Tight oil is an unconventional oil resource. It has the following characteristics: hydrocarbon source rock is in the oil-generating window, source and reservoir are interbedded or adjacent, overburden matrix permeability is not greater than 0.1×10^-3 μm² (air permeability is less than 1×10^-3 μm²), there is no natural production single well or the natural production is below the lower limit of commercial oil production, but commercial oil production can be obtained under certain economic conditions and technical measures^[1]. China is rich in tight oil resources, with a geological resources of 178.20×10⁸ t^[1]. The Ordos, Sichuan, Junggar, Qaidam and Jiuquan basins in central and western China account for 74% of China's tight oil resources^[2,3]. The study on the mechanism and main controlling factors of tight oil accumulation in central and western China is of great theoretical significance and practical value for the efficient exploration and development of tight oil in China.

As for the mechanism and main control factors of tight oil enrichment in central and western China, many scholars in China and abroad have conducted in-depth discussions and summarized the following understandings: The first is the source reservoir assemblage, which is considered as the basic factor for the formation and enrichment of tight oil^{[4,5,6,7,8,9]}. The second is source-reservoir differential pressure, which emphasizes that the continuous strength source-reservoir differential pressure is the power source of oil and gas charging and enrichment^{[10,11,12,13,14]}. The third is source-reservoir quality and scale, which indicates that the scale of tight oil enrichment is closely related to source-reservoir quality and scale^[15,16,17]. The fourth is natural fractures, mainly structural fissures, which are not only the main channels for tight oil migration and accumulation, but also the important space for tight oil enrichment^[18,19]. Many scholars also believe that the accumulation of tight oil is the result of the joint control of multiple factors, such as the "source control", "reservoir control" and "band control" combined control mode of tight oil established by Wang Wenguang et al.^[20,21]. In addition, reservoir thickness, pressure coefficient and natural gas yield all affect the enrichment of tight oil^{[22,23,24,25,26]}. In terms of the mechanism of tight oil accumulation, many scholars believe that the narrow pore throats of tight reservoirs determine that oil flows mainly through non- buoyancy driven, non-Darcy flow and diffusion flow^{[27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]}.

As for the problems of unclear filling mechanism and control factors of tight oil, unclear accumulation rules and differential accumulation mode of tight oil in large freshwater lake clastic rocks in central China and small salty lake diamictite in western China, the physical simulation experiment, combined with typical examples, make a synthesis research, get the following conclusions and understanding.

In China, there are nine basins in which tight oil has been found, including Songliao, Bohai Bay and Sichuan, of which seven are discovered in central and western China (Fig. 1). The 6^th to 8^th members of the Triassic Yanchang Formation in the Ordos Basin are typical freshwater lakes. The Permian Lucaogou Formation in Jimsar sag of Junggar Basin and Lower Cretaceous Xiagou Formation in Qingxi sag of Jiuquan Basin have typical salt lake tight oil characteristics.

The Ordos Basin in central China, with an area of about 25×10⁴ km², is a large stable depression basin with relatively simple structure. The basement was formed from early Archean to late Archean, and the whole basin experienced multiple periods of tectonic uplift and subsidence and depression migration. Longitudinal fractures developed in the Yanchang Formation of the Zhongnan sag, which is the first set of source-reservoir series of freshwater deposition developed after the formation of the Zhongnan sag. The source rocks are mainly distributed in the ninth and seventh members of the Yanchang Formation (“Chang 9” and “Chang 7” for short). They are mainly composed of dark mudstone, oil shale, carbonaceous mudstone, among which the Chang 7 sedimentary period is the peak period of lake basin development, and dark mud shale and oil shale of deep lacustrine facies are the most important hydrocarbon source layers. The tight oil is mainly developed in the tight reservoirs of the semi-deep lacustrine and deep lacustrine facies belt at the lower part of the Chang 6 to Chang 8 members of the Yanchang Formation, and in the sedimentary formation at the front end of the delta. Among them, the tight sandstone reservoirs in the Chang 7₁ to Chang 7₂ sub-members of the Chang 7 Member, the 8₁ sub-member of the Chang 8 Member, and the Chang 6₄ and 6₃ sub-members of the Chang 6 Member are adjacent to the high-quality hydrocarbon source rocks in the Chang 7 Member. They have distinct and symbiotic source and reservoir and are the main layers of tight oil enrichment.

The Jimusar sag located in the southwest of the eastern uplift of Junggar Basin, with an area of about 1278 km², is an extruded depression. The Lucaogou Formation of the Permian System is distributed in the whole sag, with an average thickness of about 200-350 m. It is generally characterized by the sedimentary characteristics of delta front, beach-bar and dolomite-sandbar under the background of salty shore-shallow lake to (half) deep lake. The structure shows a gentle west dip monocline with high east and low west. The faults are rare but the stratifications and bedding fractures are developed. The Lucaogou Formation is the main hydrocarbon source layers in the sag. The area with the thickness of the source rock greater than 200 m is 806 km². The lithology is mainly dark gray mudstone, gray black mudstone and dolomitic mudstone. The Lucaogou Formation is divided from bottom to top into two sets of tight oil source-reservoir assemblages of the first (P₂l₁) and second (P₂l₂) salty lake diamictite. The tight oil is enriched in the upper and lower sweet spots of The Lucaogou Formation. The thickness of P₂l₂² is 13.3-43.0 m, with an average value of 33 m. It is mainly developed on the eastern slope of the depression. The lithology is mainly gray doloarenite, feldspathic lithic siltstone, and dolomite lithic sandstone with gray mudstone and dolomite mudstone. The thickness of P₂l₁² is 17.35-67.5 m, with an average value of 42.8 m, which is developed in all the depressions. The lithology is mainly gray (containing) dolomitic siltstone with gray mudstone or gray (containing) dolomitic siltstone, muddy siltstone and gray mudstone interbedded.

The Qingxi sag is located in the west of Jiuxi Depression, Jiuquan Basin, with an area of about 170 km². It is a strong extrusion type depression. The Lower Cretaceous sediments are mainly controlled by concave faults, and the maximum thickness of the sedimentary rocks is 7000 m. The sedimentary features of the detrital facies of shore-shallow lake fan delta - (half) deep lake dolomitic diamictite facies are mainly from the background of fresh water deposition to saline deposition. The tight oil is distributed around the Liugouzhuang fractured oilfield in the center of the depression. Vertically, it is mainly enriched in the tight reservoir of diamictite of clayey dolomite, dolomite mudstone and dolomite in the Xiagou Formation, and the buried depth is 3700-4610 m. The Xiagou Formation is also the main hydrocarbon source layers in the depression, mainly consisting of organic rich mud shale and dolomitic mudstone. And the content of organic carbon is 1.41%-3.54%, and the R_o value is 0.8%-1.2%. They are combined with tight reservoirs such as clayey dolomite and dolomite. The source-reservoir boundary is unclear, and the whole has the characteristics of source-reservoir assemblage. The bedding fractures and structural fractures are developed, and the oil and gas display section are up to 500 m. The matrix pores of tight reservoirs include secondary pores and micropore fractures, with porosity of 2%-6%, average value of 3.89%, permeability of (0.63-66.40)×10^-3 μm², average value of 1.94×10^-3 μm².

The enrichment process and mechanism of tight oil include the power of tight oil charging, filling channel, accumulation mode, and the flow state of tight oil. The characteristics of tight reservoirs with nanopores-fractures as the migration and accumulation space determine its enrichment process and mechanism different from conventional oil. Simulation experiment is undoubtedly an important way to reveal the process and mechanism of tight oil filling and enrichment.

2.1.1. Starting pressure of oil filling and ladder-like filling process

2.1.1.1. Experimental devices and samples

The core holder oil filling physical simulation experimental device independently developed by the Research Institute of Petroleum Exploration and Development is used. The experimental samples are taken from typical well mudstone cores of Xiagou Formation (K₁g) in Qingxi sag, and the reservoir physical properties parameters and experimental results are shown in Table 1.

The simulated oil is injected into the core sample through a injection pump, and the pressure and the simulated oil flow data through the core sample are statistically analyzed.

2.1.1.2. Analysis of experimental results

Filling of tight oil is a necessary process of tight oil enrichment, which is related to lithology to some extent. However, as a tight reservoir, the filling process and characteristics of different lithologies have certain commonalities. Although the experimental samples are limited, the following experiments represent the general filling characteristics of tight oil. Fig. 2 shows the injection amount (flow rate) obtained by simulating the filling experiment of No. 1, 2, 3 and 4 cores with simulated oil, and then the change curve of it with the filling pressure. It reflects the relationship between simulated oil injection amount and filling pressure (equivalent to pressure difference).

Starting pressure is the minimum pressure required to cause tight oil to flow from source rock into a tight reservoir. Under laboratory conditions, a certain initial charging pressure is required, namely, starting pressure, and then tight oil can suddenly enter and flow in tight reservoirs in large quantities. For the 4 samples shown in Fig. 2, when the filling pressure is increased to 0.5 MPa, the flow rate increases sharply. For example, for sample No. 4, the filling pressure is less than 0.5 MPa, and the amount of oil entering the sample is very small, less than 0.005 mL/s. After the pressure reaches 0.5 MPa, the amount of oil injected into the sample increases sharply, and the filling pressure ranges from 0.5 MPa to 0.7 MPa. The oil flow rate charged into the sample suddenly increases from less than 0.005 mL/s to 0.038 mL/s. The flow rate changes of samples 2 and 3 also have this trend, which reflects that only under a certain source-storage pressure difference (filling pressure) can a large amount of oil enter the tight reservoir. That is to say, the higher the charging pressure, the higher the flow rate in the tight reservoirs. All show a trend of alternating fast and slow, showing a step-like growth, and all show a three-stage stepped growth characteristic of slow-steep- slow. The better the physical property (permeability) (from No. 1 to No. 4), the lower the filling pressure required for oil enrichment. As the pressure increases, the more oil is charged into tight reservoirs.

2.1.2. Tight oil surface filling and enrichment

2.1.2.1. Experimental model and process

The experimental device is a plexiglass cylinder with a size of 72 cm×3 cm×6 cm filled with oil from multiple filling holes at the bottom to the source and reservoir above it (Fig. 3a). Located on the upper part is 72 cm long, 3 cm wide and 3 cm high. 70 mesh (grain size of 0.212 mm, median grain size 0.185 mm) glass beads are placed to simulate a relatively high porosity and permeability reservoir section (conventional reservoir). The middle part is also 72 cm long, 3 cm wide, and 3 cm high. Glass beads with a median particle size of 0.125 mm are placed to simulate tight reservoirs. The lowermost part is a series of injection ports (filling holes) for upward charging of oil, which are used to simulate the hydrocarbon expulsion process of the source rock filling the upper surface of the tight reservoir. The simulated oil is kerosene dyed red (addition of oil-soluble red), and its viscosity is about 2.24 mPa·s. The simulation experiment is carried out in the physical simulation laboratory of oil and gas accumulation of Research Institute of Petroleum Exploration and Development, and the physical simulation experiment device is used to expel the hydrocarbon from the source rock into the reservoir. The pressure is increased by the oil pump to produce a pressure difference between the oil injection port and the tight reservoir. Fill the tight reservoir with oil evenly from the lower oil injection pipe through multiple oil injection ports. Carefully observe the changes of the red simulated oil migration and accumulation state, migration direction, form, and oil saturation (color). Record and photograph the experimental phenomena at key time points and analyze them. Fig. 3a is a photo of the experimental phenomenon from the beginning of the experiment to 37, 43, 47, …, 1060 min, continuous filling and stopping filling. The phenomenon and process explanation of oil migration, accumulation and losing at these critical times are shown in Fig. 3b.

2.1.2.2. Analysis of experimental results

Summarizing the above experimental phenomena and experimental results, the following understandings can be drawn: (1) The filling migration of tight oil in a large area is a diffuse short-distance migration. The filling strength of petroleum and the heterogeneity of reservoir physical property determine the difference of large area short distance advance speed and the enrichment from local to large area, and then finally losing dynamic balance. (2) Close to the source rock and early filled tight reservoirs are preferentially enrichment. The quality of the direct cover of the source-reservoir symbiosis obviously affects the preservation of tight oil. (3) Large area filling coexists of tight oil has the characteristics of "overall advancement, diffuse penetration, short-distance migration, large-area accumulation, local differential enrichment, dynamic balance between source rock hydrocarbon expulsion filling and cover leakage and losing", etc. The filling, migration and accumulation of tight oil have obvious difference, stage and order characteristics.

The tight oil filling enrichment requires starting pressure (difference), and the fluid saturation (enrichment degree) into the tight reservoir shows a ladder-like increase trend with the increase of starting pressure.

The geological implications of the previous experiments are described in a tight oil source reservoir assembly system. In this system, petroleum generated from mature source rocks, driven by the difference in source and reservoir pressure, migrates to the reservoir space of tight reservoirs through the seam-pore-throat carrier system. And oil eventually accumulates and enriches in the pore of tight reservoirs. The geological model can be represented as Fig. 4. The black boxes on the lower side represent mature source rocks, and the upper boxes represent tight reservoirs. The red part represents the oil discharged from the source rock and entering the tight reservoir under the pressure differential Δp.

The study shows that the oil saturation formula obtained in the process of tight oil migration and accumulation in oil displacing water is as follows:

It can be seen that oil saturation of tight reservoir (core) is functionally related to viscosity, permeability and pressure difference. In other words, the oil saturation of tight reservoir (core) is related to oil viscosity, permeability of tight reservoir and pressure difference of source-reservoir. In a specific tight oil transport and accumulation system (at certain μ and K), oil saturation is positively correlated with the pressure difference between source and reservoir. The higher the pressure difference, the higher the oil saturation and the higher the enrichment of tight oil.

Due to the size of pore structure is hierarchical, the filling and enrichment of tight oil presents the mode of step filling, large area short distance overall advance and local enrichment in parallel. Fig. 5a shows the relationship between oil saturation and filling time of sample 4 under different filling pressures. The oil saturation increased in a step-wise manner with the increase of filling pressure difference and the increase of filling time, indicating that the filling of tight oil was controlled by the pore structure of tight reservoirs. The starting pressure represents the lower limit of the first large amount of oil entering a tight reservoir, and corresponds to the maximum pore diameter (d_max) of the tight reservoir. When the oil injected into the maximum pore structure is saturated, the oil will be filled with the next pore structure (d_mid) of a smaller scale. The oil cannot enter the tight reservoir as the pore throat becomes smaller and capillary pressure increases. When the pressure difference of the source-reservoir increases beyond the capillary force at this pore scale, such as 1.5-7.0 MPa, oil can be injected into the pore space at this scale until it reaches saturation. If we continue to fill the smaller pore structure (d_min), it will require greater pressure to break through the capillary force of the pore structure at this scale, such as 8.0-20.0 MPa, until the pore space at this scale is fully saturated, eventually leading to a step-like growth pattern in the filling process of tight oil (Fig. 5b). If we continue to fill the smaller pore structure (d_min), it will require greater pressure to break through the capillary force of the pore structure at this scale, such as 8.0-20.0 MPa, until the pore space at this scale is fully saturated, eventually leading to a step-like growth pattern in the filling process of tight oil (Fig. 5b). Locally, tight oil is injected from large pores to small pores in sequence with the increase of filling pressure, but at the macro level, it is still injected as a whole.

To sum up, the filling and enrichment of tight oil have the mechanism of "starting pressure starting, differential pressure advancing, stepped filling, uneven large area overall migration, and continuous and orderly differential enrichment". The tightness of the reservoir leads to the existence of start-up pressure, and the multi-scale layering of pore structure is the fundamental reason for the stepped filling of tight oil. The gradual change of reservoir physical properties within the same scale determines the overall filling of tight oil.

Source rocks with high hydrocarbon generation strength are the basis for tight oil enrichment. Whether it is a large freshwater clastic lake basin or a small salinized cisternstone depression, mature and high-quality source rocks distributed widely and continuously are the primary conditions for tight oil enrichment.

The source rock of Chang 7 is the main source rock of the tight oil enrichment formation of Chang 6, Chang 7 and Chang 8 in the south-central sag of Ordos Basin. The effective source rock (include dark mudstone and oil shale) in The Chang 7 Member is continuously distributed on a large area in the central and southern Ordos Basin, with an area of more than 8.5×10⁴ km², thickness of the source rock being 50-110 m, the main peak value of the content of TOC being 5%-10% and the average value being 3.75%. The average TOC of oil shale is 13.81%, and the favorable area with hydrocarbon generation strength greater than 400×10⁴ t/km² is over 3.0×10⁴ km^{2 [11]}.

The area of Qingxi sag in Jiuquan Basin is about 170 km². The Xiagou Formation is the main hydrocarbon source rocks. Organic matter type is given priority to with I₂ and type II. The source rocks are widely and continuously distributed in the semi-deep lake area, covering an area of up to 91 km². The TOC value is mainly concentrated in 0.5%-2.5%, with a maximum of over 4%. Source rocks have strong hydrocarbon generation and expulsion ability, since source rocks are formed in a saline environment with high hydrocarbon conversion rate, the source layer is in an environment of overpressure with a pressure coefficient of over 1.36^[10].

Similar to Qingxi sag, source rocks of the Lucaogou Formation are widely distributed in Jimusar Depression, with an area of 806 km² with a thickness of more than 200 m. The average TOC content was 5.16%, the kerogen type is given priority to with II₁, R_o value of 0.6%-1.7%. The average intensity of hydrocarbon generation is as high as 8.5×10⁶ t/km². The upper sweet spot pressure coefficient is 1.27, which belongs to abnormal high pressure and has high filling power^[12].

In the above three areas, the tight oil accumulation degree is high, all of which benefit from the source rocks with high hydrocarbon generation strength. High quality source rocks provide large scale of oil sources. The high hydrocarbon generation intensity reflects the high pressure of source-reservoir, and the high pressure of filling oil into tight reservoir is high. Statistics show that the hydrocarbon generation intensity of high-yielding tight oil Wells is greater than 3.0×10⁶ t/km² in the Chang 7 Member of Ordos Basin, the Da’anzhai Member of Sichuan Basin, and the Lucaogou Formation of Jimusar basin. Therefore, the source rocks with high hydrocarbon generation strength covering a large area provide enough oil sources for the enrichment of tight oil on the one hand, and sufficient charging and migration and accumulation forces on the other hand to promote the high enrichment of oil generated and discharged from source rocks in the sweet spots of tight reservoirs.

3.2.1. Source-reservoir combination and tight oil accumulation unit

3.2.1.1. Source-reservoir combination

Source-reservoir combination refers to a group of stratigraphic units that are spatially adjacent to or adjacent to a set of source rocks and a set of reservoir rocks with definite hydrocarbon supply and accumulation relationships. According to the spatial matching relationship between hydrocarbon sources and reservoirs in the tight oil basins in central and western China, it can be summarized into five types: “lower source and upper reservoir”, “upper source and lower reservoir”, “sandwich”, “source-reservoir integration”, and “thin interbedded reservoir”. The first four are often found in the tight oil accumulation units of freshwater lakes, and the source-reservoir integration is mainly developed in the tight oil accumulation units of saline lakes.

(1) Lower source and upper reservoir (A): A type of combination in which source rocks lie beneath reservoirs and are in close contact over large areas. A typical example is the type of source-reservoir combination where tight oil is located in the Chang 6₄ sub-member of the Ordos Basin, in which Chang 7 is the source rock and the overlying Chang 6₄ sub-member is the reservoir.

(2) Upper source and lower reservoir combination (B): A type of combination in which source rocks lie above reservoirs and are in close contact over large areas. A typical example is the type of source-reservoir combination in which tight oil is located in the Chang 8₁ sub-member in the Ordos Basin. The Chang 7₃ sub-member is the source rock and the underlying Chang 8₁ sub-member is the reservoir. Because of the capping effect of the overlying source rocks, such combinations tend to have good oil-bearing properties.

(3) Sandwich combination (C): The overlying strata and the underlying strata of the reservoir are all composed of source rocks and the three are in close contact in a large area. A typical example is the type of source-reservoir combination where the tight oil is located in Chang 7 of the Ordos Basin, in which the organic-rich shale in Chang 7 Member is the source rock, and the tight clastic rock distributed in Chang 7 is the reservoir. Sandwich combinations have a good oil content due to the oil provided in both the upper and lower directions.

(4) Thin interbedded type combination (E): A type of source-reservoir combination formed by longitudinal interaction between thin-layer hydrocarbon source rocks (usually less than 1 m) and thin-layer reservoirs. The formation of thin interbedded combination is the result of frequent water inlet and retreat and usually occurs in the transition zone of slope source-reservoir in freshwater lake basin. Because the source rocks are far away from the deposition center, the thickness of the source rocks is small and the quality is poor, and the reservoirs are thin, therefore, the oil content of thin interbedded combinations is often poor.

(5) Source-reservoir integration combination (D): A type of source-reservoir combination that source rocks and reservoirs are mixed together and it is difficult to distinguish them. It is usually developed in and around the sedimentary center of the saline lake. The fine grain mixers are developed that formed by the mixing of clastic rocks (shale, argillaceous siltstone, siltstone, etc.) and carbonate rocks (dolomite, limestone, etc.). This kind of combination is located in the sedimentary center of rich organic matter, and carbonate rocks are developed by both dissolution and fracture, so they often have good oil content. The most typical example is the upper and lower sweet spot interval of the Lucaogou Formation in Jimusar sag.

3.2.1.2. Tight oil accumulation unit

Unlike conventional reservoirs, where the accumulation units are usually characterized by traps, there is no concept of traps for tight oil accumulation, whereas the source-reservoir combination is the basic unit to characterize tight oil accumulation. A source-reservoir combination unit can be defined as a spatial combination unit consisting of one or more sets of hydrocarbon source rocks with hydrocarbon origin related to a certain tight reservoir as the target reservoir for oil accumulation. The source rock provides oil and the reservoir accumulates oil, and they are relatively independent.

According to the concept of source-reservoir combination unit, a set of source-reservoir combination is a relatively independent tight oil combination unit, which determines the range and degree of enrichment of tight oil enrichment. A tight oil basin often develops multiple sets of source-reservoir combinations, corresponding to the formation of multiple tight oil accumulation units.

3.2.2. Dominant source-reservoir combination

The combination of dominant source-reservoir is favorable for tight oil enrichment. Theoretical research and experimental simulation have confirmed^[27] that “sandwiches” and “upper source and lower reservoir” in freshwater lake, and source-reservoir integration combination of saline lake have a good oil content, which are the most common three types of dominant source-reservoir combination. Actual drilling has also confirmed that the dominant source-reservoir combination controls the degree of tight oil enrichment.

There are four types of source-reservoir combination in the tight oil sedimentary system of clastic rocks in Freshwater lake, including sandwich, upper source and lower source, lower source and upper source, and thin interbedded combinations. The “sandwich” and “upper source and lower reservoir” combinations usually obtain commercial oil flow, while the “lower source and upper reservoir” and “thin interbeds” are mostly low production or no oil and gas display. The well-connecting profile of Tie 34-Sheng 1-Lou 22 well in Yanchang exploration area of Ordos Basin shows that oil shale is a high-quality source rock. In the tight oil accumulation unit consisting of multiple source and storage combinations with oil shale, “sandwich” and “upper source and lower reservoir” can obtain commercial oil flow. Tie34 well revealed the source and storage combination of two sandwich sets in Chang 7 Member. The oil flow with a daily output of 2.97 t was obtained in the upper “sandwich” source-reservoir combination, and the large oil reservoir was revealed in the lower “sandwich” source-reservoir combination. Daily 3.1 t oil flow was obtained from the upper source and lower reservoir combination of Chang 7 and Chang 8 reservoirs in Well Lou 22. The oil content of the lower source and upper reservoir combination is poor.

The lithology of the saline lake is mainly mixed rock. The source-reservoir integration is the main type of source-reservoir combination, and the oil content is usually very good. The upper and lower sweet spot sections with the highest enrichment of tight oil in Jimusar Depression are typical source-reservoir combinations, which are composed of mud shale, silty mudstone, arenaceous dolomite, dolomite sandstone, micritic limestone and feldspar debris and fine sand rocks that are both source and reservoir rocks. It has a significant integrated feature of source and reservoir, and there are also four types of source-reservoir combination: upper source and lower reservoir, upper source and lower reservoir, sandwich and thin interbeds in Xiagou Formation in Qingxi sag. The oil production of the source-reservoir combination, sandwich and upper source and lower reservoir is relatively high, while the other two types of source and reservoir combinations are relatively low. We have analysis of the relationship between the oil yield and the combination of source and reservoir in more than 20 Wells, such as Well Liu 4 and Well Liu 106. It shows that the oil yield and medium-high yield of integrated source and reservoir are 43% and 26% respectively, sandwich is 19% and 12% respectively, upper source and lower reservoir are 13% and 8% respectively, thin interbeds are 19% and 5% respectively, lower source and upper reservoir are 7% and 3% respectively.

In conclusion, different types of source-reservoir combinations have different levels of tight oil enrichment, and dominant source-reservoir combinations, namely source-reservoir combinations with high degree of oil injection are sandwich, source-reservoir integration, upper source and lower reservoir combinations, etc.

The simulation results show that fractures and bedding are conducive to the local rapid and short distance migration of tight oil.

3.3.1. Experimental procedure and parameters

A tight oil accumulation unit is formed from a proximal or adjacent source. A short distance filling and transmission system shall be provided between the oil source and the accumulation unit. In addition to the pore-pore throat transmission system, fractures (mainly structural and bedding joints) and bedding are also important filling and transmission systems (Fig. 6).

The experiment was conducted in a relatively closed sand box device (Fig. 6d). Vertical and horizontal fractures represent high angle structural fractures and near-horizontal bedding fractures (including bedding), respectively. Metal mesh is used to replace the crack, and black rectangular plastic block is used to simulate the filled drain joint. The three substrates were simulated by quartz sand of different particle sizes. The particle size of the substratum is the largest, 0.6-0.7 mm (simulated coarse sand), representing the source rock or oil layer. The middle layer is 0.05-0.10 mm (simulated fine sand), representing the tight reservoir. The upper layer is 0.25- 0.30 mm (simulated medium sand), representing the conventional reservoir. An oil filling port is arranged at the bottom of the lower layer. The filling and migration characteristics of oil can be analyzed, through the filling port, the oil can be filled upward from the substratum matrix to observe the color change in the experimental apparatus.

3.3.2. Experimental phenomena and mechanism explanation

The oil was injected into the lower matrix in two ways, rapid filling (0.2 mL/min) and stable slow filling (0.1 mL/min), and the migration and enrichment characteristics of oil in the experimental model were observed and described. Fig. 6d shows the experimental phenomena at 6, 30, 50, 70, 90, 110 h, respectively, and the geological interpretation of petroleum filling and migration trends. Through the interpretation of experimental phenomena and experimental results, the following conclusions are drawn:

(1) Longitudinal and lateral fractures (and bedding) are the dominant channels for filling and transporting tight oil along the reservoir. After saturation, under the action of pressure difference, the oil continues to fill, permeate, diffuse and accumulate in the matrix around the fracture.

(2) Pressure difference is the main driving force for oil migration along the vertical and horizontal fractures. The longitudinal and transverse fracture intersection zones are tight oil enrichment zones.

(3) The mechanism and model of petroleum migration and accumulation in tight reservoirs with fractures are “differential pressure charging, rapid local oil transport along dominant channels in both vertical and horizontal fractures, diffuse and unbalanced migration along a large area of pore-throat network, and concentrated accumulation in intersecting zones of fractures in both horizontal and vertical directions”.

Like conventional reservoirs, the pore structure and fracture network of tight reservoirs are the space for oil storage^{[15, 18, 32]}. It has multi-scale characteristics such as micron and nano, and there are many combination relations between pores and fractures of different scales. Clearly, the combination of favorable pores and favorable fractures (favorable fracture-pore coupling) is one of the factors most conducive to tight oil enrichment. Fig. 6 shows the evidence that fracture-pore coupling restricts the accumulation of tight oil, and shows the characteristics of concentrated distribution of petroleum along criss-cross structural fractures, stratified fractures and their pore pores, and the greatest accumulation of petroleum (including asphalt) at the intersection of longitudinal and transverse fractures. It is shown that the fracture-pore coupling controls the accumulation of tight oil in tight reservoirs. Fractured-pore reservoirs are often developed in tight oil production. For example, Wells Qing1-2, Long102, Liu3 in Xiugou Formation, Qingxi sag, Jiuquan Basin, obtain high yield oil flows with a daily output of 9.72 t, 42.56 t, 23.77 t, respectively. Core and well logging interpretation show that fractures and pores are developed in the reservoir, which is a favorable sweet spot for fractured tight reservoirs. The 4.5 t/d and 3.1 t/d industrial oil flows obtained from Wells Qiaotan 17 and Xin 91 in the Chang 8 Member tight reservoir in the Yanchang exploration area of Ordos Basin. It is also related to the good fracture-pore coupling characteristics of the tight reservoir.

The enrichment of tight oil is different from conventional oil in its dynamic mechanism and efficiency of migration and storage. The driving force for tight oil enrichment is the pressure difference between the source rock and the reservoir, which has low transport and storage efficiency, while buoyancy is the driving force for conventional oil enrichment, and the transport and storage efficiency is relatively high. This difference means that 4 optimal factors in terms of high-quality source rocks, good reservoirs, appropriate large-area fractures and good source-reservoir configuration are essential for the enrichment of tight oil.

A wide and continuous high-quality source rock, such as the oil shale and black mudstone of the Chang 7 Member of the Ordos Basin, and the black shale and carbonate rocks of Lucaogou Formation in Jimusar sag, has strong hydrocarbon generation potential and large pressure difference between source rock and reservoir. High-quality source rock embodies the characteristics of “source-controlled and high-pressure enrichment mode” and is the material basis for tight oil enrichment.

Good reservoirs refer to those large and continuous sets of tight layers next to the source rock, with relatively high physical properties (porosity, permeability), appropriate pore structure, and large brittleness index is more conducive to the accumulation of shale oil. For example, the silt-fine sandstones of the Chang 7₁, Chang 7₂ and the Chang 7₃ sub-member of the Chang 7 in the Ordos Basin, and the feldspar silt fine sandstone in the upper sweet-spot of the Lucaogou Formation in the Jimusar sag reflect the “facies-controlled and high-porosity enrichment model”.

Appropriate large-area fractures provide good advantageous transport conditions for tight oil enrichment. such as the vertical fractures in the Chang 6 to Chang 8 members of the Ordos Basin, the horizontal and vertical fractures in the Xiagou Formation in the Qingxi sag, and the bedding cracks and microcracks in the Lucaogou Formation in the Jimusar sag. It is worth mentioning that excessive development of fractures may also lead to the loss of tight oil if the fracture zone crosses the tight oil accumulation unit.

"Good source-reservoir configuration" refers to the good matching of source and reservoir in time and space. The favorable spatial relationship between source and storage includes “sandwich” type, source reservoir integration, etc. The favorable source-reservoir time relationship is the match between source rock hydrocarbon generation and expulsion and reservoir pore development period.

The tight oil blocks currently under development, such as the Xin'anbian area in the Ordos Basin and the Changji area in the Jimusar sag, all have these four characteristics: high-quality source rocks, good reservoirs, appropriate large-area fractures and good source-reservoir configuration. The enrichment of conventional oil also requires these four conditions, but the degree is far less than that of tight oil.

There are two types of tight oil enrichment areas: clastic tight oil in freshwater lake basins and tight oil in saline lacustrine diamictite rocks in central and western China. Differences in sedimentary media, basin (sag) scale, and lithology, etc. lead to different tight oil enrichment models.

4.2.1. Enrichment model of tight oil from clastic rocks in a large freshwater lake basin

Tight oil in large freshwater lake basins is mainly distributed in the central and eastern regions, such as the Ordos and Sichuan Basins in the middle and the Songliao and Bohai Bay Basins in the east. The characteristics of tight oil in freshwater lake basins are as follows:

(1) The tight oil sedimentary system in the freshwater lake basin is large in scale and the source rocks are distributed in a wide area. They all have relatively stable tectonic backgrounds, gentle strata, and large-area multi-cycle delta front-predelta-turbidite fan fine-grained sedimentary systems, which are conducive to the accumulation of tight oil. The Zhongnan sag of the Ordos Basin experienced two stable tectonic movements from the Chang 9 Member to the Chang 6 Member during the deposition period. Semi-deep lake (Chang 9 Member), delta front (Chang 8 Member), deep lake (Chang 7 Member) and delta front (Chang 6 Member) are formed in the central and southern part of the basin. The area of the lake basin exceeds (3-5)×10⁴ km² at each stage, and the area of the delta front-pre-delta-turbidite fan also reaches 10⁴ km², which is conducive to the large-scale enrichment of tight oil.

(2) Multiple sedimentary cycles have resulted in multiple sets of large-area mature oil shale and high-quality source rocks closely connected with tight reservoir^{[3-5, 7, 10]}. Multiple sets of complete and high-quality source rocks and reservoir configurations have been formed, such as the “Down-source and Upper-reservoir” combination between the mudstone of the Lijiapan Formation of the Chang 9 Member and the silt fine sandstone of the Chang 8₂ sub-member in the Zhongnan sag of the Ordos Basin, the “Upper-source and Down-reservoir” combination between the silt fine sandstone of the Chang 8₁ sub-member and the high-quality source rock of the Chang 7 Member, the "Sandwich source- reservoir" combination in the Chang 7 high-quality source rock and internal silty fine sandstone tight reservoirs, providing good "four excellent" conditions for the enrichment of tight oil in the Chang 6 to Chang 8 members of the Zhongnan sag, Ordos Basin.

(3) Tight oil is continuously enriched on a large scale. The source rocks of the Chang 7 and Chang 9 members in the Zhongnan sag of the Ordos Basin have abnormally high pressures during the accumulation period. The pressure difference between the fracture and the reservoir is the main driving force for the accumulation of oil in the Chang 6 to Chang 8 members. The mature oil produced and expulsion from the Chang 7 Member and Chang 9 Member is filled longitudinally along regional structural fractures and laterally along pore-fractures (bedding fractures and micro-fractures) in the Chang 6 to Chang 8 reservoirs. It is worth mentioning that the physical properties of the Chang 6₁ and Chang 6₂ sub-segments of the Chang 6 Member are better, and the buoyancy is also the driving force for the enrichment of tight oil (Fig. 7).

(4) Tight oil is mainly enriched around the deposition center and its periphery. Tight oil from clastic rocks in large freshwater lake basins is mainly concentrated in areas where high-quality source rocks overlap with high-quality reservoirs in the delta front-predelta-turbidite fan depositional system. The 4 optimal factors all exist in these areas. For example, the tight oil in the Dingbian, Zhidan, Wuqi, Xin'anbian areas of the Ordos Basin is distributed in the underwater distributary channel microfacies of the delta front on the north side of the source rock of the Chang 7 Member, the tight oils of Huachi, Heshui, and Zhengning are located in the underwater distributary channel microfacies of the delta front on the south side of the source rock of the Chang 7 Member, and both are distributed around the hydrocarbon generation and expulsion center of the source rock. In addition, some turbidite sand tight oil is distributed in the center of the source rock^{[43,44,45,46,47]}.

In summary, the tight oil enrichment model of large freshwater lake clastic rock basins is "source reservoir symbiosis, 4 optimal factors control accumulation, around central distribution, and large-scale enrichment".

4.2.2. Enrichment model of tight oil in a small salty lake diamictite depression

The tight oil of saline lakes is mainly distributed in western China, such as the Jimusar sag in the Junggar Basin, the Qingxi sag in the Jiuquan Basin, the Malang and Tiaohu sags in the Santang Lake Basin, and the Yingxi sag in the Qaidam Basin. The enrichment model of tight oil in saline lakes is very different from that in freshwater lakes, as follows:

(1) The hydrocarbon-generating sag is small but enrich, with high resource abundance. The area of the Qingxi sag is 170 km², and the area of the Jimusar sag is 1287 km². Although the area of the western salty lake tight oil basins (sags) is relatively small, its high resource abundance is conducive to the enrichment of tight oil. The main reason is the large thickness of the source rock. For example, the continuous thickness of the source rock of Lucaogou Formation in Jimusar sag exceeds 350 m, and the continuous thickness of source rock of Xiagou Formation in Qingxi sag exceeds 500 m, which makes up for the disadvantage of small area. The saline sedimentary medium has increased the conversion rate of organic matters to hydrocarbons^[43], and increased the hydrocarbon generation and expulsion intensity of source rocks. The average hydrocarbon generation intensity exceeded 500×10⁴ t/km², and the highest reached (900-1000)×10⁴ t/km². Deeper burial, higher hydrocarbon-generating swelling force, and higher abnormal high pressure result in high charging pressure, which is extremely conducive to the enrichment of tight oil.

(2) The salty lake has a strong annoying dissolution and has secondary pores and fractures. Salty lake diamictite reservoirs contain a lot of limestone, dolomitic rock and other carbonate rocks with good brittleness. It is easy to fracture to form structural fractures, bedding fractures and other vertical and horizontal fractures, and it is also easy to be generated by organic acids. Dissolution, forming secondary dissolution pores and cracks, provides transport conditions and enrichment space for the massive migration and enrichment of tight oil. The secondary dissolution pores and fractures in the reservoirs of the Xiagou Formation in the Qingxi sag and the Lucaogou Formation in the Jimusar sag are very well developed. The average porosity of the tight reservoirs in the upper sweet spot of Jimusar is 11%, and the highest is 26%. The fracture density of the tight reservoirs of the Xiagou Formation in the Qingxi sag is 0.2-44.7 per meter, with an average of 4.1 per meter.

(3) The source and reservoir of the diamictite tight reservoir are integrated, with clastic rocks and carbonate rocks developed. The development of transitional rocks, such as mudstone, argillaceous dolomite (lime) rock, dolomitic (lime) mudstone, argillaceous siltstone, silty mudstone, etc., causes the gradual transition and integration of source rocks and reservoirs. The tight oils of the Lucaogou Formation in the Jimusar sag and the Xiagou Formation in the Qingxi sag mainly develop in the sweet spots of the "source-reservoir integrated" and "sandwich" combinations.

(4) Tight oil from high-quality source and reserves is concentrated in the deposition center. The salty lake basin has arid climate and insufficient supply of terrestrial clastic sources. Carbonate rocks such as organic-rich mudstone, siltstone clastic rocks, limestone or dolomite are only developed in the deposition center. The quality of source rock deteriorates from the deposition center to the edge of the depression, lacking relatively coarse- grained continental clastic sediments. Exploration experience shows that high-yield tight oil wells in the Jimusar sag, Qingxi sag, and Tiaohu sag are all distributed within the depositional center.

In summary, the enrichment mode of tight oil in saline lake is "source-reservoir integration, 4 optimal factors control accumulation, central distribution, Small but rich" (Fig. 8).

This article mainly discusses the enrichment conditions, main controlling factors and models of tight oil in clastic rocks and diamictite rocks. The enrichment of tight oil in carbonate rocks has similar characteristics (tight oil in the Da'anzhai Member of the Ziliujing Formation of the Lower Jurassic in the Sichuan Basin).

Two types of tight oil from large freshwater lake clastic rocks and small saline lacustrine diamictite rocks are developed in central and western China, and have a high strategic position.

Tight oil enrichment generally has an enrichment mechanism of "start-up pressure start, pressure difference advancement, diffuse speed alternation, stepped large-area filling and rapid migration of fracture dominant channels, and dominant fracture-pore coupling enrichment". The pore structure is the root cause of stepped tight oil filling. Vertical and lateral cracks provide fast migration channels for tight oil filling.

High-quality source rocks, good reservoirs, appropriate large-area fractures and good source-reservoir configuration are essential for the enrichment of tight oil.

There are two modes of tight oil enrichment. The mode of large freshwater lake clastic rocks is "source-reservoir symbiosis, 4 optimal factors control accumulation, circular distribution, and large-scale contiguous", and the mode of saline lake diamictite rock is "source-reservoir integrated, 4 optimal factors control accumulation, central distribution, small but rich".

During the writing process of this article, we have received the help of Gong Yanjie, PetroChina Research Institute of Petroleum Exploration and Development, Sun Weifeng, China University of Mining and Technology, and Li Yaohua, Fan Chunyan, Yao Limiao, Kaohsiung Xiong China University of Petroleum (Beijing), and others. We express our sincere gratitude.

d_max—maximum pore diameter of tight reservoir, nm;

d_mid—the diameter of the pore structure with a pore diameter smaller than d_max, nm;

d_min—the diameter of the pore structure with a pore diameter smaller than d_mid, nm;

K—tight reservoir (core) permeability, μm²;

L—length of tight reservoir (core), m;

p₁—reservoir fluid pressure, Pa;

p₂—source rock pressure, Pa;

p_c—capillary force, Pa;

Δp—pressure difference between source and reservoir, Pa;

S—oil saturation, %;

t—filling time, s;

μ—viscosity, Pa^.s;

μ₁—viscosity of water in tight reservoirs, Pa^.s;

μ₂—oil viscosity, Pa^.s.