Tight oil and gas is a new field in the oil industry, which is a very important type of unconventional resource in the world^[1,2], and a major force to replace conventional oil and gas energy and support oil and gas revolution^[3]. Currently, the tight oil and gas is defined as the oil and gas enriched in non-shale rocks such as clastic or carbonate rock reservoirs with overpressure permeability of less than 0.1×10^-3 μm^2[4]. The definition is also the standard to judge whether give the producer tax subsidy. The standard has two characteristics. One is considering from the perspective of economic benefits, and the other is selecting the reservoir permeability as the key evaluation parameter.

Tight reservoir is the typical feature of tight oil and gas. Compared with conventional oil and gas, tight oil and gas is close to the source rock, accumulates continuously on a large scale with no obvious trap boundary, and is less affected by structures. Occurring in reservoirs with poor physical properties and strong heterogeneity, it is low in reserve density ratio (oil and gas reserves per unit rock volume) and low resource grade, making it difficult to select enriched region and predict effective reservoirs. Moreover, since the reservoir has poor flowing capacity, wells have low production and high decline rate, and the oil and gas fields have low recovery factor, high difficulty to keep stable production, and poor economic benefit. The successful exploration and development of tight oil and gas depend on: (1) the breakthrough in the formation and accumulation theory and the progress in sweet spot area selecting technology; (2) upgrading of fracturing technology for tight reservoirs; (3) low-cost development, matching technology of enhancing recovery factor and innovation in management system.

Research and development of tight oil and gas was originated in the North America^[5,6]. Successful development cases include tight gas in the San Juan and Alberta basins, tight oil of Bakken in the Williston Basin and in Eagle Ford, Texas. China's tight oil and gas industry started late but developed rapidly. In this paper, by reviewing the development history, summarizing theoretical progress, comparing the tight oil and gas industry of China with that of the United States, we clarified the development characteristics of China's tight oil and gas and sorted out the differences between tight oil and gas of the America and China in geological conditions, development idea and technology process. The research may be helpful for the future development of China tight oil and gas.

Tight gas exploration and development in China began in 1972 and has entered into a rapid development stage since 2006.

Tight gas in China is mainly distributed in the Ordos, Sichuan, Songliao, Tarim, Bohai Bay, Tuha and Junggar basins. Looking back, the development history of tight gas in China^[7-8] can be divided into 3 stages: the initial exploration, the scale discovery and the rapid development (Fig. 1). (1) The initial exploration stage (before 2000): in the Zhongba region of northwest Sichuan Basin in 1972, a tight gas field was found for the first time in the 2^nd Member of Xujiahe Formation in Triassic System (Well Zhong 4), then several small tight gas fields were found successively. Since these tight gas reservoirs were mistakenly regarded as low permeability ones and no effective methods of enrichment region selection and reservoir stimulation were available at that time, their development process was slow. In this stage, the concept of tight gas didn’t emerge yet. (2) The scale discovery stage (2000-2005): after a great breakthrough was made in the exploration of the Upper Paleozoic in the Ordos Basin, some large gas fields such as Sulige and Daniudi were discovered consecutively. But restricted by geological knowledge, technological level and economic conditions, the production growth was slow. (3) The rapid development stage (2006 to present): the organization and management innovation marked by the "5+1" cooperative development mode of the Sulige gas field, as well as the maturity of low-cost development ideas and main matching technologies, promoted the development of tight gas represented by Sulige into a stage of rapid development. In 2009, the Cretaceous Denglouku Formation gas field in Changling district of Songliao Basin was put into production. In 2014, the Sulige gas field produced 235×10⁸ m³ of gas, making it the largest gas field in China. At the same time, China national standard of "geological evaluation method of tight sand gas" was issued in February 2014^[9], stipulating the tight sand gas reservoir is sandstone gas layers with overburden pressure permeability of less than or equal to 0.1×10^-3 μm², which has no natural productivity or natural productivity lower than the lower limit of industrial gas flow, but can obtain commercial gas production under certain economic conditions and technical measures (generally including fracturing, horizontal well, multi-branch wells, etc). In the standard, the definition of tight gas reservoir, resource evaluation and productivity evaluation criteria are made clear. The promulgation of this standard marks China's tight gas entering into large-scale industrialization stage. Since “the 13th Five- year Plan” began, the discovery and development of tight gas in the Shenmu, Yichuan and Huanglong gas fields in Ordos Basin have accelerated the development of tight gas in China.

China's tight oil started late but developed rapidly. To date, several scale reserves areas of tight oil have been discovered in the Ordos, Songliao, Junggar and Bohai Bay basins (Fig. 2, the development history of China's tight oil). With 2014 as the time node, the tight oil development can be divided into two stages: the stage of exploration and discovery and the stage of industrial test and production. (1) The stage of exploration and discovery (before 2014): In 2010, based on the introduction and development of the concept of "continuous oil and gas accumulation", tight oil was identified as a hot issue and important research area for unconventional oil. From 2012 to 2013, PetroChina Company Limited (abbreviated as PetroChina) held two tight oil exploration and development conferences, promoting the exploration of tight oil in Ordos and other basins. (2) The stage of industrial test and production (2014 to present): a major breakthrough of tight oil exploration was made in the Mesozoic of Ordos Basin, and Xian’anbian oilfield, the first terrestrial tight oil field was discovered and exploited. Furthermore, six development demonstration areas have been set up in the Ordos, Songliao and other basins. The Research Center of National Energy for Tight Oil and Gas was established in 2014, and a China national standard of "geological evaluation method of tight oil" was issued in November 2017. The standard defines that tight oil is the oil stored in tight sandstone or carbonate reservoirs with the overburden pressure permeability of less than or equal to 0.1×10^-3 μm², or the oil with mobility of less than or equal to 0.1×10^-3 μm²/(mPa•s) but not belonging to heavy oil. The tight oil reservoir is near organic rich source rock, while wells have no natural productivity or natural productivity below the cut-off of commercial oil production, but can obtain commercial oil production under certain economic conditions with some technical measures. Meanwhile, the 3-level evaluation system for tight oil sweet spots was also established in the standard (Table 1).

Theory and technology innovations are the cornerstones that support the rapid development of China's tight oil and gas. After years of scientific research, it is recognized that the stable slope deposition system in large basin, the large-area ‘sandwich’ source-reservoir combination, the match of reservoir densification and main accumulation period are favorable conditions for the formation and distribution of tight reservoirs^[10]. The reservoir with high quality, local structure and fracture are the main geological factors controlling the distribution of “sweet spots”. Reservoir formation theories for different types of basins with different exploration focuses have been established. In the terms of development, theories such as ‘multi-stage pressure drop’ and ‘man-made reservoir’ have been proposed, and several technology series including enrichment regions selection, well pattern deployment, single well production and recovery factor enhancement and low cost development have been innovated and integrated, promoting the rapid rise of both reserves and production of tight oil and gas.

2.1.1. Distribution of tight oil and gas in China

Large scale and continuous accumulation of tight oil and gas in China has broken the traditional conventional oil and gas geology theories^[11]. The distribution mainly has the following characteristics:

(1) The stable slope deposition system in a large basin is the basis for the formation and distribution of tight oil and gas in large scale^[12]. For example, Ordos Basin has been a stable Cratonic basin since the Permian, where a river-delta sedimentary system of tens of thousands of square kilometers was formed: the sandstone group in the tight gas layer of the 8th Member of the Permian Lower Shihezi Formation is 13.0×10⁴ km², the sandstones in the tight oil layer of the 7th Member of the Triassic Yanchang Formation is 6.0×10⁴ km². Sichuan Basin was an open-flow shallow-water large lake basin with an inherited water system. The reservoir groups are huge in scale: the main water system of the 4th Member of the Xujiahe Formation is 13.5×10⁴ km², and the reservoirs are 10.2×10⁴ km^2[13].

(2) The large-area “sandwich type” combination of source and reservoir is the guarantee for the continuous and scale distribution of tight oil and gas. Widespread source rocks and large-scale sand bodies are interbedded, forming favorable combinations of source and reservoir symbiosis. The source rocks of the 1^st, 3^rd and 5^th members of the Xujiahe Formation in Sichuan Basin with gas generation intensity of more than 20.0×10⁸ m³/km² are about 8.0×10⁴ km². The superimposed area of the reservoirs in 2^nd, 4^th and 6^th members of Xujiahe Formation is about 6.0×10⁴ km².

(3) Tight oil and gas in large-scale and continuous distribution is likely to occur when the reservoir densification matches well with the main accumulation period. Tight oil and gas generally migrate a short distance from the source rock along relatively high-permeability channels and charge into reservoir in plane continuously (Fig. 3). If the reservoir densification matches the main accumulation period well, then the reservoir densifies and accumulates side by side. Although the densification reduces the permeability of the reservoir on one hand, it improves the preservation capacity of the reservoir on the other hand, which is conducive to the formation of large-scale and continuous distribution of the tight oil and gas to some extent.

(4) The reservoirs with relatively high quality, local structures and fractures jointly control the distribution of “sweet spots”. Among them, source rock is the material basis for forming sweet spot. The distribution of the reservoir and the distance from the source rock affect the accumulation range and quality. The structure fluctuation controls the differentiation of oil, gas and water. The fracture zone greatly improves the transport capacity of tight oil and gas, on the other hand, may cause oil and gas leakage.

2.1.2. Formation theories of tight oil and gas in China

Nano-scale pore-throat connectivity system is fundamental to tight oil and gas accumulation mechanism^[14]. Because of the gas absorption on the rock surface and the interaction among gas molecules, for hydrocarbon to charge, migrate and accumulate, the tight reservoir need to have a lower limit of pore and throat diameter^[15]. The size, structure and combination of pores and throats are the key factors affecting the permeability of the reservoir. By using experimental analysis methods of environmental scanning electron microscopy, high pressure mercury injection, nuclear magnetic resonance, nanotechnology simulation, the lower limit of tight oil and gas pore throat diameter has been determined at 20-50 nm, falling between the lower limit of shale pore throat diameter (5 nm) and conventional oil and gas pore throat diameter (1000 nm)^[14].

For different types of basins, different reservoir formation theories have emerged, pointing out different exploration focus in them, which has important guidance for exploring tight gas. In craton basins with gentle and stable structure, oil and gas mainly charge vertically into reservoirs near-source rock. Selection of favorable filling area is the key, and the elements for tight gas accumulation are shown in Table 2. For example, the source rock formations in the central area of Sulige are Carboniferous-Permian coal measures, with hydrocarbon generation intensity of (16-28)×10⁸ m³/km², on average 24×10⁸ m³/km². The source rock layers and reservoir layers are interbedded, and gas accumulates near source, forming a favorable filling zone of 1.6×10⁴ km² with a gas saturation of generally more than 60%. In the western area of Sulige, the hydrocarbon generation intensity is lower at (10-18)×10⁸ m³/km² and 14×10⁸ m³/km² on average, so gas filling is not sufficient, resulting in no clear gas-water differentiation and an obvious gas-water transitional zone.

In fault basins, rifts come in clusters, so source rocks vary widely in distribution, and the source-reservoir combination is vital. The elements for tight gas accumulations in this kind of basin are shown in Table 3. For example, source rocks in the Lower Cretaceous Shahezi Formation of the Anda area of Daqing Oilfield include lake mudstone and coal; with source and reservoir interbedded, accumulation near source was formed, with no bottom or edge water. In contrast, the source and reservoir of the 4th Member of the Yingcheng Formation of the Cretaceous in the Xingcheng area of Daqing Oilfield are separated, the reservoir is formed through fault transport, with bottom and edge water.

Foreland basins have large dip angles and high oil and gas column height. In them, traps and preservation conditions are the key to the formation of tight gas accumulation. The elements for tight gas accumulation in them are shown in Table 4. For example, Qigu gas field of Junggar Basin is located in the thrusting nappe belt, with thrust faults and folds developed. Through overpressure filling and efficient transport of fractures, structural gas reservoirs are formed in anticline and fault block traps. The gas reservoirs are distributed at the structural highs, with boundary controlled by the structural contour, bottom and edge water, and obvious gas-water contact. Dibei gas field of Tarim Basin is located in the piedmont slope area, where the gas reservoirs are mostly in lithologic and tectonic-lithologic traps, with boundary not controlled by the structural contours, no obvious gas-water contact, and the water layer above the gas layer.

Tight oil develops in the black shale sedimentary system and exists in the micro-nano pore-throat system. There exists “sweet spot areas and sections” of tight oil under large area of accumulation. “Sweet spot areas” are unconventional oil and gas areas with high-production on the plane. “Sweet spot sections” are in the black shale layers with source and reservoir in symbiosis, which can produce unconventional oil and gas at high production rate after stimulation. A “six characteristics” evaluation method has been proposed to analyze the quality of the “sweet spot areas and sections” of tight oil in terrestrial facies. Based on the key parameters such as the reserves density ratio (reserves per unit volume of rock) and the brittleness index, the sweet spot areas and sections are comprehensively evaluated in the “six characteristics”, source rock, lithology, physical properties, electrical properties, brittleness and geo-stress characteristics^[16], and the “sweet spot areas and sections” are classified into different grades and types, to provide a basis for producing tight oil resources.

2.1.3. Development theories of tight oil and gas in China

On one hand, both tight oil and tight gas reservoirs require fracturing to improve reservoir permeability and fluid mobility, and they both need manual intervention to achieve scale development. On the other hand, there are big differences in the development methods between tight oil reservoirs and tight gas reservoirs. Oil reservoirs are often developed with supplementary energy. Injection-production system make the continuity, connectivity and heterogeneity of the reservoir the key contents of study. Development technologies of fine water injection, chemical flooding and deep profile control require detailed layer correlation, from plane and interlayer heterogeneity to inlayer heterogeneity and internal characterization of single sand body. Gas reservoirs are mostly developed by depletion, and the range of pressure drop is the core of the description. Scale and geometry of the reservoir units are the focuses of research, which determine the gas drainage area and well pattern etc. The development theories of “multi-stage pressure drop” and “man-made reservoirs” are established for tight oil and gas.

Tight gas reservoirs of Ordos Basin are mostly in terrestrial braided river sediments. Under the dual effects of sedimentation and diagenesis, effective sand bodies are limited in distribution, appearing in the binary structure of limited effective sand in wide distributed matrix sand. The effective reservoirs are characterized by poor continuity, low permeability and weak pressure conductivity. Permeability of the near well zones is improved after fracturing, but the reservoir heterogeneity is aggravated. Based on the geological characteristics and development mode for tight gas, the development theory of “multi-stage pressure drop” has been established. According to this theory, the reservoir is developed by making full use of formation energy, the pressure drops in multiple steps from the artificial fracture zone to the matrix zone, from the effective sand body to the matrix sand body, from the micro-scale pore to the nano-scale pore gradually, to enlarge the swept volume of pressure drop, make the gas flow in high permeability zone set the gas in low-permeability zone in motion, to realize the production of gas in different parts step by step. Dynamic reserves of gas wells developed in this pattern vary in three stages with the extension of production time: rapid ascending stage, slow ascending stage and stable production stage, respectively reflecting the sweep of man-made fractures near gas wells, matrix far away from gas wells, and boundary. Based on this theory, a production mode of pressure control in early stage and production proration optimization has been proposed, to achieve the relative equilibrium pressure drop of the near-well fracture zone reservoir, the far-well matrix reservoir and the other poor reservoir, which improves the single well production and development economics. It has promoted the scale development of tight gas.

In view of the low seepage ability, no naturally stable and commercial production, fast energy drop and difficult energy replenishment of tight oil and gas, academician Zou Caineng of Chinese Academy of Engineering proposed the theory of “man-made reservoir” in 2016-2017, and systematically elaborated its theoretical connotation, key technologies and application practices^[17]. “Man-made reservoir” takes “sweet spot area” as target unit, and makes high permeability zone and reshape seepage field by deploying appropriate well groups and adopting integrated fracturing, injection and production, to change stress field, temperature field, chemical field of rock, and the wettability and fluidity of oil and gas. The aim is to achieve effective and scale development of unconventional oil and gas through artificial intervention. During the fracturing process, seepage field underground changes, and the change of fluid pressure in fracture leads to the change of the width and length of the fracture. This progress also brings about changes in the stress field, and changes in the far field stress and the fracture induced stress also form the constraint for the fracture width and fluid pressure in the fracture. During the fracturing process, the acid-rock reaction acts as a heat source, which affects the change of the temperature field of the “man-made reservoir”, and the temperature change also affects the chemical reaction rate and the chemical stability of the mineral reaction process. The heat source entering the formation with the fracturing fluid is different from the reservoir temperature. Temperature change causes changes in thermal stress and temperature-dependent rock mechanical properties. Seepage of the fracturing fluid in the fracture and matrix drives the heat transfer, forming convective heat transfer and affecting the change of temperature field. Change of the temperature field affects fluid properties such as fluid density and viscosity related to temperature.

Changing the relations of “four fields”, seepage field, stress field, temperature field and chemical field is an important way to establish flow system of fracture network with large well cluster for the “man-made reservoir”. Within the specific volume of the “sweet spot area” unit, the fracture control of reservoir in large area can be realized by changing the “four fields”. Within the influence scope of single well, “man-made reservoir” in the well-control scope is realized by making “artificial high permeability zone” through volume stimulation. Within the influence scope of single fracture, oil recovery can be enhanced by measures of imbibition replacement and fluid upgrading. Through researches and practices, "man-made reservoir" development has formed five core technology series: three-dimensional seismic and geologic "sweet spot area" evaluation technology based on big data, "man-made reservoir" technology of large group of well cluster, artificial intelligence fracturing technology for volume stimulation, replaced flooding of oil and energy complement technology, and "man-made reservoir" intelligent management technology based on cloud computing. Among them, artificial intelligence fracturing technology combines the fine fracturing with intelligent materials, which forms two kinds of fracturing methods. The first one is the subdivision cutting stimulation method based on “fast drilling bridge plug and clustering perforation”, which is suitable for the tight oil reservoirs difficult to form complex fractures. Through multi-clusters fracturing in stages, the reservoir can be cut finely by fractures. The second one is the fracturing stimulation method to make complex fractures, which mainly aims the brittle reservoirs with developed natural fractures. Large displacement, temporary blocking steering and fracture spacing optimization in horizontal wells are adopted to create complex fracture system. Various stimulation intelligent materials are added at the end, inside and entrance of fractures in different reservoirs to change the wettability of the reservoir rock and realize the artificial fracture steering at fixed points.

“Man-made reservoir” is a systematic technology integrating exploration, development, engineering, production and information. Industrial tests of large-scale liquid injection, energy supplementation and imbibition replacement stimulation were conducted, 235 well-times of pilot tests were carried out, resulting in tight oil production 2 times of the conventional technologies, showing good application prospects. This technology is of great significance for promoting the beneficial and sustainable development of unconventional and low-grade oil and gas resources.

Based on the theoretical breakthrough of tight oil and gas exploration and development, 4 technology series have been innovated and integrated: enrichment area selection and well pattern deployment, production increase of single well, enhanced oil recovery and low-cost development, improving the production and economics of tight oil and gas significantly.

2.2.1. Technology series of enrichment area selection, sweet spot area evaluation and well pattern deployment

According to the reservoir characteristics of tight gas reservoirs, enrichment area selection technology with seismic gas detection as the core and well pattern deployment technology with the large-scale composite sand body classification configuration as the core have been established^[18,19,20]. Development practices show that the effective sand bodies in Sulige tight gas field are mostly distributed in the gritstone sediments of central and lower part of the channel bar or the bottom of the channel filling. The classified description of large-scale composite sandstone is suitable for the characterization of highly migratory fluvial facies reservoirs under the sedimentary background of braided river system. Its connotation is to carry out fine descriptions of regional channel system, braided river system, complex zone of channel, single channel, sedimentary micro-facies by level by using multiple technologies jointly to identify outer lithological boundary and inter physical boundary of the reservoir, divide the “blocking zone” grade in the reservoir, study the influence of different grades of interlayers on the fluid seepage, analyze the matching relationship between well pattern and distribution, scale and frequency of effective sand bodies. During the development process, the information obtained is continuously enriched, the research precision is gradually improved, and the acquired understandings tend to be more and more accurate. However, spacing of gas wells is still relatively larger than that of oil wells. Since the well pattern has limited control on the reservoir, hierarchical configuration description in inter-well areas is subjective to some extent. Therefore, predicting the effective reservoir distribution by integrating hierarchical configuration description of composite sand body and seismic gas detection has achieved good results. In Sulige gas field, by using this technology, the enrichment area of 1.6×10⁴ km² was selected, proved reserves of 2.0×10¹² m³, and about 1.2×10⁴ development wells were deployed, boosting the scale and effective development of the large tight sandstone gas field. In this gas field, the I+II wells increased from 40% at the initial development stage to more than 75% in 2017^[21], the skeleton well pattern has been adjusted from 600 m×1200 m to 600 m×800 m, and the recovery has been improved from the early 20% to 32% currently^[22]. Daqing Oilfield has improved the resolution of seismic data and the prediction accuracy of reservoirs by tackling high-resolution 3D seismic data processing and interpretation techniques. Currently, they can identify 3m fault throw, 3 m-thick thin layers and 22 m wide channels.

2.2.2. Technology series for increasing single well production

For tight gas reservoirs, fracturing stimulation technologies such as finite stages open-hole packer + sliding sleeve, infinite stages hydraulic injection + annulus sand adding for vertical wells and horizontal wells have been developed. The horizontal wells generally have a lateral section of 1000-2000 m, in which up to more than 20 stages of fracturing have been realized. After fracturing, initial daily production of a horizontal well increased to more than 5×10⁴ m³ from 1×10⁴ m³ of the vertical wells. The cumulative production of a horizontal well is (0.6-1.0) ×10⁸ m³, which is more than 3 times of a vertical well.

For tight oil wells, horizontal well volume fracturing is used to form a fracture network, while micro-seismic monitoring is used to monitor the “man-made” volume stimulation effect. Well Hengping 1 is a typical tight oil well in Daqing Oilfield of Songliao Basin. The well has a total depth of 4300 m and horizontal section length of 2660 m. It was fractured in 11 fracturing stages in total, with fracturing fluid of 1.5×10⁴ m³ and sand of 1724 m³. The stimulation effect is ideal. It had a daily oil production of 71.3 t after fracturing, and has an oil production of 3.4t/d at present, and cumulative oil production of 2.58×10⁴ t.

2.2.3. Technology series enhancing oil recovery

Technologies enhancing oil recovery such as well pattern infilling, repeated stimulation, old well sidetracking, drainage of low pressure and low production well have been established. Combined with geological analysis, interference well test and production performance data, it is concluded that the average control range of a vertical well in Sulige gas field is 0.20-0.25 km², so the current skeleton well pattern of 600 m×800 m has no sufficient control on reserves and there is still infilling room. Well pattern of the enrichment area in the gas field can be infilled from 2 wells/km² to 3-4 wells/km², and the recovery can be increased from 32% to more than 45%. Combined with drainage gas recovery, layer check and reperforation, repeated stimulation, old well sidetracking, optimization of production measures etc recovery enhancement technologies, it is predicted the recovery can be increased to 50%.

2.2.4. Technology series of low-cost development

With low resources grade and poor development benefits, the tight oil and gas should be developed by low-cost strategies. For this purpose, the rapid drilling technology with polycrystalline diamond composite drill bit as the core, middle-low pressure gas gathering with a downhole choke as the core, factory operation with large well clusters and multi-well types, digital production management technology have been developed. The low-cost development technologies make the comprehensive cost of a vertical well in Sulige gas field reduce to 8 million yuan from 14 million yuan in the early days. Under the condition that the reservoir quality gradually declines, they support the sustained scale economic development of tight oil and gas. In Fuyang tight reservoir of Jilin Oilfield, the application of large factory operation of 48 wells in large well pads has effectively reduced the cost, saved resources and improved the economic benefit.

Supported by progress in theory and technology, tight gas of China has developed rapidly since 2005^[23], reserves and production have maintained fast growth, and the tight gas province with reserves of trillion cubic meters has been explored and developed in the Ordos Basin^[24]. Exploration and development of tight oil has also made great breakthroughs since 2014.

According the 4th round resource assessment of PetroChina, tight gas in China has a favorable exploration area of 32.46× 10⁴ km², geological reserves of 21.85×10¹² m³ and recoverable reserves of 10.92×10¹² m³ (Table 5). Since the beginning of the 12^th Five Year Plan, proven reserves of tight gas have grown 0.32×10¹² m³ annually on average. By the end of 2017, proven reserves of tight gas in China are 4.6×10¹² m³, accounting for 33% of total gas reserves; and production of tight gas is 350×10⁸ m³ in 2017, accounting for 24% of total gas production. Ordos Basin is the biggest tight gas production province in China, where the proven reserves of tight gas in the Upper Paleozoic are 3.3×10¹² m³, and the production of main tight sandstone gas fields (Sulige, Daniudi, Shenmu, Yanchang and eastern of Ordos Basin) reached 310×10⁸ m³ in 2017, accounting for 89% of China’s total tight gas production.

To date, a number of tight oil provinces have been found in China, including the Ordos, Songliao, Junggar and Bohai Bay basins (Fig. 4). According to the 4th round of resource assessment of PetroChina, tight oil in China has geological reserves of 125×10⁸ t, recoverable reserves of 13×10⁸ t and proven reserves of nearly 3×10⁸ t. The Ordos Basin and Songliao Basin are the main tight oil provinces^[25]. Tight oil geological reserves of Ordos Basin are 34.2×10⁸ t, and proved, probable and possible reserves are 9.2×10⁸ t in total. By the end of 2017, production capacity of the basin was 137.8×10⁴ t, and annual oil production was 53.8×10⁴ t^[21] (Table 6). Tight oil reservoirs in Songliao Basin are mainly distributed in Daqing and Jilin oilfield. To date, 92 horizontal wells and 23 vertical wells have been deployed, with an average daily oil production per well of 8-10 t. Production of Fuyu reservoir was 28.6×10⁴ t in 2017. By the end of 2017, production capacity of tight oil in China was nearly 200×10⁴ t, and the production reached 103.1×10⁴ t (Table 6).

4.1.1. Geological conditions and resources

Tight oil and gas in North America is mainly distributed in marine large-scale flat cratonic basins, where the structure is stable and source rocks and reservoirs are widespread; the source rocks have higher TOC value and higher maturity; the reservoirs have good connectivity and large scale, good physical properties, higher porosity and reserve abundance, and moderate burial depth, most of them are overpressure, and have fractures in local parts^{[26,27,28,29]}. In contrast, China tight oil and gas reservoirs generally have the characteristics of multi-cycle structural evolution, and are mainly in continental sedimentary environments (Tables 7 and 8). Therefore, they feature great facies changes, unstable formation distribution; complex surface conditions, mostly in mountainous, hilly and desert areas, with big operation difficulty; mostly lacustrine shale source rocks with wide variation in TOC value and low maturity; higher density and viscosity of the tight oil; thin reservoirs with strong heterogeneity, big variation and limited distribution, poor physical properties and fracture development, low porosity and reserves density, big burial depth and low pressure coefficient. In general, China tight oil and gas has poorer economic conditions, posing a big challenge to its economic development.

According to EIA data, tight gas in the US has geological reserves of 28.0×10¹² m³ and recoverable reserves of 12.6× 10¹² m³, which are basically equivalent to those of China. But tight oil in the US has recoverable reserves of nearly 81.2×10⁸ t^[30], which is six times more than that of China^[30].

4.1.2. Development technology

The tight oil and gas development technologies of China lag somewhat behind those in the US in the accuracy of reservoir identification and sweet spot prediction, rapid drilling, large cluster horizontal wells, multi-layer multi-stage fracturing process etc^[31]. Reservoir identification and “sweet spot” prediction answer the question of where to exploit oil and gas, and rapid drilling and reservoir fracturing solve the problem of how to effectively develop.

In terms of reservoir identification and sweet spot prediction, North America can identify thin sand bodies of 5 m thick through high-precision 3D seismic technology, and is gradually putting advanced cutting-edge technologies such as big data, cloud computing and virtual reality into geological modeling. The success rate of sweet spot prediction is 65% to 95%. China can identify 5-10 m faults by simulating 3D sand body and seismic pre-stack inversion technology. The identification accuracy of 10 m thick sand bodies is 70%-80%^[32], and the accuracy of sweet spot prediction is 50%-85%.

In terms of drilling, North American EOG's daily average footage at Eagle Ford increased from 291 m/d in 2011 to 786 m/d in 2018, significantly shortening the drilling cycle. Gas wells with an average depth of 5 500 m (2 500-3 500 m vertical section and 1 100-3 200 m horizontal section) can be finished in only 6-8 days. Due to the increase in drilling speed, drilling costs have declined year by year. At present, the unit drilling cost is 2 500 RMB yuan/m, and the average cost per well is 14 million RMB yuan^[33]. In comparison, in the Sulige gas field in China, horizontal wells with about 5 000 m deep (3 000 to 3 500 m vertical section and 500 to 1 500 m horizontal section) have a drilling period of 25 to 35 days, daily average footage of 167 m/d, drilling unit cost of 5 000 RMB yuan/m, and an average cost of 25 million RMB yuan per well.

In reservoir fracturing, North America has greatly improved reservoir permeability and single well production through large well clusters, multi-well clusters, and high density fractures. In Bakken tight oil area, the number of fracturing stages in double-branched wells has reached up to 80, the initial production reached 100 t/d, and the production in stable period is about 20 t/d ^[34]. In the Rulison and Jonah gas fields, vertical wells can be fractured up to 50-80 layers, and 20 to 30 horizontal wells can be drilled from one well pad. The two gas fields have a recovery of 48% to 55%, and producing rate of greater than 70%^[35,36]. In contrast, in the tight oil layer of the Chang 7 Member of the Ordos Basin in China, the single-well daily production exceeds 20 t/d after large-scale mixed hydraulic fracturing. In the Sulige gas field, vertical wells usually have less than 10 layers fractured, horizontal wells have up to 20 fracturing stages, often 5 to 20 horizontal wells are drilled from one well pad, and the recovery and reserves producing rate of the gas field are 32% and less than 50% respectively.

4.1.3. Production scale

Tight oil and gas reservoirs feature poor reservoir physical properties, low gas well production, fast decline speed, slow energy supplement and high development cost. Low-cost development is the key to the scale development of tight oil and gas.

In the 1970s, the United States explored the development of tight oil and gas. After 30 years of exploration and preparation, it successfully broke the conventional geological development theory and technology, realizing the "first revolution" of conventional oil and gas to unconventional oil and gas. In 2008, the peak production of tight gas exceeded 1 913×10⁸ m³ (Fig. 5a), accounting for 34% of the annual US natural gas production. In recent years, under the low oil price, the unconven-tional oil and gas represented by shale gas and tight oil is undergoing the "second revolution" through technology and management innovation. Specifically: firstly, main and supporting technology have been vigorously researched and developed to improve single well production and recovery, aiming to reduce development costs through technological innovation. Secondly, drilling wells but not fracturing wells and only producing the high-yield wells in the “sweet spot” etc. measures have been taken to reduce development costs through construction method innovation. Thirdly, innovations in management such as scale layoffs and full adoption of market mechanisms have been taken to reduce costs. The shale gas revolution in the United States is in the rise, resulting in a slight decline in tight gas production. However, the tight gas production in 2017 was still 1 200×10⁸ m³. US tight oil production in 2017 exceeded 2.4×10⁸ t, accounting for 50% of its total oil output. In fact, it is exactly the sharp increase in tight oil production that has reversed the decline in US oil production^[37].

In China, tight gas production increased from less than 10×10⁸ m³ in 2000 to 350×10⁸ m³ in 2017, accounting for 23.5% of the country's natural gas production. It is currently the best-developed type of unconventional natural gas in China (Fig. 5b), and a powerful support for China's annual natural gas production to enter the 1 500×10⁸ m³. However, in comparison, the tight gas production in China only accounts for 29% of the tight gas production in the United States during the same period. China's tight oil reservoirs are small in scale, low in single well production, and high in development costs. The output in 2017 was 1.0×10⁶ t, which only accounted for 0.5% of the national annual oil production of that year. The tight oil development in China is still in its infancy, lagging far behind the United States. If the technologies such as in-situ conversion and volume fracturing make breakthroughs, the future development potential of tight oil will be huge.

Controlled by geological conditions, development techniques, stimulation process and development concepts, single well production and oil and gas field production of tight oil in China are much lower than those in the United States. The comparison of geological conditions, development techniques and stimulation process has been stated above. In terms of development concept, North America has a higher degree of marketization degree and flexible institutional mechanisms. At the same time, there are thousands of companies involved in the development of tight oil and gas^[38]. They pursue the economic benefits as the first goal, and production wells mostly take the constant pressure production mode. However, China's oil and gas industry has a high barrier to entry, low level of development, and fewer qualified and competitive enterprises. In view of the large investment in the early stage of oil and gas fields and large occupation of social resources, oil and gas fields should hold stabilize production for a certain period of time after put into production, and controlled pressure production mode is employed for most production wells. It should be pointed out that China's oil and gas industry is responsible for safeguarding national energy security and maintaining social stability. It does not take profit and economic benefits as the only goal. Thus, it is obviously unrealistic to reduce costs by only producing from high-yield wells in the “sweet spot” and scale layoffs. It is necessary to explore the path of tight oil and gas development suitable for China's national conditions.

China's tight oil and gas development faces problems such as the trend of worsening resource quality, need to improve effective development and enhancing oil recovery technologies, and high development costs. In the future, China should adhere to the low-cost development strategy and focus on resources, technology, organization and management, and benchmark against the international advanced level to promote the leap-forward development of tight oil and gas. The production of tight gas in China is expected to reach (400-430)×10⁸ m³ in 2020 and reach (550-800)×10⁸ m³ in 2035. The output of tight oil is expected to reach (150-200)× 10⁴ t in 2020, and 1 500×10⁴ t in 2035.

4.2.1. To strengthen resource evaluation

We should work hard to systematically reveal the law of tight oil and gas enrichment, strengthen resource exploration in the old and new district and in the new strata, and objectively evaluate China's tight oil and gas resources and recognize its strategic position^[39]. Unlike the millimeter-micron scale pore-throat system of conventional oil and gas reservoirs, tight reservoirs mainly contain nanoscale pore-throat systems. The tighter reservoir limits the role of buoyancy in hydrocarbon migration and accumulation. The oil and gas move in the forms of seepage and diffusion mainly, which is non- Darcy flow, with a short migration distance from the source rock. The tight oil resources are large, but the proportion of effective resources available for development is low. It is necessary to deepen and refine the evaluation of resource scale and structure, carry out comprehensive classification of resources, establish an orderly succession sequence, and clarify the mobilization and utilization conditions of various resources.

4.2.2. To promote development and engineering
technology advancement

Technological breakthroughs and large-scale applications are the keys to support the economic development of tight oil and gas. The first is to improve the reservoir hierarchical structure description, high-precision 3D seismic technology, and reservoir fine characterization. The second is to improve technologies such as slim well-hole completion, horizontal well drilling, and digital intelligent management, etc., to accelerate the pace of production capacity construction and the application level of localized materials to reduce development costs. The third is to upgrade the reservoir stimulation technologies, promote the application of mature supporting technologies like high-density fracturing, soluble bridge plugs, platform factory operations, etc., to greatly improve the reservoir stimulation effect. The fourth is to strengthen research on recovery enhancement technology, including on mechanism enhancing tight oil and gas recovery. At the same time, the pilot of dense well pattern should be set up to optimize and adjust the development well pattern, and to confirm the development indicators such as interference rate and single well EUR. In addition, methods such as layer check for reperforation, repeating stimulation, lateral horizontal well drilling and production system optimization should be taken jointly to tap the remaining reserves and enhance the utilization degree of the reserves and the recovery factor.

For the two “large basins” of Ordos and Sichuan with multi-types of oil and gas reservoirs, deepening and enriching the theory and technology of three-dimensional development of “artificial oil and gas reservoirs” is the main technological direction for improving development benefits. The future development goal is to optimize the development sequence according to the pressure and fluid properties of different types of gas reservoirs in the vertical direction, share the underground well pattern system, the above-ground well site and the gathering and transportation system to maximize the recovery factor and development benefit^[40].

4.2.3. To optimize organization and management mode and champion for national favorable development policy

Within oil and gas companies, we need to establish an innovative mechanism to improve the enterprise efficiency and employee initiative. It is necessary to revitalize assets through the transfer of mineral rights. Also, there should be a matching science and technology incentive policy to enhance the staff's responsibility and enthusiasm for work. At the national level, we should strengthen the cutting and guiding of profits in the national policies of production, transportation and sales, champion for the increase of upstream profits proportion in the industrial chain, and preferential fiscal and tax subsidies to maximize the liberation of low-grade reserves. Taxation originates from the people and is used for the people. It is a favorable tool for the state to allocate social resources and support the national economic construction. Under the Chinese political system, reasonable taxation and preferential policies are not only manifested as “concentrating power to do big things”, but also reflecting the support and inclination of macro-policies on certain industries and fields. Taking tight gas as an example, without subsidies, China's remaining available reserves can support stable production of 350×10⁸ m³/a. If the subsidy is 0.2 yuan/m³, it can support the production of 400×10⁸ m³/a by well pattern infilling in the remaining available reserves area. If the subsidy is 0.4 yuan/m³, the low-abundance type-I reserves area can be developed to support the production of 500×10⁸ m³/a. If the subsidy is 0.6 yuan/m³, the low-abundance type II reserves area can be produced to boost the production up to 600×10⁸ m³/a.

In the recent decades, China has innovated exploration and development theory of tight oil and gas, integrated several technology series such as single well production enhancement and low-cost development, promoting the rapid rise of both reserves and production of tight oil and gas. However, compared with the advanced level of tight oil and gas development, China lags behind in some aspects such as “sweat pot” identification and prediction, large clusters of long horizontal wells drilling and reservoir fracturing technology.

It is particularly important to note that, in terms of time, tight gas development in China is later than that in the United States. But in terms of stage, the significant increase in tight gas production in China started when conventional gas production increased, while the United States did not begin to develop tight gas until conventional gas was in decline. This superposition effect needs to pay full attention. The following points should be focused in the tight oil & gas development in the future: to deepen understanding of the law of tight oil and gas enrichment and optimize resource evaluation methods; to develop key engineering technologies such as high-precision three-dimensional seismic, large clusters of horizontal wells, artificial oil and gas reservoirs, and intelligent engineering to support the development of tight oil and gas; to optimize the organization and management mode and champion tight oil and gas tax subsidy and exemption policy to develop the low-grade reserves to the greatest extent; to innovate and develop a new generation of theories and technologies for increasing single well production and oil recovery to promote the leap development of tight oil and gas.