Occurrence space and state of shale oil: A review

Highlights

• Occurrence space and state of shale oil within shale reservoirs are reviewed.

• Occurrence state of shale oil needs further experimental characterization.

• Ratio of free oil to adsorbed oil in shale reservoirs requires accurate estimation.

• Study on occurrence of shale oil in closed pores of shales needs close attention.

• Microwave irradiation is a promising option to produce shale oil.

Abstract

Shale oil is a promising alternative unconventional energy to conventional fossil fuels. Its exploration and production are critical to improve the present energy consumption pattern and mitigate dependence on foreign energy. Particularly, the key factors determining shale oil occurrence are the most important issues for its production. In this review, we briefly summarize the dominant space of shale reservoirs for accommodating shale oil. We also provide a comprehensive overview of occurrence state of shale oil and analyze in detail the factors such as pore size, organic matter, inorganic minerals of oil-bearing shale and reservoir burial depth. Finally, we discuss the existing challenges and perspectives on how to explore and address occurrence of shale oil in practical shale reservoirs to promote the future shale oil production. An accurate description of shale oil occurrence through viable research methodology, advanced instrument characterization and theoretical simulation is crucial for estimating shale oil reserves, optimizing and enhancing shale oil production.

Introduction

Energy is of great significance to sustainable development of human society and global economy. The worldwide consumption of primary energy in 2019 was estimated as 583.9 EJ (BP, 2020). Among them, the consumption of crude oil reached up to 191.45 EJ with approximately 33.06 percent share of the total consumption of primary energy (BP, 2020). That is, the crude oil is crucial in energy consumption. It mainly includes conventional oil and unconventional oil. Given continuously decreasing conventional oil production, unconventional oil resource with considerable proven reserves has been gaining ever-increasing interest (Zou et al., 2013). The unconventional oil typically includes heavy oil, tar sand and carbonate fracture-cavern oil, tight oil, shale oil, and oil shale (Fig. 1). Particularly, shale oil accounts for 20–50% of the total oil reserves around the world, thus making it competitive in exploration, production and utilization (Zou et al., 2013). The worldwide technically recoverable reserves of shale oil are estimated as 4.69 × 10¹⁰ tons (Zhao et al., 2018). Among them, that of Russia ranks the first (1.05 × 10¹⁰ tons), followed by the U.S. (6.72 × 10⁹ tons) and China (4.48 × 10⁹ tons) (Zhao et al., 2018). Shale oil is liquid hydrocarbon originating from thermal evolution of organic matter. The accumulated shale oil is trapped in organic-rich shale reservoirs (Zou et al., 2013). Similar to gas-bearing shale reservoirs, oil-bearing shale reservoirs are characterized by self-generation and self-accumulation. However, there are some differences in geochemical properties between these two reservoirs. Generally, oil-bearing shale reservoirs have low vitrinite reflectance (R_o) of 0.7–2.0% (Zou et al., 2013), while that of gas-bearing shale reservoirs is 1.1–1.5% (Rezaee, 2015). That is, oil-bearing shale reservoirs are in lower thermal evolution than gas-bearing shale reservoirs. According to occurrence space and mining conditions, shale oil is divided into matrix-type shale oil, fracture-type shale oil and interbed-type shale oil (Ning, 2014; Chen, 2017). Matrix-type shale oil mainly refers to oil in nanopores of organic matter and clay minerals, making it difficult to produce. This type of shale oil is profitable only for target reservoirs with high oil content. Fracture-type shale oil always occurs in microfractures, thereby possessing good mobility. Thus, it is of high commercial value. Interbed-type shale oil has undergone short-distance migration. It mainly accumulates in interbed of sandstone, carbonate rock, volcanic rock, etc. Its exploration and production greatly depend on reservoir brittleness and thickness.

Up to now, shale oil reservoirs have been frequently discovered in China. Nonetheless, they have not produced massive shale oil industrial flow yet. For instance, the shale oil production from well L57 in Santanghu Basin, Shugu well 165 in Bohai Bay Basin and HF1 well in Nanxiang Basin are only 22.20 m³/d, 24.00 m³/d and 23.60 m³/d, respectively (Liang et al., 2012; Jiang et al., 2014). These undesirable data are owing to extremely low oil-bearing shale reservoir porosity and permeability (Zou et al., 2013). Therefore, reservoir stimulation is employed to promote shale oil migration to enhance its recovery. In general, the mobility of shale oil lies on its occurrence in reservoirs. The understanding on occurrence rule of shale oil not only benefits ‘sweet spot’ selection, but also promotes optimization and innovation of shale oil production technology. Therefore, it is necessary to conduct in-depth research on shale oil occurrence. Till now, the shale petrology characteristics, sedimentary environment, resource potential evaluation, and research and development (R&D) of production technology have been widely reported (Lu et al., 2018; Li et al., 2019d; Zhang et al., 2020b; Zhou et al., 2020). However, the occurrence of shale oil within reservoirs has not been well understood. Firstly, the insight into occurrence space of shale oil on a microscopic scale is a core issue. It involves the contribution of closed pore space to shale oil occurrence, and distinguishment of shale oil occurrence between organic matter and inorganic minerals. Secondly, the occurrence state of shale oil includes free state, adsorbed state, and dissolved state (Li et al., 2018b). The ratio of free shale oil to adsorbed shale oil is widely used as a critical indicator to shale oil production. The experimental methodology and theoretical simulation on accurately determining the ratio require further investigations. Lastly, the objective of clarifying occurrence space and state of shale oil within reservoirs is to develop highly efficient, cost effective and environmentally friendly technology for producing shale oil. Although hydraulic fracturing method has achieved commercial production of shale oil in recent decades, its implementation exposes the following weaknesses (Norris et al., 2016). Specifically, hydraulic fracturing method injects large quantities of water blended with chemicals into the target shale reservoirs. However, only 20–40% of fracturing fluid can flow back to the ground, implying huge waste of water resources. Furthermore, the flowback fluid usually carries methane (CH₄) to the atmosphere during oil production, which may aggravate greenhouse effect. Finally, the residual fracturing fluid probably causes groundwater and soil contamination. Therefore, developing novel technology for producing shale oil based on insight into shale oil occurrence is also a core issue.

Overall, the dominant space of shale reservoirs for accommodating shale oil is briefly summarized in this review to give a detailed reference on shale oil production. Moreover, the occurrence state of shale oil and its influencing factors are presented. Finally, the existing challenges and perspectives on further understanding occurrence of shale oil in practical shale reservoirs to promote shale oil production are discussed.