Efforts have been put into exploration and development of shale oil, inspired by successes in exploration and large-scale development of shale gas in China^{[1⇓⇓⇓-5]}. Two stages of major lacustrine transgression occurred in the central depression zone of Songliao Basin, resulting in the deposition of the first and second members of the Cretaceous Qingshankou Formation (abbreviated as the Qing-1 Member and Qing-2 Member, respectively) and the first and second members of the Cretaceous Nenjiang Formation organic-rich shale (abbreviated as the Nen-1 and Nen-2 Members, respectively). Among them, the Qing-1 Member with higher organic matter abundance and thermal maturity is the primary target for near- and medium-term shale oil exploration^[6-7]. After over sixty years of exploration, the Songliao Basin has entered into the stage of fully exploration of unconventional oil and gas^[8⇓-10]. In the Jilin Oilfield, the exploration for shale oil in the Qing-1 Member has been accelerated since 2018^[11], with significant discoveries in practice. However, the understanding on the shale oil enrichment mechanism fell behind the exploration practice. In the central depression zone of the Songliao Basin, the Qingshankou Formation shale depositing in a large-scale depression lake basin filled with terrigenous clasts different from other basins has unique mineral composition and lithologic and lithofacies assemblages. Continental shale is more climate-sensitive than marine shale, hence shows quicker lateral and vertical variations in lithofacies, and contains silica materials dominated by terrigenous clastic quartz. Unlike small-scale fault depression endogenous lake basin groups, large-scale depression terrigenous clastic lake basins usually show homogeneous overall subsidence and simple and flat lakebed topography. Due to the abundant sediment supply of shallow-water delta and mainly fresh-water in paleo-water system, largely coarse-grained and clastic-rich facies deposited at the margin of the lake basin, and clay-rich pure shale poor in carbonate mineral deposited in the semi-deep to deep lacustrine area. Thus, the shale oil exploration in the Qingshankou Formation of the Songliao Basin can’t follow the experiences gained in shale oil exploration in marine fine-grained sedimentary system and salinized lake basin mixed system simply. It is very important to study the spatial distribution of lithofacies, reservoir space types and shale oil enrichment models in the semi-deep to deep lacustrine shale widespread in the lake basin filled with terrigenous clasts of large depression on the basis of lithofacies and lithology analysis.

In the Songliao Basin, the great transgression happened during the deposition of the Qing-1 Member, resulting in a widespread, semi-deep to deep, large-scale depression lake basin, in which thick organic- rich dark mudstone and shale deposited. This member is both the primary source rock and the major reservoir of shale oil^[10]. In the southern part of the basin, the Qing-1 Member was supplied by provenances located in the west, southwest and southeast^[12], and the peripheral sandstone bodies vary in range and scale due to the differences in provenances. The western provenance was perpendicular to the long axis of the basin and proximal to the lake, and the flow channel terminated to the west of Da’an. The southeast provenance covered a relatively small area as a result of expansion of lake basin at the early and middle sedimentary periods of Qingshankou Formation. The southwest provenance, in general, was parallel to the long axis of the basin, far from the lake and distributed in the Tongyu, Baokang, Changling and Qian’an areas (Fig. 1).

The Qing-1 Member shale in the central depression zone with main types of Type I and II₁ kerogen, high organic matter abundance (TOC of 1.0% to 6.0%), and higher thermal maturity (R_o of 0.5% to 1.3%, averaging 1.05%)^[6,9] is the key target for shale oil exploration. According to the sweet spot evaluation criterion set up based on 3 shale traits and 7-property relationships^[11], the favorable area of shale oil with TOC of over 2%, S₁ (free hydrocarbon content) of over 1 mg/g, R_o of over 0.7% and pressure coefficient of over 1.21 has been confirmed at 5000 km². Under the joint control of provenance and sedimentary environment, multiple continuously distributed shale beds were formed in the southern Songliao Basin, extending from the delta front and pro-delta subfacies in the Daqingzijing area to the deep-lacustrine sub-facies in the Songyuan- Da’an area. These shale beds can be divided into two types of shale oil reservoirs: (1) the delta front interbedded shale oil reservoir in Daqingzijing, characterized by sandy laminae and thin sandstone interbeds, with single layer thickness of less than 5 m and sand to formation ratio of less than 20%; and (2) the pure shale oil reservoir in Qian’an-Da’an, characterized by absence of sandstone or sandy laminae. The favorable areas for these two types of reservoirs are 1300 km² and 3700 km². In the Jilin Oilfield, exploration wells were drilled targeting these two types of reservoirs and stimulation tests were conducted. The maximum oil production rate after fracturing was 10 m³/d, indicating a good shale oil potential in southern Songliao Basin.

The Qing-1 Member in southern Songliao Basin consists mainly shale, with laminar sandstone and ostracod limestone present locally (Fig. 2). The shale is mostly grey black to black and have three distinct sedimentary structures from core observation: (1) crushed structure with rich lamellation and cracks along horizontal beddings; (2) massive structure tight in texture with evenly distributed minerals; and (3) millimeter-scale sandy lamina structure, with evident thin bedding (0.10 to 0.01 m thick) and micro-bedding (less than 0.01 m thick). The fine-grained sedimentary rocks with lamellation or micro-beddings less than 0.01 cm thick and granularity of less than 62 μm (Fig. 2a, 2b) are named shale in this study, to distinguish from homogeneous massive mudstone (Fig. 2c). The sandstone is predominately light grey fine sandstone, which comes in two types of major sedimentary structures from core observation: some samples are in massive structure and others containing small number of shale laminae with bedding.

The Daqingzijing area in Southern Songliao Basin has mainly clay-bearing felsic shale, with brittle minerals (including quartz, feldspar, carbonate mineral and pyrite) content ranging from 65% to 80% and clay mineral content from 20% to 35%. The Qian’an-Da’an area has largely mixed shale and clayey shale, with brittle mineral content ranging from 40% to 65% and clay mineral content from 35% to 60%. Illite/smectite mixed layer accounts for 40% to 70% of the clay minerals, and the smectite content of the shale is estimated at 10%.

Statistics show organic matter abundance, mineral composition and sedimentary structure of the shale are related with each other. With the increase of clay mineral content, the sandstone transforms to shale, and the laminar and massive structure transform to leaf-like bedding (Fig. 3a). TOC value is positively correlated to clay mineral content when the clay mineral content exceeds 20% (Fig. 3b). Based on the criterion of “organic matter abundance, mineral composition and sedimentary structure”^[7], the Qing-1 Member can be divided into five types of lithofacies, with the thresholds of 1% and 2% in TOC, 20% of clay mineral content and corresponding sedimentary structure: the lamellar shale rich in organic matter, massive mudstone with medium organic matter content, laminated shale with medium organic matter content, laminated shale with low organic matter content, interbedded sandstone with low organic matter content (Figs. 3 and 4). The correlation between TOC-HI (hydrogen index) shows that (Fig. 3c) with the increase of TOC,HI of all the shale lithofacies increase rapidly first and then stabilize at about 700 mg/g. It shows that the lamellar shale with high organic matter content has single source of organic matter and mainly Type I kerogen. The shale with medium-low organic matter content has a large range of HI, suggesting mixed sources of organic matter. The interbedded sandstone with low organic matter content has high HI of more than 400 mg/g, clearly deviating from the variation trend between TOC and HI of shale, suggesting the contribution of migrated hydrocarbons to the HI.

This kind of shale is mostly black, fine-grained and rich in lamellation. It has mainly clay minerals (over 30%), and high organic matter content, with TOC of over 2%. In this kind of shale, feldspar and quartz particles are clay-sized, accounting for less than 30%, and the carbonate minerals make up less than 20%. Under microscope, the shale is tight and very small in grain size, and has horizontal micro-bedding structure with minerals and organic matter in directional arrangement (Fig. 4). This type of lithofacies is widespread the Xinbei area along the Songhuajiang River and is formed mainly in lacustrine transgressive, highstand systems tract (HST) lentic environment.

It is predominately dark grey, massive and tight. It has a clay mineral content range from 25% to 35%, slightly lower than that of lamellar shale with high organic matter content; higher organic matter content (TOC of 1% to 2%), quartz and feldspar content from 60% to 75%, and carbonate mineral content usually less than 15%. Under microscope, it contains evenly distributed minerals and has coarser grain size and higher clast content than organic-rich lamellar shale. This type of lithofacies was formed in lentic reducing environment with high sedimentation rate (Fig. 4).

It is made up of interbedded dark grey to black organic-rich argillaceous laminae and light grey organic-lean sandy laminae in micro-bedding structure. This type of lithofacies is similar with silty mudstone from the perspective of traditional grain size classification scheme. It has higher organic matter content, with TOC of 1% to 2%, quartz and feldspar content from 65% to 90%, carbonate mineral content of less than 15%, and clay mineral content from 15% to 30%. Observation under microscope shows it has obvious interbedded bright and dark laminae (Fig. 4). This type of lithofacies was formed mainly in lentic environment with alternating seasonal suspension and undercurrent sedimentation.

It is made up of grey interbedded sandy thin layers and dark grey to black argillaceous laminae. This type of lithofacies is similar with argillaceous siltstone from the perspective of traditional grain size classification scheme. It has lower organic matter content, with TOC of less than 1%, feldspar and quartz content from 65% to 90% which is higher than that of laminated shale with medium organic matter content shale, carbonate mineral content from 5% to 25%, and clay mineral content from 15% to 30%. Observation of thin sections under microscope shows that the sandy laminae are much thicker than argillaceous laminae (Fig. 4). This type of lithofacies was formed in flowing water environment with alternating seasonal suspension and undercurrent sedimentation.

It is predominately light grey massive fine-grained sandstone, with occasional argillaceous laminae and charcoal and beddings. It has low organic matter content, with TOC of less than 1%, quartz and feldspar content of generally 85%, carbonate mineral content of less than 10%, and clay mineral content of less than 20%. Observation of thin sections under microscope shows that it has many coarse-grained clastic particles scattering in matrix. This type of lithofacies was formed mainly in deep water turbidite or pro-delta sand bodies.

It is difficult to effectively recognize the shale lithofacies using conventional well logging through cross-plot plates. In this study, the TOC derived from log data was combined with GR, compensating density (DEN), compensated neutron log (CNL), acoustic time difference (AC) and lateral logging resistivity (LLD) values to conduct an orderly dimensionality reduction for dataset with adjacent index and nuclear representative index using the image-based multi-resolution cluster analysis (MRGC)^[13], to determine the optimal number of partition by searching the abrupt change in data variation curve, and effectively recognize the lithofacies of the Qing-1 Member.

Through geologic analysis, 1% and 2% in TOC were taken as the thresholds dividing low, medium and high organic matter contents. The log response features of the lithofacies are as follows: the lamellar shale with high organic matter content shows high AC, high CNL, high LLD, medium GR and low DEN; the massive mudstone with medium organic matter content exhibits high GR, medium AC, medium DEN, medium CNL and medium LLD; the laminated shale with medium organic exhibits medium AC, medium GR, medium DEN, medium CNL and medium LLD; the laminated shale with low organic matter content shows high GR, medium AC, medium CNL, low DEN and low LLD; and the interbedded sandstone with low organic matter content exhibits low AC, low GR, low DEN, low CNL and high LLD. It is therefore speculated that, the hydrodynamic force became weaker from south to north along the Daqingzijing-Qian’an- Da’an areas (Fig. 1c), along with the progradation of shallow-water braided river delta towards the lake, resulting in the gradual decrease in terrigenous input carried by undercurrent and the transition in facies combination from inner delta front (well 6#: laminated shale with medium organic matter content intercalated with massive mudstone with medium organic matter content, with interbedded sandstone) to outer delta front (wells 5# and 4#: laminated shale with low organic matter content dominates the middle and lower parts of the Qing-1 Member) and then to semi-deep to deep lacustrine subfacies (wells 3# and 2#: lamellar shale with high organic matter content intercalated with massive mudstone of medium organic matter content and laminated shale of medium organic matter content) (Fig. 5).

The isopach maps of various lithofacies also reveal the variation law of lithofacies in depression lacustrine fine-grained sedimentary system filled with terrigenous clast (Fig. 6). The sedimentation within the lake basin was essentially controlled by two provenances. The provenance to the south fed a shallow water braided river delta extending over a long distance into the lake. In this context, the sandstone deposited thins gradually basinward and laminated shale with low organic matter content is present at the outer delta front (Fig. 6a). The Qian’an and Da’an areas were situated in the semi-deep to deep lacustrine zone for a long period. The interdistributary bay deposited in delta system features massive mudstone with medium organic matter content. Laminated shale with medium organic matter content is widespread within the semi-deep lake zone and lamellar shale with high organic matter content is present mainly in the deep lake zone (Fig. 6b). These lithofacies vary in mineral composition, organic matter abundance and sedimentary structure orderly, reflecting the characteristics of lithologic and lithofacies zonation in the depression lake basin filled with terrigenous clast, such as overall lake basin subsidence, gradual decrease of hydrodynamic intensity and compensation of terrigenous clasts from the coast to non-compensated clastic deposits in the center of the lake. The presence of various shale lithofacies provided the material basis for differential enrichment of shale oil.

Shale of the Qing-1 Member has an effective porosity of 3.4% to 8.4% and a horizontal permeability of (0.01- 1.62)×10^-3 μm². Except the laminated shale with medium organic matter content with much higher porosity (on average 6.8%, about 2% higher than other lithofacies) and higher permeability (on average 1.2×10^-3 μm², about 5 to 10 times higher than other lithofacies), the other lithofacies don’t differ much in physical property parameters. But the lithofacies have various types of reservoir space and the pore structure is strongly controlled by sedimentary facies, providing favorable reservoir condition for shale oil enrichment.

Reservoir space can be quantitatively assessed and qualitatively characterized with pore size and occurrence^[14]. In this study, shale oil reservoir space is classified as inorganic pore and organic pore according to its origin. The inorganic pore is then further divided into intergranular (intercrystalline) pore and intragranular (intracrystalline) pore (Fig. 7).

Intergranular pores are usually narrow and long, or polygon-shaped, and commonly occur in cluster around hard brittle minerals. The primary intergranular (intercrystalline) pores in the Qing-1 Member sedimentary rock diminished during the shallow burial stage under effects of compaction and later cementation, as the Qing-1 Member with fine-grained mineral particles (less than 62 μm) and a large amounts of plastic minerals has low resistance to compaction. The residual intercrystalline pores are present mainly between hard brittle particles, such as quartz. The acidic fluid dissolved minerals along the primary intergranular (intercrystalline) pores at later stage, forming dissolution marginal pores and intergranular dissolved pores of 100 nm to 3 μm in diameter.

With the diameter of 10 nm to 1 μm, intragranular pores can be divided into mineral aggregate interior pores and intragranular dissolved pores. Clay mineral aggregate interior pores are sheet-like and arranged in parallel with the sedimentation orientation, and pyrite aggregates contain triangular- or polygon-shaped intercrystalline pores, occasionally filled with organic matter. Intragranular dissolved pores are present mainly inside soluble mineral particles, such as K-feldspar.

Organic matter pores are mostly strip- or ellipse- shaped, and can be divided into organic matter marginal pores (fissure) and organic matter interior pores, usually less than 100 nm in diameter.

SEM- and EDX surface scanning-based in-situ automatic quantitative statistical results show that (Fig. 7) different types of lithofacies in the Qing-1 Member of southern Songliao Basin contain different types of high-quality reservoir spaces. Lamellar shale with high organic matter content and massive mudstone with medium organic matter content have the highest clay mineral content and poorest compaction resistance, and have mainly intragranular pores (accounting for 75% on average). With sandy laminae, laminated shale with low-medium organic matter content has mainly intergranular pores (accounting for 80% of total porosity). With little clay minerals and pyrite aggregates, the interbedded sandstone with low organic matter content has intergranular pores in absolution dominance, accounting for up to 100%. Ostracod limestone occasionally seen in this kind of shale has largely intergranular pores formed by alteration of carbonate minerals between calcareous shell space and shell interior, accounting for 85% of total porosity. It is generally considered that organic pore is the most important reservoir space for shale oil and gas^[15]. In this study, however, statistics show organic pores are under-developed, making up less than 15%. This is because the following two reasons: (1) Organic matter of the Qing-1 Member is mainly of Type I kerogen, the hydrocarbon-generating material is algae, which has become laminated algae as a result of ductile deformation under strong compaction^[16], and the majority of pores generated during hydrocarbon-generating process have been damaged, and the very few pores left are eclipse- or slot-shaped. (2) The development of organic pore is influenced more strongly by the thermal evolution degree of source rock besides the type of organic matter. A large number of studies suggest that oil-prone source rock generates substantial organic pores only when R_o exceeds 1.2% (i.e., the oil cracking phase)^[17]. The Qing-1 Member in southern Songliao Basin is currently at oil generation peak and thermally less mature than the shale rich in organic pore. At this stage, residual space left by hydrocarbon-generation of organic matter is mostly filled with bitumen, resulting in the rareness of organic pore.

To characterize the pore structure characteristics of the Qing-1 Member shale in southern Songliao Basin, a test combining high-pressure mercury injection with low-temperature nitrogen adsorption^[18] was conducted (Fig. 8), and based on test result, pore structure can be classified into five types^[19].

Type 1 of pore structure occurs mainly in massive mudstone with medium organic matter content. Typical characteristics are as follows: the isotherm hysteresis loop is H2 type (indicating existence of spherical mesopore), the straight and flat section of the capillary pressure curve at over 100 MPa is longer, most of the pores are less than 7.3 nm in diameter, and the mercury ejection volume is low (less than 20%), development features of spherical mesopore. The distribution of pore diameter shows that the pores are generally less than 200 nm in diameter, and 2 to 50 nm mesopores take the majority.

Type 2 of pore structure occurs mainly in lamellar shale with high organic matter content. The typical characteristics are as follows: the isotherm hysteresis loop is H2 type (indicating the existence of spherical mesopore), the adsorption curve rises significantly as p/p₀ approaches 1, and the absorption quantity is higher than Type 1, this is because of adsorption of fissures between large-sized laminae. The straight and flat section of the capillary pressure curve at over 100 MPa is longer (but shorter than that of Type I), most of the pores are less than 7.3 nm in diameter, the mercury ejection volume is low (less than 20%), but still indicating the presence of spherical mesopore, and the capillary curve has another straight and flat section at pressure less than 0.02 MPa, which is resulted from the mercury invasion into pores and fissures in laminae over 36 μm in diameter. The distribution of pore diameter shows that the pores are generally less than 400 nm in diameter, 2 to 50 nm mesopores take the majority, and a large number of 10 nm fissures between laminae.

Type 3 of pore structure occurs mainly in laminated shale with low-medium organic matter content. The typical characteristics of this type are as follows: the isotherm hysteresis loop is H3 type, indicating aggregate of non-rigid flaky particles, that is couplet consisting of sandstone and mudstone laminae, the pores are mainly narrow fissure-type. The capillary pressure curve is gentle and flat at the pressure over 40 MPa, indicating the presence of substantial mesopores, but the pores invaded by mercury at less than 40 MPa are poorly sorted, and the pressure curve appears as an inclined line at 45°. The distribution of pore diameter shows that the pores range from 2 μm to 100 μm, 2-200 nm pores are mostly pores inside argillaceous laminae, 200 nm to 1 0000 nm pores are mostly pores in sandy laminae, and pores more than 10 μm are mostly fissures between lamina.

Type 4 of pore structure occurs mainly in interbedded sandstone with low organic matter content. The typical characteristics of this type are as follows: the isotherm hysteresis loop is H3 type, indicating that the pore network consists of mainly macropores over 50 nm in diameter not completely filled with cement. The capillary pressure curve is gentle at pressure over 1 MPa, indicating that the sample is rich in macropores and most of which are less than 735 nm in diameter. The distribution of pore diameter shows that most of the pores range from 100 nm to 1000 nm in diameter and can be classified as macropore.

Analysis of 3D pore structure connectivity by using digital core further confirms that the type of lithofacies under the influence of sedimentary structure has significant control on pore structure in shale oil reservoir (Fig. 9). Pores in massive mudstone with medium organic matter content are mostly ellipsoidal, small in individual volume, and mostly scattered and isolated with poor connectivity (Fig. 9a). Lamellar shale with high organic matter content contains abundant bedding fissures (Fig. 9b). Pores in laminated shale with low-medium organic matter content are flaky, better in connectivity, and formed by directional arrangement of rigid mineral particles (such as fine-grained quartz, feldspar and calcite) along beddings in sandy laminae (Fig. 9c). Under the confining pressure of 10-21 MPa, the horizontal permeability of laminated shale measured along the bedding surface is (0.1-1.2)×10^-3 μm², 100 to 1000 times higher than the vertical permeability of 0.000 06-0.003 00)×10^-3 μm² measured perpendicular to the bedding surface. The vertical permeability varies in two-section pattern with the increase of confining pressure, that is, the slope is pretty large at first, then becomes small. The closing pressure of horizontal micro-fracture is estimated at 12-15 MPa (Fig. 9d).

In conclusion, lamellar shale and massive mudstone rich in clay minerals have mainly mesopores and relatively poor matrix reservoir physical properties. But lamellation fissures improve the reservoir physical properties of lamellar shale lithofacies. With relatively well-developed macropores and a certain amount of bedding fissures at the lamina boundaries, the laminated shale shows pore-fissure binary structure and high horizontal permeability. The interbedded sandstone has most developed macropores.

A large-scale lacustrine transgression took place during the sedimentation of the Qingshankou Formation. The depocenter of southern Songliao Basin, at that period, was located in the Changling sag, where the Qingshankou Formation deposited slowly under a long-term stable deep lacustrine setting and thus is rich in organic matter and horizontal bedding. A relatively closing fluid environment was formed as the shale experienced compaction continuously under the deep lacustrine setting, and ductile materials, such as organic matter and clay minerals were arranged directional to form beddings. When organic matter was thermally mature and generated a large amount of hydrocarbons, the discharge of hydrocarbons was hindered, resulting in abnormal overpres-sure (formation pressure of 25-32 MPa) of the Qing-1 Member in the Changling sag. The basin was tectonically inverted at the end of the Nenjiang and Qingshankou sedimentary periods. As a result, some areas in the Fuxin uplift zone were uplifted up to 600 m according to the relation of R_o and burial depth. Tectonic uplifting would lead to the variation in deformation mode of shale^[20-21], and generation of structural fractures^[22-23]. As a result of tectonic uplift, ductile rock became brittle^[21,24], and the effective stress declined gradually, making the mudstone over-consolidated and break^[25]. Due to the presence of preexisting horizontal beddings, fractures propagated laterally in response to the point effect of organic matter and other clastic minerals arranged along beddings, forming a large number of bedding fissures (Fig. 10). The geologic stress condition of the Qing-1 Member was calculated using the well logging and measured data from wells 1# and 3#. The results show the current fluid pressure has already reached the critical fracture pressure of shale due to the effects of abnormal pressure and tectonic uplifting of the Qing-1 Member. According to the com-pression coefficient of crude oil under formation conditions and the present residual overpressure value, it is inferred that the bedding fissures formed by hydraulic fracturing can make the shale reservoir space increase 1 to 1.2 times, equivalent to the porosity increase of 2% to 3%.

The lamellar shale has TOC ranging from 2% to 6%, of which 73% has TOC of over 2.0%, featuring high organic matter content. The massive mudstone, with TOC ranging from 1% to 4%, and only 12% having TOC of over 2.0%, features medium organic matter content. The shale thin sections show strong light blue fluorescence mainly on the lamellation surface. Oil saturations of the samples measured by 2D NMR test reach up to 80% and average at 54%, and the oil content can reach up to 5 μL/g. The mudstone thin sections basically show no fluorescence, apart from weak dark yellow fluorescence locally. The mudstone samples have oil saturation of less than 20% and oil content of less than 2 μL/g in general. Thus, it is believed that in the absence of terrigenous clastic lamina, lamellar shale is the favorable lithofacies for shale oil enrichment and massive mudstone can serve as the top and base seal for shale oil reservoir.

Unlike conventional oil and gas, shale oil and gas are characterized by “integrated source and reservoir” and “continuous accumulation”^[26]. Shale oil enrichment model of organic-rich lamellar shale oil reservoir in the Qian’an-Da’an area can be summarized as follows: the Qing-1 Member source rock began to generate and expel substantial hydrocarbons at the end of the Nenjiang and Mingshui sedimentary periods^[27], which was the period of tectonic inversion induced by ES-trending compression^[28], and the tectonic inversion led to the reactivation of some T2 faults, increasing the pressure coefficient of the Qing-1 Member and forming bedding fissures^[29]; meanwhile the stress release resulted in tectonic fractures. Therefore, the enrichment of “lamellation-type” shale oil also needs the top and base sealing provided by massive mudstone with medium organic matter content and tight texture and high breakthrough pressure.

Also, exploration practices suggest that shale oil enrichment in deep lake zone is closely related to lamellation fissure. Two examples can be evidence: (1) Six wells drilled before 1999 obtained industrial oil flow of 1.70-6.55 t/d from the Qing-1 Member shale reservoir, which was considered fractured oil & gas reservoir^[20], and the cores taken recently all contain oil in the fractures and bedding fissures. (2) A large number of densely distributed, “V”-shaped S-N trending small faults are present at the base of the Qing-1 Member (T2 horizon). Previous testing results show that wells penetrating the pure shale layers in fault zones have higher oil production rates, while wells outside fault zones have low production rates. These findings suggest that the large number of structural fractures associated with faulting improve quality of the shale reservoirs, and lamellation fissures formed in deep lacustrine mudstone under the later-stage tectonic uplifting provide favorable reservoir space and enable shale oil enrichment. But the complementary distribution of shale oil and conventional oil and gas on the plane in southern Songliao basin indicates that faults remaining active during the inversion period might act as pathways for vertical migration of hydrocarbons if fault-associated fractures are very dense, which is unfavorable for shale oil enrichment. Therefore, this kind of shale oil can enrich in sequences and zones with ultra-low permeability massive mudstone as the top and base sealing layers, good preservation condition, high pressure coefficient, and developed organic-rich lamellar shale.

The laminated shale with medium organic matter content, although slightly lower in organic matter abundance than the lamellar shale, but with abundant sandy laminae and higher horizontal permeability, it has quite high hydrocarbon-generating potential. In general, it has better matrix physical properties than lamellar shale and massive mudstone, and is characterized by the integration of source and reservoir at micron- to millimeter-size.

Sandy laminated shale commonly deposits in delta front environment. The Qingshankou Formation deposited under the context of the basin subsidence, the first expansion of lake basin and subsequent contraction. Along with lake level fluctuations, deep lake shifted periodically and frequent undercurrent activity at the outer delta front, resulting in ripple cross lamination and parallel bedding, and alternative deposition of argillaceous lamina and sandy lamina^[30].

QEMSCAN analysis results of laminated shale samples show that the shale is composed of mainly quartz (32.86%) and illite (27.52%), followed by albite (18.93%), K-feldspar (3.41%) and calcite (2.61%, as shown in Fig. 11). The sandy lamina has a quartz content of 34.47%, illite content of 22.87%, albite content of 20.83%, K-feldspar content of 3.43% and calcite content of 2.38%, showing higher felsic content than the whole rock. The argillaceous lamina is dominated by illite (39.46%), followed by quartz (25.31%), albite (17.03%), K-feldspar (2.42%) and pyrite (2.27%). CT scanning of the two types of pore structure at the resolution of 2.5 μm reveals that the sandy lamina has a computed porosity of 4.247%, an average pore diameter of 3.801 μm, a pore number of 30 090, a total pore volume of 17.91×10⁶ μm³, an average throat length of 29.54 μm, and a connected pore volume proportion of 8.882%; and the argillaceous lamina has a computed porosity of 2.114%, an average pore diameter of 3.496 μm, a pore number of 25 066, a total pore volume of 8.92×10⁶ μm³, an average throat length of 30.16 μm, and a connected pore volume proportion of 5.829%.

In general, the sandy lamina is felsic, with quartz and feldspar content exceeding 60%, grain size ranging from 1 μm to 60 μm, mainly micron-sized pores (28.4% to 63.3%, averaging 40%) 1 μm to 3 μm in pore throat diameter. In contrast, the argillaceous lamina is clayey, with quartz and feldspar less than 45% in content and less than 5 μm in grain size, largely nano-sized pores (accounting for 36.7% to 71.6% of total porosity, 60% on average) 100 nm to 300 nm in pore throat diameter.

Laser confocal fluorescence 3D visualization analysis shows that there's rich oil between fine-grained clastic mineral particles, and the occupation of fluorescent organic fluid to pores reveals intergranular pores appear in patches and are in good connectivity (Fig. 12). According to fluorescence spectrum features, the fluorescence organic matter can be divided into light component (showing leading peak on fluorescence spectrum) and heavy component (showing tailing peak on fluorescence spectrum). The light and heavy components were characterized in 3D and quantitatively counted respectively^[10]. Light component in laminated shale occurs mainly in laminae formed by fine-grained feldspar and quartz with an oil content of 12.64%. Pores holding light component show two peaks on the distribution curve of diameter, with a leading peak at 5-8 μm, and a tailing peak at 40-50 μm. This suggests that as the Qing-1 Member entered the oil window, substantial hydrocarbons were generated by organic matter in argillaceous lamina and migrated over a micro-distance into the sandy lamina in close contact with higher porosity and higher permeability, resulting in in-situ micro-migration and enrichment feature. Oil content of the argillaceous lamina is as low as 4.3%. Pores occupied by light component show a leading peak at 2-5 μm. In addition, compared with argillaceous lamina, the sandy lamina has a ratio of light component to heavy component of up to 2.65, indicating higher mobility.

After separate-layer network fracturing, vertical wells targeting laminated shale oil reservoirs at the outer delta front in the Daqingzijing area all obtained industrial oil flow. The wells have produced stably for over 200 d, showing good potential of economic development. The shale oil producing area and the Qing-1 Member tight sandstone oil are in close proximity on the plane, piecing together in distribution. According to the shale lithofacies filling succession under the control of depositional environment, different types of unconventional oil and shale oil under different enrichment models show continuous accumulation feature (Fig. 13).

Based on distribution features of sedimentary facies and high-quality source rocks, formation pressure characteristic, predominant lithofacies distribution and tectonic uplifting effect, favorable areas of lamellation-type and laminated-type shale oil have been sorted out (Fig. 14). According to the enrichment conditions of them, three favorable shale oil exploration zones have been identified, i.e., the favorable shale oil exploration zones of lamellation-type, lamellation+laminated-type and laminated-type shale oil (Fig. 14).

Lamellation-type shale oil favorable area is distributed mainly in the northwestern part of the Fuxin uplift zone, with lamellar shale of high organic matter content developed. This prospective area was defined according to pressure coefficient of over 1.2, predominant lithofacies thickness of over 40 m (lamellar shale with high organic matter content), TOC of over 3% and quartz content of over 30%, and is characterized by high organic abundance, high pressure coefficient, moderate to low maturity (R_o of 0.6% to 0.9%), and lower brittleness index than laminated shale oil. The laminated shale oil favorable area is distributed at the outer front of Da’an and Daqingzijing areas, with laminated shale with medium organic matter content mainly. This prospective area was defined according to pressure coefficient of over 1.0, predominant lithofacies thickness (lamellar shale with high organic matter content and laminated shale with medium organic matter content) of over 45 m, TOC of 1% to 2% and quartz content of over 30%, and is characterized by moderate organic abundance, moderate pressure coefficient (1.0 to 1.2), moderate to high maturity (R_o of 0.85% to 1.20%), and higher mineral content and brittleness index than lamellation-type shale oil. A transition zone between the lamellation-type and laminated-type favorable areas is located in the Da’an area, which is characterized by high organic abundance (over 2%), moderate pressure coefficient (over 1.0), moderate maturity (R_o of 0.65% to 1.00%) and brittleness index between that of laminated-type and lamellation-type shale oil. The thickness of predominant lithofacies of lamellar shale with high organic matter content and laminated mudstone with medium organic matter content exceeds 50 m. This area is a prospective area rich in both lamellation-type and laminated-type shale oil.

The distribution of different shale oil prospective areas reveals the richness of shale oil resources in the southern Songliao Basin, which is of great strategic significance. But as the criteria for selecting geological and engineering sweet spots vary for different areas, it is urgently required to develop tailor-made stimulation technique for each type of shale oil reservoir, strengthen the integrated geologic-engineering technology study, and clarify the proper development mode, to expedite the economic development of shale oil resources.

Based on organic matter abundance, mineral composition and sedimentary tectonics, the Qing-1 Member shale in southern Songliao Basin is divided into five types of lithofacies, i.e., lamellar shale with high organic matter content, massive mudstone with medium organic matter content, laminated shale with medium organic matter content, laminated shale with low organic matter content, and interbedded sandstone with low organic matter content. These lithofacies vary greatly in reservoir physical properties. The massive mudstone with scattered spherical isolated pores has the worst physical properties. With lamellation fissures, the lamellar shale with high organic matter content has better reservoir physical properties. Laminated shale contains well-developed horizontal beddings, which create stripe-shape pore network within the coarse-grained clastic laminae and has a good oriented connectivity in the presence of bedding fissures. The interbedded sandstone is rich in macropores. Lamellar shale with high organic matter content dominates the lithofacies depositing in the deep lacustrine envi-ronment that remained stable over a long period with low sedimentation rate. Affected by tectonic inversion and uplifting, this type of lithofacies has abundant lamellation fissures, and has nano-sized pores and bedding fissures as the major reservoir space for oil and gas storage. The ultra-low permeability massive mudstone at the base and top of lamellar shale with high organic matter content can act as sealing layers, resulting in good preservation condition and high pressure coefficient, and thus is favorable for formation of lamellation-type shale oil. Under the periodic shifting of deep lacustrine setting, organic matter accumulated in argillaceous laminae, and intergranular macropores developed well and connected directionally in sandy laminae, with laminated shale with medium organic matter content dominating the lithofacies. Oil generated is likely to accumulate in-situ after micro-migration, forming laminated-type shale oil.