Great changes of the global energy industry have been caused by the rapid development of unconventional oil and gas. It is necessary to deeply consider the profound influence of the unconventional oil and gas revolution on the classical petroleum geological theory and to review geological conception of oil and gas accumulation elements and theoretic framework of petroleum system, giving the petroleum geology a new academic connotation. The author summarizes the significant progresses of global unconventional oil and gas exploration and development, and points out that the unconventional oil and gas revolution not only has a significant economic significance of oil and gas resource increment, but also brings great innovation to the theory of petroleum geology, thus having important scientific significances. This paper summarizes the core contents of four aspects of hydrocarbon generation, reservoir, distribution and development in classical petroleum geology, and comprehensively reviews the five important nodes in the developmental history of petroleum geology, which include anticline and trap theory, hydrocarbon generation from organic matter and petroleum system theory, continental petroleum geology, marine deepwater petroleum geology, continuous hydrocarbon accumulation and unconventional oil and gas geological theory. Unconventional oil and gas has made a great breakthrough to classical petroleum geology on the basic theoretical concepts such as trap, reservoir, caprock, resource distribution, and enrichment, thereby promoting the basic research on petroleum geology to transform into the whole process of hydrocarbon generation, whole type of reservoir, and whole genetic mechanism, deepening unconventional petroleum geology theory, promoting the development and reconstruction of petroleum geology system, representing great significances to the strategic development from conventional to unconventional oil and gas in China or even in the world.

This paper deals with the main scientific problems, academic connotation, progress and prospects of reservoir development geology. The reservoir development geology involves the key scientific problems of reservoir connectivity, flow ability, and changeability through time. Its research focus on the forming mechanism and distribution model of geological factors controlling the reservoir development, the control mechanism of geological factors to oil and gas production, the rule of reservoir dynamic evolution during development, and the reservoir characterization and modeling technology. Important progress has been made on theory and technology of reservoir development geology in high water-cut reservoirs, low permeability and tight shale reservoirs, fracture-cavity reservoirs, which makes the reservoir development geology grow as an independent academic subject already. With the development expansion in areas of deep-strata, deep-water, and unconventional hydrocarbon reservoirs, and the increasing difficulties of high water-cut reservoir development, the theory and technology of reservoir development geology remain to be developed in order to support efficient and economic development of hydrocarbon fields with a sustainable growth.

According to the actual demands of petroleum exploration and development, this paper describes the research progress and application of artificial intelligence technology in the field of petroleum exploration and development, and discusses the applications and development directions of artificial intelligence technology in the future. Machine learning technology has preliminary application in lithology identification, logging curve reconstruction, reservoir parameter prediction and other logging processing and interpretation, and has shown great potential. Computer vision is effective in the seismic first breaks picking, fault identification and other seismic processing and interpretation. Deep learning and optimization technology has been applied to reservoir engineering, and realized the real-time optimization of waterflooding development and prediction of oil and gas field production. The application of data mining in drilling and completion, surface engineering and other fields has formed intelligent equipment and integrated software. The potential development directions of artificial intelligence in the field of petroleum exploration and development are intelligent production equipment, automatic processing interpretation and professional software platform. The focus of development is digital basin, fast intelligent imaging logging tool, intelligent node seismic acquisition system, intelligent rotary steering drilling, intelligent fracturing technology and equipment, real-time monitoring and control engineering of separate layer injection and production.

The development of natural gas in China has entered a golden and leap-forward stage, which is a necessary bridge to clean energy. This in-depth study on the status quo, theory, technology and prospect of natural gas development shows: (1) The global remaining proven recoverable reserves of natural gas are 186×10¹² m³, and the reserves-production ratio is 52.4, indicating a solid resource base for long-term and rapid development. (2) Ten formation and distribution laws of conventional and unconventional natural gas reservoirs have been proposed. In terms of exploration geology, the theory of conventional “monolithic” giant gas fields with different gas sources, and an unconventional gas accumulation theory with continuous distribution of “sweet spots” in different lithologic reservoirs have been established; in terms of development geology, a development theory of conventional structural gas reservoirs is oriented to “controlling water intrusion”, while a development theory of unconventional gas is concentrated on artificial gas reservoirs. (3) With the geological resources of 210×10¹² m³ (excluding hydrates) and the total proven rate of the resources less than 2% at present, the natural gas in China will see a constant increase in reserve and production; by 2030, the proven geological reserves of natural gas are expected to reach about (6 000-7 000)×10⁸ m³, the production of conventional and unconventional natural gas each will reach about 1 000×10⁸ m³, and the gas consumption will reach 5500×10⁸ m³. The dependence on imported natural gas may be 64% by 2030, and 70% by 2050. (4) Ten measures for future development of natural gas have been proposed, including strengthening exploration in large-scale resource areas, increasing the development benefits of unconventional gas, and enhancing the peak adjusting capacity of gas storage and scale construction of liquified natural gas.

Based on the understandings on enrichment rules of marine shale gas in southern China and data obtained from exploration and development in Fuling shale gas field, this article discusses the key controlling factors on shale gas enrichment and their relationships, it also discusses further the theory of Two-Factor Enrichment of marine shale gas in southern China. The bases for shale gas enrichment are shale gas generation and accumulation, the shale gas reservoirs of deep-water shelf are characterized by high TOC, high porosity, high gas content and high siliceous content, with high hydrocarbon-generation intensity, they are rich in organic pores, favorable for reformation, so they are the base for large scale hydrocarbon accumulation. Preservation conditions are vital to the formation and enrichment of shale gas reservoir, good top and base layers can effectively prevent hydrocarbon from escaping vertically at the beginning of hydrocarbon generation. Shale gas preservation conditions depend on the intensity and duration of tectonic movements, good preservation conditions are key geological factors for shale gas accumulation, shale reservoirs have high gas content, high porosity and high pressure and are likely to form high yield area of shale gas.

Based on the comprehensive interpretation of cores, loggings and 2D/3D seismic data of Shenhu GMGS3 drilling area in the northern South China Sea, the distribution characteristics, differential accumulation mechanism and reservoir forming mechanism of diffusion type natural gas hydrate with high saturation discovered from clayey silt reservoirs were investigated. The following findings are reached through the research: (1) Gas hydrate with high saturation often displays high resistivity, low interval transit time, and strong bottom-simulating reflectors (BSRs), and accompanies with fluid seepage phenomena beneath BSRs, such as mud diapiric structure and gas chimney. (2) The gas hydrate reservoirs are dominated by fine grained clayey silt sediments, and the reservoirs have higher porosity and permeability in local parts. (3) The gas hydrate is largely type I, whereas type II gas hydrate may exist below the type I gas hydrate. (4) The gas sources are mixed microbial and thermogenic gases, and the thermogenic gas originated from the deep formation in the center of Baiyun Sag migrated into shallow strata through faults, mud diapirs and gas chimneys, then was mixed with microbial gas in situ and continued to migrate until they accumulated in the temperature and pressure stability zone and formed diffusion type gas hydrate with high saturation finally. (5) The fluid migration system influenced and controlled the differential distribution of gas hydrate with high saturation.

Through reviewing the development history of tight oil and gas in China, summarizing theoretical understandings in exploration and development, and comparing the geological conditions and development technologies objectively in China and the United States, the progress and stage of tight oil and gas exploration and development in China have been clarified, and the future development orientation of theory and technology, process methods and development policy for tight oil and gas in China have been envisaged. In nearly a decade, relying on the exploration and development practice, science and technology research and management innovation, huge breakthroughs have been made in the exploration and development of tight oil and gas in China. The laws of formation, distribution and accumulation of tight oil and gas have been researched, the development theories such as “multi-stage pressure drop” and “man-made reservoirs” have been established, and several technology series, including enrichment regions selection, well pattern deployment, single well production and recovery factor enhancement, and low cost development, have been innovated and integrated. All those promote the rapid rise of both reserves and production of tight oil and gas. However, limited by the sedimentary environment and tectonic background, compared with North America, China’s tight oil and gas reservoirs are worse in continuity, more difficult to develop and poorer in economic efficiency. Moreover, there are still some gaps in reservoir identification accuracy and fracturing technology between China and North America. In the future, boosting the rapid development of tight oil and gas, Chinese oil and gas companies should further improve the resource evaluation method, tackle key technologies such as high-precision 3D seismic interpretation, man-made reservoir, and intelligent engineering, innovate theories and technologies to enhance single well production and recovery rate, and actively endeavor to get the finance and tax subsidy on tight oil and gas.

The discovery of the giant Anyue gas field in Sichuan Basin gives petroleum explorers confidence to find oil and gas in Proterozoic to Cambrian. Based on the reconstruction of tectonic setting and the analysis of major geological events in Mesoproterozoic-Neoproterozoic, the petroleum geological conditions of Proterozoic to Cambrian is discussed in this paper from three aspects, i.e. source rocks, reservoir conditions, and the type and efficiency of play. It is found that lower organisms boomed in the interglacial epoch from Mesoproterozoic-Neoproterozoic to Eopaleozoic when the organic matters concentrated and high quality source rocks formed. Sinian-Cambrian microbial rock and grain-stone banks overlapped with multiple-period constructive digenesis may form large-scale reservoir rocks. However, because of the anoxic event and weak weathering effect in Eopaleozoic-Mesoproterozoic, the reservoirs are generally poor in quality, and only the reservoirs that suffered weathering and leaching may have the opportunity to form dissolution-reconstructed reservoirs. There are large rifts formed during Mesoproterozoic-Neoproterozoic in Huabei Craton, Yangtze Craton, and Tarim Craton in China, and definitely source rocks in the rifts, while whether there are favorite source-reservoir plays depends on circumstance. The existence of Sinian-Cambrian effective play has been proved in Upper Yangtze area. The effectiveness of source-reservoir plays in Huabei area depends on two factors: (1) the effectiveness of secondary play formed by Proterozoic source rock and Paleozoic, Mesozoic, Cenozoic reservoir rocks; (2) the matching between reservoirs formed by reconstruction from Mesoproterozoic- Neoproterozoic to Eopaleozoic and the inner hydrocarbon kitchens with late hydrocarbon generation. As for Tarim Basin, the time of Proterozoic and the original basin should be analyzed before the evaluation of the effective play. To sum up, Proterozoic to Cambrian in the three craton basins in China is a potential exploration formation, which deserves further investigation and research.

Seismic, drilling, logging and production performance data were analyzed to study the key geological factors such as reservoir properties and gas/water distribution, which influence the development of the Lower Cambrian Longwangmiao reservoirs in the Moxi block, Anyue gas field, Sichuan Basin, and the development strategies are established. The results indicate that: (1) Four stages of grain shoals developed longitudinally and two main shoals with one trench formed laterally in the development area; (2) Three types of reservoirs are identified, which are reservoir with millimeter sized dissolved vugs, reservoir with solution pores, and reservoir with inter-particle/inter-crystal pores; (3) Low matrix porosity but mid-to-high permeability affected by developed high-dip fractures; and (4) Three gas/water contacts (GWC) can be defined and step down from west to east, with a unique gas/water contact of -4 385 m in the target developing area. Based on geologic characterization, reservoir simulation and case study of similar gas reservoirs, the development strategies are made: to select the main grain shoal area as the prior production area; to locate wells on the structural high to delay edge water breakthrough and prolong stable production; to drill horizontal wells enhancing single well deliverability; to keep reasonable production of wells for improving the production effect.

On the basis of comprehensive analysis of drilling, gas testing, laboratory analysis and testing data, the characteristics and genesis of tight carbonate reservoirs in Ma5₁₊₂ Member of Ordovician Majiagou Formation, eastern Yi-Shaan slope, Ordos Basin were examined, and the potential of natural gas exploration and development were analyzed. The tight carbonate reservoir is defined as the reservoir with a porosity of less than 2% and permeability of less than 0.1×10^-3 μm². The Ma5₁₊₂ reservoirs are dominantly gypsum mud dolomite, muddy dolomite and Karst-breccia dolomite and has strong heterogeneity, pore types being mainly composed of fracture-dissolution pores and fracture-intercrystalline pores, and thin reservoir layers are distributed in a large area. The unconformity structure adjustment at the top of the Ordovician caused pore creation and pore filling effects, and the joint effect of dissolution pore increase and pore reduction by filling is the major reason for extensive reservoir densification. The thin tight dolomite reservoirs and the overlying adjacent coal source rock in the Upper Paleozoic formed extensive tight carbonate gas with shallow depth (1900-2500 m) and formed a three-dimensional gas containing pattern combined with the Upper Paleozoic tight sandstone gas. The eastern Yi-shaan slope in the Ordos Basin has great exploration and development potential.

With the development of gas exploration of the Sinian-Cambrian strata in the Sichuan Basin, creative understandings of geology were achieved. An N-E trending intracratonic rift was found in the Wanyuan-Dazhou area, northeastern Sichuan Basin. Based on seismic interpretation data, outcrop data and analysis of regional structural geological background, studies on the boundary, distribution, formation and evolution history of Wanyuan-Dazhou rift were carried out, and the significance on exploration was discussed. The following findings were obtained. (1) The seismic section indicated that a steep-slope belt was developed in the first and second members (Z₂dn₁-Z₂dn₂) of the Dengying Formation of the Sinian System, showing platform edge facies. The rift generally strikes in NE direction. (2) The thicknesses of the first and second members of the Dengying Formation are thicker than the third and fourth members (Z₂dn₃-Z₂dn₄) at the rift periphery, while it came to the opposite conclusion at the inside of the rift. (3) The rift formed in Z₂dn₁-Z₂dn₂ of Sinian deposition period. Filling and subsidence occurred in Z₂dn₃-Z₂dn₄ of Sinian deposition period. The shrinkage stage was the deposition period of the Early Cambrian Maidiping Formation - Qiongzhusi Formation. The formation of the rift was controlled by the Nanhua rift and regional uplift. The finding of the Wanyuan-Dazhou rift changed the traditional understanding of the Sinian - Early Cambrian tectonic sedimentary framework, and the rift will be the significant exploration direction in the future due to its superior natural gas accumulation conditions in the Sinian.

Aiming at analyzing the issues of non-uniform growths of multiple hydraulic fractures caused by stress shadowing, a numerical model considering elasto-hydrodynamic, stress interference and flow distribution into different fractures was built. Based on the model, the effects of perforation friction, perforation cluster spacing, Young modulus of rock and fracturing fluid viscosity on the growth of multiple fractures were investigated. The simulation results show that the growths of hydraulic fractures are relatively uniform with adequate perforation friction; the reduction of perforation cluster spacing, increase of Young modulus or fluid viscosity will cause the reduction of some fracture width and uneven flow distribution into these fractures, thus aggravating non-uniform growth of multiple fractures. Since appropriate perforation friction is conducive to the uniform growth of fractures, a convenient quantitative optimization method to calculate the needed perforation friction for uniform growth was proposed. By estimating interfracture induced stress during fracturing, the perforation friction coefficient needed to maintain uniform growth of fractures inside a stage is calculated, and reasonable engineering parameters of perforation can be selected based on this. The perforation parameters of a horizontal well were calculated with the proposed method, and the simulation results and actual fracturing performance show that the optimized perforation parameters can effectively keep uniform growth of fractures.

To find out the origin of dolomite, the precipitation of primary dolomite, and the formation of pores in dolomite, petrologic and geochemical characteristics of typical samples from Sichuan and Tarim Basin were analyzed based on the previous understandings, and three aspects of results were achieved. (1) A classification of dolomite origins based on petrologic features, forming environment and time sequence was proposed, which shows clear boundaries of diagenetic and characteristic realms and evolved clues between different types of dolomite. (2) Petrographic and geochemical identification marks for different types of dolomite were presented, revealing that the orderly geochemical variation of different types of dolomite is the response to the change of forming environment of dolomite during continuous time sequence. (3) The contribution of dolomitization to the formation of porosity was re-evaluated, revealing that the porosity in dolomite was mostly attributed to the primary pores and supergene dissolution and burial dissolution, and early dolomitization was conducive to the preservation of primary pores. These understandings are of great theoretical significance for identifying the origins and types of dolomite, and can guide the prediction of dolomite reservoirs.

The development history of carbon capture, utilization and storage for enhanced oil recovery (CCUS-EOR) in China is comprehensively reviewed, which consists of three stages: research and exploration, field test and industrial application. The breakthrough understanding of CO₂ flooding mechanism and field practice in recent years and the corresponding supporting technical achievements of CCUS-EOR project are systematically described. The future development prospects are also pointed out. After nearly 60 years of exploration, the theory of CO₂ flooding and storage suitable for continental sedimentary reservoirs in China has been innovatively developed. It is suggested that C₇-C₁₅ are also important components affecting miscibility of CO₂ and crude oil. The mechanism of rapid recovery of formation energy by CO₂ and significant improvement of block productivity and recovery factor has been verified in field tests. The CCUS-EOR reservoir engineering design technology for continental sedimentary reservoir is established. The technology of reservoir engineering parameter design and well spacing optimization has been developed, which focuses on maintaining miscibility to improve oil displacement efficiency and uniform displacement to improve sweep efficiency. The technology of CO₂ capture, injection and production process, whole-system anticorrosion, storage monitoring and other whole-process supporting technologies have been initially formed. In order to realize the efficient utilization and permanent storage of CO₂, it is necessary to take the oil reservoir in the oil-water transition zone into consideration, realize the large-scale CO₂ flooding and storage in the area from single reservoir to the overall structural control system. The oil reservoir in the oil-water transition zone is developed by stable gravity flooding of injecting CO₂ from structural highs. The research on the storage technology such as the conversion of residual oil and CO₂ into methane need to be carried out.

Carbon dioxide is an important medium of the global carbon cycle, and has the dual properties of realizing the conversion of organic matter in the ecosystem and causing the greenhouse effect. The fixed or available carbon dioxide in the atmosphere is defined as “gray carbon”, while the carbon dioxide that cannot be fixed or used and remains in the atmosphere is called “black carbon”. Carbon neutral is the consensus of human development, but its implementation still faces many challenges in politics, resources, technology, market, and energy structure, etc. It is proposed that carbon replacement, carbon emission reduction, carbon sequestration, and carbon cycle are the four main approaches to achieve carbon neutral, among which carbon replacement is the backbone. New energy has become the leading role of the third energy conversion and will dominate carbon neutral in the future. Nowadays, solar energy, wind energy, hydropower, nuclear energy and hydrogen energy are the main forces of new energy, helping the power sector to achieve low carbon emissions. “Green hydrogen” is the reserve force of new energy, helping further reduce carbon emissions in industrial and transportation fields. Artificial carbon conversion technology is a bridge connecting new energy and fossil energy, effectively reducing the carbon emissions of fossil energy. It is predicted that the peak value of China’s carbon dioxide emissions will reach 110×10⁸ t in 2030. The study predicts that China's carbon emissions will drop to 22×10⁸ t, 33×10⁸ t and 44×10⁸ t, respectively, in 2060 according to three scenarios of high, medium, and low levels. To realize carbon neutral in China, seven implementation suggestions have been put forward to build a new “three small and one large” energy structure in China and promote the realization of China's energy independence strategy.

Based on the fine characterization of weathering crust paleokarst landform of the Ordovician Yingshan Formation in Lungu area, Tarim Basin, the size, number and reservoir controlling characteristics of karst monadnocks have been investigated quantitatively. Impression method and karst monadnock volume formulas were employed to characterize the various levels of karst microtopography, to work out the number, relative relief, area, volume and other parameters of monadnocks, and analyze the reservoir development situation at different parts of monadnock. The results showed that the weathering crust karst reservoirs in Lungu area are 140-160 m in relative altitude, and thinning from karst highland to karst slope and terrace. Two hundred fifty-two karst monadnocks have been identified in the study area. Among them, typeⅠand typeⅡmonadnocks are fewer in number, but larger in bulk volume, and are favorable reservoir development zone. The monadnocks have a high coincidence ratio with current structural high points of 96.42%, and the monadnock morphology has remained basically unchanged during the late tectonic movement, thus “karstification-reservoir-accumulation” configuration has been kept good. In the main part (core) of karst monadnocks, there are cave, vug, fracture-vug, and fracture reservoirs, thicker high quality reservoir, and all reservoir parameters are better than those of karst monadnock wings. It is found that the relative amplitude of karst monadnock is positively correlated with single well productivity, and the scale and position of monadnock are closely related with productivity.

The progress in pore structure characterization, hydrocarbon occurrence state, mechanism of oil and gas accumulation, main controlling factors and high production model of unconventional oil and gas is reviewed. The unconventional oil and gas geological research developed from observation of the nanopores to quantitative full scale and 3D pore structure characterization, from macroscopic occurrence state study to microscopic occurrence state evolution discussion, from differential pressure drive and preferential channel migration to staged accumulation and wettability preferential migration, from accumulation controlled by source to accumulation jointly controlled by source-reservoir assemblage and preservation conditions, from accumulation model to enrichment and high production model, revealing the research progresses and future trends of unconventional oil and gas geology. Challenges are presented in unconventional oil and gas geological theory, enrichment conditions and recoverable resources potential of deeply buried unconventional oil and gas, combination of unconventional oil and gas geological research and engineering technique, and basic geologic research for joint mining of different unconventional oil and gas resources.

Based on seismic, logging, formation testing, core and lab test data, this study analyzed the sequence division, facies features of deep water deposits and modes, development of large-scale gravity flow, reservoir physical properties and their main controlling factors, and proposed a classification standard and prediction method of favorable exploration areas in deep water area of the Bin1 oil layers of the lower sub-member of the first member of Paleogene Shahejie Formation in Banqiao-Qibei slope zone of Qikou sag, Bohai Bay Basin. The Bin1 oil layers can be divided into three fifth-order sequences, each less than 100 m thick; a set of gravity flow deposits were formed under deep water background in the slope zone, which contains sedimentary micro-facies such as main channel, distributary channel, channel margin, inter-channel mudstone, and turbidite sand sheet in areas without channels, and, in space, has inherited and constructive development features of multistages. It is a sedimentary sequence of fan delta - distal subaqueous fan - deep lake, and every distal subaqueous fan formed by gravity flow can be divided into inner-, middle- and outer fans. The cross-facies transported sands which are sourced from higher-sand-content major sands of delta front can form high quality reservoirs with an average porosity of 15.1% and geometric average permeability of 5.1×10^-3 μm². The main channel and distributary channel of distal subaqueous fan are the most favorable exploration zones.

The Ordovician Wufeng Formation-Silurian Longmaxi Formation organic-rich shales distributed widely and stably in Southern Sichuan Basin were investigated based on drilling data. Geological evaluation of wells show that the shale reservoirs have good properties in the Yibin, Weiyuan, Zigong, Changning, Luzhou, Dazu areas, with key parameters such as TOC, porosity, gas content similar to the core shale gas production zones. Moreover, these areas are stable in structure, good in preservation conditions and highly certain in resources. The shale reservoirs have a burial depth of 4 500 m or shallow, a total area of over 2×10⁴ km² and estimated resource of over 10×10¹² m³, so they are the most resource-rich and practical areas for shale gas exploitation in China. Through construction of the Changning-Weiyuan national demonstration region, the production and EUR of shale gas wells increased significantly, the cost of shale gas wells decreased remarkable, resulting in economic benefit better than expected. Moreover, the localized exploration and development technologies and methods are effective and repeatable, so it is the right time for accelerating shale gas exploitation. Based on the production decline pattern of horizontal wells at present and wells to be drilled in the near future, at the end of the 13th Five Year Plan, the production of shale gas in southern Sichuan Basin is expected to reach 10 billion cubic meters per year. The resources are sufficient for a stable production period at 30 billion cubic meters per year, which will make the South Sichuan basin become the largest production base of shale gas in China.

Researches were carried out on the origin of gas hydrate samples from the tundra in the Qilian Mountain, Pearl River Mouth Basin in the northern South Sea and the continental slope of Taixinan Basin in China. Gases of the gas hydrate samples from the Jurassic Jiangcang Formation in the Muli County in Qilian Mountain are mainly of oil-derived origin, characterized by self-generation and self-preservation. δ¹³C₁ values range from -52.7‰ to -35.8‰, and the δ¹³C₂ values vary from -42.3‰ to -29.4‰. There was a small amount of coal-derived gases, which might source from the coal-bearing Middle-Jurassic Muli Formation with δ¹³C₁ of -35.7‰ - -31.3‰ and δ¹³C₂ of -27.5‰ - -25.7‰. Gases of the gas hydrate samples from the Pearl River Mouth Basin and Taixinan Basin are dominated by bacterial origin of carbonate reduction, with δ¹³C₁ of -74.3‰ - -56.7‰ and δD₁ of -226‰ - -180‰. A trace amount of thermogenic gases were also found in these basins with δ¹³C₁ of -54.1‰ - -46.2‰. This study combined the geochemical data of gas hydrates from 20 areas (basins) in the world, and concluded that thermogenic gases of the gas hydrates in the world can be either of coal-derived or oil-derived origin, but dominated by oil-derived origin. A small amount of coal-derived gas was also found in the Qilian Mountain in China and the Vancouver Island in Canada. The coal-derived gas has relatively heavy δ¹³C₁ ≥ -45‰ and δ¹³C₂ > -28‰, while the oil-derived gas has δ¹³C₁ from -53‰ - -35‰ and δ¹³C₂ < -28.5‰. Gas hydrates in the world mainly belong to bacterial origin of carbonate reduction. Methanogensesis of acetate fermentation was only found in some gas hydrates from the Baikal basin in Russia. Bacterial gases of carbonate reduction have relatively heavy δD₁ ≥ -226‰, while gases of acetate fermentation have δD₁ < -294‰. The bacterial gas of gas hydrates in the world has the highest δ¹³C₁ value of -56.7‰ and lowest of -95.5‰, with a peak range of -75‰ - -60‰. Gas hydrate in the world has the highest δ¹³C₁ of -31.3‰ and lowest of -95.5‰ and the highest δD₁ of -115‰ and lowest of -305‰.

In-situ conversion processing (ICP) of shale oil underground at the depth ranging from 300 m to 3 000 m is a physical and chemical process caused by using horizontal drilling and electric heating technology, which converts heavy oil, bitumen and various organic matter into light oil and gas in a large scale, which can be called “underground refinery”. ICP has several advantages as in CO₂ capture, recoverable resource potential and the quality of hydrocarbon output. Based on the geothermal evolution mechanism of organic materials established by Tissot et al., this study reveals that in the nonmarine organic-rich shale sequence, the amount of liquid hydrocarbon maintaining in the shale is as high as 25% in the liquid hydrocarbon window stage (R_o less than 1.0%), and the unconverted organic materials in the shale interval can reach 40% to 100%. The conditions of organic-rich shale suitable for underground in-situ conversion of shale oil should be satisfied in the following aspects, TOC higher than 6%, R_o ranging between 0.5% and 1%, concentrated thickness of organic-rich shale greater than 15 meters, burial depth less than 3 000 m, covering area bigger than 50 km², good sealing condition in both up- and down-contacting sequences and water content smaller than 5%, etc. The shale oil resource in China’s onshore region is huge. It is estimated with this paper that the technical recoverable resource reaches 70-90 billion tons of oil and 60-65 trillion cubic meters of gas. The ICP of shale oil underground is believed to be a fairway to find big oil in the source kitchen in the near future. And it is also believed to be a milestone to keep China long-term stability of oil and gas sufficient supply by putting ICP of shale oil underground into real practice in the future.

Through analysis of the problems in the production of Gulong shale oil in the Songliao Basin and the scientific exploration of the preliminary basic research, the special characteristics of Gulong shale oil in terms of reservoir space, phase distribution, flow pattern and mineral evolution are proposed, and six basic scientific problems currently faced are concluded, including: (1) The source of organic matter, mechanism of hydrocarbon generation and expulsion, and key factors affecting shale oil abundance; (2) The types and structural characteristics of effective reservoir space and their contribution to porosity and permeability; (3) The genesis and evolution of minerals and their control on reservoir availability, sensitivity and compressibility; (4) The rock mechanical characteristics and fracture propagation law; (5) The shale oil products, phase change law and main control factors of adsorption and desorption conversion; (6) The shale oil-liquid solid-liquid gas interaction mechanism and enhanced oil recovery mechanism. Three key research suggestions are proposed for realizing the large-scale economic utilization of the Gulong shale oil: (1) Deepen research on the mechanism of oil and gas generation and discharge, storage and transportation, to guide the selection of geological sweet spots of shale oil; (2) Deepen research on the compressibility and fracture initiation mechanism to support the selection of engineering sweet spots and optimization of engineering design; (3) Deepen research on the fluid action mechanism under formation conditions, to guide the optimization of development schemes and the selection of technologies for enhancing oil recovery.

Oil and gas resources in low permeability and unconventional reservoirs are playing more and more important role in global energy supply, and are confronted with pressing problems in hard development, quick production decline, low recovery efficiency and high exploitation cost. Therefore, new development concept “man-made reservoirs” and a complete set of techniques and methods are proposed. With “sweet spots” as units, an integrated way of fracturing, injection and recovery is presented for the low permeability oil and gas resources to reconstruct the underground seepage field and petroleum output system and finally to realize enhancement of the recovery efficiency. Well-group development, fracturing and targeted fluid injection are applied to change the underground seepage field, supplement the formation energy, and form “man-made high permeability area” and “reconstructed seepage field”. By integration of information technology including big data, cloud computing, artificial intelligence etc., an integrated information management platform of “man-made reservoirs” including geology, development, production, management and decision has been set up, and large-scale, effective and sustainable development of this kind of resources are realized. Five series techniques are developed including 3D seismic geological evaluation for sweet spot area, well-group platform development, intellectual volume fracture, imbibition displacement and energy complement development, and intellectual management development based on cloud computing for “man-made reservoir”. In China, five blocks of shale gas and tight oil have been tested 235 times, and the effect of tight oil fracturing and output was 2 times better than that before, has achieved business development and showed bright perspectives.

A discrete fracture model for multi-scale flow in large-scale fractured tight oil reservoirs is proposed considering the compressibility of reservoir rock and fluid, and the non-linear flow in the tight matrix. Validation of the model is performed, followed by the field application of the model. The two-point flux-approximation scheme is adopted in the model to calculate conductivity, and small grids at the fracture intersections are eliminated by the “star-delta” transformation method to improve the computational stability. The fully implicit discretization scheme is performed on the temporal domain. Automatic differentiation technique which can improve model establishment efficiency and computational accuracy is applied in the model to solve the numerical model. The model is validated with the simulation results of Eclipse and the historical production data of a long fractured horizontal well in a tight oil reservoir in Xinjiang oilfield. Simulation results of a field-scale reservoir show that the model proposed can simulate reservoirs with large-scale complex fracture systems; well productivity is positively correlated with the scale of the stimulated reservoir volume, and the difference in planar fracture density and fracture connectivity are proved to be the key factors that lead to the heterogeneous distribution of remaining oil in tight oil reservoirs.

Completely taking into account the interferences between fractures as well as the friction effects on injection allocations, a fully coupled finite element method inherited from a verified one is proposed to discuss fracture propagation laws and analyze their impacts on fracture conductivities. Simulations show that although fractures have similar injection allocations that fluctuate around the allocation averaged by fractures, interferences between them lead to their different propagation rates and some fractures even stop propagating for a while. Shorter fractures generally have higher pressure and smaller pressure gradients than longer ones. The pressure differences between fractures result in long fractures having bottlenecking zones far away from the wellbore, and make them vulnerable to screen-out at the inlets and the bottlenecking zones. The effects of the propagation laws on fracture conductivities include: (1) the conductivities in short fractures are weakened by rapid proppant settlement in them; (2) long fractures may lost their conductivities due to screen-out near the wellbore; (3) the conductivities in long fractures decrease because of screen-out at the bottlenecking zones.

Weiyuan shale gas play is characterized by thin high-quality reservoir thickness, big horizontal stress difference, and big productivity differences between wells. Based on integrated evaluation of shale gas reservoir geology and well logging interpretation of more than 20 appraisal wells, a correlation was built between the single well test production rate and the high-quality reservoir length drilled in the horizontal wells, high-quality reservoir thickness and the stimulation treatment parameters in over 100 horizontal wells, the dominating factors on horizontal well productivity were found out, and optimized development strategies were proposed. The results show that the deployed reserves of high-quality reservoir are the dominating factors on horizontal well productivity. In other words, the shale gas well productivity is controlled by the thickness of the high-quality reservoir, the high-quality reservoir drilling length and the effectiveness of stimulation. Based on the above understanding, the development strategies in Weiyuan shale gas play are optimized as follows: (1) The target of horizontal wells is located in the middle and lower parts of Longyi ₁¹ (Wei202 area) and Longyi ₁¹ (Wei204 area). (2) Producing wells are drilled in priority in the surrounding areas of Weiyuan county with thick high-quality reservoir. (3) A medium to high intensity stimulation is adopted. After the implementation of these strategies, both the production rate and the estimated ultimate recovery (EUR) of individual shale gas wells have increased substantially.

A function expression of the oil-water relative permeability ratio with normalized water saturation at high water saturation was proposed based on statistics of measured oil-water relative permeability data in oilfields. This expression fits the later section of conventional relative permeability ratio curve more accurately. Two new water drive characteristic curves at the ultra-high water cut stage (f_w>90%) were derived by combining the new oil-water relative permeability ratio expression and reservoir engineering method. Then, the numerical simulation results of five point well pattern and production data of Yangerzhuang Oilfield and Liuzan Oilfield were used to verify the adaptability of the new water drive characteristic curves. The results showed that the new water drive characteristic curves are more accurate than conventional water drive characteristic curves after A type and B type water drive curves rise, and can be used to predict production performance at ultra-high water cut stage, ultimate recovery efficiency and recoverable reserves.

The source rock quality, organic pore structure, occurrence state and sealing mechanisms of shale gas in the Ordovician Wufeng-Longmaxi Formation (O₃w-S₁l), Fuling region, Sichuan Basin were studied using ultra-microscopic organic maceral identification, FIB-SEM, high temperature/pressure isothermal adsorption and isotopic age dating of noble gas. The results show that: (1) O₃w-S₁l organic-rich shale was mainly formed in a sedimentary environment with high productivity in surface water and hypoxia in bottom water, it can be divided into two sections according to TOC, of which the lower section (TOC≥3%) is mainly composed of graptolite, phytoplankton, acritarch, bacteria and solid bitumen, among them, graptolite is the main contributor to TOC, but the shale gas is mainly derived from phytoplankton, acritarch and other hydrogen-rich organic matter, as well as the pyrolysis of liquid hydrocarbons produced by this kind of organic matter. (2) Organic pores, as principal reservoir space for shale gas, exist in hydrogen-rich organic matter and solid bitumen. The graptolites and plenty of other organic matter stacking distribution in lamina provide more reservoir space for shale gas, and effective pathways of connected pores for fluid flow. (3) Shale gas in Fuling region is in supercritical state and dominated by free gas; the match of formation time of closed shale gas system and gas-generation peak, as well as slight alteration degree of sealing conditions in the later stage, are key factors controlling the retention and accumulation of shale gas in the regions with high thermal maturity and complex structural areas; adsorption, capillary sealing and slow diffusion of shale are the main microscopic mechanisms for the retention and accumulation of shale gas. It thus can be seen that the generation and accumulation of marine shale gas with high thermal maturity in complex structure areas is controlled jointly by anoxic depositional environment, excellent hydrocarbon rock quality, superior reservoir space and favorable sealing conditions.

The development theories of low-permeability oil and gas reservoirs are refined, the key development technologies are summarized, and the prospect and technical direction of sustainable development are discussed based on the understanding and research on developed low-permeability oil and gas resources in China. The main achievements include: (1) the theories of low-permeability reservoir seepage, dual-medium seepage, relative homogeneity, etc. (2) the well location optimization technology combining favorable area of reservoir with gas-bearing prediction and combining pre-stack with post-stack; (3) oriented perforating multi-fracture, multistage sand adding, multistage temporary plugging, vertical well multilayer, horizontal and other fracturing techniques to improve productivity of single well; (4) the technology of increasing injection and keeping pressure, such as overall decreasing pressure, local pressurization, shaped charge stamping and plugging removal, fine separate injection, mild advanced water injection and so on; (5) enhanced recovery technology of optimization of injection-production well network in horizontal wells. To continue to develop low-permeability reserves economically and effectively, there are three aspects of work to be done well: (1) depending on technical improvement, continue to innovate new technologies and methods, establish a new mode of low quality reservoir development economically, determine the main technical boundaries and form replacement technology reserves of advanced development; (2) adhering to the management system of low cost technology & low cost, set up a complete set of low-cost dual integration innovation system through continuous innovation in technology and management; (3) striving for national preferential policies.

Carbon dioxide capture, EOR-utilization and storage (CCUS-EOR) are the most practical and feasible large-scale carbon reduction technologies, and also the key technologies to greatly improve the recovery of low-permeability oil fields. This paper sorts out the main course of CCUS-EOR technological development abroad and its industrialization progress. The progress of CCUS-EOR technological research and field tests in China are summarized, the development status, problems and challenges of the entire industry chain of CO₂ capture, transportation, oil displacement, and storage are analyzed. The results show a huge potential of the large-scale application of CCUS-EOR in China in terms of carbon emission reduction and oil production increase. At present, CCUS-EOR in China is in a critical stage of development, from field pilot tests to industrialization. Aiming at the feature of continental sedimentary oil and gas reservoirs in China, and giving full play to the advantages of the abundant reserves for CO₂ flooding, huge underground storage space, surface infrastructure, and wide distribution of wellbore injection channels, by cooperating with carbon emission enterprises, critical technological research and demonstration project construction should be accelerated, including the capture of low-concentration CO₂ at low-cost and on large-scale, supercritical CO₂ long-distance transportation, greatly enhancing oil recovery and storage rate, and CO₂ large-scale and safe storage. CCUS-EOR theoretical and technical standard system should be constructed for the whole industrial chain to support and promote the industrial scale application, leading the rapid and profitable development of CCUS-EOR emerging industrial chain with innovation.

Based on the geochemical characteristics of condensates and gases, in combination with geological background, the origin and formation of condensate and oil-gas accumulation mechanism of the deepwater Lingshui 17-2 gas field in the Qiongdongnan Basin, western South China Sea are discussed. The condensate from Lingshui 17-2 gas field has the features of low density, low wax content and high Pr/Ph value. The condensate is generated at mature stage while the co-existing gas is dominated by high mature coal-derived gas. The oil and gas are derived from the Oligocene Yacheng Formation source rocks. The formation of the condensate is related not only to the source rock, but also to the later “gas-washing”. The light oil and gas which was generated at oil-window stage and accumulated in reservoirs have strong reaction of evaporation fractionation during “gas-washing” by high mature natural gas injected largely at later period. The Yacheng source rocks provided sufficient oil and gas, diapiric fractures formed migration paths for oil and gas, and two stages of hydrocarbons charged and accumulated. The Miocene Huangliu Formation turbidite sandstones lithologic trap and Paleogene structural trap close to source Kitchen in the central canyon have favorable oil-gas exploration potential.

Based on outcrop, core, logging and mud logging data, and modern deposition analog, the sandbodies genetic types and spatial distribution in different facies belts of the shallow water delta in the first Member of Cretaceous Yaojia Formation (K₂y₁) of Qian’an area in Changling sag of southern Songliao Basin were analyzed, and the sedimentary model of shallow water delta was established according to deposition process. Active channel sandbodies, point bar sandbodies and bifurcation bar sandbodies in distributary channels and complex sandbodies formed the main sandbodies in the delta plain, while terminal distributary channels and mouth bar sandbodies formed the main sandbodies in the delta front. The delta prograded gradually under the construction of river mouth and reconstruction of river erosion, the early delta front evolved into delta plain and was reformed by distributary channels, and new delta fronts were formed at the river mouth ahead constantly. Under the control of the above deposition process, the size of main distributary channels in delta plain-delta front declined, while mouth bars developed increasingly. The sandbodies of different genetic types formed framework of river-dominated shallow water delta. The point bar and active channel sandbodies in main distributary channels of the delta, with the best reservoir quality, are lithologic reservoir targets in future exploration.

Nine sets of sand-box experiments were designed according to actual geologic data to investigate the evolution pattern and development mechanism of simple shear strike-slip fault zone using the fault CT scanning technique. The experiment results show that R (Riedel) shear faults were developed early and more in number, and one set of these faults intersect with the principle displacement zone (PDZ) at lower angle; the P shear faults (being in symmetrical distribution with the R shear faults) and Y shear faults (subparallel to PDZ) were developed later than the R shear faults, and the fault zone was through-going only after the formation of Y shear faults. The through-going process of strike-slip fault zone can be divided into four stages: embryonic stage, R (Riedel) shear fault development stage, P shear fault and Y shear fault development stage, and through-going stage of fault zone. In the process, the faults developed progressively from the basement to the top cover, the faults spread upward in the profile at embryonic stage, and spread outward at first and then converged toward the PDZ on the plane at R shear faults development stage. The modeling demonstrates that the en echelon structure developed in “helicoidally drag” pattern, the length of the en echelon fault grew linearly at two times the growth rate of its depth, and the fault intersection angle with the PDZ decreased in quadratic function with the increase of its depth. The analysis reveals that cover thickness and the maximum principal stress direction are the main factors causing the difference in “helicoidally drag” structure. The modeling provides guidance for the strike-slip fault interpretation and evolution study, and for layering and segmentation of faults in the marine carbonates of the Tarim Basin.

The Chang7 Member of the Triassic Yanchang Formation in the Ordos Basin is a typical continental tight oil province. The geological conditions and the main controlling factors for the formation of tight oil province in Chang 7 Member were studied based on extensive core analysis data, laboratory simulation tests and practical work of tight oil exploration and development in the basin. The tight oil in the Chang 7 Member is characterized by wide distribution, excellent source rock conditions, tight sandstone reservoirs, complicated pore-throat structure, poor physical properties, high oil saturation, high quality oil, and low pressure coefficient. During the depositional period of Chang7 Member, the bottom shape of the basin was steep at southwest and gentle at northeast, the tectonic movements were active, favorable for the deposition of source rock and reservoir; the widespread high quality source rock can provide sufficient oil supply for the large tight oil province; the large scale sand bodies provide good reservoir condition for the large tight oil province; the abundant structural fractures in the tight reservoir act as pathways for tight oil migration; and the stable deposition and tectonic evolution of the basin provide good preservation conditions for the tight oil province. The main controlling factors of Chang7 Member tight oil enrichment are as follows: (1) good configuration of source rock and reservoir and constant charging are the key to the formation of the tight oil province; (2) abundant micro-scale pores are the premise of tight oil enrichment; (3) strong and sustained charging guarantees the enrichment and high yield of oil in the tight reservoirs.

Carbonate reservoirs in China have the characteristics of diversified accumulation pattern, complex structure and varying reservoir conditions. Concerning these characteristics, this article tracks the technical breakthroughs and related practices since the 1950s, summarizes the developed theory and technologies of carbonate reservoir development, analyzes their adaptability and problems, and proposes their development trend. The following theory and technologies have come into being: (1) carbonate reservoir formation mechanisms and compound flow mechanisms in complex medium; (2) reservoir identification and description technologies based on geophysics and discrete fracture-vuggy modeling method; (3) well testing analysis technology and numerical simulation method of coupling free flow and porous media flow; (4) enhanced oil recovery techniques for nitrogen single well huff and puff, and water flooding development techniques with well pattern design in spatial structure, changed intensity water injection, water plugging and channel blocking as the core; (5) drilling and completion techniques, acid fracturing techniques and its production increasing techniques. To realize the efficient development of carbonate oil and gas reservoirs, researches in four aspects need to be done: (1) complex reservoir description technology with higher accuracy; (2) various enhanced oil recovery techniques; (3) improving the drilling method and acid fracturing method for ultra-deep carbonate reservoir and significantly cutting engineering cost; (4) strengthening the technological integration of information, big data, cloud computation, and artificial intelligence in oilfield development to realize the smart development of oilfield.

Through a comprehensive review of the development status of workover technology of PetroChina Company Limited (PetroChina), this paper presents the connotation of workover operation under the background of the new era, the latest progress of workover operation in the respects of equipment, tools, technology and the construction of information and standardization. The gaps between PetroChina and foreign counterpart in workover technology are as follows: the level of automation and intellectualization of tools and equipment is relatively low, the snubbing operation in gas wells characterized by HT/HP and high H₂S is lagged behind; water plugging in the long horizontal wellbore needs to be further developed, coiled tubing and its relevant equipment for ultra-deep well operation has to be optimized; informationization, standardization and big data application of workover operation need to be started. Based on this as well as the development status of workover technology in China and the technical difficulties faced in the future, eight suggestions for future development are put forward: (1) strengthen the dynamic understanding of reservoir and improve the pertinence of workover schemes; (2) develop the general overhaul technology in a systematical way to tackle issues of seriously problematic wells; (3) put more efforts into the research of horizontal well workover operation and develop relevant technology for long horizontal section operation; (4) improve the snubbing technology and extend its applications; (5) expand the capacity of coiled tubing operation and improve the level of special operations; (6) develop automatic workover technology into the field of artificial intelligence; (7) promote clean operation in an all-round way and build green oil and gas fields; (8) perfect the informationization construction to realize the sharing of workover resources.

For typical blocky, laminated and bedded mudrock samples from the Paleogene Shahejie Formation in the Dongying Sag of Bohai Bay Basin, this work systematically focuses on their structure characterization of multiple micro-nano pore networks. A use of mercury injection capillary pressure (MICP) documented the presence of multiple μm-nm pore networks, and obtained their respective porosity, permeability and tortuosity. Different sample sizes (500-841 μm GRI fractions, 1 cm-sized cubes, and 2.54 cm in diameter and 2-3 cm in height core plugs) and approaches (low-pressure N₂ gas physisorption, GRI matrix permeability, MICP, heliumpy cnometry, and pulse decay permeameter) were used to measure pore size distribution, porosity and permeability. The average porosity and matrix permeability determined from MICP are (6.31±1.64)% and (27.4±31.1)×10^-9 μm², the pore throat diameter of pores is mainly around 5 nm, and the median pore throat diameter based on 50% of final cumulative volume is (8.20±3.01) nm in shale. The pore-throat ratios decrease with a decrease of pore size diameter. Moreover, the permeability of shale samples with lamination is nearly 20 times larger than matrix permeability. The geometrical tortuosity of the nano-scale 2.8-10.0 nm pore networks is 8.44 in these shales, which indicates a poor connectivity of matrix pore network and low flow capability. Overall, the variable and limited pore connectivity of shale samples will affect hydrocarbon preservation and recovery.

Continental shale oil has two types, low-medium maturity and medium-high maturity, and they are different in terms of resource environment, potential, production methods and technologies, and industrial evaluation criteria. In addition, continental shale oil is different from the shale oil and tight oil in the United States. Scientific definition of connotations of these resource types is of great significance for promoting the exploration of continental shale oil from "outside source" into "inside source" and making it a strategic replacement resource in the future. The connotations of low-medium maturity and medium-high maturity continental shale oils are made clear in this study. The former refers to the liquid hydrocarbons and multiple organic matter buried in the continental organic-rich shale strata with a burial depth deeper than 300 m and a R_o value less than 1.0%. The latter refers to the liquid hydrocarbons present in organic-rich shale intervals with a burial depth that in the "liquid window" range of the Tissot model and a R_o value greater than 1.0%. The geological characteristics, resource potential and economic evaluation criteria of different types of continental shale oil are systematically summarized. According to evaluation, the recoverable resources of in-situ conversion technology for shale oil with low-medium maturity in China is about (700-900)×10⁸ t, and the economic recoverable resources under medium oil price condition ($ 60-65/bbl) is (150-200)×10⁸ t. Shale oil with low-medium maturity guarantees the occurrence of the continental shale oil revolution. Pilot target areas should be optimized and core technical equipment should be developed according to the key parameters such as the cumulative production scale of well groups, the production scale, the preservation conditions, and the economics of exploitation. The geological resources of medium-high maturity shale oil are about 100×10⁸ t, and the recoverable resources can to be determined after the daily production and cumulative production of a single well reach the economic threshold. Continental shale oil and tight oil are different in lithological combinations, facies distribution, and productivity evaluation criteria. The two can be independently distinguished and coexist according to different resource types. The determination of China's continental shale oil types, resources potentials, and tight oil boundary systems can provide a reference for the upcoming shale oil exploration and development practices and help the development of China’s continental shale oil.

China’s continental oil and gas geology theory occupies an important academic position in the world's academic circle of petroleum geology. China’s oil and gas resources are dominated by continental resources. Chinese geologists have successfully explored and developed complex continental oil and gas, and developed a continental oil and gas geological theory system. This paper summarizes the development history and theoretical achievements of continental oil and gas geological theory since the 1940s and proposes that the development of this theory should be divided into three stages (i.e., proposal, formation and development). The China’s continental oil and gas geology theory has formed a basically perfect theoretical system consisting of five parts, i.e., continental basin structure, continental lake basin sediments and reservoirs, continental oil generation, continental basin oil and gas accumulation, and continental sandstone oil and gas field development geology. As an advanced geological theory, it has a universal significance globally. This paper focuses on the major discoveries of oil and gas exploration and development and the production growth in the basins of the Central and Western China in the past 30 years as well as the major advances in the continental oil and gas geological theory, including the continental basin tectonics of Central and Western China under the compression background, special reservoir geology such as various types of lake basin sedimentary systems and deep conglomerate, new fields of continental hydrocarbon generation such as coal-generated hydrocarbons, continental oil and gas enrichment regularity such as foreland thrust belts and lithologic-stratigraphic reservoirs, continental unconventional oil and gas geology and continental low-permeability oil and gas development geology. These major advances have greatly developed and enriched the continental oil and gas geology theory and become an important part of it.

The mechanism model of both static and dynamic imbibition considering capillary pressure and gravity was presented based on the imbibition mechanisms and seepage theory. The validation of the model was performed using published experiment data. Then, this model was employed to study the impacts of oil viscosity, matrix permeability, core size, interface tension, and displacement rate on imbibitions. The results show that, the recovery decreases as oil viscosity increases, and the initial imbibition rate is much faster for lower viscosity oil. Imbibitions recovery is positively related to matrix permeability, the differences of oil recovery for low-permeability to tight oil reservoirs are obvious. Imbibitions effect is negatively related to core size. If the interface tension is low, imbibitions cannot occur without consideration of gravity. But it can occur even in very low interface tension scenario with consideration of gravity. On the whole, the recovery first increases and then decreases as the interface tension decreases. The gravity and capillary play different roles at different ranges of interface tension. There exists an optimal value range of displacement rate in fractured reservoir, which should be optimized with a sufficient oil production rate to achieve higher recovery.