In recent years, with transformation and innovation of oil and gas exploration concepts, the shale oil resources in hydrocarbon source rock have received extensive attention^[1]. The generalized shale oil is divided into three main types^[2]: (1) matrix shale oil, e.g. Barnett shale in the Fort Worth Basin in the U.S. and the 3rd Member of Paleogene Hetaoyuan Formation in Well HF1 in the Biyang Sag, China; (2) shale intercalated with brittle oil-bearing layer (sandstone or carbonate interbed), e.g. Niobrara shale and Bakken shale in the U.S. and the Permian Fengcheng Formation in the Junggar Basin, China; (3) fractured shale oil, such as Monterey shale and Pierre Shale in the U.S., Bazhenov shale in Russia and China's fractured mudstone oil reservoir formations confirmed in the 20th century. The dark mudstone and shale in the first member of Cretaceous Qingshankou Formation (referred to as the Qing-1 Member) in the Gulong Sag, northern Songliao basin, with large thickness, high TOC, moderate maturity and overpressure, is a set of high-quality source rock with strong oil generation ability, which is favorable for forming the commercial shale oil and gas^[2,3]. Since the 1980s, systematic investigation and exploration have been implemented in the fractured mudstone oil reservoirs of the Gulong Sag^[4]. The oil testing was performed in 35 exploration wells, and the commercial oil flow was obtained from 10 exploration wells. Commercial oil and gas were also successively obtained from the sandstone intercalated in the shale, but breakthrough has not been made in the matrix-type shale oil.

Based on the current exploration status of the Qing-1 Member shale oil, this paper aims to: (1) investigate the petrology of mudstone and shale sedimentary sequence and establish a lithofacies filling evolution sequence within the high-resolution isochronous framework; (2) clarify the reservoir space type and reservoir physical property of different lithofacies; (3) analyze the oil storage capacity of shale and investigate the lithofacies conditions for enrichment of different types of shale oil.

The Gulong Sag is located in the west of the Central Depression of the Songliao Basin, adjacent to the Daqing placanticline on the east and the Longhupao-Da’an terrace on the west. It is a monoclinic structure high on the northwest and low on the southeast on the whole with an area of around 3700 km² (Fig. 1). The Qingshankou Formation is dominated by lacustrine sediments formed under warm and humid conditions. Large-scale water-transgression occurred in the sedimentary period of Qing-1 Member, so the depositional environment was semi-deep to deep lake anoxic, and during this period, two cycles of micro-amplitude uplift and subsidence happened, associating with moving of the delta toward the lake basin to different extents, and deposition of black thick mudstone intercalated with grey siltstone and fine sandstone (Fig. 1). The Qing-1 Member deep water sediments in the Gulong Sag provide a material basis for establishment of fine- grained sedimentary cycles.

Organic content, mineral composition and sedimentary structure are main factors able to reveal the diversity of organic-rich shale petrological characteristics and also important bases for identifying lithofacies types according to sedimentary genesis.

The Qing-1 Member mudstone in the Gulong Sag has TOC of up to 7.5%, on average 1.9% and mainly between 1.4% and 2.5%, and the hydrocarbon generation potential (S₁+S₂) of 4.7-11.2 mg/g, on average 7.9 mg/g. The mudstone with organic matter dominated by Type I and Type II₁ kerogen, is typical oil-prone lacustrine organic-rich mudstone. With R_o value mainly ranging between 0.70% and 1.13%, 0.89% on average, the mudstone is in the mature stage overall. The relationship between the original hydrogen index (HI_o) and TOC of immature-low mature shale in the study area^[5] indicates that when TOC <1%, HI_o is very low, when TOC is 1%-2%, HI_o has a positive correlation with TOC, when TOC >2%, HI_o is stablized and does not increase with the increase of TOC. Thus, for the terrestrial organic-rich shale, TOC <1% can be defined as low organic matter content, TOC between 1% and 2% is defined as the medium organic matter content, and TOC >2% is defined as high organic matter content.

The mineral compositions of the Qing-1 Member shale are mainly quartz (19%-43%, on average 34%), plagioclase (8%-65%, on average 31%) and clay minerals (2%-52%, on average of 23%), followed by a small amount of K-feldspar, calcite, dolomite and pyrite. The clay minerals are dominated by illite, followed by the illite-montmorillonite mixed-layer and kaolinite. The carbonate minerals are concentrated in the ostracod limestone (the total rock content is higher than 30%, and mostly higher than 50%). In the silty laminae within fine sandstone and mudstone, calcite cement can be seen filling in between debris particles (2%-20% in whole rock, on average 9%). According to "clay-carbonate-debris contents (quartz and feldspar)", the fine-grained rocks with a particle size of less than 62 μm (including siltstone, mudstone, limestone) are preliminarily classified into five categories^[6]: I limestone facies, II clayey marlstone facies, III siliceous marlstone facies, IV clayey mudstone facies, and V siliceous mudstone facies (Fig. 2). The samples in the study area are dominated by siliceous mudstone and limestone lithofacies, and other litho-facies are rarely seen.

The fine-grained rocks have massive, layered and laminated sedimentary structures. The massive structure does not show obvious variation of color, grain size or mineral composition in the vertical direction. The rock is homogeneous, with a particle size of less than 62 μm. The layered structure has a grain size of less than 62 μm and single layer thickness of more than 1 mm and is intercalated with fine sandstone or ostracod limestone. The laminated structure shows abrupt variation of color, grain size and mineral composition at sedimentary horizontal planes, and has a particle size of less than 62 μm and single layer thickness of less than 1 mm, and the laminae appear alternately and repeatedly^[7].

The results of core observation and measurements show that as the hydrodynamic conditions in the sedimentary environment weakens, the TOC and clay mineral content of the rock increase (Fig. 2a), meanwhile the rock sedimentary structure transits from the layered to laminated structure and to the massive structure (Fig. 2b). According to classification of five types of rock facies mentioned above, according to TOC, the fine-grained rock was classified into the one with high organic matter content (TOC >2%), medium organic matter content (TOC betwen 1% and 2%) and low organic matter content (TOC <1%), and then, based on the division of massive, layered, and laminated sedimentary structures and considering the sandstone and limestone intercalated in fine-grained sediments, the Qing-1 Member in Gulong Sag was divided into 7 types of lithofacies (Table 1): (1) the high TOC laminated clayey mudstone (i.e. oil shale); (2) the high TOC massive siliceous mudstone; (3) the medium TOC massive siliceous mudstone; (4) the medium TOC laminated seliceous mudstone; (5) the low TOC laminated siliceous mudstone; (6) the low TOC layered sandstone; and (7) the low TOC layered limestone. Among them, the sample of high TOC laminated clay mudstone is abundant in lamellation and highly fragmented, and thus, its reservoir properties could not be analyzed.

The logging data has high vertical resolution and can continuously records the rhythmic characteristics of the measured strata, and even fine-grained sedimentary profile without obvious vertical petrological variation (such as successive semi- deep lake-deep lake sediment) can be divided into sedimentary cycles of different scales through wavelet transformation^[8]. The low-frequency large-scale decomposition curve corresponds to long-period component with long sedimentary cycle and large cycle thickness; the high-frequency small- scale decomposition curve corresponds to short-period component with short sedimentary cycle and small cycle thickness.

The GR value of the Qing-1 Member is 78-167 API. Considering multiple spectrum records of GR curve, the Qing-1 Member was divided into 10 layers with the Daubechies wavelet (abbreviated as db wavelet) with large scale and offset distance, and 10 wavelet transformation curves of different levels were obtained^[9]. According to spectrum analysis, the dominant frequency with relatively concentrated energy was selected as the basis for the cycle stratigraphic division, and the corresponding wavelength was obtained. Among them, the cycles resulted from wavelet transform based on the wavelet coefficients of 32, 64 and 128 are equal to or close to the cycle ratio of the Milankovitch cycle and respectively correspond to the long-term, medium-term, and short-term base level cycles. This indicates that the sedimentary cycles of the Qing-1 Member well correspond to the Milankovitch cycles^[10] and are mainly controlled by the long eccentricity cycle of 405 ka. Thus, the Qing-1 Member was subdivided into 3 medium-term cycles (SSC1-SSC3 from bottom to top) and 10 short-term long eccentricity cycles, which laid the basis for division of stratigraphic cycle (Fig. 3). During a long eccentricity cycle, the eccentricity fluctuation caused variation of sunshine amount on the surface, which made the seasonality of the short-term cyclic sedimentary periods of the Qing-1 Member enhance, and controlled the vertical rhythmic variation of gradual increase of TOC in the rising half-cycle and gradual decrease in the descending half-cycle.

As the sedimentary area of the Qing-1 Member semi-deep to deep lake facies was far from source supply, the deposition of Qing-1 Member was controlled by the Milankovitch cycle and shows obvious vertical cyclicity and sequence (Fig. 4). The vertical combination of mudstone lithofacies was controlled by the mid-term cycle, and the lithofacies type and coupled rhythmic variation were controlled by long eccentricity cycle. In the semi-cycle under the dry-cold climate, as there was relatively little precipitation, little source material was fed into the lacustrine water , so the layer formed then is low in TOC and mostly in layered-massive structure. If evaporation caused the water level to drop, the carbonate would precipitate. In the semi-cycle under warm and humid climate, due to increase of paleo-water depth, abundant source material and rich organic matter of diverse origins were carried into the water, forming a sedimentary stratum with high TOC. Due to the seasonal effect, the sedimentary strata take on layered-laminated structure^[11]. As a result, the medium TOC laminated siliceous mudstone facies and the high TOC massive siliceous mudstone facies developed in the sedimentary zone of the deep lake in the SSC1 sedimentary period, and the high TOC laminated clayey mudstone deposited on the maximum flooding surface and then filled with a high TOC massive mudstone. Laminated siliceous mudstone developed in SSC2 and SSC3 during the initial water transgression, forming the high TOC siliceous mudstone on the maximum flooding surface. During low-water level, the cycle ended with medium-high TOC laminated siliceous mudstone. The low TOC layered ostracod limestone facies was observed in the upper part of descending semi-cycle in both cycles.

The western slope of the Qing-1 Member of the Gulong Sag was influenced by the deposition of shallow water braided river delta^[12] and characterized by the different lithofacies patterns within the lacustrine sedimentary area (Fig. 4). In the delta sedimentary area, the vertical lithofacies combination of mid-term cycle SSC1-SSC3 is characterized by mutual intercalation of low TOC layered siltstone and fine sandstone and low TOC laminated siliceous mudstone. In the delta-lacustrine transitional sedimentary area where the sand body only developed in top SSC1 during delta progradation, the organic matter content gradually increases from the delta sedimentary area to the lacustrine sedimentary area, and the sedimentary structure also transits from layered and laminated ones to massive one, forming gradual variation of lithofacies.

It can be seen that the continental lake basin was controlled by both cycle and source provenance supply, resulting in regular variation of organic matter content, rock sedimentary structure and mineral composition, and an orderly variation of lithofacies types.

According to adsorption-desorption isothermal curve and scanning electron microscopy observation, the meso-pore structure (pore diameter of 2-50 nm) is divided into 5 types^[13,14] (Fig. 5). The Type I meso-pore has the convex desorption curve, with obvious desorption and decondensation, and wide hysteresis loop, indicating the slim-neck wide-body ink bottle-like pores in particles/crystals^[14]. They have BET specific surface area of more than 10 m²/g, total pore volume of 12-30 μL/g, and average pore diameter of mostly 4-8 nm, and occur in the medium TOC massive mudstone facies. The Type II meso-pore has the concave desorption curve, with obvious desorption and decondensation, and fairly large hysteresis loop, indicating slim-neck wide-body ink bottle-like pores and open parallel-wall slit-like pores, representing combined reservoir space of intragranular/intragranular pores and horizontal bedding micro-seam. They have BET specific surface area of mostly 2-3 m²/g, total pore volume of 9-12 μL/g, and an average pore diameter of 4-20 nm, and exist largely in the high TOC massive mudstone. The characteristics of the curves of the Type III meso-pore are similar with those of the Type II meso-pore, but with smaller hysteresis loop, indicating that the reservoir space is dominated by open parallel wall-slit horizontal bedding micro-seam. They have BET specific surface area of 2-3 m²/g, total pore volume of 9-12 μL/g, and an average pore diameter of 12-16 nm, and are found in the medium TOC laminated mudstone facies. The Type IV meso-pore has the concave desorption curve, with small hysteresis loop and no obvious decondensation, indicating the reservoir space combined of the open wedge-like pores and the curved slit-like horizontal and small wavy bedding micro-seam and the dissolution pores. They have BET specific surface area of 0-1 m²/g, total pore volume of 3-9 μL/g, and average pore diameter of 24-28 nm. The Type IV meso-pore is found in low TOC laminated mudstone and low TOC layered sandstone. The curve of the Type V meso-pore doesn’t show evaporation and decondensation, indicating the reservoir space dominated by slit-like pores with one end closed. This kind of pore has a BET specific surface area of 0-1 m²/g, total pore volume of 0-6 μL/g, and an average pore diameter of 40-50 nm, and is seen in low TOC layered limestone facies.

According to the capillary pressure curve and the pore diameter distribution characteristics, the macro-pore structures (pore diameter greater than 50 nm) are divided into four types (Fig. 6). The Type A macro-pore has the capillary pressure curve with a slight decline, a peak pore diameter of 7-8 nm, and a meso-pore proportion of 50%-96% and macro-pore proportion of 4%-50%, and exists in high TOC massive mudstone facies and medium TOC massive mudstone facies. The Type B macro-pore has capillary pressure curve with a significant sudden decline, a peak pore diameter of 15-20 nm, and meso-pore proportion of 52%-79% and macro-pore proportion of 21%-48%, and is found in medium TOC laminated mudstone facies. The Type C macro-pore has capillary pressure curve with a significant sudden decline, a peak pore diameter of 200-600 nm, and meso-pore proportion of 4%- 18% and macro-pore proportion of 82%-96%, and is seen in the low TOC laminated mudstone facies and the low TOC layered sandstone facies. The Type D macro-pore has a relatively flat capillary pressure curve, and mercury saturation of 99.50% at the mercury inlet pressure of 34.56 MPa. This type has a pore diameter of 60-400 nm, a meso-pore proportion of 0-9%, and macro-pore proportion of 91%-100%. The Type D macro-pore is seen in low TOC layered limestone facies.

In summary, the pore diameter distribution of high TOC massive mudstone facies and the medium TOC massive mudstone are dominated by meso-pores, the medium TOC laminated mudstone facies shows meso- and macro-pore “double peak” distribution, and the low TOC laminated mudstone, low TOC layered sandstone facies, and low TOC layered limestone facies are dominated by macro-pores.

The abnormal formation pressure is common in the Qing-1 Member of the Songliao Basin and is divided into high pressure anomaly (1.0-1.2) and overpressure (>1.2) according to the pressure coefficient^[15] (Fig. 7a). The acoustic velocity and density calibrated by organic matter content show overpressure is caused by fluid expansion resulted from hydrocarbon generation^[16], and the high pressure anomaly in the Qing-1 Member occurred at the end of the Nenjiang Formation sedimentation when the large-scale hydrocarbon generation occurred (Fig. 7b). Affected by the later structural activities, the continuous hydrocarbon generation conditions, preservation conditions or structural inversion uplift vary significantly in different regions and cause variation of current formation pressure coefficient.

The pressure coefficient difference of 19 wells (i.e. the current pressure coefficient minus the pressure coefficient at the end of the Nenjiang Formation sedimentation) was calculated to reflect the pressure evolution history. The relationship between the pressure coefficient difference and the shale oil type and productivity shows that the strong pressure increase is due to a large amount of in-situ retention oil caused by hydrocarbon generation and is the high-pressure of retention origin. The strong pressure increase corresponds to matrix and fractured shale oil, but the current production is not high, which is possibly related to current low technological level of production test and reservoir stimulation for shale^[17]. The slight pressure increase is caused by short-distance micro-migration after oil and gas generation and is a high-pressure of conduction origin. The slight pressure increase corresponds to shale intercalated with brittle layered shale oil, and its productivity has a good positive correlation with the magnitude of pressure increase (Fig. 7c). During the evolution of formation pressure, the water and dry layers saw pressure decrease largely. It can be seen that shale oil is enriched by two mechanisms, retention and micro-migration. The shale oil production is not directly related to the current formation pressure coefficient, but the history of pressure coefficient variation in geological history.

Considering the mechanisms of shale oil enrichment, retention and micro-migration, the pore structure and oil-bearing properties of each lithofacies were further characterized by CT scan and laser confocal technique according to the TOC value (Fig. 8), and the favorable lithofacies was determined.

The matrix pore connectivity and oil content vary significantly in the high TOC massive mudstone, medium TOC massive mudstone and medium TOC laminated mudstone facies with higher TOC. In the high TOC massive mudstone facies, the pore morphology is mostly spherical and ellipsoidal (consistent with the gas adsorption pore model). The pores are small in volume individually, mostly isolated and not connected, but large in number, which is related to the high contents of clay minerals and organic matter. The high ratio of long-wave fluorescence within the pores indicates high ratio of heavy components in the hydrocarbon, which is related to the hydrocarbon expulsion and differentiation (Fig. 8a, 8b). The characteristics of the medium TOC massive mudstone facies are similar with those of the high TOC massive mudstone facies. For the medium TOC laminated mudstone facies, the clayey lamina has higher TOC, and the siltstone lamina has lower clay mineral content. The sample of medium TOC laminated mudstone shows the pores are in microtubule bundle, with the pore throat radius slightly larger than of massive mudstone. The pores are distributed along the light-colored lamina bedding and connected with each other into sheet, forming a pore and throat network. The fluorescence of the pores is dominated by short-wave, indicating lighter hydrocarbon composition. Bedding seam connection is seen occasionally and results in better permeability (Fig. 8c, 8d).

The pores are well developed in the low TOC layered sandstone, low TOC layered limestone and low TOC laminated mudstone facies with lower TOC (Fig. 8). In comparison, the low TOC layered sandstone facies is dominated by intergranular pores and show the superimposed fluorescence wavelength of short-wave and long-short wave, with the single long-wave fluorescence rarely observed (Fig. 8e, 8f). The low TOC layered ostracoda limestone with obvious shell structure and high calcium content, is dominated by inter-shell calcium dissolution pores in poor connection. The hydrocarbon components emit short-wave fluorescence and some long-wave fluorescence (Fig. 8g, 8h). The low TOC laminated mudstone has low potential of hydrocarbon generation. A small amount of clayey laminae in it reduces its permeability and may prevent the micro-migration of external hydrocarbon, so it has lower oil abundance in general. However, the correctness of the conclusion remains to be further verified due to limited number of samples.

In summary, the medium TOC laminated mudstone facies has higher hydrocarbon generation potential, well- developed reservoir space and macro-pore proportion of 21%-48%, and is the favorable matrix shale oil facies with shale oil enriched by in situ retention. The low TOC layered sandstone and low TOC layered limestone facies are dominated by macro-pore and can be favorable shale lithofacies of shale intercalated with brittle layered shale oil, under the background of original permeability and formation pressure evolution favorable for hydrocarbon micro-migration.

The preservation conditions for shale oil and gas are evaluated from two aspects, structural uplift and lithological trap^[18,19]. On the one hand, after the Qingshankou Formation sedimentation, the basin experienced strong structural inversions at the end of the Nenjiang Formation deposition, at the end of the Mingshui Formation deposition and during Paleogene. The structural inversions gave rise to some structural traps such as small compresso shear or compressive anticlines and thrust high fault blocks, on the other hand, they caused formation of faults of different scales, or led to re-activity of early growth faults, which resulted in damage and adjustment to the reservoirs formed earlier. Compared with the southeastern uplift area with strong structural inversion intensity, the Gulong Sag in the western central depression belt had weak structural inversion intensity, so only small nose-uplift reversal anticlines were generated in the northern part of the study area, accompanied with a large number of fractures on both wings of the reversal anticline, making this part a favorable fractured shale oil exploration area (Fig. 9). On the other hand, the top and bottom conditions of the shale layer are critical for shale oil preservation. If the vertical sealing conditions are poor, the oil and gas could directly migrate to the shallow layer through the connected sandbodies, leading to reduction of in situ retention hydrocarbon volume. The high hydrocarbon expulsion efficiency and reduction of the pressure coefficient are not favorable for shale oil preservation. Compared with the medium TOC laminated mudstone facies with high hydrocarbon generation potential and favorable reservoir conditions, the high TOC massive mudstone facies formed during the maximum lacustrine transgression of SSC2 and SSC3 has poor physical properties and stable distribution on the top and bottom of favorable lithofacies, which effectively slowed hydrocarbon expulsion and migration out of the source rock and provided the favorable conditions for shale oil preservation, so they are favorable intervals for matrix shale oil exploration.

Under the continuous abnormal high pressure background, the medium TOC laminated mudstone facies developed in SSC2 top-SSC3 bottom has stable distribution laterally in the far source area and a continuous thickness of over 30 m, and the high TOC massive mudstone facies and the medium TOC massive mudstone facies in the middle of cycle as the roof and floor seal respectively, constituting favorable conditions for matrix shale oil (Fig. 9). The evaluation results^[20] show that the matrix shale oil in the Qing-1 Member of the Gulong Sag has effective resources of 23.5×10⁸ t and is an important area for future oil and gas exploration in the Songliao Basin.

The shale in the Qing-1 Member of the Gulong Sag is dominated by Type I and Type Ⅱ₁ kerogen, indicating it is a typical oil-prone lacustrine source rock generally in mature stage. The mineral composition is dominated by quartz, plagioclase and clay minerals, followed by K-feldspar, calcite, dolomite and pyrite.

The Qing-1 Member fine-grained sediments are divided into 7 lithofacies: high TOC lamellated clayey mudstone facies, high TOC massive siliceous mudstone facies, medium TOC massive siliceous mudstone facies, medium TOC laminated siliceous mudstone facies, low TOC laminated siliceous mudstone facies, low TOC layered sandstone facies and low TOC layered limestone facies. The vertical filling evolution of lithofacies is influenced by the Milankovitch cycle and sedimentary source supply and has obvious sequence.

The medium TOC laminated massive mudstone facies has high potential of hydrocarbon generation and shows meso- and macro-pore “double peak” distribution. The reservoir space is dominated by intergranular and intercrystalline pores and bedding micro-seam, forming interconnected laminated reservoir network. The dominant lithofacies is intercalated between the roof and floor seal of high TOC massive mudstone, and has continuous distribution laterally and abnormal high pressure, which are favorable conditions for matrix shale oil accumulation. The resource potential is huge.