The concept of shale oil is inconsistent all over the world up to now, including mainly two kinds, shale oil in the general sense and narrow sense respectively. The former refers to all the adsorbed or free state oil accumulating in source rock, tight sandstone or carbonate reservoirs near source rock that has not experienced long distance migration^[1]. Whereas the latter refers to hydrocarbon in adsorbed or free state occurring in organic rich shale within source rock^[2,3]. In this paper, shale oil refers to liquid hydrocarbon in adsorbed or free state occurring in organic rich shale (TOC > 2%) , or carbonate and thin interlayered sandstone (single layer < 2 m, cumulative thickness account for less than 30%) within the organic rich shale series. After more than 10 years of exploration, several breakthroughs of shale oil productivity have been made in different regions and different layer systems in China, such as Lucaogou Formation of Permian in Junggar Basin, the 2^nd member of Kongdian Formation of Eogene in Huanghua Depression in Bohai Bay Basin, and the upper submember of the 4^th member of Shahejie Formation (Es4s for short) to the lower submember of the 3^rd member of Shahejie Formation (Es3x for short ) of Eogene in the Jiyang Depression, multiple wells have realized industrial production^[4,5,6,7]. Oil and gas shows have been found in more areas and different layer systems, such as the 7^th member of Yanchang Formation of Triassic in Ordos Basin, Qianjiang Formation of Eogene in Jianghan Basin, and Hetaoyuan Formation of Eogene in Nanxiang Basin, etc^[8,9,10]. All these indicate that shale oil has huge exploration potentia in China, will be one of the main alternative energy resources, and is realistic replacement for conventional oil in mature exploration regions in eastern China. In recent years, experts and researchers in China have carried out many studies on depositional rules of terrestrial shale, occurrence and accumulate mechanisms, reservoir properties and oil-bearing features of sweet spots, and mobility of shale oil^[11,12,13]. But research in shale oil in China is still in its infancy, factors controlling terrestrial shale oil accumulation, elements sustaining stable yield are still unclear, and comprehensive evaluation method system is to be perfected. Shale oil in Eogene in the Jiyang Depression in Eastern China is taken as the study object in this work. Based on database of 40 wells with industrial shale oil yield, and elaborate description of systematic coring samples, the elements controlling shale oil sweet spots are analyzed, an enrichment model of shale oil is established, and comprehensive evaluation method system of shale oil is set up, in the hope of providing theoretical basis for exploration and development of terrestrial shale oil in eastern China.

According to exploration practice, exploration in shale oil in the Jiyang Depression can be divided into two stages with 2013 as dividing point. The first stage is featured by regional assessment and exploratory well drilling, the second stage is featured by a new round fundamental research and pilot test.

During this stage, exploration deployment was guided by the geological understanding of continuous accumulation and local enrichment. After trap and target assessment, 4 coring wells, L-69, NY1, LY1, and FY1, were drilled successively, from which 1 010.26 m long core was taken systematically in total. Then systematical tests of 10 000 sample-times were done, providing detailed data and information for evaluation of shale oil in the Jiyang Depression. By the end of 2018, oil and gas shows had been detected in shale formations of 320 wells, and shale sections in 66 wells were tested, among which 40 wells had obtained industrial oil flow, with cumulative oil production of 11×10⁴ t. Based on the above data, Es4s and Es3x in Dongying sag and Bonan subsag were sorted out as favorable targets in the Jiyang Depression. Then another 4 exploratory wells focusing on shale oil, BYP1, BYP2, BYP1-2, and LY1HF, were deployed. Among them, Well BYP1 was to confirm productivity of class I and II oil-bearing shale in fracture zone, and the well trajectory was designed to penetrate a class II layer (about 10 m thick) between 2 class I layers. Well BYP1-2 was to confirm productivity of slump section with active oil and gas shows encountered in Well BYP1, but failed to encounter. Well LY1HF was to confirm productivity of class I oil-bearing shale. It has been completed with screen pipe, but hasn’t been fractured yet. Besides, 5 exploratory wells with shale oil as secondary exploration target, Y182, Y186, Y187, L758, and N52, were deployed and tested. The horizontal specialized exploratory wells and vertical concurrent exploratory wells all obtained oil flow of 2.3-154.0 t/d. The horizontal wells are generally low in productivity, with the highest daily oil production of 2.30-9.48 t/d, while the vertical concurrent wells are higher in productivity, with daily oil production of 5.81-154.00 t/d. All these prove shale oil in the Jiyang Depression has huge exploration potential, but there are also many problems need to be solved.

Exploration practice in this stage indicates that terrestrial shale oil in the Jiyang Depression has its own characteristics compared with shale pays in North America, including Eagle Ford, Wolfcamp, and Woodford. Shale oil in Jiyang depression features strong heterogeneity, and high density, high viscosity, and poor mobility of oil, and strong plasticity of shale section making fracturing difficult (Tables 1 and 2). Influenced by highly variable sedimentary environment in terrestrial lake basin, there developed various kinds of shale with complex distribution; the lithofacies can change in lateral extension of just several kilometers, and even faster vertically, with cyclic variation in meter scale and even in centimeter scale. Influenced by sedimentary environment and evolution of organic matter, shale oil in the Jiyang Depression has high sulphur content (averagely 2.3%), high nonhydrocarbon content (averagely 39.7%), high asphalt content (averagely 11.2%), high density and high viscosity (57.7 mPa•s at 50 °C surface condition). Influenced by both sedimentary environment and diagenesis, terrestrial shale in the Jiyang Depression is characterized by high clay content, short burial history, and weak diagenesis, adding difficult to fracturing. The horizontal well aiming at the class II oil-bearing section failed to establish connection channel with class I sections above and below, and fracturing technology has not yet realized the scale of 10000 m³ fracturing fluid and 1 000 m³ sand.

During this stage, exploration deployment was guided by the geological understanding of local enrichment and high yield in sweet spot. Relying on achievements in National "973" project “Mechanism and distribution rules of terrestrial shale oil enrichment in Eogene of eastern China”, and National major special project “Target evaluation for exploration and development of shale oil in Jiyang Depression”, and key project of Sinopec Group, based on pilot test of fracturing in old wells, and in view of strong heterogeneity of terrestrial shale, sedimentary mechanism of fine grained shale has been studied, and distribution rules of different lithofacies have been figured out: (a) Provenance supply has a significant control on the lithofacies in shallow lacustrine environment, where bedded or bulk limy mudstone lithofacies, silty mudstone develop due to suspension effect of low density laminar flow. (b) seasonal effect has obvious control on lithofacies in semi-deep lacustrine environment, where organic rich lamellar argillaceous limestone or limy mudstone lithofacies develop due to flocculation effect. (c) the area neighboring steep slope belt in deep lake is the site where multiple provenances in different directions come together, with high content of external material, where bedded limy mudstone lithofacies due to flocculation effect and suspension effect develop. In view of high density, high viscosity, and poor mobility of oil, shale oil occurrence state was investigated to find out the evolution pattern of hydrocarbon fluid physical properties. The results show that controlled by hydrocarbon generation process vertically: (a) Shale buried at less than 3 000 m has high content of I/S mixed layer, shale oil occurs in adsorbed state, and is mainly composed of nonhydrocarbon and asphaltene with poor mobility. (b) In shale at buried depth of 3 000-3 400 m, shale oil has part in free state, higher content of light hydrocarbon, and lower content of nonhydrocarbon and asphaltene, moreover, in the shale of this depth range, microfractures start to develop and I/S mixed layer transforms, making shale oil mobility increase. (c) In shale formations more than 3 400 m deep, the shale oil is largely in free state, and has higher more light hydrocarbon than nonhydrocarbon and asphaltene, lower density, and even higher mobility^[14]. In order to solve the problem of difficult fracturing of terrestrial shale, proceeding from improving swept volume and flow conductivity of fractures, a new volumetric fracturing technology has been proposed. In line with the horizontal stress characteristics of shale in the Jiyang Depression, the fracturing technology of large displacement + multistage alteration between low viscous and high viscous fluid + multiple times of artifical temporary plugging has been worked out, which can overcome the maximum and minimum horizontal stress difference, realize diversion of fracture, connect artificial fractures with natural ones. This technology together with micron scaled proppant can increase significantly the effective swept volume. During main sand- adding stage, pulsed sand-adding is applied to change the placement form of the proppant in fractures from planar suport to columnar suport, so as to increase flow conductivity and effective length of main fracture, and construct complex artificial fracture network with better underground permeability and much bigger swept volume (Fig. 1). This is the technology has been used in the Jiyang Depression to make effective complex fracture network in carbonate rich source rock. Based on the above technology and undersandings, pilot tests have been carried out in old wells such as Y176 in Bonan subsag, F159 in Boxing subsag, and GX26 in the south slope of Dongying sag, etc., and all the 6 wells tested obtained industrial oil flow after fracturing, with daily oil production of 6.3-44.0 t/d during testing. After several months of production, they have been stable in productivity, boosting shale oil exploration in the Jiyang Depression.

2.1.1. Organic rich laminated lithofacies

Lithofacies controls reservoir property and oil-bearing property of shale to a large extent, and even affects shale oil mobility to some extent^[15,16]. Systematic analysis and assay results show that organic rich laminated lithofacies has the best reservoir property and oil-bearing property. According to analysis of core samples from more than 20 wells, Es4s-Es3x in the Jiyang Depression has more than 20 kinds of lithofacies, among which, 5 kinds of lithofacies with organic abundance of more than 2%, lamina thickness of less than 1 mm, and continuous distribution, namely organic rich laminated argillaceous limestone lithofacies, organic rich laminated limy mudstone lithofacies, organic rich bedded argillaceous limestone lithofacies, organic rich bedded limy mudstone lithofacies, and organic rich bulk limy mudstone lithofacies are most developed. Systematic tests of reservoir property and oil-bearing property show that organic matter rich laminated lithofacies and organic rich bedded lithofacies have the main peak of porosity of 7%-11% and 5%-8% respectively. Permeability of the organic rich laminated lithofacies has two peaks in general, the former peak is consistent with that of organic rich bedded lithofacies at (0.02-0.04)×10^-3 μm², and the latter peak is at (0.1-0.4)×10^-3 μm², and the corresponding T2 in NMR also has two peaks too. Organic rich laminated lithofacies and organic rich bedded lithofacies both have high organic matter abundance and free hydrocarbon (S₁). Organic matter abundance and S₁ peak of organic rich laminated lithofacies are 3%-7% and 3-8 mg/g respectively, and those of organic rich bedded lithofacies are 2%-5% and 2-5 mg/g respectively. Free state oil content of organic rich laminated lithofacies peaks at 4-13 mg/g, and that of organic rich bedded lithofacies peaks at 3-7 mg/g.

Multiple technical means such as argon ion scanning electron microscope, SEM, and NMR, etc were used to reveal the structure features of reservoir space in different types of lithofacies. The results show that organic rich laminated lithofacies has both pores and fractures, larger reservoir space in better connectivity, and higher saturation of movable oil. Specifically, intercrystalline pores in calcite of organic rich laminated argillaceous limestone/limy mudstone lithofacies have radius of 240-825 nm and 560 nm on average, which is larger than those of organic matter rich bedded argillaceous limestone/limy mudstone lithofacies and organic rich bulk limy mudstone/ mudstone lithofacies (Table 3). 2D throat pore coordination numbers of intercrystalline pores in calcite of organic rich laminated argillaceous limestone/limy mudstone lithofacies range from 1.7 to 2.8, higher than that of organic rich bedded argillaceous limestone/limy mudstone lithofacies of 1.5-2.3 and that of organic rich bulk limy mudstone/mudstone lithofacies of 0.5-0.9. Intercrystalline pores in calcite of organic rich laminated argillaceous limestone/limy mudstone lithofacies have better sorting and heterogeneity than those in organic rich bedded argillaceous limestone/limy mudstone lithofacies and organic rich bulk limy mudstone/mudstone lithofacies. The corresponding micropores in clay minerals show the same features. Different lithofacies differ greatly in combination of reservoir space. Shale of laminated lithofacies has net-like fractures, fractures along bed, intercrystalline pores and dissolved pores, which combine into complex pore-fracture network, resulting in good reservoir property and a porosity of 8.72%. Shale of bedded lithofacies has bed-crossing fracture, bed parallel fractures, and some intergranular pores, poorer connectivity between pores and fractures than the laminated lithofacies, and a porosity of 5.23%. Bulky lithofacies has irregular fractures and intercrystal pores in clay minerals, poorest connectivity between pores and fractures, and a porosity of 2.28% (Fig. 2). Generally, the better the reservoir property, the better the oil-bearing property, and there is obvious positive correlation relationship between storage capacity and movable oil saturation.

Exploration practice shows that shale oil produced by conventional measures mainly come from laminated lithofacies with well-developed micropores and microcracks. Statistics on shale oil producing wells show that organic rich laminated lithofacies, organic rich bedded lithofacies and other lithofacies account for 70%, 27%, and 3% of total pay intervals respectively. By means of APPI-high resolution mass spectrometer, molecular weight distribution of weakly polar compounds, composition and distribution of heteroatomic compounds, DBE (the sum of naphthenic rings and double bonds) and carbon number distribution of different chemical compounds in shale oil and core extract from the tested 3 199 m- 3210 m interval in Well-FY1 were systematically analyzed, and the results showed that the shale oil is mainly produced from laminated lithofacies, as carbazole content (DBE) and distribution of different compounds in extract from laminated lithofacies core are similar to those in shale oil, while different greatly from those in extract from bulk lithofacies^[17]. All the above evidences indicate that the organic rich laminated lithofacies is geologic sweet spot lithofacies of shale oil.

2.1.2. Microcrack is a key factor for enrichment and high yield of shale oil

Microcrack can improve reservoir storage capacity to a certain extent and dramatically increase shale permeability, and provide seepage pathway for shale oil^[18]. Multi-scales of microcrack network system can greatly increase connectivity of storage space in shale and improve fracturing effect. During diagenesis and hydrocarbon generation process, multiple kinds and scales of microcracks would be created due to many factors such as tectonic stress, compaction, dehydration and contraction, hydrocarbon generation pressurization, and recrystallization etc^[19] (Fig. 3). These microcracks include nanometer scale shrinkage joints due to hydrocarbon generation (20-800 nm wide), grain edge fractures (10-1 000 nm wide), micron scale shale bedding fractures (0.1-2.0 μm wide), dissolution fissures (0.5-10.0 μm wide), pressure solution fractures (0.5-2.0 μm with), and millimeter scale tectonic fractures (>1 mm), etc. Under certain geological conditions, fractures of different kinds and different scales developed massively, forming complex pore-fracture network system together with different kinds of pores. The system provides storage space for shale oil, and more importantly, it connects different kinds of storage space and greatly enhance the permeability of shale. This is in consistent with the microcrack functions in shale gas flow model put forward by LIU Weixin et al^[20] which can be summarized as nanometer scale pore as storage space, grain edge fracture as connection path, and shale bedding fracture as seepage pathway. Generally, fractures of various kinds all possess good oil-bearing property, and are filled with light oil. For example, during vacuuming process of No. 1469 sample from Well-FY1, oil was seen seeping through grain edge fractures more than 10 nm wide under microscope, indicating that grain edge fractures are main storage space and seepage pathways for continuous phase free state shale oil. Other examples such as oil filled in bedding fractures of shale from 3 295.1 m depth of Well-NY1, bitumen filled in abnormal pressure fissures in shale from 3177.39 m depth of Well- FY1, bitumen filled in dissolution fissures of calcite from 3 041.1 m depth and pressure-solution fractures in shale from 3 098.2 m depth of Well-L69, and bitumen filled in tectonic fractures in shale from 3 023.5 m depth of Well- NY1. They are all evidences that microcracks are main storage space and seepage pathways for shale oil. Furthermore, they can communicate with other storage space, constituting an integrated storage system. According to statistics of 556 samples of Well-L69, samples with microfractures and without microfractures don’t differ much in average porosity, (5.82% and 5.05% respectively), but the average permeability of samples with microfractures is 54.9 times that of samples without microfracture, and it can be concluded that microcrack has great impact on shale permeability. Moreover, shale oil is usually produced by large scale fracturing, early microcracks as stress weak surfaces are easy to open again during fracturing process, they can improve extension of fracture network, and can also increase fracture density. Existence of microcracks is a favorable natural geologic condition for large scale volume fracturing.

Exploration practice in the Jiyang Depression shows that oil wells with high yield are mostly located in the areas with active tectonic movement, for example, 4 wells in warped fault block and gravity sliding anticlinal structural belt tested had an average daily oil production of 94.9 t/d per well, while 7 wells located in area with weak tectonic movement usually had an average daily oil production of 2.4/t per well in formation testing. These are good evidence that microcrack is a key factor for enrichment and high yield of shale oil.

2.1.3. Thin interlayers provide favorable conditions for stable seepage of shale oil

Thin interlayer within good source rock serves as favorable storage space for enrichment of shale oil, and also seepage pathway. There are mainly two kinds of thin interlayers within good source rock in the Jiyang Depression, namely thin carbonate interlayer within shale in Es4s and Es1, and thin sandstone interlayer in Es4s and Es3x shale. Thin carbonate interlayers mainly occur in slope structural belts in Dongying sag and Bonan subsag, etc., and are 0.5-2.5 m thick each, mostly less than 2 m thick. Influenced by dissolution and metasomatism, they have many dissolution pores in calcite and intergranular pores in dolomite (Fig. 4), and an higher average porosity of about 6.78% and average permeability of about 8.36×10^-3 μm². They provide storage space and filtration pathway for shale oil, and have active oil and gas shows. The thicker the interlayer, and the higher the degree of dolomititsation, the higher the shale oil productivity will be. Single well productivity is positively correlated with permeability. For example, the 3443.0-3 495.5 m interval in Es3x of Well-Y182 had an equivalent oil production of 156 t/d with 5 mm choke in November, 2012. The well was put into production in May, 2013, and produced for 778 days, with cumulative oil output of 7681 t and cumulative water output of 256 m³. In present viewpoint of profit exploration, thin interlayer carbonate within good source rock is the most realistic exploration target. Thin sandstone interlayers are widespread in the Jiyang Depression, but usually small in scale and poor in continuity, and influenced by diagenesis, have limited storage space, and low porosity similar with neighboring shale, but permeability quite different from neighboring shale (Fig. 5). According to simulation experiment, when permeability of sandstone is 10 times that of shale, the proportion of shale oil flowing through sandstone thin interlayer will increase remarkably (Fig. 6). Analysis on saline water inclusions from 3 562.7 m depth of Well-JYC1 shows the pressure coefficient during the reservoir forming process was 1.0, suggesting that the thin sandstone interlayer was smooth migration pathway for shale oil. White fluorescence analysis shows that the purer the sandstone and the bigger the grain size, the better the oil-bearing property of the sandstone interlayer will be. The fine sandstone gives off blue fluorescence, argillaceous siltstone gives off yellow fluorescence, while shale has no obvious fluorescence. The above data and analysis prove that thin interlayer is not only good storage space for shale oil but also seepage pathway.

Shale oil exploration in the Jiyang Depression shows that most shale oil sections discovered so far have rich thin interlayers, oil producing sections with interlayers account for 53% of the total oil producing sections. Several wells, such as F159, F143, and GX26, etc. obtained high production after refracturing recently all have rich interlayers. Of them, the section of 3139.0-3 239.5 m with abundant interlayers in Well-F159 was fractured by "volume fracturing" + "high conductive pathway fracturing", with sand volume of 51 m³, fracturing fluid volume of 1 920 m³, and discharge capacity of about 12 m³/min. The fracturing has achieved good results. The well has been stable in productivity after fracturing, with daily shale oil production maintaining at 5 m³/d after production test for half a year. Clearly, the organic rich shale with rich interlayers is not only geologic sweet spot, but also sweet spot for fracturing, so it is important target for further exploration.

2.1.4. Abnormal pressure ensures enrichment and stable production of shale oil

Evolution of abnormal pressure system can reflect the evolution process of storage space and shale oil accumulation. Abnormal pressure occurs universally in Es4s-Es3x shale of the Jiyang Depression, and the hydrocarbon generation threshold of the two series of source rock is coincide with depth at which abnormal pressure occurs, indicating that the abnormal pressure is mainly caused by hydrocarbon generation^[10]. According to principle of hydrocarbon generation pressurization, fluid volume tend to increase during kerogen degradation, while the volume of kerogen would decrease, pressure of overlying formation acting on the kerogen would transfer to interstitial fluid. If the fluid can’t be discharged in time, overpressure would be formed. Hence, the forming process of abnormal pressure is also the process of kerogen degradation, and accumulation of acid fluid and shale oil. The support of abnormal pressure provided protection for early storage space, and acetic acid and formic acid formed during this period would dissolve feldspar and carbonate minerals, giving rise to large amounts of secondary pores to improve reservoir properties. Meanwhile, wettability of minerals reversed from water wet to oil wet. This reversal promoted oil and gas accumulation, slowed down diagenesis especially cementation and metasomatism, and preserved storage space. Therefore, hydrocarbon generation process, evolution of storage space and abnormal pressure is the coupling evolution of abnormal pressure, storage property and oil-bearing property in essence. In the Jiyang Depression, this is manifested as good coincidence between abnormal pressure, residue hydrocarbon peak and high quality reservoir interval when hydrocarbon generation reached a certain extent (Fig. 7).

Abnormal pressure can reflect shale hydrocarbon generation and preservation condition of shale oil. Hydrocarbon accumulates constantly during maturation of shale, pressure continuously builds up, giving rise to overpressure storage system. The better the sealing ability of shale and the more intense the abnormal pressure, the more enriched the continuous state shale oil will be. Exploration practice in the Jiyang Depression shows that the wells obtaining shale oil flow so far all have abnormal pressure, with pressure coefficient of more than 1.2. Analysis of inclusions from shale at 3 058.35 m depth of Well-L69 shows that pressure during the capture of the inclusions was high, with pressure coefficient of 2.22- 2.89, and the shale is a closed abnormal pressure system. In contrast, in Well-D95 located near a fault, interval of 3 280- 3307 m has a pressure coefficient of 1.16 and low productivity of 0.36 t/d, and the oil has a density of 0.957 g/cm³ and viscosity of 314 mPa•s, all which show the shale pressure system has been destroyed, preservation condition for shale oil is poor, the shale is a semi-open system, and shale oil has been degraded.

Abnormal pressure has great impact on oil phase state and productivity too. In the Jiyang Depression, heavy oil usually occurs in normal pressure shale at depth of less than 2 500 m, conventional oil and heavy oil occur in weakly abnormal pressure shale between depth of 2 500-3 000 m, conventional oil occurs in abnormal overpressure shale at the depth of more than 3 000 m, and volatile oil and condensate gas occur in strong overpressure shale at depth more than 4 200 m. Abnormal pressure due to hydrocarbon generation is the main driving force for shale oil migration. Early exploration practice shows that pressure coefficient is positively correlated with daily oil production. Shale oil usually accumulates near the margin of pressure compartment, indicating that shale oil tend to move toward and accumulate near the margin of pressure compartment under the drive by abnormal pressure. Moreover, shale oil productivity decreases obviously with the decrease of formation pressure, suggesting that abnormal pressure is the driving force for shale oil production.

Based on analysis of data of 66 wells in the Jiyang Depression, and comparison with Barrnett shale in Fort Worth Basin, Niobrara Formation in Rocky Mountains, and Montery shale in San Joaquin Basin^[21,22,23], according to storage space, exploitation conditions and economic effect and tectonic location, shale oil in the Jiyang Depression is classified into 3 types, namely matrix, interlayer and fracture types.

Matrix shale oil occurs largely in organic rich lamellar lithofacies within oil window in the low lying area below 3400 m, and the oil is mainly stored in organic pores in shale, intercrystalline micropores of carbonate minerals, intergranular micropores in clay minerals, and intergranular pores, shale bedding fractures as well as grain edged fractures in detrital minerals. The enrichment degree of shale oil is dependent on lithofacies, organic matter abundance and maturity. This kind of shale oil is difficult to produce, only when the shale reaches higher maturity, has well-developed network of organic pores, and the oil is good in properties, can this kind of shale oil be profitably exploited. For example, the oil producing area of Barnett shale has a Ro of higher than 0.9 and organic pores accounting for 95.2% of total porosity, where light oil and condensate oil has been developed in industrial scale. Source rock in the Jiyang Depression is generally low in thermal evolution degree, with a Ro range of 0.4%-1.0%, and has poorly developed organic pores and only shrinkage pores around organic matter, and developed organic matter pores only at buried depth of more than 3 400 m. Affected by parent material and thermal evolution degree of source rock, matrix shale oil in the Jiyang Depression is mostly conventional oil with a density of 0.88 g/cm³, viscosity of 57.7 mPa·s at 50 °C, and production of 2 t/d per well (natural productivity).

Interlayer shale oil largely exists in organic rich lamellar lithofacies, bedded lithofacies and thin sandstone and carbonate interlayers in slope structural belt of more than 3 000 m deep. This kind of shale oil usually occurs in intergranular pores in clastic debris, intercrystalline pores in carbonate and shale microfractures. The enrichment degree of interlayer shale oil is dependent on development degree of interlayer, lithofacies, organic matter abundance and evolution degree. This kind of shale oil has higher enrichment degree and better recovery conditions, and thus can be recovered at higher degree. Many successful exploration cases abroad have been reported, such as Niobrara shale oil in the Rocky Mountains and Eagle Ford shale oil in Gulf of Mexico. So far, interlayer shale oil in the Jiyang Depression has achieved good exploration results, several wells with shale oil as the secondary target and old wells for fracturing test have all obtained stable and fairly high yield. For example, interval of 3 440.42-3 504.47 m in Es3x of Well-Y187 had an oil yield of 154 t/d, water yield of 3.22 m³/d, gas yield of 13 400 m³/d with 5 mm chock during testing, and cumulative oil output of 7 681 t, cumulative water output of 256 m³, and cumulative gas output of 18.71×10⁴ m³ in 3 years. This proves this kind of shale oil has good prospects and is the most favorable and realistic type for exploration.

Fracture shale oil is mainly distributed in organic rich lamellar lithofacies and bedded lithofacies in stepped fault zone, nosing structural belt, and central uplift zone. Fracture shale oil usually occurs in fracture network and microcracks induced by tectonic movement, diagenesis and hydrocarbon generation. The enrichment degree of fracture shale oil is dependent on development degree of microcracks, lithofacies, abundance of organic matter, and thermal evolution degree. This kind of shale oil has good enrichment and development conditions. For example, 2 828.13-2 861.00 m interval of Es3x in Well-L42 had an open flow oil production of 79.9 t/d and gas production of 7 746 m³/d with 6mm chock with gas/oil ratio of 97 m³/m³. The well had a cumulative oil output of 13 605 in 2 years. Restricted by backward fracture prediction technology, this kind of shale hasn’t been put into scale development. To this day, the most successful case of industrial development of this type shale oil is Montery shale in San Joaquin Basin. But EIA downgraded its reserves two times in 2012 and 2014 respectively by up to 96%, which indicates the exploitation of this kind of fracture shale oil still has many uncertain factors.

Geological factors controlling sweet spots of shale oil in the Jiyang Depression include lithofacies, microcrack, thin interlayer, and abnormal pressure. Through case study, comprehensive analysis, and exploration practice, a comprehensive evaluation system of shale oil sweet spots based on single factor and multiple factors has been set up, and favorable shale oil enrichment zones have been sorted out.

3.1.1. Favorable lithofacies evaluation and prediction

Shale oil in the Jiyang Depression is mainly produced from organic rich lamellar lithofacies and organic rich bedded lithofacies, and these two lithofacies intervals producing oil account for 70% and 28% of the total oil producing intervals. According to analysis results of reservoir properties, oil-bearing property and oil mobility, lithofacies in Jiyang depression is divided into three types, organic rich lamellar lithofacies is type I, organic rich bedded lithofacies is type II, and the others are type III. Among them, lithofacies with TOC higher than 2%^[23] corresponds to mature source rock with S₁ of more than 2 mg/g^[24]. Since rock composition, sedimentary structure, and organic matter control reservoir property, oil-bearing property and oil mobility of shale oil, a comprehensive classification scheme for shale lithofacies has been proposed. By means of logging data simulation, the distribution of lithofacies in the Jiyang Depression was extrapolated, and it is concluded that shale lithofacies is influenced by terrestrial debris supply laterally, perpendicular to the long axis direction of the basin, and toward basin center, organic-bearing bulk mudstone lithofacies, organic rich bedded limy mudstone lithofacies, organic rich lamellar limy mudstone lithofacies, organic rich lamellar argillaceous limestone, organic rich lamellar limy mudstone lithofacies, organic rich bedded limy mudstone lithofacies, and organic bearing bulk mudstone lithofacies appear in symmetrical rings in turn.

3.1.2. Evaluation and prediction of microcracks

Shale formations in the Jiyang Depression have different types of microcracks. In this work, tectonic fractures, interlayer fractures and overpressure fractures were evaluated based on core sample analysis, conventional well logging data, and imaging logging, etc., and combined with distribution of oil producing wells and well with oil and gas shows, we classified fractured area into 3 types. The class I fractured area has 3 kinds of fractures developed, the class II fractured area has 2 of the 3 kinds of fractures, and others are classified as the class III fractured area. The tectonic fracture was identified by calibrating imaging logging with core sample, and then calibrating conventional logging with imaging logging. Logging responses of interval with well-developed fractures are characterized by hole enlargement, high DT, high neutron porosity, high resistivity, low density and low GR. Since the lamellar lithofacies has the most developed interlayer fractures and is located in slope belt of semi-deep lacustrine facies, the distribution of interlayer fractures was predicted by using the relationship between formation occurrence and interlayer fracture development degree together with lamellar lithofacies. It is found from large amounts of statistic data that interlayer fracture begin to occur massively when formation dip is more than 6°, so formation dip and laminated lithofacies distribution were used to predict areas rich in interlayer fractures. Formation breakdown pressure and organic matter characteristics were used to predict distribution of overpressure fracture. According to measured data and by means of Inton formula, lower limit of pressure coefficient at for breakdown of organic rich shale lithofacies in the Jiyang Depression was calculated at 1.38. Therefore, overpressure fracture was predicted by using the formation breakdown pressure coefficient of over 1.38 and TOC of more than 2%. Based on identification of 3 kinds of fractures, microcrack development areas were predicted. Taking the 2^nd layer group in Es4s of Dongying sag as an example, class I fractured area is mainly distributed in central structural belt, class II fractured area is mainly distributed in Boxing lowlying area, Chenguanzhuang area, and Shengbei fault belt.

3.1.3. Evaluation and prediction of thin interlayer development degree

Among all wells producing shale oil in the Jiyang Depression, wells producing from carbonate interlayers account for 82.7% all wells producing from interlayers, and most of them have more than 3 interlayers and cumulative interlayer thickness of more than 4 m. Wells producing from sandstone interlayers only account for 17.3% of the wells producing from interlayers, but have more interlayers and larger cumulative thickness of interlayers than wells with carbonate interlayers. Therefore, interval with carbonate interlayers is classified as class I shale, and interval with sandstone interlayers is classified as class II shale. Based on massive mud logging and well logging data, distribution of interlayers in the study area was predicted. Thin interlayer features mid-low GR (50-80 API), CNL, DT and RHOB curves close with each other, and obvious increase of resistivity. Taking the 2^nd layer group of Es4s in Dongying sag as an example, thin interlayers are mainly distributed in the gentle slope belt and the steep slope belt.

3.1.4. Evaluation and prediction of abnormal pressure

Since pressure coefficient in wells obtaining shale oil is more than 1.2, and breakdown pressure coefficient of shale is 1.38 in the study area, abnormal pressure is classified into two classes, the class I has pressure coefficient higher than 1.4, and the class II has pressure coefficient between 1.2 and 1.4. Since there is few measured pressure data of shale interval available, acoustic data and pressure measured in lithologic reservoir during oil testing were used to simulate formation pressure by multiple means such as equivalent depth method, stratigraphic factor method, and empirical relationship method, etc. Taking the 2^nd layer group of Es4s in Dongying sag as an example, abnormal pressure mainly occurs largely in the lowlying structural belt and gentle slope belt, and center of abnormal pressure is in consistence with depocenter.

Based on evaluation of geological factors influencing development of shale oil sweep spots, combined with distribution of shale oil resources, mobility and fracability of shale oil, and exploration experience of shale oil at home and abroad, an assessment system of terrestrial shale oil in the Jiyang Depression has been established (Table 4), to find out the distribution rules of shale oil sweet spots by overlapping multiple factors. Free state shale oil resources were calculated at 41×10⁸ t by subtracting adsorbed oil from retained oil. Shale oil is mainly distributed in Dongying sag and Zhanhua sag laterally; vertically, shale oil largely occurs in 3 000-4 000 m (accounting for more than 85% of the resources), more specifically in the 3^rd layer group of Es3x and the 2^nd layers of Es4s. Based on main factors controlling development of shale oil, matrix shale oil in the Jiyang Depression is mainly distributed in Lijin subsag and Bonan subsag, with an overlapping prospect area of about 231.38 km². Interlayer type shale oil is mainly distributed in the gentle slope belt and steep slope belt in Dongying sag, the steep slope and lowlying area in Bonan subsag, the slope belt in Chezhen sag, lowlying area in Huimin sag, with an overlapping prospective areaarea of about 2 062.65 km². Fracture type shale oil is mainly distributed in the central uplift zone in Dongying sag, nose like structural belt in Bonan subsag, stepped faulted belt in Chezhen sag, and the central uplift zone in Huimin sag, with an overlapping prospect area of about 763.28 km².

In view of high heterogeneity of terrestrial shale in Eogene of the Jiyang Depression, depositional mechanism of fine grained sediments was studied to find out distribution rules of different lithofacies. In view of high density, high viscosity, and low mobility of shale oil, occurrence state of shale oil was investigated to reveal evolution pattern of hydrocarbon fluid physical properties. In consideration of difficult fracturing of terrestrial shale, complex network fracturing technique characterized by "acid etching induced pore and fractures + volumetric fracturing for branch fractures + main fractures with high conductivity" was adopted to realize effective fracturing of shale with high clay content, short burial time, and weak diagenesis.

The elements controlling shale oil sweet spot in Eogene of the Jiyang Depression include lithofacies, microcrack, thin interlayer, and abnormal pressure. Among them, lithofacies is the basis for shale oil enrichment, shale oil mainly occur in organic rich laminated lithofacies with better storage property and oil-bearing property. Microcracks provide storage space for accumulation of free state shale oil and also serve as shale oil seepage pathway. Thin interlayer is a favorable condition for stable seepage of shale oil and also favorable section for fracturing. Abnormal pressure is the guarantee for accumulation and stable production of shale oil. Abnormal pressure not only is a factor for forming good reservoir properties and oil-bearing property, but also provides driving force for shale oil accumulation and production, and reflects occurrence state and preservation condition of shale oil directly.

There develop 3 types of shale oil in Eogene of the Jiyang Depression, namely matrix type, interlayer type and microcrack type. Based on detailed case analysis, systematic study, and combined with exploration practice, evaluation criterion of elements controlling shale oil enrichment has been set up, and a comprehensive evaluation system for shale oil sweet spots has been established. It is pointed out that interlayer shale oil is the most realistic type for exploration in the Jiyang Depression, and is mainly distributed in gentle slope belt and steep slope belt in Dongying Sag, steep slope belt and low lying belt in Bonan subsag, gentle slope belt in Chezhen sag, and low lying belt in Huimin sag.