After decades of large-scale exploration and development, conventional oil and gas is getting more and more difficult in exploration. Seeking breakthroughs in the field of unconventional oil and gas exploration is an important means to achieve an orderly replacement of resources^[1,2]. Marine shale oil revolution took place in the United States in the early 21st century after decades of technical exploration and the world’s energy map has been changed^[3]. The International Energy Agency (IEA) predicted that China had 45×10⁸ t of shale oil recoverable resources, only behind the United States and Russia, ranking third in the world, with huge exploration potential^[4,5]. However, shale oil in China is mainly continental, compared with marine shale oil overseas, as ancient continental lake basins are small in area and greatly influenced by climatic conditions, sediments have rapid change in sedimentary facies and stronger heterogeneity, and the basins have higher requirement for shale oil enrichment and much more complicated law of shale oil enrichment.

In recent years, thick bed (with sandstone to stratum thickness ratio greater than 48%, and single sand body thickness greater than 10 m) and interbedded (with sandstone to stratum thickness ratio between 30% and 48%, single sand body thickness between 6 and 10 m) shale oil reservoirs represented by Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin, NW China have been developed profitably, while there is no industrial breakthrough in the pure shale oil (with sandstone to stratum thickness ratio of less than 10% and single sand body thickness of less than 2 m). In the Cangdong Sag of the Bohai Bay Basin, more than 100 wells encountered shale in the second member of Kongdian Formation (Ek₂), and oil test was made in some intervals with good shows, resulting in daily oil production of 5 to 30 t. But the test production rate declined rapid, and the cumulative oil production was only hundreds of tons. Improving the output of single well and reducing development costs are urgent problems to be solved in pure shale oil development currently^[6,7].

In 2018, by applying three key technologies for exploration and development, continental shale oil sweet spot identification method, precise drilling trajectory design, and low-cost fracturing, pilot oil wells (GD1701H, GD1702H) obtained industrial oil flow from the Kong 2 Member of the Cangdong Sag. Subsequently, twenty-six horizontal wells were newly drilled and completed in 2019, with a sweet spot encountering rate of 100%. Among them, 5 horizontal wells have been put into production, with a stable daily oil production of over 80 t. This is a major breakthrough in exploration and development of continental pure shale oil^[8,9], and can provide technical reference for the exploration and development of medium-maturity pure shale oil reservoirs. Based on fine description of cores taken systematically from some wells and joint analysis data, the enrichment laws of continental shale oil are investigated, and key technologies for exploration and development of continental shale oil are worked out and put into engineering practice.

The Cangdong Sag is in the southern part of the Huanghua Depression of the Bohai Bay Basin, with an exploration area of 1700 km². The source rock of the Kong 2 Member (Ek₂) is the main contributor to the discovered oil and gas reservoirs. During the depositional period of the Kong 2 Member, the whole lake basin was in stable subsidence stage (Fig. 1), showing the characteristics of ring-shaped deposition on the plane^[9,10]. The outer ring zone (Area A) is a delta front coarse-grained facies zone, with structural-lithologic oil and gas reservoirs developed. The middle ring zone (Area B) is the transition zone where coarse and fine grained sediments formed by the distal delta front-prodelta, and tight sandstone and dolomite reservoirs occur. The inner ring zone (Area C) is a fine-grained facies zone in the distal prodelta-semi-deep lake area, with thick mud shale developed, which is a favorable zone for shale oil exploration. The Kong 2 Member is mainly gray black-black mud shale, with a small amount of light gray silty sandstone, with a thickness of 400-600 m. Observation of thin slices shows that the massive mudstone section has invisible micron-scale laminated texture. The mudstone with no lamellation is mainly homogeneous block texture with poor fluorescence shows, and the shale with lamellation has mainly laminated texture and obvious fluorescence shows along bedding^[11].

The Kong 2 Member shale is characterized by large thickness, good hydrocarbon-generating material, high organic matter abundance, and high conversion rate. The samples from the Kong 2 Member have an organic matter abundance of above 2.00% in general, 3.61% on average and 11.92% at maximum; hydrocarbon generation potential of 22.18 mg/g on average and 73.00 mg/g at maximum. According to the geo- chemical evaluation method of terrestrial source rocks (SYT 5735-1995)^[12], the organic matter in the Kong 2 Member is divided into three major types based on pyrolysis data, (of which the second type is subdivided into 2 sub-types). The Kong 2 Member has mainly type I and type II₁ organic matter (accounting for 78%) and a small amount of types II₂ and III. The fine-grained sedimentary rock in the Kong 2 Member has a porosity range of 2% to 5% through conventional test. Microanalysis reveals that the shale interval has dense nano- scale intercrystalline pores, organic pores and micro-cracks, with pore throats generally between 450 and 1500 nm in diameter. Pore throats with diameter larger than oil molecules account for over 80%, which is conducive to oil molecule migration^[13]. The Kong 2 Member shale in the Cangdong Sag has basic geological conditions for shale oil formation.

In the past exploration of the Cangdong Sag, more than 100 exploration wells encountered the Kong 2 Member shale and had active oil and gas shows and high gas logging values. Oil test results show that more than 10 wells obtained oil flow, but they had poor production test results, with short stable production time (10 to 30 d) unable to sustain commercial development. The main reason is that the Kong 2 Member is a typical shale section with high organic matter abundance, and good oil and gas shows during drilling process. But microscopic identification shows that the reservoirs have strong heterogeneity, and the development mechanism of sweet spot is not clear. Before this study, there was no theory on the evaluation and selection of favorable target areas to guide the deployment of horizontal wells, making it difficult to enhance and keep stable of single well production.

Wells drilled reveal that the Kong 2 Member in the middle- inner ring zones is a set of gray and gray-black fine-grained shale. Whole rock X-diffraction analysis by close sampling reveals that the Kong 2 Member shale is composed of terrestrial debris, carbonate, clay and other minerals, with felsic mineral contents of 17.0% to 48.0%, 38.6% on average, carbonate mineral contents of 10.0% to 58.0%, 32.6% on average, and clay contents of generally less than 30.0%, 16.4% on average. The samples have analcite laminae rich in local parts, and an average analcite content of over 10.0%.

Although the fine-grained sedimentary rocks have complex components, the mineral composition of each type has a good correlation with the corresponding acoustic wave and density logging data^[13]. Felsic and other minerals have low-density and high-frequency acoustic responses and carbonate minerals have high-density and low-frequency acoustic response characteristics. The contents of felsic (quartz, feldspar) and carbonate minerals can be calculated based on the distances between the acoustic waves and densities on log curves:

And then, the contents of felsic and carbonate minerals can be worked out by the fitted formulas below^[11]:

Quantitative identification and brittleness evaluation of fine-grained facies area can be realized based on the calculated felsic and carbonate mineral contents (Fig. 2). Furthermore, the frequency structure formed by the intersection of acoustic wave and density logs is in positive correlation with the development degree of laminae. The shale sections can be divided into thick bed, interbedded-layer cake and thin interlayer types according to the thickness of layers. The thick bed shale has no laminae, while interbedded-layer cake and thin interlayer types have abundant laminae. Therefore, the Kong 2 Member shale series can be divided into four fabric types, namely, laminar felsic, laminar mixed, thin-layer limy dolomitic, and thick bedded limy dolomitic ones.

Continental shale oil is a general term for liquid petroleum hydrocarbons and many types of organic matter that exist in continental organic-rich shale formations, including petroleum hydrocarbons that have formed underground, various types of asphaltenes, and solid organic matter that has not been thermally degraded and transformed^[5]. At present, the major exploration object is still the part of shale oil that has been generated but not yet expelled out of the shale. From the perspective of oil and gas generation and expulsion, after the shale enters the oil generation threshold, the generated oil and gas must first meet the demand of kerogen adsorption itself. With the increase of the evolution degree, the amount of hydrocarbon generated and expelled gradually increases. When entering the highly mature stage, the amount of liquid hydrocarbon generated gradually decreases until no liquid hydrocarbon is generated anymore. It can be seen that the liquid hydrocarbons that have not been discharged from the source rock after generation include two parts, one part is the retained oil adsorbed by the kerogen, and the other is the free oil remaining between mineral particles around the organic matter, and the latter is the movable hydrocarbon, which is most meaningful for shale oil and gas exploration. In other words, only when the amount of retained hydrocarbons exceeds the maximum adsorption capacity of kerogen, does the cross-over effect occur, and the cross-over effect determines the degree of shale oil enrichment^[5].

The cross-over effect of shale can be approximately characterized by the ratio of pyrolysis hydrocarbon generation potential to organic matter abundance (OSI). The potential of pyrolysis hydrocarbon generation in pure shale mainly reflects the amount of residual hydrocarbon that has been generated per unit of rock. When OSI>100, it is considered that there is a cross-over effect. To find out the magnitudes of the cross- over effect of shales of different fabrics, the cross-over effects and oil contents of shale samples of four different fabrics were tested and calculated (Table 1). The laminar felsic and mixed shales are the most favorable facies, with an average OSI of more than or equal to 40. Among them, the laminar felsic shale samples with OSI values greater than 100 account for 66%. The mixed shale samples with OSI value greater than 100 account for 47%. They have the most significant cross-over effect. The fluorescent thin section observation shows they have higher oil contents, in other words, are richest in oil (Fig. 3). Thin laminated limy dolomitic shale and thick bed limy dolomitic shale have OSI of no more than 100 in general, and hardly have cross-over effect. Therefore, they have lower oil content and only sporadic fluorescence shows.

Shale oil "sweet spots" are the comprehensive reflection of geological and engineering sweet spots. The sweet spots can be quantitatively evaluated based on six parameters, intersection frequency of acoustic wave and density curves (indicating rock fabric), TOC (indicating hydrocarbon generation capacity), OSI (indicating cross-over effect), brittle mineral content (indicating engineering brittleness), porosity (indicating reservoir property), and R_o (indicating shale thermal maturity). The distribution ranges of parameters of different types of sweet spots have been established according to the oil and gas shows during drilling process and the tested production rate (Table 2).

Since the six factors are correlated to some extent, the parameters of different types of sweet spots exist a certain crossing. Therefore, the weight of each parameter needs to be considered in the comprehensive evaluation, and the influencing factors in Table 2 are considered comprehensively to establish the comprehensive evaluation index of sweet spot:

Sweet spots with comprehensive evaluation indexes of greater than 1.6, between 1.2 and 1.6, and less than 1.2 are classified as class Ⅰ, class Ⅱ and class Ⅲ respectively. 21 sub- layers in the Kong 2 Member were evaluated with equation (4) and the evaluation criterion quantitatively. The results show that the laminar felsic shale and laminar mixed shale are the most favorable fabric patterns, and are mainly class Ⅰ sweet spots; thin-layered limy dolomitic shale comes second, and is mainly class Ⅱ sweet spot; thick bed limy dolomitic shale is the worst fabric and can be taken as a potential shale oil exploration target.

It is difficult to get stable industrial production from shale reservoirs with vertical wells as shale is low in porosity and permeability. Only through drilling horizontal wells precisely into sweet spot and fracturing, can productivity breakthrough be made^[14,15]. Continental faulted basins have complex structures, and shale sweet spot sections predicted could have large lateral variations. Hence, it is difficult to meet the precise drilling needs only by geophysical predictions, and it is necessary to work out shale oil horizontal well trajectory design and field tracking adjustment method, to adjust and optimize the borehole trajectory in time during drilling and ensure that the horizontal section enters the sweet spot section precisely.

On the plane, it is better for the horizontal well trajectory to go along the direction where the formation occurrence is flat and straight and has no flexure structure, which is helpful to track drilling along the formation and avoid faults. The distance of the window entry point to the fault is preferably greater than 150 m, to prevent leakage and pressure loss along the fault during fracturing that would have negative effect on the fracturing result. Engineering experience shows that the fracturing effect is the best when the horizontal well trajectory is perpendicular to the direction of formation principal stress, therefore the angle between the horizontal well trajectory and the maximum principal stress should be 90° or no less than 35°. Affected by the complex geological structure in the study area, the horizontal section should not be too long. Considering factors such as not passing faults, distance before landing in target and trajectory angle, horizontal sections in most regions should be 600 to 1000 m. The shale of the Kong 2 Member is about 400 m thick, and the sweet spot sections are generally 10 to 40 m thick. There are two sets of strong reflection events on the seismic section. The distribution of the sweet spot sections cannot be accurately identified, making it difficult to optimize the design of the horizontal well trajectory.

The C1 sweet spot section in the G1608 well area of the Guandong region is thick (70-80 m), with high free hydrocarbon content, high oil test output, obvious gas logging anomalies, and stable lateral distribution. Therefore, this area was chosen to illustrate the procedure of precise trajectory design in sweet spot section. In vertical well G1608, the C1 layer is divided into four sub-layers (Fig. 4), with single layer thickness of 10 to 25 m. Among them, two sub-layers ② and ③ have comprehensive evaluation indexes of greater than 2.4 and high oil content, so the horizontal section should go along them. From the logging curves, it can be seen that the sub-layer ② shows "W" shape on resistance and acoustic curves, while the sub-layer ③ shows sawtooth shape, making them easy to tell apart. But the whole C1 layer is between two strong reflection events, with small differences in resistance and acoustic travel time, so it is difficult to distinguish the four sub-layers on the conventional inversion profile. Since the amplitude attribute is sensitive to lithological changes, and the frequency is sensitive to the lithological combination, therefore, the frequency difference is amplified by power to highlight the contribution of the frequency attribute and show the internal differences:

Through the inversion of sensitive attributes, the distributions of the four sub-layers in the high-resistance shale area were made clear (Fig. 4). No less than 3 key control points were set for the identified sub-layers ② and ③ to optimize the horizontal well trajectory further, increase the horizontal section footage in the optimal sweet spot sub-layers, and guarantee the maximum drilling rate of the optimal sweet spot sections.

Restricted by the seismic data resolution, the error between the interpreted depth and actual depth of the target layer is tens of meters in general or even larger, so it is very difficult to encounter the sweet spot section in a large set of shale. During the drilling of a shale oil horizontal well, electrical logging while drilling and geochemical logging data must be analyzed comprehensively to realize on-site tracking and adjustment of well trajectory to maximize the drilling rate of sweet spot.

When drilling to the horizontal section, due to slight variations in structural relief and planar facies changes, etc., 20-30 m thick shale sections can be easily missed, resulting in a low drilling rate of sweet spot. In on-site tracking, geochemical logging information such as element data has some delay, and mud logging and logging while drilling curves should be combined and compared in real-time to correct the trajectory and improve the drilling rate of high-quality sweet spot. The horizontal section of Well GD1701H is mainly in the middle part between the ② and ③ sublayers of the C1 sweet spot section. The interface between the two sublayers features sharp reduction of resistivity and gas logging values, and increase of carbonate content. Through comprehensive analysis of resistivity while drilling, gas logging and element logging data, the horizontal trajectory can be controlled in real time to effectively prevent it from deviating out of the sub-layer ③ prematurely, ensure the horizontal section to pass through the middle of the sub-layers ② and ③, and maximize the drilling rate of class I sweet spot.

With the help of the trajectory optimization and tracking technology, the horizontal wells in the Cangdong Sag had achieved significantly higher drilling rates of sweet spot section. The 26 horizontal wells with 800 m to 1200 m horizontal sections all had a sweet spot drilling rate of 100%, and an average drilling rate of class I sweet spot of over 75%.

Continental shale formations have deep burial depth and complex rock mineral compositions, while shale oil has high requirements on the conductivity of propped fractures. Research and practice have confirmed that large-scale volume fracturing on many favorable cluster points would result in better stimulation effect, but higher cost. By designing fracturing cluster points properly, the fracturing cost can be cut down, meanwhile, the single well production can be effectively enhanced.

The shale oil sweet spot area encountered by a horizontal well still has strong heterogeneity, so the selection of fracturing cluster points for a horizontal well has a great influence on its production. A comprehensive evaluation index of favorable fracturing cluster points is established by considering 5 parameters including organic carbon content and free hydrocarbon content etc. jointly:

A comprehensive evaluation index curve of fracturing cluster points can be obtained by multiplying the normalized organic carbon content, free hydrocarbon content, porosity, gas logging total hydrocarbon value, and brittleness index by their corresponding weights and then summing them up. The points with a comprehensive evaluation index greater than 0.5 are the preferred cluster points, and then favorable fracturing segments can be picked according to the favorable cluster points^[16].

The data of 5000 to 5150 m horizontal section in Well GD1701H, including organic carbon, free hydrocarbons, porosity, gas logging total hydrocarbons, and brittleness index were normalized, and substituted into equation (6) to calculate the comprehensive evaluation index. The points with a comprehensive evaluation index greater than 0.5 (reference value) were taken as preferred cluster points (Fig. 5). Since the distance of 10 to 20 m between any two cluster points in horizontal well can result in good fracturing effect, the differential design and selection of cluster points were conducted by considering the comprehensive evaluation index and the distance between cluster points jointly. The segment with higher overall evaluation index can have denser cluster points designed^[17,18].

Shale oil horizontal well fracturing needs a large amount of fracturing fluid and proppant. Traditional guar gum fracturing fluid and ceramsite proppant are expensive, making it difficult to realize profitable exploration and development. With the advance in fracturing technology, the fracturing process of slick water and quartz has been used on large scale shale gas development in the United States, enabling the low-cost commercial development of shale gas.

Two pilot horizontal wells, GD1701H and GD1702H, in the Cangdong Sag were treated with volume fracturing of "slick- water+quartz sand", and the key concept was to treat with "slick water+low damage" fracturing fluid^[18,19,20], and to add sand continuously in the slick-water. The fracturing fluid system for GD1701H well was a hybrid system of polymer slick-water and low-damage fracturing fluid, with 80% of slick-water. The formula of the slick water was 0.08% resistance reducing agent+0.30% anti-swelling agent, and the resistance reducing rate of the slick-water was greater than or equal to 70% in field site. The low-concentration and low-damage fracturing fluid was a low-concentration modified guar solution, with a guar concentration of 0.3% and a residue content of 177 mg/L. It had good compatibility with the formation.

According to the pilot test results and the characteristics of shale formations, the formula of low-resistance slick-water was further optimized, and the proportion of slick-water was gradually increased, and a full-range slick water fracturing technology has been developed to meet the demand of large- scale volume fracturing of shale oil reservoirs. In the fracturing of horizontal wells in 1^# platform and 2^# platform in the Cangdong Sag in 2019, the fracturing fluid used 100% of slick water, with the maximum volume of 3×10⁴ m³.

The effective closure pressure of the Kong 2 Member in the Cangdong Sag is 60 MPa, and accordingly a silt proppant with conductivity comparable with 0.106 to 0.212 mm (70 to 140 mesh) ceramic powder was selected^[21]. The ceramic powder price is 2800 yuan/t, whereas the silt price is 850 yuan/t, only 30% of that of ceramic powder, making it highly cost effective. In order to have better fracture network conductivity, sand adding technique of matching proppant type with fracture type was designed in the fracturing: 0.106 to 0.212 mm (70 to 140 mesh) silt was used to support microfractures, 0.212 to 0.425 mm (40-70 mesh) ceramsite to support secondary fractures, and 0.300 to 0.600 mm (30-50 mesh) ceramsite to support primary fractures, to create a complex fracture network and improve reservoir flow capacity.

After breakthrough in pilot test wells, the proportion of quartz sand used in new wells treated in 2019 was greatly increased. The quartz sand used in 5 wells of the Guanye 2^# platform accounted for 64.26% to 69.13%, 66.54% on average. By using slick water fracturing fluid and quartz sand instead of ceramsite in large-scale fracturing, the fracturing costs of these wells reduced by 23.0% to 31.0%, 26.4% on average.

Continuous cores of 565 m were taken from Well G108-8 and GD14. Over ten thousand samples were tested for lithologic, physical, hydrocarbon generation, oil-bearing, and engineering mechanic properties, etc. Based on the test results, 7 sweet spot sections were picked out, with a total area of 260 km² and resources of 6.8×10⁸ t. At the end of 2017, 2 horizontal wells (GD1701H, GD1702H) were deployed and drilled in the Guandong area. With horizontal section of 1200 to 1400 m, these two wells had an oil reservoir drilling rate of 100%, a maximum daily oil production of 75.9 m³, have a stable production of 20.0-30.0 m³/d at present, and have produced for over 500 days stably, marking a major exploration breakthrough^[22]. On this basis, a development plan of building 100×10⁴ t production capacity in 2023 has been made, with producing geological reserves of 2×10⁸ t.

In 2019, 26 horizontal wells were deployed in the Cangdong Sag, with a new deliverability of 15×10⁴ t. The main production layer is C1 sweet spot section in the GD1701H well area. At the same time, preliminary exploration was carried out in the Duanliuba and Wangguantun areas to evaluate C5 and C3 sweet spot layers. By adopting the development scheme of multi-layer series and super-large well group, factory-like drilling and fracturing were conducted on the 1^#, 2^#, 5^#, 7^# and 9^# platforms, cutting down the average period of well construction by 17%; meanwhile, uninterrupted zipper mode cyclic fracturing was implemented, at the fracturing speed of 3.2 stages on average daily and 4 stages a day at maximum, and high-pressure pumping time of 11.5 h a day. The waiting time between two stages was cut down to 2 to 3 h, and this waiting time was used for equipment maintenance, fuel addition and other jobs. As a result, the fracturing efficiency doubled than traditional fracturing, greatly improving the fracturing time efficiency. Up to now, 5 wells have been fractured and put into production. They all have obtained stable oil production of over 80 t per day. The ultimate recoverable reserves of a single well are expected to be 3.1×10⁴ t. Economic evaluation shows that economic indicators of all the wells reach the oil industry standards, suggesting economic development.

The enrichment of continental pure shale oil in the Kong 2 Member of the Cangdong Sag is mainly controlled by two factors: dominant fabric facies and cross-over effect of retained hydrocarbons. The Kong 2 Member has four types of favorable shale fabric facies: laminar felsic, laminar mixed, thin-layer limy dolomitic and thick-layer limy dolomitic facies. The shale oil sweet spot identification and evaluation method proposed can predict sweet spot areas of continental shale oil accurately.

The sweet spot precise trajectory design and tracking- while-drilling and trajectory optimization techniques considering geological and engineering factors jointly, have been tested and achieved good results, with the drilling rate of sweet spot of 100% and drilling rate of class I sweet spot of over 75% on average.

Low-cost volumetric fracturing with “slick water+high quartz sand” has been used in some continental shale oil horizontal wells to create multi-stage fracture network in the horizontal section, greatly improving productivity and effectively reducing the overall project cost by 26.4%.

a, b—weighting coefficient, dimensionless;

A_p—amplitude attribute, dimensionless;

A_t—attribute fusion inversion coefficient, dimensionless;

B_I—normalized brittleness index, dimensionless;

C_C—carbonate mineral content at the point measured, dimensionless;

C_F—felsic mineral content at the point measured, dimensionless;

E_i—comprehensive evaluation index, dimensionless;

F_re—frequency attribute, dimensionless;

GAS—normalized gas logging total hydrocarbon value, dimensionless;

i—No. of sweet spot evaluation parameter;

n—total number of sweet spot evaluation parameters;

OSI—ratio of pyrolysis hydrocarbon generation potential to organic matter abundance, mg/g;

P_i—measured value of the evaluation parameter;

P_Ⅱ,max—maximum value of evaluation parameter for class II sweet spot;

Q_i—weight coefficient of the evaluation parameter, dimensionless;

R_o—shale thermal maturity, %;

S₁—normalized free hydrocarbon content, dimensionless;

S_EI—comprehensive evaluation index of favorable fracturing cluster point in horizontal well, dimensionless;

TOC—normalized organic carbon content, dimensionless;

ΔL—the distance between overlapping density logging curve and acoustic logging curve at the point measured, dimensionless;

Δt—value of the interval transit time at the point measured, μs/m;

Δt₁—minimum value of the selected range of the acoustic transit time curve, μs/m;

Δt₂—maximum value of the selected range of the acoustic transit time curve, μs/m;

ρ₁—minimum value of the selected range of the density curve, g/cm³;

ρ₂—maximum value of the selected range of the density curve, g/cm³;

ρ_b—density of the point measured, g/cm³;

ϕ—normalized porosity, dimensionless.