Accumulation Conditions and Pattern of Tight Oil in the Lower Submember of the Fourth Member of the Shahejie Formation in the Damintun Sag, Bohai Bay Basin

Abstract

To determine the accumulation conditions and pattern of tight oil in oil shales in the Damintun Sag, Bohai Bay Basin, this study investigated the basic geological conditions of the source rocks and reservoirs in the sag using methods such as organic carbon analysis, whole-rock XRD analysis, and field emission scanning electron microscopy. The results show that: (1) The high-quality source rocks in the lower submember of the fourth member of the Shahejie Formation (E₂S₄²) in the Damintun Sag have high organic matter abundance, favorable organic matter types, high hydrocarbon expulsion efficiency, and high fluidity. Therefore, they provide sufficient oil sources for tight oil accumulation.; (2) During the burial of organic-rich shales, the thermal degradation of organic matter produces large amounts of organic acids, which can dissolve carbonate minerals. In this way, secondary pores are formed.; (3) The special microscopic pore structure that connects fractures to pores is the key to the enrichment of tight oil is a key factor for the high oil saturation of pores in oil shales; (4) The breakthrough pressure (up to 100 MPa) and specific surface area of dolomitic mudstones in the E₂S₄² submember are significantly higher than those in other horizons. As a result, the dolomitic mudstones can effectively seal the underlying tight reservoirs; (5) Compared with the tight oil in tight sandstones, the tight oil in the oil shales in the study area has significantly superior geological conditions for reservoir formation, such as the favorable arrangement of hydrocarbon expulsion channels, low filling resistance, and the presence of reservoir spaces.

1. Introduction

Tight oil refers to the oil accumulated in or immediately adjacent to the tight reservoirs in high-quality oil source strata without experiencing long-distance migration on a large scale. It is an unconventional oil resource, whose reservoirs have low porosity and low permeability [1,2,3,4,5,6,7]. Compared with other reservoirs with particularly low or ultra-low permeability, tight oil resources have more complicated accumulation mechanisms, finer pore throats, and higher interstitial material content and thus are more difficult to explore. Driven by both the dramatic rise in oil price and technical advances, tight oil and gas have entered a new stage of rapid development. In particular, the successful exploitation of tight oil by the United States has imposed significant impacts on energy supply and geopolitics [8,9]. China started late in its development of tight oil and gas. Over the past few years, China has witnessed a significant increase in single-well production and a great decrease in cost due to the advancement in horizontal drilling and fracturing technologies and their large-scale applications. Accordingly, China has achieved significant progress in the exploration and exploitation of tight oil and gas and successively discovered a number of tight oil and gas areas [10,11,12,13,14], including large-scale tight oil and gas provinces, such as Sulige in the Ordos Basin, the Xujiahe Formation in the Sichuan Basin, and the Kuche Sag in the Tarim Basin, and tight oil provinces, such as the Chang 7 Member in the Ordos Basin, the Permian Lucaogou Formation in the Jimsar Sag in the Junggar Basin, and the Fuyang oil layer in the Songliao Basin. The tight oil industry in China has formally transitioned from the theoretical and technical preparation stage and policy promotion stage to the golden stage of rapid development, achieving substantial growth in tight oil and gas yields [15,16].

Tight oil in China mainly occurs in continental deposits, and its occurrence horizons differ greatly in different basins. Unlike the predominant marine tight oil in North America, the tight oil in China has significant characteristics including multiple reservoir types, poor physical properties, and strong heterogeneity [17,18,19]. In recent years, technical challenges of the exploration and exploitation of unconventional oil and gas in the Liaohe Oilfield have been constantly addressed. Specifically, volume fracturing and HIWAY techniques have been applied to the tight oil exploration in the Damintun Sag for the first time, achieving a breakthrough. Although industrial oil flow has been produced from multiple wells such as An-95, Sheng-14, and Shen-238, the exploitation and production of tight oil resources in the Damintun Sag are still in their early stages, and the basic geological conditions such as the source rocks and reservoirs of tight oil are yet to be ascertained. Therefore, there is an urgent need to investigate the accumulation characteristics and patterns of tight oil in the Damintun Sag, which is significant for further determining new exploration strata and expanding oil prospecting fields in the sag [20,21].

Overview of the Study Area

The Damintun Sag, which is in the shape of an irregular triangle (wide in the south and narrow in the north), is located in the northeast of the Liaohe fault depression in China. It is controlled by boundary faults and covers an area of about 800 km² [22]. This sag is a composite tensional and strike-slip sag developed from an early rift. It experienced four stages of tectonic evolution, namely, the Paleocene germination stage as a rift, the Eocene rifting—deep depression stage, the continuous rifting—shrinkage stage during the Oligocene, and the overall depression stage during the Neogene. Laying in the structural background of a dustpan-shaped half-graben that is high in the east and low in the west overall, the Damintun Sag has a tectonic framework consisting of the uplifted central tectonic zone sandwiched between the Rongshengbao and Santaizi sub-sags in the south and north, respectively (Figure 1). The sedimentation in the Damintun Sag is dominated by alluvial and lacustrine processes, forming sedimentary systems such as alluvial fans, rivers, deltas, fan deltas, lakes, and subaqueous fans. In the early stage, target horizon E₂S₄ was characterized by the development of anoxic lacustrine subfacies, and the Damintun Sag had a warm and humid middle subtropical climate and was covered by shallow water during this stage. The lacustrine basin of the sag had a brackish water-reducing environment, in which subaqueous organisms such as bacteria, algae, and lower plankton were very abundant. Moreover, a set of oil shales with extremely high organic matter content were continuously deposited in the sag. In the middle and late sedimentary stages, E₂S₄ had widely developed semi-deep to deep lacustrine subfacies. Thickly laminated dark mudstones with high hydrocarbon generation potential were deposited in the sag, with a sublacustrine fan system composed of coarse clastics developing locally [23].

2. Samples and Experiments

2.1. Sampling

This study focused on the oil shales in E₂S₄², which have always been considered source rocks. It was not until the introduction of the concept of “tight oil” that E₂S₄² was studied in detail as a tight reservoir. In this study, 40 oil shale samples were collected from E₂S₄² at several key exploration wells, such as wells Shen-352, An-1, and An-17. The cores from the wells exhibited pronounced schistosity and were extremely brittle, with numerous fractures visible.

2.2. Experiments and Analyses

The cores collected in this study were loose and brittle, making it impossible to obtain regular columnar or blocky samples. As a result, rate-controlled mercury penetration experiments and mercury injection capillary pressure (MICP) experiments cannot be conducted, causing difficulties with the quantitative characterization of reservoir parameters. Considering the specificity of the samples and the need to explore source rocks and reservoirs, this study conducted experiments such as organic carbon pyrolysis, whole-rock X-ray diffraction (XRD), scanning electron microscopy (SEM), low-temperature nitrogen adsorption, and confocal laser scanning. These experiments require only crushed rock samples and can quantitatively yield microscopic reservoir parameters, such as the organic carbon content, clay mineral composition, average pore size, and specific surface area. Moreover, these experiments are simple to perform, require a few samples, and boast mature technologies and high analytical accuracy. The basic processes of these experiments are as follows.

2.2.1. Organic Carbon Analysis

Organic carbon content is a basic indicator for measuring the abundance of organic matter and is the basic data for evaluating source rocks. Only rocks with certain organic carbon content can generate enough oil and gas to accumulate and form reservoirs, and it can be directly determined that rocks with low organic carbon content cannot generate oil. Therefore, organic carbon analysis is essential for oil and gas exploration. In this study, a CS-230 Carbon/Sulfur Determinator manufactured by the American company LECO was employed to carry out experiments according to GB/T 19145-2003. This determinator has the advantages of high stability, high accuracy, low analysis cost, low failure rate, and quick analysis. The rock samples for organic carbon analysis were soaked in hydrochloric acid at 60–80 °C for more than two hours to completely remove inorganic carbon. Then, the samples were fully burned and decomposed at a high temperature. Finally, the total organic carbon (TOC) content of the rock samples was determined using a high-sensitivity CO₂ detector. It is noteworthy that hydrochloric acid should be added to the rock samples at an appropriate rate and that the heating and combustion of the samples should guarantee that the organic matter in the samples is fully oxidized and completely converted to CO₂ [24,25].

2.2.2. Whole-Rock XRD Analysis

The whole-rock XRD analysis has been widely used in the quantitative analysis of clay minerals in sedimentary rocks. It can determine both the chemical composition and the relative content of materials [26]. A D8 DISCOVER polycrystalline X-ray diffractometer was employed in this study. This diffractometer has an angle accuracy better than 0.02° (2θ) and operates at a voltage of 30–45 kV, a current of 20–100 mA, a scanning rate of 2°/min, a sampling step width of 0.02°, an emission slit and a scattering slit of both 1°, and a receiving slit of 0.3 mm. The clay mineral samples with grain sizes of less than 10 μm and less than 2 μm were extracted using the suspension separation method. The clay minerals with a grain size of less than 10 μm were used to determine the total relative content of clay minerals in the protolith, and those with a grain size of less than 2 μm were used to determine the relative contents of various types of clay minerals.

2.2.3. Field Emission Scanning Electron Microscopy (FE-SEM)

SEM has the advantages of high resolution, high magnification, and excellent three-dimensional visualization and can accurately reflect the structural characteristics of rock minerals. Therefore, this technology has been widely applied in geosciences [27,28,29]. In this study, an ordinary scanning electron microscope was first used to observe and describe the pore structure of tight reservoirs. The shale layers as target horizons have tiny pores and a very dense structure, which cannot be effectively characterized using a conventional scanning electron microscope. Therefore, a field emission scanning electron microscope with a higher resolution was used to carry out experiments according to standard SY/T 5162-2014 using a Quattro S environmental scanning electron microscope manufactured by the American company, FEI. This microscope has magnifications of 6–2,500,000 and a maximum resolution of 1.0 nm at an accelerating voltage of 30 kV in high vacuum mode. The main steps of the FE-SEM experiment are as follows: (1) The sample was pretreated to make it nonmagnetic, non-toxic, pollution-free, radioactive, stable, dry, conductive, and nonluminous without heating. The original surface morphology and structure of the sample should be kept as long as possible, and the sample should not be squeezed to avoid changing its real appearance. The whole process should be clean, and the sample and instrument should not be touched directly to avoid polluting the vicinity of the electronic channel. The sample can be cleaned with alcohol or ultrasonic in advance to reduce its pollution degree; (2) The sample was prepared according to the specification requirements of the scanning electron microscope slice. In other words, the glass slice should not be covered, and its thickness should be 0.03 mm; (3) The sample was sent to the sample table, throughout which gloves should be worn; (4) The machine was turned on, and the joystick was moved to take photos after the image was adjusted clearly.

2.2.4. Confocal Laser Scanning Microscopy

Confocal laser scanning microscopy (CLSM) is a combination of microscopy, high-speed laser scanning, and image processing technology; and it has the advantages of high resolution, easy sample preparation, and strong penetration. This technique can be used to study micropores in tight reservoirs and provides quick, intuitive, and accurate information such as pore structure and plane porosity of micropores [30]. In this study, a Zeiss LSM700 confocal laser scanning microscope manufactured by the German company Carl AG was employed, which supports full-spectrum imaging and has three confocal channels, including two single PMT fluorescence detectors and one transmitted-light channel. It is equipped with laser beams of 405 nm, 488 nm, 555 nm, and 635 nm and can comprehensively analyze three-dimensional information such as the micro-morphology and roughness of the sample surface. It can provide images with a resolution of up to 2048 × 2048 pixels, the largest linear scanning field, a high scanning speed, high-sensitivity detectors, and a short optical path design from samples to detectors. The main steps of the CLSM experiment are as follows: First, samples were prepared. Since LSCM images are obtained by measuring the fluorescence signals excited by laser, attention should be paid to two aspects in the preparation of the cast sheet: (1) the fluorescence intensity of the cast material should be appropriate; (2) to make micropores fill with the casting body as much as possible, the pressure and time for sample preparation should be appropriate. Next, the machine was preheated, appropriate laser intensity and grating were selected, and the appropriate image scanning times and objective lens multiples were determined. Finally, micropores were measured, and the laser intensity and color display were adjusted according to the macropores. In this way, the macropores that run through the thickness of the thin slice can be distinguished from the micropores with thin pore throats and filled by clay minerals from images. Then, the representative micropores were selected to be locally enlarged, and the real structure of the micropores was restored using multiple-layer scanning and three-dimensional reconstruction technology. Digital image analysis technology was used to determine the structure and porosity of micropores. In this study, 15 samples were selected for the statistical analysis of pore quantities, the maximum and minimum pore radii, and plane porosity in two vision fields using confocal laser scanning microscopy. As a result, two-dimensional scanning images and three-dimensional pore images were obtained.

2.2.5. Low-Temperature Nitrogen Adsorption

The low-temperature nitrogen adsorption experiment can be used to measure the specific surface area of rocks and evaluate the pore size distribution in reservoirs based on the changes in the adsorption curves with pressure [31,32]. In this study, the low-temperature nitrogen adsorption experiment was carried out using a Surfer specific surface area and pore size analyzer manufactured by Thermo Fisher Scientific in the United States. This analyzer uses static volumetric gas adsorption to measure the characteristic parameters of the microstructures of solids and powders under controlled temperature and pressure. It can measure pore sizes in the range of 1.5–200 nm and a minimum specific surface area of 0.0005 m²/g. To remove the residual bound water and capillary water from samples, all the samples were pretreated at 300 °C for three hours in advance. Then, using nitrogen gas with a purity higher than 99.999% as an adsorbent, the nitrogen adsorption capacity at different relative pressures was measured at a temperature of 77 K.

3. Results and Analysis

3.1. Source-Reservoir Conditions of Tight Oil

3.1.1. Source Rocks of Tight Oil

The presence of source rocks in the E₂S₄ member in the Damintun Sag of the Bohai Bay Basin has long been confirmed through exploration and research on petroleum geology [33]. E₂S₄ was formed during the initial depression—deep depression stage of deep fault depressions in the Bohai Bay Basin. Its upper part consists of thickly laminated greyish-brown and dark-gray mudstones interbedded with medium-thick layered greyish-white pebbled sandstones, sandstones, and siltstones, with a general thickness of 300–800 m. Its lower part comprises interbeds consisting of variegated glutenites, sandstones, and mudstones, with a thickness of 0–500 m. E₂S₄ mainly has TOC content of 2–12%, with high-quality source rocks (TOC content > 1%) and excellent source rocks (TOC content > 2%) accounting for more than 95% and about 85%, respectively. The E₂S₄¹ submember shows a relatively concentrated distribution of the TOC content, which is generally less than 4%. However, high-quality source rocks present a high proportion, and excellent source rocks account for about 30% of this submember. (Figure 2) The organic matter types are determined (Figure 3). Higher hydrogen index (HI) corresponds to more favorable organic matter types. In the case of a low degree of evolution of source rocks, HI > 580, 580 > HI > 300, 300 > HI > 150, and HI < 150 indicate type I, II₁, II₂, and III kerogen, respectively. Tmax represents the degree of evolution of source rocks, and a higher Tmax value suggests a higher degree of evolution. The two parameters jointly determine the type of organic matter. Based on the commonly used standard for classifying kerogen types in the industry (Figure 3), the E₂S₄¹ submember does not contain type I organic matter, and type II₁, II₂, and III organic matters account for approximately 10%, 50%, and 40%, respectively. In contrast, the E₂S₄² submember contains mainly type I and II kerogens, with type I, II₁, and II₂ kerogens accounting for more than 75%, approximately 15%, and approximately 5%, respectively. Meanwhile, this submember does not contain type III organic matter. Therefore, the organic matter type of the E₂S₄² submember of the Damintun Sag is significantly superior to that of the E₂S₄¹ submember. Regarding the maturity of organic matter, the vitrinite reflectance (R_o) of kerogen changes regularly as the thermal evolution of organic matter intensifies. The source rocks in the Damintun Sag have R_o values of up to 0.5% at a depth of approximately 2400 m and up to 0.75% at a depth of approximately 3300 m, with the maturity gradient decreasing with an increase in the depth. Overall, the source rocks in the target horizons have a high abundance, favorable types, and moderate maturity, indicating favorable source rock conditions for tight oil.

3.1.2. Tight Oil Reservoirs

The fourth member of the Shahejie Formation in the Damintun Sag can be divided into two submembers, i.e., E₂S₄¹ and E₂S₄². The oil shales in the E₂S₄² submember are presently important horizons with distinct oil and gas shows discovered during the tight oil exploration and are the focus of the reservoir evaluation in this study. The E₂S₄² submember has a distinct sandwich structure according to its vertical lithological distribution and can be divided into beds I, II, and III. Figure 4 shows the lithological histogram at Well Shen-352 in the Damintun Sag. According to this figure, beds I and III are mainly composed of oil shales, while bed II consists mainly of dolomitic mudstones and siltstones.

This study systematically evaluated the reservoir spaces of tight reservoirs in the E₂S₄² submember through multiple experiments, including CT scanning, FE-SEM, confocal laser scanning, low-temperature nitrogen adsorption, argon ion polishing, and casting thin sections and conducted a comparative analysis of the reservoir space types in beds I, II, and III of E₂S₄². The results are as follows. The reservoir spaces developing in the E₂S₄² submember include matrix pores, organic pores, dissolution pores, dissolution fractures, and microfractures. The tight reservoirs in beds I, II, and III of the E₂S₄² submember show greatly different diagenetic degrees and mineral compositions, as well as different reservoir spaces. The reservoir spaces in bed I mainly include matrix pores and microfractures, with micro-dissolution pores of minerals such as carbonates developing locally. The reservoir spaces in bed II are also dominated by matrix pores and fractures, with a small quantity of micro-dissolution pores of carbonates visible. The reservoir spaces in bed III mainly include matrix pores, fractures, and dissolution pores, with organic pores and biological cavity pores developing locally. Therefore, the pores and fractures in bed III have significantly larger occurrence scales than those in other beds, with partial dissolution pores up to 200 μm in diameter (Figure 5). Therefore, bed III is more favorable to tight oil accumulation.

3.2. Accumulation Conditions of Tight Oil

3.2.1. Effective Source Rocks in the E₂S₄² Submember Have High Organic Matter Abundance, Favorable Types of Organic Matter, and High Hydrocarbon Expulsion Efficiency, Providing Sufficient Oil Sources for Tight Oil Accumulation

Regarding the geochemical characteristics of high-quality source rocks, the source rocks in the E₂S₄² submember (beds I and III) contain original organic matter with high abundance and favorable types, providing sufficient oil sources for tight oil accumulation. For example, oil shales in bed III of E₂S₄² have TOC content of 0.4–10.63% (average: 3.70%), and more than 72% of the samples from this bed had TOC content greater than 2% and were thus excellent source rocks. Moreover, the organic matter in the E₂S₄² submember is mainly of type I, while type II₁ organic matter is present in only a few data points. In addition, the whole E₂S₄ member is in its primary oil generation stage, and the E₂S₄² submember has a higher burial depth and, accordingly, a slightly higher degree of maturity than the other horizons, creating favorable conditions for the generation of crude oil. As revealed by the hydrocarbon expulsion evaluation of high-quality source rocks, there is a distinct inflection point in the hydrocarbon expulsion vs. original organic carbon plot. The inflection point is accompanied by the improvement of organic matter types of source rocks (roughly types I and II₁) and the surge in oil discharge, and the organic matter abundance corresponding to the inflection point is the lower limit of organic matter abundance of high-quality source rocks.

Bed III of the E₂S₄² submember also has high hydrocarbon expulsion efficiency. The high maturity of the organic matter results in relatively light and low-viscosity crude oil generated in this bed. Therefore, the crude oil is freer and more prone to discharge than that in beds I and II (Figure 6). As a result, shales in bed III of E₂S₄² have higher hydrocarbon expulsion efficiency, which is even up to a maximum of more than 90% and is significantly higher than that of shales in beds I and II. Therefore, the crude oil in bed III of E₂S₄² is more prone to be recovered than that in the other beds.

3.2.2. During the Burial of Organic-Rich Shales, the Thermal Degradation of Organic Matter Produces Large Amounts of Organic Acids Which Dissolve Carbonate Minerals and Produce Massive Secondary Pores, Providing Favorable Reservoir Space for Tight Oil Accumulation

Secondary pores act as the main reservoir space for tight oil and consist mainly of dissolution pores and microfractures [34]. As shown by the microscopic evaluation of the tight reservoirs, bed III of the E₂S₄² submember has the most favorable pore structure and the most developed secondary pores, which are closely related to the sedimentary environment, evolutionary degree, and mineral composition of this submember. During the burial of organic-rich shales, the thermal degradation of organic matter produces large amounts of organic acids, which dissolve carbonate minerals. In this manner, massive secondary pores are formed.

The E₂S₄² submember is located in a reducing sedimentary environment overall, especially bed III, which has the strongest reducibility and high Sr/Ba and Cu/Zn ratios. The reducing background is favorable to the deposition of source rocks. The organic matter in this background is distributed in clay minerals in the form of dispersion, bedding enrichment, local enrichment, and biological residues, thus coexisting with and being inseparable from clay minerals. The organic matter adsorbed by clay minerals is mature. Meng Y.L. (2008) studied the vertical distribution of organic acids in the shales of sags in the western Liaohe oilfield. By comparison with these sags, the Damintun Sag is in the zone with peak organic acids in shales [35]. As shown by the results from the whole-rock mineral composition analysis, bed III of the E₂S₄² submember has large quantities of soluble minerals, including dolomites, calcites, and feldspars. These unstable minerals can form numerous secondary pores when exposed to organic acids, thus providing favorable reservoir space for tight oil accumulation.

3.2.3. Special Microscopic Pore Structure That Connects Fractures to Pores Is the Key to the Enrichment of Tight Oil

The tight oil in oil shales mainly migrates and accumulates through fractures and dissolution pores. Fractures and dissolution pores are inseparable in the E₂S₄ member in the Damintun Sag, with dissolution pores mostly distributed along fractures. The special microscopic combination of fractures and dissolution pores can reduce the filling resistance of tight oil, and this is the main factor for the high oil saturation in bed III of the E₂S₄² submember.

Figure 7a shows a FE-SEM image of oil shales at a depth of 3294.3 m at Well Shen-352, which clearly presents the favorable combination of pores and fractures. Specifically, the fractures are half filled with carbonates, and the dissolution pores have a diameter of greater than 150 μm. Figure 7b shows a three-dimensional confocal laser scanning image of oil shales at a depth of 3335 m at Well Shen-352, which shows that both fractures and pores are well developed and that dissolution pores are directionally distributed along fractures. Therefore, compared with beds I and II of the E₂S₄² submember, bed III has numerous fractures, which act as the main migration and accumulation channels for tight oil, reduce its filling resistance, and increase the oil saturation of the reservoirs.

3.2.4. Dolomitic Mudstones in the E₂S₄² Submember Have Significant Higher Breakthrough Pressure (Up to 100 Mpa) and Specific Surface Area Than Other Horizons, thus Effectively Sealing the Underlying Tight Reservoirs

The tight oil in fractured oil shales is typically self-generating and self-storing. Owing to the extremely developed fractures, the preservation condition is also a key factor for tight oil accumulation. Bed II of the E₂S₄² submember has high carbonate content of up to 40–60%. Moreover, rocks in bed II have significantly higher specific surface areas (up to 30 m²/g) than those in beds I and III (generally less than 10 m²/g; Figure 8), indicating that rocks in bed II have fine-grained particles and that the matrix of this bed has poor physical properties. As shown in Figure 8b, bed II of the E₂S₄² submember has a high breakthrough pressure of up to a maximum of 150 MPa, which is much higher than that of beds I (7 MPa) and III (15 MPa). This result indicates that the rock pores in bed II are poorly connected, and it is difficult for crude oil to flow through them. In sum, the pore structure with high micropore content and poor connectivity makes it difficult for crude oil to break through bed II. Therefore, the calcareous mudstones in bed II of the E₂S₄² submember seal the oil and gas in bed III as high-quality cap rocks, thus contributing to the effective preservation of the oil and gas in bed III.

3.3. Establishment of Tight Oil Accumulation Pattern

Regarding the oil generation, residual oil, oil expulsion, and reservoir characteristics, bed I of the E₂S₄² submember is characterized by more generated oil and more residual oil, while bed III has the characteristics of more generated oil, less residual oil, high oil expulsion efficiency, and high reservoir performance. In other words, compared to bed III, bed I has a higher oil content but the oil is less free and more difficult to recover. Despite more generated oil and less residual oil in bed III, tight reservoirs of this bed have a high brittleness index and well-developed sweet spot areas.

Compared with tight oil in sandstones in successful exploration areas in China, that in fractured oil shales unique to Damintun Sag has a significantly different accumulation pattern (Figure 9), and the microscopic differences are as follows (

Table 1. Comparison of accumulation factors between the tight oil in oil shales in the Damintun Sag and that in sandstones in other areas.

Type of Tight Oil	Reservoir-Forming Factors
	Source-Reservoir Configuration	Hydrocarbon Expulsion Channel	Hydrocarbon Expulsion Efficiency	Organic Matter Abundance	Fracture Density	Filling Resistance
Tight oil in oil shales	Self-generation and self-storage	Fractures	Relatively high	High	High	Low
Tight oil of sandstones	Superimposed sources and reservoirs	Pore throats	Relatively low	Low	Low	High

): (1) In terms of source-reservoir configuration, the tight oil in oil shales is typically self-generating and self-storing, while that in sandstones is significantly characterized by lower generation and upper reservoirs; (2) In terms of hydrocarbon expulsion channels, the tight oil in oil shales directly enters the reservoir spaces through fractures in the shales, while that in sandstones enters the reservoir space through nano-scale pore throats; (3) In terms of filling power, the tight oil in oil shales is connected by fractures, thus featuring low filling resistance, while that in sandstones needs to break through the nano-scale pore throats and is subject to high capillary resistance. Therefore, a higher power is required for the tight oil in sandstones to form reservoirs; (4) In terms of reservoir spaces, fractures and dissolution pores are more developed in oil shale reservoirs, while the sandstone reservoirs have a high skeleton density, retain partial primary pores, and hold poorly developed fractures in gentle structural zones. Moreover, bed II of the E₂S₄² submember in Damintun sag is a set of extremely tight dolomitic mudstones and acts as the direct cap rocks of the tight oil in oil shales of bed III. As indicated by the above comparison results, the tight oil in fractured oil shales enjoys more favorable accumulation conditions, and its various key accumulation factors are superior to those of the tight oil in sandstones.

4. Conclusions

(1)

Beds I, II, and III of the E₂S₄² submember differ greatly in residual organic carbon. Bed II has obviously lower organic matter abundance than other beds, while beds I and III differ slightly in the restored original organic carbon. All three beds of the E₂S₄² submember mainly contain type I organic matter and small amounts of type II₁ and II₂ organic matter. Moreover, the target horizons all have entered the hydrocarbon generation stage and are mature.

(2)

The oil shales in the E₂S₄² submember are the main target horizons for tight oil enrichment in the Damintun Sag. They can be divided into beds I, II, and III from top to bottom, showing a distinct sandwich structure. Beds I and III consist mainly of dolomitic mudstones and siltstones, while bed II is primarily comprised of oil shales. The dissolution pores and microfractures in bed III have significantly higher density than those in the other beds, and dissolution pores are mostly distributed along fractures.

(3)

The tight oil in the Damintun Sag is typically self-generating and self-storing in oil shales, and its enrichment mainly depends on the oil-generation and reservoir conditions. The high-quality source rocks in the E₂S₄² submember have high original organic abundance, favorable original organic types, high hydrocarbon expulsion efficiency, and high fluidity, providing sufficient oil sources for tight oil accumulation. During the burial of organic-rich shales, the thermal degradation of organic matter produces large amounts of organic acids, which dissolve carbonate minerals. In this way, massive secondary pores are formed. The special microscopic pore structure that connects fractures to pores is the key factor for the high oil saturation of oil shale pores. The dolomitic mudstones in the E₂S₄² submember have significantly higher breakthrough pressure (up to 100 MPa) and specific surface area than other horizons, thus effectively sealing the underlying tight reservoirs.