The Williston Basin spanning the U.S. and Canada has an area of approximately 340× 10³ km². The Bakken petroleum system comprises of the Upper Devonian Three Forks Formation and the Upper Devonian-Lower Mississippian Bakken Formation, and it is one of the key strata for the exploration and development of tight oil in the U.S. The Bakken Formation can be divided into 4 members from top to bottom: the upper Bakken Member, the middle Bakken Member, the lower Bakken Member, and the Pronghorn Member. Among them, the organic-rich shales in the upper and lower Bakken members, deposited in deep, anoxic water at relatively high sea level, are major source rock formations. The middle Bakken Member and Pronghorn members, composed of mainly sandstone, siltstone, and dolomite, were deposited in shallow subtidal zones and open sea environments, and are major reservoirs^[33]. The tight oil exploration and development targets of the Williston Basin are the middle Bakken Member and the Three Forks Formation. In the Sanish, Parshall and Billings Nose oil fields in North Dakota, the Pronghorn Member is the main oil production stratum. However, this member doesn’ t exist or hasn’ t been developed in other areas of the basin. Through a study of the whole Williston Basin, Skinner et al.^[34] and Rebecca L J^[35]found that the Pronghorn Member is wide spread across the basin, with a maximum thickness of more than 15 m, hydrocarbons generated from organic-rich shales in the lower Bakken Member can migrate to and collect in large scale in the adjacent Pronghorn Member. Reservoirs in this member have an average porosity of 5-6%, average permeability of (0.4-0.6)× 10^-3 μ m², and average oil saturation of 31-32%.

The Whiting Petroleum Corporation (WLL), based in the Wilston Basin, conducted detailed research and evaluation on the Bakken Formation and discovered a new oil-bearing series in the Pronghorn Member. The first two exploration wells had initial daily production of 286.3 t/d and 267.5 t/d, respectively. By the end of 2015, WLL had drilled 80 wells, with promising exploration prospects.

The economic efficiency of sweet spot resources is what the oil companies concern most. At present, the economic evaluation of the tight oil sweet spots in North America lays stress on the size of resources, reservoir quality, and production capacity^{[4, 23, 31, 32, 33, 34, 35]}. Based on the data of Shell, Marathon Oil Corporation^[4] and China’ s national standards for geological evaluation of the tight oil^[30], the qualitative parameters applied in the assessment of tight oil sweet spots are as follows: the source rock is greater than 2% in total organic carbon (TOC), in the oil generating window-gas condensate window, with ideal value R_o of greater than 0.9%; the effective reservoir should be greater than 15 m thick, ideally greater than 50 m thick, greater than 70% in storage ratio, and greater than 8% in average porosity; oil saturation of greater than 40%, ideally greater than 60%; the oil should have a surface density of less than 0.85× 10³ kg/m³, and gas-oil ratio (GOR) of greater than 100; the reservoir should have a Poisson’ s ratio of less than 0.3, elastic modulus of greater than 20× 10³ MPa, and natural fractures; the burial depth should be less than 4 100 m and ideally less than 3 500 m. The pressure system should be normal or over-pressured.

From the histogram of statistical results on the cost prices of tight oil in different regions (Fig. 1), it can be seen there are great differences in the economic efficiency of tight oil in different areas of North America, even in the same tight oil zone, due to the combined effect of heterogeneity and other factors, and there are also big differences in the cost price of different series of strata. Taking the tight oil of Eagle Ford for example, the cost price in Dewitt County, Texas, is only USD 168/t (USD 23/bbl), while that in Dimmit County, Texas, is the highest, reaching USD 423/t (USD 58/bbl), 2.5 times the former^{[26, 32, 36]}. Therefore, when oil companies deploy exploration and production, they often prefer to develop zones with lower cost price. According to the statistical results of the IHS Cambridge Energy Research Institute^[28], of the four key tight oil zones in North America at present, under the premise of guaranteeing a 10% internal rate of return (IRR), the Wolfcamp tight oil in the Permian Basin, Eagle Ford tight oil, Bakken tight oil and Niobrara tight oil have the minimum cost of USD 160/t (USD 22/bbl), USD 182/t (USD 25/bbl), USD 277-292/t (USD 38-40/bbl) respectively. Therefore, IHS CERA forecasts the investment in the Permian Basin in 2021 will reach USD 40× 10⁹, accounting for 35% of North America’ s onshore oil and gas exploration investment.

Fig. 1.

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	Fig. 1. Histogram of the cost prices of tight oil in different regions of the U.S. (Modified according to Reference [32]. TX— Texas; NM— North Montana).

With the development of tight oil, major changes have taken place in the development thinking of oil companies, shifting from the eagerness to expand leases in the past to maintaining or reducing leases^{[36, 37, 38, 39, 40, 41, 42]}. In existing leases, one measure is to strengthen the repeated stimulation of existing production wells, with repeated fracturing and three-dimensional (3D) fracturing becoming important technological innovations. Taking the Eagle Ford tight oil for instance, Carrizo Oil & Gas, Inc. further reduced the effective cluster distance from 110 m to 83, 67, and 55 m using the above two innovative technologies, and thus increasing the corresponding producing degree of recoverable tight oil reserves by 20%, 45% and 80% respectively^[37]. On the other hand, the stimulation is further focusing on development strata and fracturing targets. In the instance of Wolfcamp tight oil in the Permian Basin, before 2015, major oil companies did all-round exploration from the upper Spraberry Formation to the Wolfcamp Formation, with the drilling depth gradually increasing and fracturing treatment scale continuously expanding. A large number of practical results have confirmed that dolomitic sandstone, marl, and lithic sandstone in Wolfcamp A and B have relatively good development potentials. Therefore, more and more developers are focusing on this interval, where several operators have obtained high-yield commercial oil flows, and the initial crude oil production was generally greater than 63 m³/d (400 bbl/d) and the highest was 254 m³/d (1 600 bbl/d). Currently, the length of horizontal section drilled for Wolfcamp tight oil is generally greater than 2 500 m, and the cumulative production in the first half of 2017 exceeded 15× 10⁶ t^[38].

Technology leads the development, so cost reduction and efficiency enhancement have become the core ideas for the tight oil industry in North America^{[36, 37, 38, 39, 40, 41, 42]}. The successful application of open hole completion, factory-like fracturing and standard development model has greatly cut the drilling and completion cycle and cost. According to statistics, facilitated by these techniques, the single well costs of Bakken tight oil in the Oasis Petroleum Inc. (OAS) decreased from USD 1 0.6 × 10⁶ to USD 7.4 × 10⁶, and the drilling cycle decreased from 24 d to 16 d, and total operating costs reduced by 35%^[39]. For the Continental Resources, Inc. (CLR), the drilling cost of Bakken tight oil well reduced by 30%, while the corresponding estimated ultimate recovery (EUR) per well increased by 45% in 2016 compared with that in 2014^[40]. The Devon Energy, adopting the staggered arrangement in Eagle Ford tight oil fracturing, increased drilling efficiency by 50% and reduced completion cost by 25% through such 3D fracturing^[41]. By employing open hole completion in Eagle Ford tight oil development, the Carrizo Oil & Gas, Inc. reduced the drilling costs and completion costs per well by 21% and 27%, respectively^[37], and shortened the drilling cycle per well to only 7.79 d at most (well depth 2 400 m; horizontal section 2 400 m). The costs per well in Niobrara tight oil development were reduced from USD 6.7× 10⁶ in 2010 to about USD 300× 10⁴ in 2015^[37] (Fig. 2).

Fig. 2.

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	Fig. 2. Variation of costs per Niobrara tight oil well [35].

The hedging of commodities by oil and gas production enterprises refers to the practice that in order to avoid the risk of commodity price fluctuations, a company designates one or more hedging instruments so that the fair value or cash flow of the hedging instrument changes, which is expected to offset the hedged items or part of changes in fair value or cash flow. The strategy is usually to buy (or sell) a futures contract that is the same as the spot market but with the opposite direction in order to compensate for the real price risk of the change in the spot market by selling (or buying) futures contracts at some time in the future. Many independent oil and gas companies had adopted their derivatives hedging contracts to lock in crude oil and natural gas prices in different degrees as early as in the first two or three years before the sharp drop in oil prices in 2014, and they also locked in the selling prices, to avoid the risk of falling prices and the loss caused by falling oil prices. For example, the Pioneer Natural Resources Company’ s crude oil has a hedge ratio up to 86% and the price range is between USD 584/t and USD 657/t (USD 80 to 90/bbl). The adoption of oil and gas hedging measures ensures that the company can continue to earn profit at the oil price of USD 292/t (USD 40/bbl). In the third quarter of 2014, its gains and losses on commodity derivatives were USD 341× 10⁶, net profit was USD 374× 10⁶; in the fourth quarter of the same year, its gains and losses on commodity derivatives rose to USD 696× 10⁶ and the net profit was USD 431× 10^6[25], evading the risk of falling prices.

Besides technical promotion of geological evaluation and exploitation of tight oil, strengthening the comprehensive development and utilization of different resources to enhance the economic benefits is also an effective measurement. During the development of Bakken tight oil, most oil companies, i.e., ConocoPhillips Oil Company recovered the natural gas and heavy hydrocarbon resources that were associated with tight oil, and re-injected the recovered gas back to oil formation to improve the oil recovery^[43]. There are two advantages: (1) it can reduce the expel of gas to avoid the waste of resources, and reduce the amount of injected water during the development of tight oil, which is friendly to the environment; and (2) the economic benefit of the project can be improved through the recovery of heavy hydrocarbon resources. HESS Oil Company has achieved zero emissions of the production liquid and gas after the optimization of the multi-round treatment of gathering and transportation facilities. ConocoPhillips Oil Company has optimized for separators, and the annual benefit of single well group can exceed 36 million us dollars^[43].

Compared with the marine tight oil in Bakken, Eagle Ford, and Wolfcamp in North America, continental tight oil in China is much more complex in geological background and tectonic setting. Deposited in various types of basins with rapid changes in lacustrine basin deposition systems and experiencing multiple stages of reconstructions and transformations, the continental tight oil reservoirs in China are unique in occurrence and distribution features (Tables 1-3).

Table 1

Table 1

Table 1 Comparison of the geologic features and formation conditions of tight oil in North America and China. Region Sedimentary basin Source rock characteristics Reservoir characteristics Tectonic setting Sedimentary setting Distribution range/km2 Lithology TOC Ro/% Main lithologies Petrophysics Reservoir distribution The U.S. Tectonically stable Mainly marine deposits (10-70)× 103 Dominated by marine shale 2-20%, higher TOC values 0.6-1.7, higher maturity Carbonate, sandstone, diamictite Relatively good petrophysical properties, high porosity, good connectivity Stable distribution and good continuity China Strong late tectonic activity Mainly terrestrial deposits Hundreds to tens of thousands of square kilometers Lacustrine mud shale 0.4-16.0%, substantial variation of TOC values 0.4-1.4, lower maturity Carbonate, tight sandstone, diamictite, sedimentary tuff Poor petrophysical properties, low porosity, strong heterogeneity, thin oil layer Strong heterogeneity Region Reservoir characteristics Fluid characteristics Economy Concentrated section thickness/m Porosity/% Formation pressure Pressure coefficient Crude density/ (g• cm-3) GOR Cumulative production per well/104t Burial depth and reserve abundance The U.S. 5-20 5-13 Mainly overpressure 1.35-1.78 0.75-0.85 Tens 2-10 Moderate burial depth, higher abundance China 10-80 3-12 Low pressure coefficient 0.70-1.80 0.75-0.92 Hundreds to thousands 1-4 Greatly varied burial depth and abundance

Table 1 Comparison of the geologic features and formation conditions of tight oil in North America and China.

Table 2

Table 2

Table 2 Statistics on parameters of tight oil formation in typical tight oil basins abroad. Basin and strata Tectonic setting Sedimentary setting Basin area/ 103 km2 Favorable area/103 km2 Stratigraphic age Lithology Source rock thickness/m Burial depth/m Bakken Formation, the Williston Basin, the U.S. and Canada Continental dome- craton Littoral- neritic facies 520.0 70 Upper Devonian - Lower Carboniferous Upper and lower sections are dominated by shale, and the middle section is dominated by dolomitic sandstone and dolomite 2-18 2 100- 3 300 Eagle Ford Formation, the West Gulf Basin, the U.S. Craton Carbonate platform 250.0 20 Upper Cretaceous Limestone and shale in upper and lower sections 20-60 1 000- 3 400 Wolfcamp Formation, the Permian Basin, the U.S. Craton Carbonate platform 223.6 Upper Permian Upper limestone, lower shale 20-150 2 250- 3 300 Duvernay Formation, the Alberta Basin, Canada Continental margin Marine facies 130.0 24 Upper Devonian Siliceous shale and limestone 35-60 2 800- 3 600 Vaca Muerta Formation, the Neuquen Basin, Argentina Back-arc rift basin Marine facies 140.0 15 Upper Jurassic - Lower Cretaceous Marlite and clastic rock 45-150 1 380- 1 950 Basin and strata Mineral composition TOC/ % Ro/% Organic type Porosity/ % Permeability/ 10-3 μ m2 Crude density/(g• cm-3) Oil saturation/% GOR Pressure gradient/ (MPa· km-1) Bakken Formation, the Williston Basin, the U.S. and Canada Quartz and carbonate content greater than 90% 5.00- 20.00 0.6-1.0 II 5.0-12.0 0.5-10.0 0.79-0.82 68 50- 375 11.30-15.82 Eagle Ford Formation, the West Gulf Basin, the U.S. Clay content 8%-20% 0.70- 9.20 0.7-1.7 II 6.0-14.0 0.4-13.0 0.79 55-85 90- 850 14.70 Wolfcamp Formation, the Permian Basin, the U.S. Clay content 10% 2.24- 7.24 0.6-1.5 I-II 8.0-12.0 0.4-10.0 0.78 65-80 356 15.82 Duvernay Formation, the Alberta Basin, Canada Clay content 26%; quartz and carbonate content 67% 2.00- 7.50 0.5-2.0 II 6.0-7.5 0.2-8.0 0.78 70 150- 650 15.80-20.30 Vaca Muerta Formation, the Neuquen Basin, Argentina Clay content 15%- 20%; quartz and carbonate content 70%-90% 3.00- 5.00 0.7-1.3 I-II 4.0-14.0 0.5-2.0 0.80-0.83 55-85 89- 1 068 15.80-22.60

Table 2 Statistics on parameters of tight oil formation in typical tight oil basins abroad.

Table 3

Table 3

Table 3 Parameters of main characteristics of continental tight oil in China. Basin Strata Hydrocarbon source Reservoir property Fluidity Resource size Economy TOC/ % Ro/% Thickness/ m Lithology Thickness/ m Poro- sity/ % Brittle mineral content/ % Crude density/ (g• cm-3) Crude viscosity/ (mPa• s) GOR Oil saturation/% Pressure coefficient Cumulative production per well/ 104 t Abundance/ (104 t• km-2) Resources/ 108 t Burial depth/ m Surface conditions The Chang 7 Member, Ordos Basin 3.0- 16.0 0.7- 1.2 10- 100 Silty fine sandstone 10- 40 7-13 > 80 0.80- 0.86 5-10 95- 125 60-80 0.70- 0.80 2-4 43-72 3.5-4.0 1 000- 2 600 Loess tableland Cretaceous, Songliao Basin 0.9- 3.8 0.5- 1.3 80- 450 Silty fine sandstone 5- 30 2-15 > 50 0.78- 0.87 12-20 10- 30 50 1.20- 1.58 2-4 9-20 2.0-3.0 1 600- 2 500 Plain, swamp Permian, Junggar Basin 4.0- 16.0 0.6- 1.1 10- 35 Dolomatic silty sandstone, argillaceous dolomite 20- 50 6-20 > 80 0.88- 0.92 55-370 80-90 1.10- 1.30 1-4 5-60 1.5-2.0 2 300- 4 500 Gobi, grassland Jurassic, Sichuan Basin 1.0- 2.4 0.5- 1.4 100- 150 Silty fine sandstone, shelly limestone 10- 60 0.2-7.0 > 60 0.76- 0.87 40-50 1.23- 1.72 1-3 1.0-1.2 1 400- 3 200 Hills Shahejie Formation & Kong- dian For- mation, Bohai Bay Basin 1.5- 3.5 0.5- 1.3 100- 300 Silty fine sandstone, carbonate 100- 200 5-10 > 50 0.67- 0.86 8-22 50 1.24- 1.80 1-4 2.0-2.5 1 800- 4 300 Plain Tertiary, Qaidam Basin 0.4- 1.2 0.7- 0.8 200- 1 200 Marl, algal limestone, silty sandstone 100- 150 5-8 > 70 0.85- 0.88 60 1.30- 1.40 2-3 0.3-0.5 2 500- 4 000 Hilly area Permian, Santanghu Basin 1.0- 6.0 0.6- 1.1 30- 60 Limy dolomite, tuffite 10- 100 3-13 > 80 0.85- 0.90 40-50 1.00- 1.20 2-3 0.09-0.12 1 800- 4 000 Gobi

Table 3 Parameters of main characteristics of continental tight oil in China.

(1) Sedimentary basins. The tight oil in North America is mainly distributed in several tectonically stable marine craton basins, such as the Williston, Permian, and West Gulf basins, with a total area of (10-70) × 10³ km²; while the tight oil of continental basins in China is mainly distributed in 7 terrestrial basins with complex tectonic sedimentary background, including rifting, depression, and foreland basins, and largely in the Mesozoic and Cenozoic strata. Tens to thousands of square kilometers, these basins feature multiple sources, multiple oil-generation sags, and limited distribution of source rock and reservoir.

(2) Source rock features. The North American marine source rocks are tens of meters thick, generally with TOC values of 2-20%, and R_o values of 0.6-1.7%. China’ s terrestrial source rocks deposited in freshwater, brackish water and salty water environment, generally have thickness of tens to hundreds of meters, TOC values of 0.4-16.0% and R_o values of 0.4-1.4%.

(3) Reservoir characteristics. The tight oil reservoirs in North America are mainly carbonate, sandstone, and diamictite in lithology, dominated by carbonate, followed by sandstone. The reservoirs are generally tens of meters thick, 5-13% in porosity, and no more than 1.0× 10^-3μ m² in permeability. There are four types of tight reservoirs in China’ s terrestrial basins, namely carbonate, sandstone, tuffite and diamictite (Table 4), which are dominated by sandstone. As the reservoirs are complex in lithology, and different greatly in sedimentary environment, diagenetic evolution and structural reformation extent, they feature large lateral variation and strong heterogeneity, and thickness of tens to hundreds of meters. The tight sandstone comes in band and sheet, is thin in single body. The tight carbonate rock is thicker, with the porosity of 3-12% and the permeability of less than 0.1× 10^-3μ m². Comparison of the content of brittle minerals (siliceous and carbonate) shows that the tight reservoirs between China and America have little difference (Table 3), but they do have some difference in fracability. After repeated sorting and washing in the long distance of transportation, the marine clastic rock has a higher quartz content and better fracability, while the clastic rock in the terrestrial basin, close to the provenance, has higher feldspar and debris contents and poorer fracability.

Table 4

Table 4

Table 4 Four types of tight reservoirs and six tight oil accumulation patterns. Reservoir type Accumulation pattern Reservoir characteristics Cases Sandstone Thick sandstone pore type Large reservoir thickness, high oil saturation, good crude oil properties, low pressure Yanchang Formation, the Ordos Basin Thin sandstone pore type Thin, strongly water sensitive, low oil saturated, universally water-bearing Fuyu Formation, the Songliao Basin; Neogene System, the Qaidam Basin; Paleogene System in Nanpi, Dagang, the Bohai Bay Basin Diamictite Thick layer dolomitic sandstone type Thicker reservoir, small horizontal stress difference, and heavier oil The Permian Lucaogou Formation in Jimsar Sag, the Junggar Basin Tuffite Tuff pore type Thin monolayer, high porosity, strong pressure sensitivity, poor crude oil properties Tiaohu Formation, the Santanghu Basin Carbonate Thin dolomite pore type Thin monolayer, high clay content, high water saturation, poor oil quality Sha 1 and Sha 4 members of the Bohai Bay Basin, Paleogene dolomite in the Qaidam Basin Marl type, limestone fracture type Substantial thickness variation, fracture development, high water saturation Marl of Shulu Sag, Huabei Oilfield, Jurassic Daanzhai Member of the Sichuan Basin

Table 4 Four types of tight reservoirs and six tight oil accumulation patterns.

(4) Fluid characteristics. The tight oil in North America is mostly lighter condensate oil, with a crude oil density of 0.75-0.85 g/cm³, a pressure coefficient of 1.35-1.78, and largely overpressure, while China’ s continental basins experienced strong late tectonic movements, that might have certain influences on the preservation conditions, so the tight oil reservoirs in China changes substantially in pressure coefficient, with the pressure coefficients of 0.7-1.8, spanning overpressure to low pressure, and vary widely in formation energy and crude oil quality, with crude oil density of 0.75-0.92 g/cm³.

(5) Economy. The marine strata in North America are generally less than 3 700 m in burial depth and more than 500× 10³ t/km² in reserves abundance; while the continental tight oil reservoirs in China have a larger burial depth of 1 000-4 500 m, poorer economic efficiency and smaller recoverable scale, big difference in burial depth, and the reserve abundance of (50-720) × 10³ t/km².

The difference in regional geological background is the root cause of the difference between continental tight oil in China and marine tight oil in North America. The core elements lie in two aspects: stable tectonic setting and continuous deposition conditions, and thermal evolution of source rock.

2.2.1. Tectonic stability and geological basis for large-scale tight oil distribution

Zou et al.^[4] summarized that six conditions are needed for the formation of large-scale continuous tight oil reservoirs. Among them, large-scale wide gentle structural background and large-scale continuous deposited sedimentary setting are the main controlling factors. The stable wide gentle large- scale structural background and the smooth structure of the original deposition are favorable for the large-scale distribution of high-quality source rocks and tight reservoirs, and better regional sealing conditions, giving rise to extensive distribution of tight oil resources in the same structural background. This is also the primary reason for the difference in the accumulation of tight oil between continental tight oil in China and marine tight oil in North America. The global oil exploration and development hotspot, the Upper Devonian-Lower Carboniferous Bakken tight oil in the Williston Basin can be taken as an example. It was deposited in a large craton sedimentary basin with an area over 340× 10³km², spanning the U.S. and Canada, including North Dakota, Montana and South Dakota of the U.S. as well as Manitoba and Saskatchewan in South-Central Canada^{[33, 34, 35]}. In the Late Devonian to Mississippian (early Carboniferous) period, the Williston Basin was located in an expansive continental shelf of the western margin of the North American continent with active settlement. The structural features of the Mississippian bottom are shown in Fig. 3, which reveals the basin was semicircular, with three distinct anticline structures: the Nesson anticline, the Billings anticline and the Cedar Greek anticline, with successive development characteristics, which ensured the stable distribution of Paleozoic sediments^{[44, 45]}, slow changes in sedimentary facies, and stable development of source rocks and reservoir in large area are the key factors for this reservoir. Two sets of shales develop in the upper and lower Bakken formations, which are distributed in the whole basin. The lower Bakken shale, for example, has the thickness of generally 5-12 m. The main reservoir of the Bakken Formation is made up of tight dolomitic siltstone formed in shallow coastal environment with a single layer thickness of 10-15 m, accumulative thickness of over 55 m, and distribution area of more than 70× 10³ km². The widely distributed tight reservoirs in close contact with the source rocks ensure the formation of continuously distributed Bakken tight oil play^{[47, 48]}. Both the Wolfcamp tight oil in the Permian Basin and the Eagle Ford tight oil in the West Gulf Basin present characteristics similar with that of the Bakken tight oil. The stable development background of the large craton basins laid a solid foundation for the large-development of marine tight oil source rocks and reservoirs.

Fig. 3.

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	Fig. 3. Location of the Williston Basin (Modified according to Reference [44]).

Compared with marine tight oil, the formation background of continental tight oil in China is more complicated. Taking the tight oil of the Chang 7 Member, the Yanchang Formation in the Ordos Basin with the best in exploration and development currently as an example, it was formed in a large-scale craton basin that was superimposed by a Paleozoic platform, a platform margin depression, and a Mesozoic-Cenozoic intraplate depression. The basin with a total area about 250× 10³km², is quite simple in internal structure, and strata are flat and completely developed, with a dip angle of less than 1° and without strong deformation. Since the Mesozoic Era, the basin has been developing stably for a long time, with little structural changes in the later period. During the deposition of the Yanchang Formation in the Middle-Late Triassic period, the Ordos Basin experienced the entire process of formation, development, heyday, recession, and extinction of lake basin. The Chang 7 Member just deposited during the peak period of the basin and thus has a wide distribution area of over 100× 10³km². Lithologically, it is a set of organic-rich dark grey oil shale and dark mudstone with thin interlayer of silty fine sandstone. The oil and gas of Yanchang Formation is located largely in the south-central slope of northern Shaanxi, accounting for more than 80% of the reserves and production^{[49, 50, 51]}. However, the terrestrial sedimentary background resulted in frequent sand-mud interbeds and rapid transition of sedimentary facies. The individual sand bodies are poor in lateral stability and small in vertical thickness. For example, the sand bodies in the Chang 7 Member of the Ordos Basin are generally 3-15 m thick each and 10-30 m thick cumulatively, and limited in lateral distribution. Fuyang oil layer in the Songliao Basin exhibits thin interbedded sedimentary characteristics, with single sand body thickness of merely 3-5 m and poor lateral continuity. The Lucaogou Formation of the Jimsar Sag in the Junggar Basin is diamictite, where the favorable dolomitic sandstone reservoirs are 2-10 m each and limited in lateral distribution; at the same time, the smaller lacustrine basin area leads to the difference in sedimentary water environment and the supply of terrigenous clastics, which then leads to rapid changes in the properties of tight oil reservoirs in the longitudinal direction and strong heterogeneity. During the deposition period of the Lucaogou Formation in the Jimsar Sag, Junggar Basin, the salinity of the water body was generally high. With the evolution of the lacustrine basin and the change of the provenance system, the temperature, depth and salinity of the water body frequently changed, resulting in the drastic changes of the contents of carbonate, silicate and clay minerals (Fig. 4) and thus rapid changes in reservoir properties, for example, the porosity of reservoir is 20% maximally, but then tumbles to 4% within a distance of less than 20 cm. During the deposition period of the Chang 7 Member in the Ordos Basin, although the salinity of the water was slightly lower than that of the Lucaogou Formation, due to the differences in the composition of original substances and diagenesis, the minerals also changed rapidly in the longitudinal direction, the reservoir has a maximum quartz content of up to 58% and a minimum content of only about 15%, maximum carbonate mineral content of 45% and a minimum carbonate content of only 2% (Fig. 4), speaking for strong longitudinal heterogeneity.

Fig. 4.

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	Fig. 4. Mineral composition of continental tight oil reservoirs identified by X-ray diffraction.

2.2.2. Differences in thermal evolution of source rocks result in differences in scale and mobility of tight oil

The geochemistry features of China’ s terrestrial tight oilsource rock and North American marine source rock is similar (Fig. 5), but North American marine source rocks generally have higher thermal evolution degree, consequently, the tight oil reservoirs in North America have abnormal overpressure, higher GOR, lighter oil, better oil mobility, and better brittleness and fracability, which are also the root causes of the better economic efficiency of tight oil in North America. For example, the Eagle Ford marl located in the southern and local central parts of Texas, has a distribution area of about 44.5× 10³ km² and formation thickness rising from 15.2 m in the northeast to more than 91.4 m in the southwest. The reservoirs are composed of marl with a burial depth of 609.6 to 4572.0 m. The Eagle Ford formation tilts southwards, and has spanned oil generation window, oil-gas transition window, and gas generation window. In shallow burial zone, it mainly produces black oil; in deeper burial zone, due to high temperature and pressure, it mainly produces natural gas^[46]. The production data of 1 306 high-yield wells in the tight reservoirs shows that when the R_o value is greater than 0.9%, the probability of high oil production of tight oil increases significantly from 9-20% to over 42%, the GOR is generally greater than 800, and the crude oil density is less than 0.788 g/cm³.

Fig. 5.

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	Fig. 5. Comparison of Ro and TOC values between marine source rocks in North America and continental tight oil source rocks in China.

China’ s continental tight oil source rocks have lower thermal evolution degree. The source rock in the Chang 7 Member of the Ordos Basin has R_o values of 0.8-1.2% (46 wells), and peak pyrolysis temperatures ranging from 440 ° C to 460° C^{[51, 52]}. The source rock of the Lucaogou Formation in the Jimsar Sag, Junggar Basin has R_o values of 0.52-1.03% and peak pyrolysis temperatures from 440 to 455 ° C. Among the samples, 53% percent have R_o value of less than 0.80%, 47 percent have R_o value greater than 0.80%, all in low mature-mature evolution stage^{[53, 54]}. On the one hand, this results in low GOR of continental tight oil, higher density and viscosity of oil, insufficient formation energy and poor mobility of tight oil; on the other hand, it leads to poor thermal stability of continental tight oil, and high content of plastic minerals, which affects the fracability of the reservoir. In the Jimsar sag of the Junggar Basin, the tight oil in the Lucaogou Formation is relatively heavy, with a surface density of 0.888-0.918 g/cm³, viscosity at 50 ° C of 73-300 mPa• s, an average wax content of 9.04%, and average freezing point of 13.49 ° C, representing medium-heavy crude oil, which determines poor mobility of oil in the Lucaogou Formation^{[53, 54]}. In the Ordos Basin, the crude oil produced from Chang 6 Member has a density of 0.718-0.786 g/cm³ and viscosity of 0.92-1.14 mPa• s, and that from Chang 7 has a density of 0.717-0.760 g/cm³ and viscosity of 0.89-1.21 mPa• s. Despite the low-density and low-viscosity of crude oil^[13], the formation energy is insufficient, with pressure coefficient of mainly 0.7-1.0. Moreover, the clay minerals in the reservoir have higher proportions of mixed-layer illite/smectite, which affects the initiation and distribution of artificial fractures in reservoir fracturing, resulting in rapid decline in production. However, it should be noted that during the drilling process, shale in the Chang 7 Member generally presents high abnormal gas logging shows, and intermittent bubbles can be seen during the in-situ soaking test, showing it has gas-bearing properties to some extent. The results of desorption gas test on sealed cores show that the average gas content of shale after a single desorption of the whole core is 1.2-1.5 m³/t. The higher gas content can increase GOR of the shale layer, and to a certain extent, improve the mobility and recoverability of the tight oil of the Chang 7 Member^[52].

Therefore, compared with the typical tight oil plays in North America, the continental tight oil in China features more complex geological characteristics because of the structural background, continuous deposition conditions and thermal evolution of the source rocks. Although there develop high-quality source rocks, the stability and continuity of the reservoirs are poorer, the oil mobility is poorer, so the overall scale and profitability of the resources are not as good as that of North America’ s tight marine oil resources, making it difficult to fulfill profitable exploration and development.

Accurate and efficient evaluation of resource potential is the key to the successful exploration and development of unconventional resources. China has rich tight oil resources, but the tight oil abundance varies substantially. The classified evaluation of tight oil resources should be carried out, to evaluate the potential of tight oil resources and the degree of resource enrichment objectively, which will provide a basis for the selection of sweet spots. In the fourth round of resources evaluation recently completed, consulting current situation of tight oil exploration in key plays of North America, and based on the actual geological conditions of China’ s continental tight oil, CNPC adopted the 3P resources evaluation scheme. In the scheme, the reservoir of the Class I resource has a porosity of greater than 8%, permeability of more than 0.08× 10^-3 μ m², and throat radius of over 200 nm, saturation of movable fluid of higher than 50% and high production, so it is the focus in recent exploration. That of Class II resource has a porosity of 5-8%, permeability of (0.03-0.08)× 10^-3 μ m², throat radius of 100-200 nm, the saturation of movable fluid of 20-50%, lower production and poor economic efficiency, which can only have exploration potential when technological breakthroughs emerge. That of Class III resource has a porosity of lower than 5%, permeability of less than 0.03× 10³μ m², throat radius of less than 100 nm, saturation of movable fluid of lower than 20%, and low production, which is not prospective for exploration and development in the near- or mid-term.

Tight oil has the characteristics of source area controlling oil area, near-source enrichment, and high production in sweet spots. The practice of large-scale profitable development of tight oil in North America has proved that high-return sweet spots are favorable targets for tight oil exploration and development and the key determining the profitability of tight oil exploration and development. Attention should be paid to the following four aspects.

First, the sufficiency and effectiveness of hydrocarbon supply by the source kitchen is the basis for the large-scale accumulation of tight oil; the source rock quality controls the planar distribution range of the sweet spot; the terrestrial source rock is highly heterogeneous, with TOC showing cyclic pattern in the vertical direction and shift on the plane. The study on the sedimentary environment and organic matter enrichment mechanism of organic-rich shale should be strengthened to sort out the intervals and areas with high TOC; the control of thermal evolution on tight oil enrichment and production should be studied; the type, abundance and maturity of organic matter should be identified through geochemical analysis of cores and cuttings to predict hydrocarbon generation and fluid properties.

Second, since reservoir space and movable fluid are guarantees for the formation of tight oil sweet spots, tight reservoirs could have strong heterogeneity and come in many types, it is necessary to develop multi-parameter digital rock evaluation techniques for complex reservoirs by means of various advanced technologies, the connectivity of micro-/nano-scaled pore throat systems should be studied by advanced experimental analysis technologies, and effective characterization techniques for reservoir structures and brittle mineral evaluation techniques should be developed to evaluate the heterogeneity of tight reservoirs and to define the pore throat structure, reservoir capacity and production performance of different types of reservoirs, and geological model should be established based on the measurements.

Third, micro-texture background and natural fractures have an important influence on the enrichment and high production of tight oil. Given that the sweet spots mostly develop in the local microstructures under a broad and gentle background, the micro-structural morphology and the development and evolution patterns of favorable plays should be studied. In-situ CT imaging should be employed to observe the fracture growth with the increase of pressure to find out the mechanism and main control factors of fracture development, establish a 3D dynamic fracture development model, and visually display the fracture development characteristics of tight reservoirs, and thus predict and evaluate the fracture development intervals.

Fourth, economic evaluation for the sweet spots is the key to the effectiveness of tight oil exploration and development. In light of this, main controlling factors, the enrichment and high yield patterns and economic evaluation of sweet spots should be carried out to pick out high profit areas. The sweet spot interval is the essence for horizontal well design, precise fracturing, and profitable development, so the study on the formation conditions and distribution rules of the sweet spot intervals should be strengthened to sort out sweet spot intervals accurately.

The practice of tight oil exploration in North America has proved that the most difficult problem in the development of tight oil is how to increase well production and enhance oil recovery. The continental tight oil reservoirs in China have small scale and low production in general. For example, the tight oil reservoir in the Chang 7 Member of the Ordos Basin features strong heterogeneity, diverse types and rapid planar change of lithology, which makes the horizontal well deployment quite challenging; as the single sand layer is thin, poor in transverse continuity and unstable in distribution, the horizontal wells often end in low drilling rate of reservoir. The break-even point (BEP) of Class I well is USD 365/t (USD 50/bbl), while the development cost of tight oil sweet spots in North America range between USD 146/t and USD 365/t (USD 20-50/bbl), on average USD 241/t (USD 33/b). It is suggested that technical research should be conducted in the following two aspects: (1) to innovate and develop tight oil volume fracturing technology to reduce engineering cost; (2) to optimize horizontal well spacing, horizontal section length and fracture cluster number to maximize the coverage area of well network, optimize operation process, control the comprehensive cost of drilling and completion, etc., and maximize the producing degree of tight oil reserves. Since tight oil reservoirs in different areas of China have different geological conditions, different drilling and completion technologies should be adopted for them. In terms of drilling, Changqing Oilfield, based on the geological characteristics of the tight oil in the Chang 7 Member, has improved the deflecting efficiency by developing spherical centralizer, large-torque screw and adjusting the length of short drill collar, as a result, the average drilling cycle is shortened nearly to that of the conventional horizontal well. In line with the characteristics of the Fuyang oil layer of small scale and small thickness of the channel sands and low controlled reserves per well, Daqing Oilfield has adopted factory-like operation, which has greatly shortened the drilling cycle by optimizing the well structure, casing program and wellbore profile. In terms of fracturing, the difference in the process in different regions is mainly manifested in the completion mode. The Xinjiang Oilfield has mainly applied the multi-stage open hole fracturing system to the Lucaogou Formation, the Changqing Oilfield mainly adopts hydraulic sand blasting technology, the Jilin Oilfield adopts the switchable casing slide sleeve fracturing method, and the Tuha Oilfield has tested three kinds of completion methods using cementing sliding sleeve, fast drilling bridge plug, and open hole packer, and proved that the quick drilling bridge plug method has the best fracturing effect. Therefore, based on the geological characteristics of the concerned area, the development of suitable drilling and completion technology is the inevitable pass to improve the efficiency of tight oil exploration and development.

Marketization is the key to achieving major breakthroughs in tight oil and gas in the U.S. The U.S has a very high degree of marketization, and the open and competitive market environment plays a key role in tight oil development. In the light of this, it is suggested to learn the advanced management experience of foreign companies and the “ 5 + 1” cooperative development model of the Sulige Gas Field in China. National demonstration plots for the development of tight oil can be built by introducing the external market competition system, to promote the successful exploration and development and management experience in the whole country, solve the key technical problems, and thus further reduce costs and realize large-scale profitable development.