Definition of Shale Oil：
Shale oil refers to petroleum stored in organic-rich, nanoscale pore-bearing shale formations. It is the abbreviation for mature organic shale oil. Shale is both a source rock and a reservoir rock for oil. Shale oil exists in adsorbed and free forms and is generally light in weight and low in viscosity. It is primarily stored in nanoscale pore throats and fracture systems, distributed along or parallel to lamellae. Organic-rich shales generally accumulate over large, continuous areas in the center of a basin, are generally oil-bearing, and have a large resource size. Key factors in evaluating the "core zone" of shale oil include reservoir space distribution, reservoir brittleness index, shale oil viscosity, formation energy, and the size of the organic-rich shale. The successful extraction of shale gas provides a technical reference for shale oil extraction. "Artificial permeability" stimulation technologies, such as horizontal well volume fracturing and refracturing, are key technologies for the effective development of shale oil. Among shale oil resources, condensate oil or light oil may be the primary types for industrial production [6,11]. Condensate and light oil molecules have a diameter of 0.5 to 0.9 nm. Theoretically, they are more easily flowable and recoverable within the nanoscale pore throats of shale under high temperature and high pressure underground.
Basic Characteristics of Favorable Shale Oil Distribution Areas:
Shale oil differs significantly from conventional oil separated from its source and reservoir and tight oil accumulated near the source in terms of accumulation mechanisms, reservoir space, fluid characteristics, and distribution, while sharing more similarities with shale gas.
Source-Reservoir Integration, Retention and Accumulation:
Shale oil is a typical example of oil accumulation characterized by integrated source and reservoir, retention and accumulation, and continuous distribution. Organic-rich shales serve as both source and reservoir layers. Unlike shale gas, shale oil is primarily formed during the liquid hydrocarbon generation phase of organic matter evolution. During the continuous oil generation phase of organic-rich shales, oil is retained and accumulated within the shale reservoir, and only disperses or migrates outward after the shale reservoir itself is saturated. Therefore, any organic-rich shale in the liquid hydrocarbon generation phase has the potential to accumulate shale oil. At present, shale fracture oil has been discovered in North American marine strata and Chinese continental strata [12], but there are no reports of basement shale oil discovery. Rich in organic matter and high maturity Organic matter is the basis for shale to be rich in oil. The TOC value of high-yield and rich shale oil layers is generally greater than 2%, and the Ro value is generally 0.7% to 2.0%, forming light oil and condensate oil, which is conducive to production. Developed nano-scale pore throats and fracture systems Shales generally develop millimeter-centimeter-scale laminae. Nano-scale pore throats are widely developed in shale oil reservoirs, with pore diameters mainly ranging from 50 to 300 nm. Micron-scale pores are locally developed, and the pore types include intergranular pores, intragranular pores, organic pores, and intercrystalline pores. Microfractures are also very developed in shale oil reservoirs, and the types are diverse. Unfilled horizontal bedding fractures are the main ones, followed by shrinkage fractures. Vertical or oblique structural fractures are developed near the fault zone. Most shales have well-developed lamellae, including clay mineral lamellae, carbonate lamellae, organic lamellae, and pyrite. Shale oil is widely distributed within these lamellae and microfractures parallel to these bedding planes.
Reservoir brittleness index is high:
The content of brittle minerals is a key factor influencing the development of microfractures in shale, oil content, and fracturing stimulation methods. The lower the content of clay minerals such as kaolinite, montmorillonite, and hydromica in shale, and the higher the content of brittle minerals such as quartz, feldspar, and calcite, the more brittle the rock becomes, making it more susceptible to the formation of natural and induced fractures under external forces, thus facilitating shale oil recovery. China's lacustrine organic-rich shales generally have high brittle mineral contents, exceeding 40%. For example, the lacustrine shale in the Chang 7 Member of the Yanchang Formation in the Ordos Basin has an average content of brittle minerals such as quartz, feldspar, calcite, and dolomite, reaching 41%. The clay mineral content is less than 50%. The shale in the Chang 72 and Chang 73 Sub-Members has a high pyrite content, averaging 9.0%.
High Formation Pressure and Light Oil:
Shale oil-rich areas are located in mature, organic-rich shale formations with extensive oil production. These areas generally have high formation energy, with pressure coefficients ranging from 1.2 to 2.0. A few low-pressure formations, such as the Yanchang Formation in the Ordos Basin, have pressure coefficients of only 0.7 to 0.9. The oil is generally light, with crude oil densities ranging from 0.70 to 0.85 g/cm³ and viscosities ranging from 0.7 to 20.0 mPa·s. The high gas-oil ratio facilitates flow and production in nano-scale pore-throat reservoir systems.
Large, continuous distribution, high resource potential:
Shale oil distribution is not structurally controlled, lacking clear trap boundaries. Instead, its oil-bearing range is controlled by the distribution of organic-rich shale within the oil-generating window. Shale-generated oil is largely retained within the shale, generally accounting for 20% to 50% of total generated oil, indicating significant resource potential. For example, the Mesozoic Chang 7 Member shale in the Ordos Basin (concentrated in the lower Chang 72 Member and most of Chang 73) contains shale oil-rich intervals. Preliminary estimates indicate recoverable shale oil resources of 10 million to 15 million tons. North American marine shales are widely distributed, have stable thicknesses, high organic matter abundance, and high maturity, favoring the generation of light-weight and condensate shale oil.
Organic-Rich Shale Sedimentary Models:
Shales can form in marine, transitional, and terrestrial sedimentary environments. The formation of organic-rich black shales requires two key conditions: high productivity and abundant organic matter supply; and conditions conducive to the preservation, accumulation, and transformation of sedimentary organic matter.
There are four main depositional models for organic-rich black shales: marine (lacustrine) transgression, water stratification, threshold, and ocean current upwelling. In continental lake basins, only three of these models occur: transgression, water stratification, and threshold. The transgression model involves relative lake level rise, resulting in widespread anoxic conditions in deep waters. This allows organic matter to be buried and preserved, forming black shales (dense sections). This is typically more widespread in depression basins. The water stratification model involves the obstruction of water circulation above and below the catchment basin due to differences in temperature, salinity, or other factors, leading to anoxic conditions in localized low-lying, stagnant areas and the formation of organic-rich black shales. Water stratification is the most common form of organic-rich shale formation. Threshold sedimentation patterns are categorized as high-threshold and low-threshold, primarily based on water depth. The high-threshold pattern occurs in deep basins, such as faulted and foreland lake basins, where a "threshold" prevents external water from influencing the deeper waters. Consequently, water stratification creates an anoxic environment, leading to the development of black shales. The low-threshold pattern occurs in shallow, stagnant water areas (such as swamps). Biodegradation consumes large amounts of oxygen, resulting in a reducing environment and the preservation of higher plant organic matter, leading to the formation of coal-bearing shales. The most defining characteristic of the low-threshold pattern is the absence of water stratification.
During periodic fluctuations in lake level, water depth and sediment input rates also vary periodically, leading to regular variations in the total organic carbon content across the sedimentary profile. Sequence boundaries are characterized by shallower water, rapid sediment accumulation, and active oxidation, often resulting in minimum total organic carbon values across the profile. Near the maximum lake flooding surface, sediment supply is slow, resulting in undercompensated sedimentation. Organic matter is relatively enriched, often with maximum total organic carbon levels. This dense interval is the most favorable organic-rich shale interval in the sequence. However, dense intervals do not form near the maximum lake flooding surface in all lake basins. Continental lake basins vary in basin type and evolutionary stage, and due to factors such as small basin area, multiple provenances, and lake level fluctuations, the vertical distribution of organic-rich shale within the sequence is complex. Dense intervals in faulted lake basins in eastern China can occur in the lower part of highstand systems tracts or within transgressive systems tracts. Basins in the Midwest are primarily located within transgressive systems tracts.
The United States is a global pioneer in shale oil production, with shale oil being primarily found in several representative basins:
Bakken Shale: Located in North Dakota and Montana, it was the first shale oil province in the United States to achieve large-scale industrial production. Oil production is primarily light crude oil with a density between 0.79–0.85 g/cm³ and low viscosity, making it easy to extract. The Permian Basin, spanning Texas and New Mexico, boasts thick, continuous reservoirs rich in light oil and condensate, making it the core area of shale oil production in the United States.
The Eagle Ford Shale, located in southern Texas, is known for its high TOC values and high maturity. Its light oil and high gas-to-oil ratio make it suitable for production using horizontal well volume fracturing.
The shale oil in these areas shares similar characteristics to those in China:
Organic-rich and highly mature (Ro is generally 0.7% to 2.0%);
Well-developed nanoscale pore throats and microfracture systems, resulting in good reservoir continuity;
Light oil and low viscosity, allowing for easy flow;
Mature fracturing stimulation technology, with horizontal wells plus multi-stage fracturing as the primary production method.
The Feasibility of Artificial Lift Production of Shale Oil:
Shale oil reservoirs are primarily composed of nanoscale pore throats, resulting in low natural permeability for oil and gas. Therefore, artificial lift is often required to increase well production. Common methods include:
Sucker Rod Pump (SRP)
Advantages: Mature technology, simple operation, suitable for small-diameter, high-viscosity wells;
Disadvantages: Efficiency is significantly affected by depth and oil viscosity, making it unsuitable for very low-permeability, nanoporous shale formations.
Electric Submersible Pump (ESP) / Progressive Cavity Pump (PCP)
ESP: Suitable for large-diameter, high-yield wells, performs best with light oil and low viscosity, and enables continuous production;
PCP: Suitable for medium-to-low-yield, high-viscosity wells, and can handle some wells with high sand or wax content.
For light US shale oil (such as the Eagle Ford and Bakken shale), tubing pumps are more suitable due to their low oil viscosity, high yields, and pump body suitability for horizontal wells or long horizontal sections.