The Lower Cretaceous Tengger Formation located in the Baiyinchagan Sag of the Erlian Basin comprises mainly deeply buried tight sandstone. The identification of high-quality reservoirs in these thickly stacked and heterogeneous units requires a comprehensive understanding of the diagenetic environmental history of the rocks. This paper reports an integrated study involving thin-section petrography, scanning electron microscopy, X-ray diffraction, fluid-inclusion analysis, and vitrinite reflectance analysis of Tengger Formation sandstones with the aim of characterizing the diagenetic conditions of the reservoir rocks and providing guidance for future petroleum exploration. Observed mineral assemblages, the distribution of authigenic minerals, and the distribution and nature of pores suggest the presence of two types of diagenetic environment, acidic and alkaline, which have varied over time and vertically through the rock column. Acidic conditions are indicated by quartz overgrowths and dissolution of both feldspar and carbonate cement. In contrast, alkaline conditions are indicated by the precipitation of carbonate cement, feldspar overgrowths, quartz dissolution, and occurrences of authigenic illite and chlorite. Changes in pore fluid chemistry controlled the evolution of the diagenetic environment. The early diagenetic environment from 110 Ma to 107 Ma was syndepositional and thus controlled by the chemistry of water in depositional centers, which is interpreted to have been weakly alkaline. Significant burial that occurred at 107 Ma induced pulses of hydrothermal fluids and petroleum into the reservoir rocks, which caused a shift to an acidic diagenetic environment. From 103 Ma to 70 Ma, subsequent episodes of uplift and burial caused periodic alternation between acidic and alkaline diagenetic environments. Three distinct episodes of oil and gas charging interpreted from petrography and the homogenization temperatures of fluid inclusions played a critical role in the enhancement of porosity through time. From 70 Ma to the present, acidic diagenesis gradually weakened because of the consumption of organic acids during the process of interaction between rocks and fluids. This study demonstrates the importance of understanding the diagenetic history of reservoir rocks and provides the basis for improved reservoir characterization and optimized hydrocarbon exploration of the Tengger Formation.

With the global decrease in conventional oil and gas production, and the development of new recovery technologies such as horizontal drilling and hydraulic fracking, tight sandstone reservoirs have become important hydrocarbon exploration and development targets [15]. Recovery is highly dependent on reservoir properties such as porosity and permeability and their spatial distribution, and these properties are controlled by the interplay of sedimentary facies, tectonic activity, and diagenesis [68]. During burial of basin sediments, diagenesis plays a crucial role in determining the quality of the reservoir. Diagenetic processes include the growth of authigenic minerals and infilling of intergranular pores by cements, leading to a reduction in porosity and permeability, and the dissolution of feldspar to form secondary pores, which contribute to increasing porosity [913].

Diagenesis is controlled by the reactions between rock minerals and diagenetic fluids [1420]. As fluid flows into rock through cracks and pores, reactions between the fluid and minerals generate new combinations of minerals and change the chemical composition of the fluid, causing the diagenetic system to shift to a new state of chemical equilibrium. Fluid chemistry can be defined by the pH value, with values of <8.5 defining acidic fluid and those of >8.5 defining alkaline fluid [2125]. In hydrocarbon systems, acidic conditions can be created by hydrogen sulfide (H2S), thermochemical sulfate reduction generated in deep reservoirs, and bacterial sulfate reduction in shallow reservoirs, as well as various acids produced by biodegradation. Bacterial sulfate reduction mostly occurs in low-temperature environments, where pyrite crystallizes rapidly form. Thermochemical sulfate reduction occurs at higher temperatures, and the iron ore formed is mostly slowly crystallizing cubes. The reservoir burial depth is greater than 4500 m, and secondary pores are still developed, which is believed to be related to the dissolution of H2S and CO2 on carbonate rocks produced by thermochemical sulfate reduction [2633]. Alkaline fluids are generally brines related to mafic rocks. Microbial reduction of organic acids and the inhibition of organic matter decarboxylation may also increase formation water alkalinity [3438]. Thus, knowledge of the evolution of the diagenetic environment produced by diagenetic fluids and minerals represents the scientific basis for estimating secondary pore formation and overall hydrocarbon reservoir quality.

Recent studies have shown that the characteristics of paleo-fluids can be interpreted from investigative methods including petrography, X-ray diffraction, fluid-inclusion analysis, and mineral spectroscopy (such as laser Raman spectroscopy) [35, 37]. Of these methods, the analysis of fluid inclusions in diagenetic minerals can give reliable information about subsurface paleotemperature and paleo-pressure and about the nature of paleo-fluids in diagenetic environments. Therefore, fluid-inclusion analysis has become a favored method for studying the diagenetic and thermal evolution of reservoirs, temperature and pressure conditions, and the nature of paleo-fluids, as well as for identifying episodes of oil and gas charging.

Previous studies of the Tengger Formation reservoir units have shown that rock samples are characterized by permeabilities of <1 mD [39, 40]. Although the complex mineral assemblages are suggested to have been caused by diagenetic processes, the precise nature of these processes, as well as the record of diagenetic fluid events and diagenetic stages, is poorly known. To improve the understanding of the origin and distribution of high-permeability reservoir intervals, a reliable interpretation of the evolution of the diagenetic environment and details of the spatial and temporal relationships between fluids, minerals, and pores is required. Accordingly, the objectives of this multi-method study of the Tengger Formation reservoir units were to (1) differentiate acidic and alkaline diagenetic environments, (2) interpret the timing of hydrocarbon accumulation and its impact on diagenetic processes, (3) examine the mechanisms/processes of diagenesis under different conditions, (4) establish a model of the evolution of diagenesis, and (5) guide the exploration for high-quality hydrocarbon reservoirs.

2.1. Geological Setting

The Erlian Basin is situated in northern China and is a fault-sag superimposed basin that developed during the early Paleozoic (Figure 1(b)). The Lower Cretaceous of the Baiyinchagan Sag is located on the western margin of the Erlian Basin (Figure 1(a)). The NNW-oriented sag is 150 km long and 15–28 km wide. Early Cretaceous active faulting of the Tara Fault caused subsidence and the consequent formation of accommodation space of the Baiyinchagan Sag. The western part of the sag constitutes the major areas of oil and gas exploration, where secondary faults have formed a typical “dustpan” fault structure comprising the Tara Fault structural zone, the western subdepression zone, and the Baiyinwengte fault structural zone.

The bedrock lithology of the Erlian Basin comprises early Paleozoic metamorphic rocks [39]. A sedimentary succession with a thickness of >5000 m includes the Lower Cretaceous Baiyanhua Group comprising the Alshan, Tengger, Duhongmu, and Saihantala formations (Figure 2). The succession is overlain by the Upper Cretaceous Erliandabusu Formation. Identified sedimentary environments include alluvial fan, northern fan delta, turbidite fan, southern braided river delta, shore–shallow lacustrine, semi-deep lacustrine, and deep lacustrine facies. This study focuses on the Tengger Formation reservoirs of the Tara Fault structural belt. The Tengger Formation is composed of sedimentary rocks of fan-delta facies, with a lithology dominated by interbeds of dolomitic mudstone, sandstone, and sandy conglomerate.

2.2. Samples and Analytical Methods

A total of 16 samples interpreted as subaqueous distributary channel deposits in a fan-delta front were collected from 7 wells at various depths and including 3 distinct tectonic zones. Detailed petrographic and fluid-inclusion data were obtained for selected samples and interpreted with respect to reservoir geology and used to refine the burial history of the studied units (Figure 3).

2.2.1. Petrography

Polarizing microscopy was used to examine thin sections of 16 samples for detrital grain composition, grain size, the nature of grain contacts, and pore types. Point counting (300–500 points) was used to reduce the statistical errors commonly caused by the uneven distribution of minerals. Alizarin Red S and potassium ferricyanide solutions were mixed at a volume ratio of 3 : 2 for dyeing analysis to differentiate dolomite from calcite cement. Scanning electron microscopy (SEM) was applied to all 16 samples to analyze the morphology, composition, and fabric of mineral grains, and particularly to distinguish the types and quantities of clay minerals, such as kaolinite, illite, and chlorite, to determine the sequence of diagenetic evolution. SEM can be used to infer the diagenetic environment as well as the sediment transport history based on the surface properties of mineral grains. A fully automatic X-ray diffractometer was utilized to analyze clay minerals. The assemblages of and changes in clay minerals can be used to interpret the diagenetic environment and geochemical background. The above experiments were completed in the Zhongyuan Oilfield Geological Research Institute.

2.2.2. Fluid Inclusions

Fluid inclusions contain important information about the temperature and composition of fluids at the time of crystallization [4145]. A total of 16 samples containing fluid inclusions were analyzed using a LINKAM THMS600-7035 temperature control stage. The instrument was set to measure temperatures ranging from −196 to 600°C, with an error of ±0.1°C. Fluid-inclusion analyses were performed at the Zhongyuan Oilfield Geological Research Institute. Inclusions in calcite and quartz were measured to obtain the homogenization temperature and freezing-point temperature (for salinity estimations). Samples were made into 0.05–0.2-mm-thick double-sided polished inclusion slices for fluid-inclusion analysis. The characteristics, types, and occurrence of inclusions were identified in detail by fluorescence and with a polarizing microscope. Representative fluid inclusions were selected under the microscope for inclusion temperature measurement and for analyses of the petrography, occurrence, size, phase state, and fluorescence characteristics of inclusions of different stages. Prior to temperature measurements of the selected fluid inclusions, artificial 25% CO2–H2O and pure H2O inclusions were used for system calibration. All measurements were conducted under temperature and relative humidity of 25°C and 30%, respectively. Homogenization temperatures were measured for the hydrocarbon-bearing saline inclusions accompanying the hydrocarbon inclusions. The accuracy of temperature measurements using the homogenization method is ±1°C. Freezing temperatures of fluid inclusions and the relationship between the melting temperature of CO2 clathrate and salinity allowed salinity data (in %) for fluid inclusions to be obtained.

The petrographic characteristics of fluid inclusions include a range of shapes and distributions, with features such as quartz overgrowths, carbonate cement, and cracks. Various fluorescence colors of hydrocarbon inclusions from red, to orange, yellow, green, blue, and bright blue reflect different levels of petroleum maturity. Secondary fluid inclusions observed within authigenic minerals provide insights into the stages of diagenetic fluids. The salinity of fluid inclusions may reflect the properties of the diagenetic fluids, and fluid inclusions that formed at the same time generally have similar salinity values [44]. The salinity of fluid inclusions can be calculated based on the freezing-point temperature (Eq. (1)).
W-salinity (%); θ-freezing-point temperature (°C).

2.2.3. Sedimentary Basin Thermal History

Although there are many available methods for restoring the geothermal history of sedimentary basins, two are most commonly applied. The first is to use paleotemperature scales to simulate the thermal history of sedimentary basins, including paleotemperature information gained from vitrinite reflectance, fluid inclusions, and apatite fission tracks. The second is to use geophysical models of basin evolution to study paleotemperature. In the present study, the paleotemperature scale method was used to restore the thermal history of the Baiyinchagan Sag. For this method, previous research and drilling data, including vitrinite reflectance data, were analyzed to estimate the amounts of denudation and/or burial for different periods. Using the resultant burial history, the paleogeothermal gradients of different periods were estimated using BasinMod software, allowing the thermal history of the basin to be reconstructed.

3.1. Petrographic Characterization

Minerals identified during this study include feldspathic sandstone, lithic feldspar sandstone, feldspathic lithic sandstone, and a small amount of feldspar quartz sandstone (Figure 4). The content of quartz is 20–70 vol.%, with an average content of 38 vol.% (Table 1). Monocrystalline quartz and a small amount of polycrystalline quartz are observed. Some quartz grains show secondary enlarged edges, and some display cracks on the surface. Feldspar is mainly plagioclase and K-feldspar, with the content ranging from 8 to 56 vol.% (average of 38 vol.%). Some feldspars are bent, and some show dissolution at the edges or interiors (Figure 5). The average content of metamorphic or sedimentary lithic fragments is about 20 vol.%. The average Q/(F + R) index is calculated as 0.68, indicating low compositional maturity. The content of mica ranges from 1% to 17%. Interstitial material is composed mostly of precipitated carbonates, silica, and clays, with minor anhydrite and pyrite. The grain size of feldspar ranges from 0.03 to 14.00 mm. Linear to jagged contacts between grains indicates a change in the level of compaction from low to high.

The pore types developed in the Tengger Formation samples are mainly residual primary intergranular pores that were preserved after compaction and cementation. Secondary pores include intragranular dissolved pores formed by the dissolution of feldspar particles or carbonate cement. According to porosity and permeability characteristics, the Tengger Formation in the study area is classified as a tight sandstone reservoir.

3.2. Characterization of Fluid Inclusions

Hydrocarbon inclusions and associated saline inclusions have clear boundaries and diameters of 3–10 μm (Table 2). Inclusions show elliptical, subcircular, and irregular shapes, and they occur mostly in groups, bands, or lines and/or as isolated patterns in mineral cracks or in the secondary growth rims of quartz grains (Figures 6(a) and 6(i)), calcite cement (Figures 6(c), 6(g), and 6(h)), or quartz-grain cracks (Figures 6(b), 6(d), 6(e), and 6(f)). Hydrocarbon inclusions exhibit varying fluorescence colors, which are inferred to reflect three distinct episodes of reservoir charge, i.e., yellow (first episode), yellow–green (second episode), and blue–white (third episode) fluorescence. Hydrocarbon inclusions of the first episode are characterized by light-brown (under transmitted light) and yellow fluorescence, nearly elliptical and elongated shapes, diameters of 3 to 9 μm (Figure 6(b)), and occurrence within cracks in quartz-grain interiors. Hydrocarbon inclusions of the second episode are characterized by blue fluorescence (under transmitted light), irregular shapes, diameters of 2 to 5 μm (Figure 6(d)), and occurrence within secondary growth rims of quartz grains. Hydrocarbon inclusions of the second episode are characterized by light-blue–white fluorescence, mostly irregular shapes, diameters of 3–6 μm, and occurrence within cracks penetrating through quartz grains (Figure 6(f)).

Homogenization temperatures reveal four stages of diagenetic fluid activity (excluding the initial sedimentary stage) and three episodes of hydrocarbon charge. The four stages of diagenetic fluid activity are as follows (Figure 7(a)): (1) the first stage of 80–100°C with a main peak of 85–95°C; (2) the second stage of 100–120°C with a main peak of 110–120°C; (3) the third stage of 120–135°C with a main peak of 125–130°C; and (4) the fourth stage of 135–155°C with a main peak of 140–150°C. The three episodes of hydrocarbon charge are as follows (Figure 7(b)): (1) 85–105°C with a main peak of 90–100°C; (2) 105–130°C with a main peak of 110–120°C; and (3) 130–155°C with a main peak of 140–150°C.

The results for salinity can be divided into two groups (Figure 8). Group I contains low-temperature, high-salinity brine inclusions with homogenization temperature ranging from 81.8 to 97.3°C and salinity ranging from 8.01% to 9.60%. Group II contains high-temperature, low-salinity brine inclusions characterized by homogenization temperature ranging from 99.4 to 141.4°Χ and salinity ranging from 3.99% to 6.30%. The shift from Group I to Group II indicates the process of shallow burial following sedimentation to diagenetic stages.

4.1. Characteristics of Acidic and Alkaline Diagenetic Environments

In general, the pH of reservoir fluids controls the diagenetic environment and resultant changes in mineral composition and porosity [4657]. For acidic pH conditions of 2–8.5, the solubility of SiO2 is constant, but for alkaline pH conditions of >8.5, the solubility of SiO2 increases [3740]. Rapid increases in pH to alkaline conditions cause dissolution of quartz and precipitation of calcite, as well as the development of feldspar overgrowths. The behaviors of quartz, feldspar, and carbonate cement can therefore indicate pH. The dissolution of quartz and precipitation of carbonate and feldspar indicate an alkaline diagenetic environment, whereas the dissolution of carbonate and feldspar, the development of quartz overgrowths, and the occurrence of authigenic kaolinite indicate an acidic diagenetic environment.

In the present study, carbonate cements coexist with quartz dissolution, indicating alkaline conditions. Carbonate cements such as micritic calcite and dolomite are developed predominantly in intergranular pores and dissolution pores (Figure 9(a)), or they replace the edges of quartz and feldspar. The contact relationships of the minerals suggest a sequential formation of carbonate cement calcite (Figures 5(c)–5(e)), Fe-calcite, and Fe-dolomite. Fe2+ and Mg2+ were likely sourced from clays or alkaline fluids from deeper strata. Feldspar overgrowths measuring 0.02–0.13 μm in thickness and albitization (Na+ and K+ isovolumetric replacement reaction) indicate alkaline conditions. During oil and gas charging, most of the pore water would have been replaced, hindering the migration of K+ and thereby inhibiting the albitization of feldspar. Quartz dissolution, as inferred from irregular serrulate or crenulate quartz grains, has an estimated ratio of dissolved area of 2%–5% for individual grains (Figure 5(c)). The dissolution occurs on the surface of authigenic quartz and is associated with feldspar overgrowths and carbonate cementation, suggesting an alkaline diagenetic environment. Authigenic illite is developed on mineral surfaces and fills intergranular pores (Figures 9(b) and 9(i)), whereas authigenic chlorite forms diaphragm-like shapes covering the margins of particles (Figures 9(c) and 9(g)). Illite was converted from kaolinite and smectite under the alkaline diagenetic environment [42, 48].

The abundance of quartz overgrowths and authigenic quartz indicates an acidic diagenetic environment (Figures 9(d) and 9(i)). Data suggest at least two stages of quartz overgrowth (Figures 5(a), 5(c), and 5(f)). Authigenic quartz in the form of fine grains or prismatic hexagonal crystals filled intergranular pores. Precipitated SiO2 was sourced from the pressure dissolution of quartz particles [44, 51], transformation of clay minerals, and dissolution of feldspar. Feldspar dissolution occurs at burial depths of 1600–1800 m and 2000–2200 m (Figure 10). Feldspar dissolution is evident from serrulate edges of minerals and from honeycomb or irregular shapes of internal cleavage joints. These secondary pores were crucial sites for subsequent hydrocarbon accumulation. By-products of feldspar dissolution were SiO2 and kaolinite. Book-leaf or worm-like occurrences of kaolinite accumulated in intergranular pores (Figures 5(e) and 9(e)). The dissolution of feldspar suggests alternation of acidic and alkaline diagenetic environments (Figure 9(f)).

The results of the present study show a negative correlation between the content of carbonate cements and permeability (Figure 10). Dissolution of the early carbonate cements at burial depths of 1400–1600 m caused an increase in porosity. The dissolution of late carbonate cements at burial depths of 1900–2100 m suggests the existence of late acidic fluids. The characteristics of diagenetic minerals of the studied samples show that the properties of Tengger Formation reservoirs changed according to changing burial depth and further confirm the vertical variation in diagenetic environmental conditions through the rock column. The evolution of the diagenetic environment was characterized by multiple stages of alternating (acidic–alkaline) conditions, reflected in the vertical variation in the diagenetic environment. From burial depths of 1000 to 2500 m, the high contents of illite and carbonate suggest a continuous alkaline diagenetic environment. From depths of 1000 to 1200 m, except for the influence of compaction, the low-quality reservoir physical properties are related mainly to the occurrence of carbonate cements that formed during the early diagenetic stage. At a depth of 1200 m, the increasing content of kaolinite is interpreted as signifying an increase in the influence of acidic fluid. However, at a depth of 1600 m, the content of kaolinite decreases, suggesting a weakly acidic environment. At depths of 1200–1600 and 2100–2400 m, the contents of quartz and kaolinite show high values, whereas the contents of feldspar and rock fragments are low. Combined with observations of thin sections, feldspar dissolution is interpreted to have occurred in these depth intervals, indicating that the diagenetic environment was predominantly acidic. At depths of 1600–2000 m, contents of quartz are low, whereas the contents of rock fragments and feldspar are relatively high, indicating an alkaline diagenetic environment.

4.2. Evolution of the Diagenetic Environment

The instantaneous heat flow method was applied using the BasinMod basin thermal history simulation system, which is suitable for modeling short-term rapid burial histories of sedimentary basins for the Cenozoic [5860]. Using this method, the burial and thermal histories of Well C2 were generated and then combined with the known stratigraphy and the main tectonic events, as well as the homogenization temperatures of saline inclusions calculated in the present study (Figures 10 and 11). The evolution of the diagenetic environment was then interpreted by integrating the burial and thermal histories with results of petrography and fluid-inclusion analysis.

The early diagenetic environment of the Tengger Formation fan-delta sedimentary deposits was controlled by formation water chemistry. Trace-element data for the Baiyinchagan Sag suggest that the paleoclimate was hot and dry from 110 Ma to 107 Ma, and that the groundwater was alkaline [39, 40]. B/Ga ratios for the Tengger Formation are mostly <4.50, Sr/Ba ratios are mostly <1.0, and Fe/Mn ratios are all >5 and range from 20 to 400. The above ratios of inorganic elements indicate that the study area was under a continental sedimentary environment. Sr/Ca ratios are mostly greater than 0.01, indicating that the study area was a saline lacustrine basin (Table 3). The early alkaline conditions are further supported by the abundance of dolomitic mudstones and micritic carbonate cements.

During burial, the Tengger Formation underwent multiple transitions and alternations between acidic and alkaline diagenetic environments (Figure 12). The pore fluids interacted with minerals along sedimentary bedding planes, fractures, and faults. The generation of organic acids and petroleum promoted the formation of acidic conditions, which caused the dissolution of feldspar and carbonate minerals, with a resultant increase in porosity. The vitrinite reflectance (Ro) of Tengger Formation source rocks ranges from 0.55% to 1.07%, indicating that most of the source rocks entered the oil window. Combining the homogenization temperatures of fluid inclusions with the burial and thermal histories of Well C2 reveals three distinct episodes of oil and gas charge: (1) In episode 1, during 105–103 Ma, Ro was 0.55%–0.70%, the source rocks had low maturity, and unstable minerals began to dissolve; (2) in episode 2, during 103–97 Ma, Ro was 0.70%–1.07%, indicating large-scale oil and gas charging, as supported the abundant hydrocarbon inclusions; and (3) in episode 3, during 97–95 Ma, oil and gas charging occurred, but more weakly than in episode two, and the decarboxylation of organic acids gradually weakened.

The alkaline fluids became enriched with alkaline ions during the diagenetic process [16, 17]. The consumption of organic acids and various diagenetic alteration reactions in the Tengger Formation sandstones resulted in gradual increases in the concentrations of Ca2+, Mg2+, and Fe2+ cations in the pore fluids and the alkaline evolution of these fluids. Late-formed, iron-bearing carbonate minerals represent the products of these alkaline diagenetic conditions (Figures 11 and 13). Furthermore, faults in the strata underlying the Tengger Formation were frequently active from 107 Ma to 97 Ma, bringing alkaline fluids from deeper, older strata up to the formation rocks. Late-stage calcite veins can be observed in cores and thin sections, indicating the intrusion of alkaline fluids.

On the basis of diagenetic characteristics and the modeled burial and thermal history of the Tengger Formation, the diagenetic stage of the Tengger Formation reservoirs is interpreted as medium diagenetic A–B stage [29, 56] (Figure 13). The specific evolution of the diagenetic conditions was as follows. From the beginning of deposition from 110 Ma to 107 Ma, alkaline sedimentary conditions provided the main control on the early diagenetic environment, and the reservoirs were in the early diagenetic A stage. Fe2+, K+, Ca2+, Mg2+, and Na+ were generated during the process of hydration, which provided material for the formation of early micritic calcite [39]. A small amount of chlorite occurs in the rims of grains, which indicates that the formation of these grains postdated compaction. The generation of chlorite inhibited the development of permeability to some extent and hindered siliceous cementation, but was conducive to the preservation of primary pores. From 107 to 103 Ma, the Tengger Formation reservoirs were in the early diagenetic B stage and entered the first stage of alternating acidic and alkaline diagenetic environments, and the formation temperatures varied from 80 to 105°C. During this period, organic matter matured gradually. The first episode of small-scale, low-maturity oil charging occurred during 105–103 Ma. During this episode, micritic carbonate minerals and feldspars underwent minor dissolution. Because of the compaction, the contact edges of some quartz grains became embayed. In addition, some quartz grains developed narrow and discontinuous growth rims. However, the short duration and weakly acidic diagenetic conditions rapidly disappeared and transitioned to weakly alkaline conditions. During this time of weak alkalinity, the margins of quartz grains were dissolved, and carbonate cements developed around the edges of quartz and feldspar grains, reflecting their later formation compared with the earlier development of quartz overgrowths. From 103 to 97 Ma, the reservoirs were in the early diagenetic B stage to middle diagenetic A stage, characterized by formation temperatures of 105–130°C and a shift to an acidic diagenetic environment caused by the influx of organic acids. The second episode of oil charging occurred between 103 and 97 Ma. After the dissolution of carbonate cements and feldspars, intergranular dissolution pores were developed, which enhanced pore connectivity and hence permeability. Another stage of development of quartz overgrowths also occurred. From 97 to 70 Ma, the Tengger Formation underwent tectonic uplift then subsidence, and micro-fractures formed. During this period, the reservoirs were in the second stage of alternating acidic and alkaline diagenetic environments. From 97 to 95 Ma, the third episode of oil and gas charging occurred, and some quartz grains developed growth rims. From 70 Ma to the present, acidic diagenesis gradually weakened because of the consumption of organic acids during the process of interaction between rocks and fluids. The occurrence of carbonate cement and authigenic illite indicates that an alkaline diagenetic environment has prevailed since 70 Ma.

Our multi-method investigation of the Tengger Formation reservoirs using evidence from minerals, pores, fluid inclusions, and the burial and thermal history of the formation reveals that the reservoirs underwent five stages of diagenesis with different environmental conditions. Fluid-inclusion homogenization temperatures and salinities allow four stages of diagenetic fluids and three episodes of oil and gas charging to be identified. In future research, we will explore the conditions, pathways, and evolution of diagenetic fluid activity in the study area through C and O isotope studies and laser Raman analysis to further understand diagenetic processes and evolution in continental faulted lacustrine basins.

All experimental data comes from the Zhongyuan Oilfield Laboratory and is not disclosed on the website due to internal confidentiality. Thank you.

The authors declare no conflict of interest that may be perceived as inappropriately influencing the presentation or interpretation of reported research results.

Dr. Milovan Fustic and Dr. Raphael A. J. Wüst from the University of Calgary are sincerely thanked for their critical reviews and constructive comments on the manuscript. This work was supported by National Major Science and Technology Projects of China [Grant No. 2016ZX05007-004-02 and Grant No. 2016ZX05046-005-01 and National Natural Science Foundation of China [Grant No. 41002032 and Grant No. 42172135].

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