The shale revolution has weaned the US from its dependence on imported oil and gas to become a net exporter. This paper reviews the history of the shale revolution in the United States, summarizes the characteristics of shale oil and tight oil and gas, analyzes the development status of the shale oil and gas revolution in China, and puts forward the basic geological theory of shale oil and tight oil and gas. The research shows that: (A) The United States has advantages such as good resource endowment, strong scientific and technological innovation ability, huge investment capacity, and strong engineering construction capacity, and has already realized the shale revolution. China’s shale revolution is now under way. (B) Compared with conventional oil and gas, shale oil and tight oil and gas have different accumulation modes and development methods. They have the characteristics of continuous large-area distribution, self-sealing oil and gas accumulations, huge development and production engineering, and the "distributed" characteristics of oil and gas field production can flexibly cope with the characteristics of production reduction and low recovery. (C) Seven conditions need to be met for the further production of shale oil and gas in China: These are(1) Clear resources; (2) Mature horizontal well and fracturing technology; (3) The cost is controllable; (4) The engineering space of enhanced oil recovery in the late stage is large; (5) Meet environmental requirements; (6) Adequate capital investment; (7) Strong engineering construction ability. (D) The whole oil and gas system has a sequence of conventional oil and gas, tight oil and gas and shale oil and gas. Shale oil and gas and tight oil and gas reservoirs have the basic characteristics of tight reservoirs, complex fluid composition and phase state, and diverse reservoir driving modes, and we are faced with the problem of unclear reservoir geological and flow models. It is necessary to carry out further theoretical research of the whole oil and gas system, and vigorously develop sweet spot evaluation technology and fracturing technology.

Although the fundamental principles behind the basic hydrocarbon accumulation and distribution patterns within a whole petroleum system (WPS) are universal, these patterns tend to vary in actual geological settings, reflecting the dialectical relationship between theoretical universality and geological particularity. To address this challenge, we systematically analyze the differences in the hydrocarbon enrichment characteristics of WPSs under varying conditions. The results indicate that the differences in the hydrocarbon accumulation characteristics are determined by three fundamental conditions, i. e., geodynamic setting, reservoir medium, and source rocks, as well as their coupling relationships. Based on the combinations of key hydrocarbon accumulation factors in the three conditions, we classify WPSs into 48 types and propose a nomenclature rule. The results highlight four practically significant WPS types: those with clastic rock sequences, carbonate sequences, coal-bearing sequences, and near-surface special sequences as reservoirs, which are particularly important for further hydrocarbon resource development. Accordingly, we systematically elucidate the fundamental characteristics, hydrocarbon enrichment patterns, and resource potential of the four types of WPSs, demonstrating their respective resource development directions and major plays. Specifically, reservoirs of clastic sequences are governed by diagenesis dominated by typical tightening laws and exhibit shallow buoyancy-driven hydrocarbon accumulation depths (BHADs). In these reservoirs, deep unconventional tight hydrocarbon resources show extensive plays, suggesting broad exploration prospects. Carbonate reservoirs are primarily of the hypergene weathering-related vuggy type, with high-quality intervals potentially preserved in deep to ultra-deep strata. These reservoirs contain a high proportion of conventional resources and hold huge potential for deep to ultra-deep resource development. In coal-bearing sequences, coals exhibit high gas generation and expulsion capacities, while natural gas in peripheral reservoirs shows a wide and continuous distribution. Furthermore, coals with high thermal maturity develop abundant cleats and exhibit anomalously high porosity and permeability, while their peripheral reservoirs demonstrate high sealing performance after gas accumulation. These factors are conducive to free gas enrichment in coal-bearing sequences. The near-surface special sequences, although characterized by complex hydrocarbon accumulation conditions, hold vast resource potential, including heavy oil bitumen, natural gas hydrates, and water-soluble natural gas. Advancing research and development of key technologies, alongside reducing exploitation costs, will facilitate the efficient utilization and rapid development of these resources.

The concept of the whole petroleum system (WPS)proposed and the ordered distribution pattern of conventional and unconventional hydrocarbon reservoirs established serve to unify petroleum geology theories, providing an entirely new theory and methods for guiding hydrocarbon exploration and exploitation. However, when applied to complex geological conditions such as those of superimposed basins in China, the WPS theory, originally formulated under general geological conditions, encounters several challenges. First, due to the lack of objective indices for quantitative characterization and the disruptions caused by late-stage tectonic activities, the idenfitication or prediction of buoyancy-driven hydrocarbon accumulation depth (BHAD) is hard in practice. Second, the formation and distribution of hydrocarbon reservoirs are governed by the combined effects of multiple driving forces, multiple stages, and diverse elements, complicating the identification of the hydrocarbon accumulation dynamics and reservoir types. Third, reservoirs such as those of the carbonate and clastic types exhibit varying wettability and interfacial tension properties, which lead to difficulties in predicting the maximum burial depth of hydrocarbon reservoirs and favorable hydrocarbon enrichment areas under actual geological conditions. Last, during the evolution of the WPS, hydrocarbon migration and accumulation are controlled by multiple factors including dynamic force types, capillary pressure differences between source rocks and reservoirs, hydrocarbon fluid properties, and tectonic activities. Consequently, establishing the hydrocarbon enrichment model of the WPS remains a challenge. To address these challenges, we explore various methods and technologies based on data from globally discovered hydrocarbon reservoirs, including their physical properties, productivity variations, and wettability. Accordingly, we develop new methods for identifying the BHAD, assessing hydrocarbon accumulation dynamics, and predicting the maximum burial depth of oil and gas resources. Furthermore, we determine the hydrocarbon enrichment model of the WPS. The results of this study offer new approaches to improve, develop, and apply the WPS theory under complex geological conditions.

Since 2021, significant breakthroughs have been achieved in the exploration of deep coalbed methane (CBM) or coal-rock gas in the Carboniferous-Permian coal measures of the Ordos Basin, revealing broad development prospects. Meanwhile, wide attention has been directed to the genetic correlations and commingling production potential between CBM gas and other natural gas, such as tight gas, across multiple Carboniferous-Permian sequences within the basin. Based on the theory and a quantitative evaluation method of the whole petroleum system, we analyze the genetic mechanisms and interrelationships of various natural gas reservoirs that have already been identified in the basin. Accordingly, we establish the accumulation model of natural gas and define the orderly distribution pattern of gas reservoirs in the whole petroleum system of the coal measures. The results indicate that the boundaries of the whole petroleum system in the Carboniferous-Permian coal measures extend across the entire basin. The whole petroleum system exhibits three dynamic boundaries from shallow to deep: the buoyancy-driven hydrocarbon accumulation depth (BHAD), the hydrocarbon accumulation depth limit (HADL), and the active source-rock depth limit (ASDL). Accordingly, three dynamic fields of hydrocarbon accumulation are developed between the Earth’s surface and the three dynamic boundaries, namely the free-hydrocarbon dynamic field, the confined-hydrocarbon dynamic field, and the bound-hydrocarbon dynamic field from top to bottom. During the evolution of the whole petroleum system, hydrocarbons expelled from source rocks migrate and accumulate predominantly under the action of buoyancy in the free-hydrocarbon dynamic field of the intermediate to shallow strata, and then enter into high-porosity, high-permeability reservoirs within traps to form conventional natural gas reservoirs. In contrast, within the confined-hydrocarbon dynamic field of the intermediate to deep strata, hydrocarbon migration and accumulation from source rocks are predominantly driven by differences in capillary pressure between the source rocks and reservoirs. Consequently, these hydrocarbons migrate into adjacent low-porosity, low-permeability reservoirs, resulting in the formation of tight gas reservoirs. In the bound-hydrocarbon dynamic field, primary hydrocarbons retained in source rocks migrate and accumulate predominantly through adsorption, contributing to the formation of CBM reservoirs characterized by integrated source rocks and reservoirs. The natural gas reservoirs in the basin generally display an orderly distribution pattern, with CBM reservoirs, tight gas reservoirs, and conventional gas reservoirs occurring sequentially from bottom to top. In the eastern Ordos Basin, the intermediate to shallow strata contain only limited conventional gas due to the overall uplift and the presence of tectonic fractures. In contrast, the intermediate to deep strata in this region demonstrate widely distributed tight gas, substantial thickness of target strata, varied gas-rich plays, and considerable resource potential, establishing them as an important target for sustainable exploration and development.

The Early Cambrian Yuertusi Formation (Є₁y) in the Tarim Basin of China deposits a continuously developed suite of organic-rich black mudstones, which constitute an important source of oil and gas reservoirs in the Paleozoic. However, its hydrocarbon generation and evolution characteristics and resource potential have long been constrained by deeply buried strata and previous research. In this paper, based on the newly obtained ultra-deep well drilling data, the hydrocarbon generation and expulsion model of Є₁y shale was established by using data-driven Monte Carlo simulation, upon which the hydrocarbon generation, expulsion, and retention amounts were calculated by using the diagenetic method. The research indicates that the Є₁y shale reaches the hydrocarbon generation and expulsion threshold at equivalent vitrinite reflectances of 0.46% and 0.72%, respectively. The cumulative hydrocarbon generation is 68.88 × 10¹⁰ t, the cumulative hydrocarbon expulsion is 35.59 × 10¹⁰ t, and the cumulative residual hydrocarbon is 33.29 × 10¹⁰ t. This paper systematically and quantitatively calculates the hydrocarbon expulsion at various key geological periods for the Є₁y source rocks in the study area for the first time, more precisely confirming that the black shale of the Є₁y is the most significant source rock contributing to the marine oil and gas resources in the Tarim Basin, filling the gap in hydrocarbon expulsion calculation in the study area, and providing an important basis for the formation and distribution of Paleozoic hydrocarbon reservoirs. The prospect of deep ultra-deep oil and gas exploration in the Tarim Basin is promising. Especially, the large area of dolomite reservoirs under the Cambrian salt and source rock interiors are the key breakthrough targets for the next exploration in the Tarim Basin.

Shale oil production is vital for meeting the rising global energy demand, while primary recovery rates are poor due to the ultralow permeability. CO₂ huff-n-puff can boost yields by enabling key enhanced oil recovery mechanisms. This review examines the recent research on mechanisms and formation factors influencing CO₂ huff-n-puff performance in shale liquid reservoirs. During the soaking period, oil swelling, viscosity reduction and CO₂-oil miscibility occur through molecular diffusion into shale nanopores. The main recovery mechanism during the puff period is depressurization with oil desorption and elastic energy release. The interplay between matrix permeability and fracture network directly determines the CO₂ huff-n-puff performance. Nanopore confinement, wettability alterations, and heterogeneity also significantly impact the huff-n-puff processes, with controversial effects under certain conditions. This work provides an integrated discussion on the mechanistic insights and formation considerations essential for the advancement of CO₂ huff-n-puff application in shale reservoirs. By synthesizing the recent research findings, we aim to spotlight the key challenges and opportunities in considering reservoirs for this process, thereby contributing to the advancement of CO₂ huff-n-puff applications for enhanced oil recovery.

Despite the significant progress made in tight gas exploration and development in recent years, the understanding of the dynamic mechanisms of tight gas accumulation is still limited, and numerical simulation methods are lacking. In fact, the gap between theory and field application has become an obstacle to the development of tight gas exploration and development. This work sheds light on the dynamic mechanisms of hydrocarbon accumulation in tight formations from the aspect of capillary self-sealing theory by embedding calculation of pressure- and temperature-dependent capillary force in a pore network model. The microscale dynamic mechanisms are scaled up to the reservoir level by geological simulation, and the quantitative evaluation of reserves based on real geological sections is realized. From the results, several considerations are made to assist with resource assessment and sweet spot prediction. Firstly, the self-sealing effect of capillary in the micro-nano pore-throat system is at the core of tight sandstone gas accumulation theory; the hydrocarbon-generated expansion force is the driving force, and capillary force comprises the resistance. Furthermore, microscopic capillary force studies can be embedded into a pore network model and scaled up to a geological model using relative permeability curve and capillary force curve. Field application can be achieved by geological numerical simulations at the reservoir scale. Finally, high temperature and high pressure can reduce capillary pressure, which increases gas saturation and reserves.

The classical source-to-trap petroleum system concept only considers the migration and accumulation of conventional oil and gas in traps driven dominantly by buoyance in a basin, although revised and improved, even some new concepts as composite petroleum system, total petroleum system, total composite petroleum system, were proposed, but they do not account for the vast unconventional oil and gas reservoirs within the system, which is not formed and distributed in traps dominantly by buoyance-driven. Therefore, the petroleum system concept is no longer adequate in dealing with all the oil and gas accumulations in a basin where significant amount of the unconventional oil and gas resources are present in addition to the conventional oil and gas accumulations. This paper looked into and analyzed the distribution characteristics of conventional and unconventional oil/gas reservoirs and their differences and correlations in petroliferous basins in China and North America, and then proposed whole petroleum system (WPS) concept, the WPS is defined as a natural system that encompasses all the conventional and unconventional oil and gas, reservoirs and resources originated from organic matter in source rocks, the geological elements and processes involving the formation, evolution, and distribution of these oil and gas, reservoirs and resources. It is found in the WPS that there are three kinds of hydrocarbons dynamic fields, three kinds of original hydrocarbons, three kinds of reservoir rocks, and the coupling of these three essential elements lead to the basic ordered distribution model of shale oil/gas reservoirs contacting or interbeded with tight oil/gas reservoirs and separated conventional oil/gas reservoirs from source rocks upward, which is expressed as “S\T-C”. Abnormal conditions lead to other three special ordered distribution models: The first is that with shale oil/gas reservoirs separated from tight oil/gas reservoirs. The second is that with two direction ordered distributions from source upward and downward. The third is with lateral distribution from source outside.