The development of petroleum geology theory experienced a process of "searching oil by oil and gas seepages", "anticline" theory, "trap" theory and "continuous oil and gas accumulation" theory^[1,2,3]. Since the beginning of this century, the development of unconventional oil and gas geology has further propelled the shift of global oil and gas exploration and development from conventional oil and gas to unconventional oil and gas^{[4,5,6,7,8,9,10,11]}. In fact, the discovery and exploration of unconventional oil and gas is associated with the development of the entire oil and gas industry. In the United States, the first shale gas well was drilled in 1821^{[3, 12]}, but in the following over one hundred years, shale gas had not received theoretical and practical attention due to technical conditions. With the improvements of oil and gas engineering technology, technologies such as horizontal drilling, staged fracturing and platform-based “factory” operation have been widely applied^[13,14,15]. The development of technology has driven the gradual change of oil and gas exploration and development objects from traditional oil sands to tight oil and gas, and then to shale oil and gas, bringing about the major transformation from “outside source” to “within source” in oil and gas exploration and development, and the revolution from “conventional oil and gas” to “unconventional oil and gas”. The oil production “within source” has grown rapidly. In 2018, the production of shale oil and shale gas in the US reached 3.29×10⁸ t and 6072×10⁸ m³ respectively, while the shale gas production in China amounted to 108×10⁸ m³.

Although there are many classification schemes for unconventional oil and gas resources in academia, most of the current standard definitions for unconventional oil and gas are still based on the technical difficulty of development engineering and the commercial value of oil and gas exploitation. Such defining standards have promoted oil and gas exploration and development at a certain stage, but they are bound to be affected by technological innovation and oil price fluctuations, and cannot maintain the relative stability and durability of the theory. In addition, the unconventional oil and gas geologic theory is a great breakthrough of the traditional petroleum geologic theory, and defining from technical and economic aspects can’t clarify the difference between the two. In summary, it is believed that the re-discovering the "unconventional" in unconventional oil and gas will be of great significance for deepening the understanding of oil and gas geology theory and exploration practice.

As shale oil and gas has become an important strategic replacement field^{[16,17,18,19,20,21,22,23]}, theoretical studies on the formation mechanism and enrichment law of typical unconventional oil and gas (shale oil and gas) are of strategic significance for the development of unconventional oil and gas theories. Therefore, taking the main shale oil and gas producing areas of Sichuan Basin and Ordos Basin in China, the Permian Basin and the Western Gulf Coast Basin in the US as research objects, our research team have continuously tracked and compared the previous research results^{[2, 16, 24]}, "re-understand" unconventional oil and gas from theoretical connotation, reservoir dynamics, reservoir lithology, pore type, and fluid characteristics, and clarified the geological definition and characteristics of unconventional oil and gas, in the hope to provide a scientific basis for the study of shale oil and gas formation mechanism, “sweet spot” evaluation and rapid development of exploration and development.

In the past, unconventional oil and gas refer to oil and gas that cannot reach natural industrial production with traditional technologies and can only be economically exploited by improving reservoir permeability or fluid viscosity with new technologies. Unconventional oil and gas include oil sands, oil shale, tight oil and gas, shale oil and gas, coalbed methane, gas hydrate and so on^[2,3]. According to the classical petroleum geological theory and the latest theoretical and exploration progress, the above-mentioned unconventional oil and gas can be divided into two categories according to their accumulation mechanism: (1) Oil and gas generated from source rocks accumulate or suffer damages in traps after a certain distance of migration under the action of buoyancy and capillary pressure difference just like traditional conventional oil and gas with different locations of source and reservoir. They could occur in any kinds of reservoirs, and they include heavy oil, oil sands, tight oil and gas, and gas hydrates; this kind of oil and gas still follow the trap formation mechanism and process described by traditional petroleum geologic theory. (2) Oil and gas accumulate within source rocks under overpressure and diffusion, including shale oil and gas and coalbed methane, which are completely different from conventional oil and gas. Although oil and gas in the first category are difficult to exploit economically, it is not different from conventional oil and gas in hydrocarbon accumulation mechanism. Therefore, the unconventional oil and gas defined in this paper belong to the second category, that is, oil and gas of in-situ accumulation and continuous distribution inside source, including shale oil, shale gas and coalbed methane (Fig. 1). This paper mainly discusses shale oil and gas, and analyzes the shale gas in the Sichuan Basin as an example. For comparison, the oil and gas reservoirs discussed in this paper are mainly sedimentary rocks.

After deposition, the organic-rich shale undergoes compaction, thermal evolution, and water drainage under geological conditions and then enters the hydrocarbon generation process. A part of the oil and gas generated from kerogen migrates to favorable reservoirs such as sand bodies and carbonate rocks through the transport system. This kind of reservoir has reservoir space dominated by micron-sized pore throats of above 0.3 μm largely, in which, affected by buoyancy and capillary force, the hydrocarbon flow is mainly Darcy flow (Table 1). The fluid in the free fluid dynamic field would move upwards to structural highs or low potential areas, forming conventional oil and gas reservoirs under trap conditions, including tight oil and gas reservoirs, which have obvious oil, gas and water interfaces after stabilization^[1,2]. Heavy oil, oil sand or hydrate could be formed when the reservoirs were destroyed and modified in later stage.

The other part of oil and gas generated from kerogen is retained and reach equilibrium inside the source rock under overpressure and binding of capillary force and molecular force, forming “in-source in-situ oil and gas accumulation”, i.e. shale oil and gas. The majority of shale oil and gas exists in the nano-scale pores with the pore throat ranging from 5 to 50 nm. Lack of buoyancy and hydrodynamic force, the shale oil and gas is in a bound fluid dynamic field and flow mainly in the non-Darcy pattern. The main driving force of this kind of oil and gas accumulation is internal overpressure, including pressurization caused by hydrocarbon generation, overpressure formed as a result of undercompaction and pressurization by tectonic stress. After the accumulation of a large amount of oil and gas, diffusion also becomes the main accumulation mode^{[1-3, 12-14 ]}. The shale reservoir contains large amount of oil and gas, with no water or only a small amount of water (mainly bound water), and the oil and gas enrichment boundary are jointly controlled by the internal overpressure and capillary forces.

Shale oil and gas are retained and accumulated in the source rock driven by overpressure. The organic-rich shale is both the source rock and hydrocarbon reservoir. Based on the exploration and development practice of shale gas in the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation of the Sichuan Basin, shale oil and gas is compared with conventional oil and gas. Shale oil and gas is different from conventional oil and gas in lithofacies, reservoir space, saturation, and occurrence state.

Shale oil and gas reservoirs are dominated by fine-grained shale rich in organic matters. The "shale" here is not pure shale considered by most researchers in China, that is, shale rich in clay minerals or siliceous minerals. The North American shale oil and gas reservoirs include organic-rich shale, argillaceous carbonate rock or argillaceous siltstone. With well-developed bedding or lamellation and rich organic matter, they are commonly referred to as "shale". Therefore, the lithofacies of a shale oil and gas reservoir is a set of fine-grained sedimentary rocks rich in organic matter, complex in lithofacies and lithologic combination. The Barnett Shale in North America is a combination of organic-rich bioclastic, carbonate and siliceous shale. The Eagle Ford shale is a combination of organic-rich argillaceous carbonate and calcareous shale^[25]. The Niobrara shale consists of chalk layers poor in organic matter and argillaceous limestone rich in organic matter. The Wufeng-Longmaxi Formation in the Sichuan Basin in China is a combination of organic-rich siliceous and calcareous shale, clayey shale, argillaceous shell limestone, and argillaceous siltstone.

Conventional oil and gas reservoirs of marine facies are mainly distributed in high-energy facies such as onshore and platform marginal slopes^[26,27] (Fig. 2). In these strong hydrodynamic environments, coarse clastic rock and reef flat carbonate often deposit, which are good reservoirs. Similar to the marine facies, conventional oil and gas reservoirs of continental facies are mainly high-energy water bodies such as rivers, deltas, and shore-shallow lakes, where coarse clastic rocks deposited, providing good reservoir space for conventional oil and gas accumulation. Conventional oil and gas reservoirs are mainly controlled by the sedimentary environment, epigenesis, and tectonism.

In contrast, the organic-rich shale of marine and continental facies are mainly formed in low-energy environments in semi-closed to closed waters^[28,29]. During the transgression of marine facies, the deep-water shelf would become an oxygen- anaerobic environment, where planktonic organisms such as algae would boom, giving rise to "marine snow" sedimentation phenomenon, making it the depocenter of the marine basin, highly organic shale in depocenter doesn’t develop. For example, the maximum total organic carbon content (TOC) of the organic-rich shale in the Wufeng Formation-Longmaxi Formation in the Sichuan Basin is 25.73%, and layers with TOC value greater than 2% accounts for 30% to 45%. Analysis of paleoenvironment and paleogeography based on trace element data shows that the organic-rich shale deposited in the semi-deep to deep shelf environment of the continental slope (Fig. 2).

Conventional oil and gas reservoirs mainly include clastic reservoirs and carbonate reservoirs. The reservoir space can be divided into primary pore, secondary pore, and fracture. Primary pore includes intergranular pore and intercrystalline pore. Secondary pore includes dissolution pore, and mold pore, etc.^[1]. All the pore pores are larger in size, mostly in micron-millimeter scale, and simple in pore structure. Shale oil and gas reservoirs also have such inorganic pores (Table 2), such as pores between quartz or feldspar particles, intercrystalline pores of clay minerals, dissolution pores of carbonates, etc. However, all these pores in shale are smaller in size, mostly nanoscale, and very complicated in pore structure^[2]. Besides, the shale oil and gas reservoir has a unique kind of reservoir space, organic matter pore, which is more developed in the “sweet spot” section with main nano-pores.

During the evolution of organic matter, the hydrocarbon generation material not only produces oil and gas, but also generates nano-scale reservoir space in the organic matter, forming a three-dimensional coupling space of mineral pore-organic matter pore-microfracture. As unconventional oil and gas can be generated almost over the entire organic hydrocarbon evolution process (Fig. 3), almost all unconventional oil and gas reservoirs have organic matter pores. Moreover, the thermal evolution degree at vitrinite reflectance (R_o) of 0.8%-3.5% is more favorable for the development of such pores. Organic matter pores are mainly divided into two types: organic matter pores in kerogen and organic matter pores in solid asphalt. The organic matter pores in kerogen are honeycomb-like or sporadic, with a pore diameter of 10 to 200 nm. The organic matter pores in solid asphalt are elliptical and arranged in bead strings, even with boundary completely fused, and 300 nm to 2 μm in diameter. With a R_o value of shale organic matter of 1.8%-3.1%, the Wufeng Formation-Longmaxi Formation shale in the Sichuan Basin is in the stage of thermal pyrolysis and dry gas generation. The organic matter such as kerogen and asphalt generated groups of “honeycomb-like” organic matter pores during primary degradation and secondary cracking (Fig. 4a), which are good storage space for oil and gas. Through petrophysical models and a large number of SEM images (Fig. 4a), it is found that the surface porosity of organic matter pores in the high-yield interval is 30% to 50%, accounting for 1/3 to 1/2 of the porosity.

It is found that shale oil and gas reservoirs are generally higher in temperature than conventional oil and gas reservoirs with oil and gas supplied by shale (Table 3). Conventional oil and gas reservoirs are mainly in the form of "source below reservoir", and are much shallower in burial depth than shale oil and gas reservoirs. Moreover, organic-rich shale in marine and continental facies mainly deposit in the relatively lower part of basins, while conventional oil and gas reservoirs are formed in the high-energy zones of the higher part of basins (Fig. 2), and shallower in burial depth, so under the same geothermal gradient, the conventional reservoirs are lower in temperature compared with shale gas layers. The downhole temperatures of the North American shale gas reservoirs are 80-100 °C and 110-130 °C^[25]; the temperatures of the shale gas reservoirs in the Wufeng Formation-Longmaxi Formation in the Sichuan Basin of China are 100-140 °C; while the temperatures of the structure gas reservoirs in the overlying Upper Carboniferous Huanglong Formation are 80-110 °C, much below the temperatures of the shale gas reservoirs supplying oil and gas for them.

On the one hand, the high-temperature and high-pressure characteristics of the shale oil and gas reservoirs squeeze out and consume a large amount of free water and bound water inside the reservoirs, forming production intervals with high oil and gas saturation; on the other hand, these characteristics bring about great challenges to engineering, thus increase in cost, so in shale oil and gas reservoirs, the instruments must withstand higher forces, and the temperature and pressure deformation of the casing during completion is far more complicated than in conventional oil and gas reservoirs.

Retained in-situ, with short-distance or no migration, the fluid in shale oil and gas reservoir is uniform in nature and relatively simple in composition and does not contain H₂S. For example, the shale gas in the Wufeng Formation-Longmaxi Formation of the Sichuan Basin is mainly crude oil cracking gas with 95% to 99% of CH₄ and less than 5% of non-hydrocarbon gases such as CO₂ and N₂, and does not contain H₂S^[30]. The shale gas in North America is similar in gas composition, but different in CH₄ content due to low thermal evolution degree (Table 4). The oil of shale oil enrichment area is lighter (0.70-0.85 g/cm³) and high in gas-oil ratio, which makes it easy to flow and exploit.

However, all of the conventional gas reservoirs in carbonate rock of the Sichuan Basin contain H₂S as the carbonate rock is prone to TSR reaction with hydrocarbon to form H₂S. The H₂S content in the natural gas of the Huanglong Formation in the eastern Sichuan Basin is 0.12%-0.79%. In comparison, shale gas exploitation is safe and environmentally friendly with less corrosion of equipment, and the shale gas can be used directly after a small amount of processing, so shale gas exploitation can save costs.

Organic-rich shale is characterized by high oil and gas saturation and ultra-low water content. Table 5 shows that the shale gas reservoirs in the United States and the Sichuan Basin of China have a gas saturation of 65% to 88% and 60% to 74% respectively. And all of them with contain low water content of 12%-35%^[31]. According to statistics, tight sandstone reservoirs and sandstone oil and gas reservoirs have a gas saturation of 30% to 55% and 50% to 75% respectively^[32].

In the process of hydrocarbon generation and expulsion, organic-rich shale is the first reservoir saturated with oil and gas. Affected by overpressure, molecular force, and high temperature, the oil and gas will displace the free water and some bound water in the shale, resulting in relatively high gas saturation. In conventional reservoirs, oil and gas accumulate under the driving of buoyancy and capillary pressure difference, but these forces are limited in effective scope, and only the free water in the dominant migration path is driven away, and the oil and gas saturation is relatively low under the restriction of relative permeability.

Therefore, shale gas reservoirs often produce less water, while conventional oil and gas reservoirs often have water channeling and waterflooding, which cause troubles for oil and gas production.

The recoverable resources potential of shale oil and gas reservoir depends on the amount of hydrocarbons that have been generated and retained by organic-rich shale, and the amount of hydrocarbons that can be produced after shale reservoir stimulation^[33]. At present, techniques such as horizontal drilling and completion and staged volume fracturing are commonly used, which can realize the effective and scale development of shale oil and gas. In order to reduce the risk of exploration and development, it is necessary to identify the “sweet spot area” on the plane and the “sweet spot interval” on the section for the shale oil and gas in a large area and continuous distribution^[34,35,36]. Based on the exploration practice of the Wufeng Formation-Longmaxi Formation, we have some new understandings on shale gas enrichment regularities and high production.

The material basis of shale oil and gas is the shale zone with high organic matter abundance, which is dependent on the lithofacies and paleogeography, paleoproductivity and water environment at the time the shale deposited. Most marine organic-rich shale formations deposit in the semi-deep to deep shelf; due to the connection with the open sea and the influence of upwelling of ocean currents, plankton was prosperous and thick organic-rich shale deposited, forming the sedimentary center of the basin, which is the favorable area for shale oil and gas exploration. Continental organic-rich shale formations mainly deposit in the semi-deep to deep lake environment with weak hydrodynamic effect and low dissolved oxygen, where the sedimentary center is relatively consistent with the subsidence center, with massive organic-rich shale built, so is often the shale oil and gas enrichment center too.

The key to the success of shale gas exploitation in North America and China is the development of high-quality organic-rich shale. The TOC content of the “sweet spot interval” in shale gas fields discovered in North America is generally greater than 4%, mostly from 5% to 10%, and the TOC content of the lower member of the Marcellus shale, the gas-producing shale largest in area, is 10% to 20%^[24,25]. The Wufeng Formation-Longmaxi Formation shale in the Sichuan Basin of China has a TOC content of greater than 2% in general, and the high-quality shale interval has a TOC content of above 3.5% and gas content of 4-8 m³/t (Table 6). High TOC content is an important material basis for the formation of shale gas “sweet spot”.

The TOC content of shale is not only related to the amount of gas generated, but also positively related to the development degree of organic matter pores. The organic matter pores in shale provide the main space for the accumulation and storage of shale gas (Fig. 5). When the thermal evolution degree of shale organic matter is moderate, the TOC content and the porosity of organic matter are positively correlated. According to the petrophysical model used to characterize porosity, the contribution of each kind of pore to the total porosity was calculated^[37]. The organic matter pores in the “sweet spot intervals” of the shale gas of Wufeng Formation-Longmaxi Formation in the Sichuan Basin are up to 30% to 50%, providing abundant and effective reservoir space for shale gas enrichment.

The gas in organic-rich shale is mainly composed of free gas and adsorbed gas. The ratio of free gas to adsorbed gas is controlled by the current temperature and pressure of the shale gas reservoir, and the free gas content is positively correlated with the shale gas production. The development practice of shale gas reservoir in the Wufeng Formation-Longmaxi Formation of the Sichuan Basin shows that the higher the free gas content of the shale gas layer, the higher the daily production of a single well will be, the higher the EUR of a single well will be, and positive structures are conducive to the accumulation of shale gas and high production. The Fuling shale gas field is a broad anticline (Fig. 6) where the strata relief at the core is no more than 10°; the internal free gas content is 60% to 80%, and the average production of a single well is 32.3×10⁴ m³/d^[38]. In Changning shale gas field, the free gas content is 55%-65%, and the original average production of a single well test is 18.6×10⁴ m³/d. The difference in original production is proportional to the free gas content.

The porosity of shale is composed of matrix porosity and fracture porosity. The pores with large volume and small specific surface area in the reservoir are the main space storing free gas (Fig. 5), while the development of bigger pores and microfractures is the key for the high production of shale gas. The high production of shale gas layer is the external manifestation of high porosity and high permeability. From the initial production distribution of shale gas wells at different depths in the Changning and Fuling blocks (Fig. 7)^[39], it is found that although the two gas fields have different tectonic backgrounds, the intervals of high-production are located in the Wufeng Formation and the lower part of the Longmaxi Formation where the porosity is higher and microfractures are more developed. The high-quality shale gas reservoirs in wells JY1, JY2, JY3 and JY4 of Fuling shale gas field have a porosity of 4.65%-6.20% and permeability of (0.13-1.27)×10^-3 μm²; moreover, the reticular fractures formed by slippage effect of the reverse faults on both sides of the gas reservoir greatly increase the storage space and flow efficiency of the free gas^[40]. In the Changning shale gas field, the main production interval is comparable to that of the Fuling shale gas field in porosity, but is slightly lower in permeability (the average value is 2 orders of magnitude lower than that of the Fuling gas field); lamellation seams and a small number of structural fractures are the main seepage channels, and the overall production is significantly lower than that of the Fuling gas field (Fig. 7). According to the petrophysical model of shale in the Wufeng Formation-Longmaxi Formation established before^[38], the porosity of the shale gas layers of the Fuling and Changning gas fields is about 4.3%-5.4%, and microfractures are generally developed. Combined with the statistics, it is confirmed that the porosity of the high-production interval is above 4.0%.

According to the accumulation mechanism, heavy oil, oil sands, tight oil and gas, and gas hydrates are classified as conventional oil and gas; unconventional oil and gas include shale oil, shale gas, and coalbed methane. The formation and accumulation mechanism of unconventional oil and gas is further clarified in this paper, and it is pointed out that unconventional oil and gas reservoirs feature source-reservoir-in- one, continuous in-situ accumulation, and accumulation under the driving forces of overpressure and diffusion.

Unconventional oil and gas reservoirs are mainly formed in the low-energy oxygen-anaerobic environment. They are simple in lithology, and rich in organic matter and clay minerals. Their main reservoir space is nano-scale and complex pore structure. Organic matter pores constitute a major part of reservoir space in them. In addition, unconventional oil and gas reservoirs have oil and gas in a uniform distribution, high oil and gas saturation, low water content, and no obvious boundaries between oil, gas, and water.

The sedimentary environment controls high organic matter abundance zone, organic matter content controls oil and gas abundance; positive structure, high porosity, bedding (lamellation) and the development of fractures control the production of shale wells. The TOC content greater than 3.0%, porosity of more than 4.0%, and densely developed microfractures are important indicators for identifying “sweet spot” of shale gas.

Re-recognition of “the unconventional” in unconventional oil and gas further enriches the unconventional oil and gas geologic theory and will promote new progress in unconventional oil and gas exploration and development.

During the research process, this study has received support from CNPC and related enterprises and the National Science and Technology Major Project 2017ZX05035. In the process of writing this article, the authors received help from Guan Quanzhong from China Petroleum University (Beijing), and Sun Shasha, Zhang Surong, Jiang Shan, Guo Wen, Shi Zhensheng, Ma Chao, Qiu Zhen and Yu Rongze from Research institute of China Petroleum Exploration and Development, and Zhang Hualing from University of Houston. Thanks for their help here.