. It represents the complete sedimentary sequence of C1. It is mainly distributed in the setting of transition from low-energy to high energy in the shallow lacustrine zones in the peripheral slopes of the Yingxi depression and the paleo-uplift within the depression (Fig. 7a). At the initial stage of the lake level decline, it was as a whole in a shallow lacustrine and low-energy environment with a high content of terrigenous grains, which were quickly mixed with the calcite precipitated by the lake water to form a massive mudstone. Afterwards, the content of terrigenous grains decreased, and the paleo-salinity increased. A large amount of calcite was precipitated and became dominated. The sulphate also reached super-saturation, with massive to plaque gypsum-bearing to gypso calcareous dolostone facies deposited. As the lake level dropped further, the wave action was strengthened and the environment was transformed into high-energy. The massive-plaque carbonate rock was broken by the waves to form sand debris, which was mixed with the terrigenous grains carried by the waves to form a granular facies. The lithofacies progradated towards the depression zone, but the supersaturated sulfate would precipitate in the inter-granular pores, resulting in enhanced sulfate cementation. The abovementioned sedimentation process might be interrupted by frequent replenishment of sources, forming an incomplete sedimentary sequence. This type of complete sequence is also rare. The mixed granular facies is often missing, but mostly the combination of massive mudstone and massive-plaque calcareous dolostone lithofacies is common.

The analysis of two types of sedimentary sequence combination of carbonate lithofacies in the study area shows that the difference in source replenishment frequency and intensity caused frequent changes in the period and rate of lake level fluctuation, making the reservoir lithofacies combination in each sedimentary sequence enter a new sedimentary cycle even if it was not complete. Therefore, the single layer thickness and combination type of various reservoir lithofacies are extremely variable in different cycle periods. The type and distribution of reservoir lithofacies under different paleo-geomorphic backgrounds were the result of high-frequency spatial-temporal superposition of the two saline sedimentary sequences. The plane distribution pattern of reservoirs with various lithofacies at the monocyclic scale is evident. When the depression was in the semi-deep lacustrine sedimentary stage, laminated dolomitic limestone reservoirs were concentrated in the depression (Fig. 7b). During the shallow lacustrine deposition stage, massive-plaque calcareous dolostone facies reservoirs were widely developed in the area, and mixed granular facies reservoirs were concentrated in the basin margin slopes and paleo-uplifts (Fig. 7c).

According to the analysis of carbon and oxygen isotopic composition and the geochemical composition of inorganic elements (Fig. 8a), in the Yingxi E₃² subsalt massive and plaque calcareous dolostone reservoirs, the ankerite content is positively correlated with δ¹⁸O, indicating that the dolomitized fluid is controlled by water salinity of lacustrine basin^[28,29]. The content of Al₂O₃ in terrigenous grain often represents the amount of terrigenous grain injected, while MgO represents the residual amount of Mg²⁺ in the lacustrine basin. Therefore, the value of Al₂O₃/MgO can be used to reflect the injection volume of lacustrine basin and the degree of evaporation and concentration in the lacustrine basin^[30]. Fig. 8b illustrates a negative correlation between δ¹⁸O and Al₂O₃/MgO, indicating that the dolomitized fluid is attributed to the evaporation and concentration of the lacustrine basin water. Therefore, the nano-scale dolomite intercrystalline pores in the Yingxi subsalt massive and plaque limestone reservoirs were formed due to the pene-contemporaneous evaporation, concentration and dolomitization. The greater the salinity of the lacustrine basin, the lower the shale content, and the more developed the dolomitized and intercrystalline pores. This is consistent with the vertical change trend of paleo-salinity and dolomitization degree of the two types of saline sedimentary sequences in the area. In other words, the sedimentary sequence and lithofacies combination control the formation and distribution of dolomite intercrystalline pores in the pene-contemporaneous period.

As mentioned above, dissolved pores are also the primary matrix pore type of Yingxi subsalt reservoirs. In this study, two potential mechanisms for the formation of dissolved pore, freshwater and organic acid dissolution during diagenesis, were analyzed by dissolution simulation experiment. The high-temperature and high-pressure dissolution kinetics simulation unit was used in two groups of control experiments designed according to the principle of dissolution^[31]. The rock samples were plaque gypsum-bearing to gypso calcareous dolostone taken from the depth section of a well. In the first group of experiments, purified water was used to react with the samples under laboratory temperature and pressure conditions to simulate the corrosion mechanism of unsaturated fluids such as freshwater. In the second group of experiments, 0.2% acetic acid solution was used to react with the sample at 108°C and 50 MPa to simulate the organic acid dissolution under the temperature and pressure in the mature stage of source rocks. During the experiments, the ion concentration of reaction product was measured in real time at a certain time interval. Micro-nano CT scanning and porosity/permeability measurement were conducted on both groups of samples to detect the difference before and after the dissolution reaction.

The simulation results (Fig. 9a, b) show that the porosity and permeability of the two groups of experimental samples were significantly increased after the reaction, indicating that the experimental fluids and the samples had evident corrosion reactions, creating a large number of dissolved pores. The two fluids generated ions basically consistent in type and concentration, which are primarily SO₄^2-, Na⁺ and Ca²⁺. Combined with the characteristics of dissolved pores on the CT images, it can be seen that the dissolved components in the sample are mainly sulfate minerals such as anhydrite (CaSO₄) and glauberite (Na₂SO₄·CaSO₄). There is no evident phenomenon of pore increase by dissolution for carbonate minerals, which is consistent with actual geological facts (Fig. 2j-k).

According to the results of simulation experiments, the formation mechanism of dissolved pores in the Yingxi subsalt strata is mainly the physical dissolution of sulfate minerals under the action of unsaturated fluids. According to the chemical reaction mechanism, the weakly acidic organic acid does not chemically react with the strong acidic sulfate mineral itself. Further, the organic acid fluid formed by the thermal evolution of source rock will quickly reach a saturated state in the formation containing a large amount of evaporite^[32,33]. At the same time, due to the high shale content in the carbonate rocks in the area and the small size and isolated distribution of matrix pores, the mobility of organic acid is limited. Collectively, it does not have the conditions for large-scale dissolution and porosity enhancement of carbonate minerals, consistent with the simulation results. Thus, the under-saturated formation water increasing porosity by dissolution can only be sourced from the precipitation in the sedimentary to pene-contemporaneous stage and the involvement of fluvial freshwater^[34,35,36]. In the lithofacies sequence, the dissolved pores of anhydrite cement in the laminated dolomitic limestone at the bottom of the low-energy sedimentary sequence were mainly formed by the desalination and dissolution of seasonal lacustrine basin water during the sedimentary period of the argillaceous lamina. For the upper lithofacies combination of the two sequences, the plaque calcareous dolostone and mixed granular lithofacies at the top of the low-high energy sedimentary sequence in the paleo-uplift area are more exposed to the leaching and dissolution of atmospheric freshwater to form crystal casting molded pores and inter-granular dissolved pores. For the plaque calcareous dolostone lithofacies at the top of the low-energy sedimentary sequence in the depression area, the injected freshwater is easy to quickly reach saturation due to the high salinity of lacustrine basin, and the strength of dissolution and pore enhancement is relatively weak. Therefore, the lithofacies type and sedimentary sequence control the formation of dissolved pores in the pene-contemporaneous period, and the intensity of freshwater leaching and injection and the action time control the development of dissolved pores in different cycles as a whole. Temporally and spatially, different lithofacies sequences lead to differences in the scale and distribution of dissolved pores. In the depression area, dissolved pores are mainly developed in the laminated dolomitic limestone lithofacies at the bottom of the sequence; in the uplift area, dissolved pores are developed in the combination of plaque calcareous dolostone and mixed granular lithofacies at the top of the sequence.

During the burial diagenesis stage, the intercrystalline pores and dissolved pores formed in the pene-contemporaneous period of carbonate lithofacies are characterized by small size and strong resistance to compression and cementation, and they were well preserved in the diagenesis period^[15]. The reservoir-controlling effect of diagenetic lithofacies is mainly reflected in the laminated dolomitic limestone facies, inside which the organic-rich argillaceous lamina create laminar fractures on the laminar plane during the compaction and dehydration process. After the pressure unloading of mudstone by compaction and dehydration, the laminar fractures are closed by the overlying formation pressure; however, the laminar fractures are activated and opened to become efficient storage spaces and migration channels under the overpressure background during the process of organic matter maturation and hydrocarbon expulsion^[11].

According to the results of previous studies, under the regional continuous tectonic compression during the Late Himalayan, the large-scale Shizigou thrust detachment-thrust fault was developed in the thick inter-salt evaporite layer in the area, and the homochromous secondary thrust system was derived and formed in the sub-salt strata^{[4, 19]}. The varying trends of the mineral composition of the two subsalt lithofacies sequences all indicate the characteristics of lower shale content and stronger brittleness upwards. Therefore, under the background of the structural transformation activity, the highly brittle mixed granular and massive-plaque lithofacies are more likely to be squeezed, crumpled, and broken to form tectonic breccia reservoirs. Regionally, the upper part of Pay zone IV is closer to the detachment fault, the structural deformation stress is strong, and the distribution of various highly brittle facies is stable. Therefore, the tectonic breccia reservoirs formed can be contiguously distributed. But in the zones in close contact with the evaporation layer, the plastic flow of the gypsum salt rock cause the brecciated fractures and vugs to be easily filled and cemented by the gypsum salt. In the lower part of Pay zone IV and the underlying layer which are far away from the regional detachment fault, the slippage crumpling effect is weak and the brecciation is also weak. Tectonic breccia reservoirs are relatively developed only within the range of strong squeezing thrust stress on both sides of the secondary faults, and they are underdeveloped and gradually disappear in the zones far away from the secondary faults^[18].

Based on the abovementioned discussion on the lithofacies types, sedimentary sequence and reservoir-controlling mechanism of the subsalt reservoirs in the upper part of the Lower Ganchaigou Formation (E₃²) in the Yingxi area, together with the structural setting in the hydrocarbon accumulation period and the well productivity in the area, the subsalt lithofacies-structural composite reservoir formation model of E₃² in the Yingxi area is established (Fig. 10), and the temporal and spatial development and distribution of different lithofacies are obtained.

Inherited depression zone (zone B in Fig. 1a): It is located in the footwall of the Shizigou large-scale detachment fault. In this zone, the reservoirs were formed under the setting of inherited sedimentary depression, and they are currently deeply buried (Fig. 10a). This zone is adjacent to the secondary thrust fault zone, so small-scale tectonic breccia calcareous dolostone fractured-vuggy high-efficiency reservoirs are developed. The laminar dolomite dissolved pore shale oil reservoirs are dominant. However, the massive-plaque calcareous dolostone has low permeability and does not have good economic energy storage and productivity at large burial depth.

Depression reversed structural zone (zone C in Fig. 1a): It is located in the Shizigou large thrust detachment fault and secondary thrust fault system area. The reservoirs were formed under the setting of depression inversion structure, and are currently shallowly buried. The massive-plaque calcareous dolostone in the tectonic brecciation interval in the upper part develop a large scale of contiguous tectonic breccia calcareous dolostone fractured-vuggy high-efficiency reservoir under the action of detachment and crumple. Below the brecciation interval, the favorable reservoirs of laminar dolomite shale oil are distributed in a large area (Fig. 10a, b).

Intra-depression paleo-uplift zone (zone D in Fig. 1a): The deposition period was represented by paleo-uplift in the depression, which is also in the area affected by the Shizigou large-scale thrust detachment fault and the secondary thrust fault system. The reservoirs were formed under the setting of low-amplitude anticline. The tectonic brecciated lithofacies interval in the upper part is mainly composed of brecciated calcareous dolostone fractured- vuggy high-efficiency reservoirs formed due to the tectonic brecciation of mixed granular rocks and massive- plaque calcareous dolostone. Coupling with the development of pene-contemporaneous dissolved pores, the storage properties are better than those in the middle part. Below the brecciation interval, there are mainly multi- phase mixed granular rocks and plaque calcareous dolostone lithofacies dissolved pore reservoirs (Fig. 10a, c).

Depression peripheral slope zone (zone A in Fig. 1a): The structural influence of this area is relatively weak, and both the deposition and hydrocarbon accumulation occurred under the slope setting. The reservoirs are mainly multi-stage mixed granular rock and plaque calcareous dolostone dissolved pore-intercrystalline pore reservoirs (Fig. 10c).

Saline lacustrine hybrid carbonate reservoirs are widely developed in the major pay zones of E₃² subsalt strata in the Yingxi area. Based on the significant differences in structure and mineral composition, five types of reservoir lithofacies are identified, including four sedimentary lithofacies (mixed granular lithofacies, massive calcareous dolostone lithofacies, plaque gypsum-bearing to gypso calcareous dolostone lithofacies, and laminated dolomitic limestone lithofacies) intensively developed in the lake level decline period, and one tectonic lithofacies, i.e. tectonic breccia calcareous dolostone reservoir formed under the action of late structural transformation.

For the E₃² subsalt in the Yingxi area, under the background of different paleo-morphology and saline deposition at high-frequency oscillation of lake level, four types of sedimentary lithofacies constitute two types of lithofacies sequence combination, which are the combination of low-energy saline sedimentary sequences and the combination of high-energy saline sedimentary sequences. The former is mainly developed in the depression zone, and the latter is in the peripheral slope and paleo-uplift zone in the depression. Affected by frequent replenishment of sources, the two types of sedimentary sequences in the area are mostly incomplete lithofacies subtypes, and the thickness of a single layer is controlled by the cycle period. The lithofacies distribution is the result of high-frequency superposition of the two saline sedimentary sequences temporally and spatially.

The formation of Yingxi E₃² subsalt reservoirs is typically lithofacies-controlled. Lithofacies control the formation and distribution of pene-contemporaneous intercrystalline pores and dissolved pores of dolomite. The laminated dolomitic limestone lithofacies of diagenesis period control the formation of large-scale laminar fractured reservoir spaces and high permeability channels. Mixed granular and massive-plaque calcareous dolostone lithofacies have low shale content and strong brittleness, and formed high-efficiency tectonic breccia calcareous dolostone lithofacies reservoir under the action of late structural transformation. The development degree of brecciated fractures and vugs is controlled by the distance from the top detachment fault or secondary fracture to the reservoir. Furthermore, the brecciated fractures and vugs contacting the evaporation salt rock have a high degree of filling and cementation and poor physical properties.

The lithofacies-structural composite reservoir formation model for the Yingxi E₃² subsalt strata is established. The temporal and spatial development and distribution of tectonic breccia calcareous dolostone lithofacies fractured-vuggy high-efficiency reservoirs, laminated dolomitic limestone lithofacies dissolved pore shale oil reservoirs, and granular-plaque calcareous dolostone lithofacies dissolved pore-intercrystalline pore tight reservoirs in different zones are clarified. These results are of great significance for enriching the theory of hydrocarbon accumulation of hybrid carbonate rocks in the saline lacustrine basin, promoting efficient development in the Yingxi area and expanding the exploration in the Ganchaigou Formation in adjacent areas with similar structural and sedimentary setting.