Since the beginning of the 21st century, discoveries of global unconventional oil and gas have entered an active period. Production of tight oil/shale oil has increased rapidly, shale gas has continued to grow, tight gas and Coal Bed Methane (CBM) have been stable, and breakthroughs have been obtained in early production tests of natural gas hydrate (NGH). In recent years, significant progress has been made in exploration and development of unconventional oil and gas in China. The production of shale gas, tight oil, and tight gas have been increasing rapidly, and the exploration and development of shale oil have risen to a national strategic level. Tight oil and shale oil resources in the mining-rights blocks of the China National Petroleum Corporation (CNPC) are abundant. Recently, exploration discoveries and breakthroughs have been made in 11 blocks, and several large-scale productivity blocks have been preliminarily established. Pilot tests of in-situ conversion of shale oil with medium-low maturity have been carried out in the Ordos Basin, and the continental shale oil revolution is being actively promoted. Currently, external dependence on oil and gas is high in China, and demand is strong. However, due to the complicated geological conditions of continental facies, large-scale benefit development of unconventional oil and gas still faces a series of challenges in geological evaluation, sweet spot prediction, drilling and completion, oil and gas production technologies, economic evaluation, and management. In order to achieve benefit exploration and development of the unconventional oil and gas resources of CNPC, key issues should be focused on and diligently addressed, such as whole life cycle management, integration operation, big data application, appropriate production allocation strategy, technology and cost, marketization, and so on, to promote high-quality development in the unconventional oil and gas industry and thereby ensure national energy security.

China is rich in geological shale gas resources. However, when compared with North America, these resources are characterized by greater geological age, deep burial depths, and high degrees of thermal evolution, as well as complex structures and surface conditions. It is therefore very challenging to achieve commercial development of shale gas in China. Since 2006, Sinopec’s shale gas exploration and development has experienced three stages: investigation and favorable area selection and evaluation; exploration breakthrough, and rapid progress of exploration and development. In 2012, major breakthroughs were made in marine shale gas exploration, and the first shale gas field in China, the Fuling shale gas field, was efficiently established. The commercial development of deep shale gas in the Weirong block has been achieved, and other exploration areas continuously expanded. The proven reserves and production of shale gas have been increasing rapidly. After more than 10 years of scientific and technological research, and development of major equipment, Sinopec has innovatively pioneered a fine description and comprehensive evaluation technology system for shale gas reservoirs, developed a stereoscopic development adjustment technology, and formed a series of supporting technologies for optimized fast drilling and long horizontal well staged fracturing in mountainous areas, as well as green development of gas fields. In the future, Sinopec will continue to strengthen geological evaluation and exploration, consolidate the resource base for sustainable development, adhere to the development idea of geology- engineering integration, strengthen the upgrading of technical equipment, promote cost decreasing and benefit increasing in the oil and gas industry by using big data and artificial intelligence, emphasize the integrated management of whole projects, and improve the development efficiency of shale gas, so as to achieve a steady increase in shale gas proven reserves and production of Sinopec.

The shale oil and gas revolution in the United States has brought about the rapid development of the unconventional oil and gas industry. The China National Offshore Oil Corporation (hereinafter referred to as CNOOC) is following the trend and is actively opening up its unconventional oil and gas business. After acquiring China United Coalbed Methane Company, CNOOC was faced with a series of challenges in unconventional gas development. For coalbed methane, resource grade is poor, reserve abundance is low, production scale is small, and capacity of productivity construction is poor. For tight gas, the reserves are present but production is low. Combining the characteristics of coalbed methane and tight gas reservoirs with the technical requirements of exploration and development, through the management innovation of exploration and development integration, CNOOC has explored the path to achieving commercial development of unconventional gas and has achieved preliminary results. Through the innovation of management modes and system construction, the rhythm and effect of unconventional gas exploration and development have been obtained, which realize the goal of exploration and drilling, pipeline construction, pilot production, and gas sales in the same year, and greatly shorten the production cycle. This not only improves exploration quality and investment efficiency, but also avoids the risk involved in direct large-scale development and accelerates the pace of increasing reserves and production. It is hoped that, through the exploring and demonstration of a single project and the reform and innovation of its corresponding systems, the healthy and orderly development of the unconventional gas industry in China will be promoted.

As a large oil and gas-rich superimposed basin, the Ordos basin develops multiple sets of Permian and Triassic continental shale strata and is abundant in shale gas resources. Compared with marine shale, continental shale is more complicated, and there are many challenges during its exploration and development which restrict its economic exploitation. In this paper, achievements in the practice of continental shale gas exploration and development in the Ordos basin are summarized in terms of geological characteristics, resources background, and exploration and development technologies. In addition, the current technical difficulties of continental shale gas development are analyzed in order to effectively, and in a timely manner, utilize opportunities and approach challenges in the development process. According to comprehensive analysis, shale gas parameters in the Yanchang exploration area, such as continental shale thickness, total organic carbon content, maturity, and gas content, are above the lower limit for favorable shale gas intervals stipulated by national standards, so the continental shale in the basin possesses the geological conditions for gas accumulation. Under the control factors of macro and micro heterogeneity, continental shale gas has different enrichment characteristics in terms of phase state and scale. There are two types of accumulation modes; adsorption gas accumulation, and adsorption + free gas composite accumulation. Understanding of the different accumulation mechanisms guides the selection of favorable zones and production from continental shale gas wells. A number of key supporting technologies have been formed during the practice of continental shale gas exploration. Logging evaluation methods suitable for strongly heterogeneous continental shale have been established. A water-based drilling fluid system and a series of supporting equipment for drilling and completion have been developed. A CO2 fracturing technology system and wellsite waste fluid recovery and treatment- utilization technology have also been formed. Meanwhile, there are still many challenges in terms of precise prediction of “sweet spots”, acceleration and efficiency of drilling, environmental protection, efficiency of fracturing, and cost reduction. At present, the exploration and development of continental shale gas in China is still in an early stage, with high costs and low initial production. It is still necessary to carry out pilot tests of exploration and development to achieve scientific and technological breakthroughs, develop continental shale gas as a resource replacement, and promote the continental shale gas industry to obtain economic benefits.

Effective development of hard-to-recover oil reserves is an important measure for the enhancement of oil development in China. The hard-to-recover reserves of low permeability and heavy oil in the Shengli oilfield are nearly 6×108 t, accounting for more than 10% of the national total. Effective development faces many challenges, such as a complex oil-water system, lack of supporting technologies, and low productivity. Sinopec has adopted the Shengli oilfield as a pilot project. After more than 2 years of practical cooperative development, a number of effective methods have been explored, such as “innovation of cooperative mechanism”, “searching for sweet spots in low-grade reserves”, “geology-engineering integration”, and “fine reservoir stimulation and increasing productivity”. This has preliminarily achieved the goal of “managing, recovering and producing” hard-to-recover oil reserves. The cumulative produced oil reserves are 4860×104 t, established productivity is over 50×104 t, the drilling cycle has been shortened by more than 60%, single well production has increased by about 40%, and the break-even oil price has dropped from 75 $/bbl to within 50 $/bbl. The initial practice of producing hard-to-recover oil reserves in the Shengli oilfield has formed a unique contracted productivity establishment plan and created an efficient model for geology-engineering integration across the whole industry chain. This has opened up a new direction for the sustainable development of mature oil areas with a high degree of exploration.

Large-scale development of shale gas in China is at the initial stage. Unscientific valuation methods present a major challenge to the optimization management of drilling investment. This paper proposes a set of optimization analysis methods for drilling investment: "valuation method integration + standard well management". The purpose of valuation method integration is to establish a valuation method system for drilling engineering which will meet the needs of the entire process of management of oil and gas exploration and development projects. The premise is to establish unified and standardized calculation rules for the bills of quantities for drilling engineering, cost composition of drilling engineering projects, and a standard valuation system for the whole process of drilling engineering. The standard well management process is to establish several standard well projects for scientific investment decision-making and organization of drilling operation. These methods were applied in the calculation of development well investment and the optimization analysis of cost reducing and benefit increasing in CNPC's special planning for shale gas development in 2021-2030. This paper introduces the analysis of drilling productivity levels and preparation of budget quotas. There are 6 main functions: rough calculation quotas; rough calculation indicators; estimation indicators; reference indicators, and the methods and results of drilling investment calculations.

In January 2020, light crude oil was obtained from Lower Cambrian dolomite with burial depth deeper than 8200 m in Well Luntan-1, which is located in the Lunnan low bulge in the Tabei uplift. The result indicated a major breakthrough in ultradeep oil and gas exploration in the old craton of Tarim. This paper describes the process of discovering Well Luntan-1 and analyzes the discovery history of the ultra-deep Cambrian subsalt oil reservoirs, with the intention of providing reference for the exploration of new zones, new strata and new types in other similar areas. According to drilling results from Well Luntan-1, a set of high-quality source rocks is developed in the Cambrian Yuertusi Formation, and there are 2 sets of reservoir-cap assemblage. One is evaporite gypsum cap rocks in the Awatage Formation and dolomite reservoir in the Shayilike – Wusonggeer Formations. The other is mudstone cap rocks in the Yuertusi Formation and dolomite weathering crust reservoir in the Sinian Qigebulake Formation. The production layer in Well Luntan-1 is the Wusonggeer Formation, which is a volatile-oil reservoir with normal temperature and pressure gradients. In addition, trace natural gas was obtained from the weathering crust of the Sinian Qigebulake Formation. Exploration of the platform-basin transitional area in the Tarim Basin has experienced 2 strategic shifts: from clastic rocks to carbonate rocks, and from carbonate rocks to pre-salt dolomite. Well Luntan-1 is an important symbol of the second strategic shift, which is of milestone significance. The favorable accumulation conditions of Well Luntan-1 are the successive development of stable paleo- uplift and the high-quality source-reservoir-cap assemblages. Through analogy, this paper analyzes the exploration prospects for the Cambrian pre-salt dolomite in the Tarim Basin, and points out that favorable areas, such as the Tazhong-Gucheng area, the south slope of the Tabei area, and the northern Maigaiti slope-Keping area, are the key areas for the next steps in exploration.

Tight oil is one of the primary targets of “exploring oil inside source kitchen” in China. Geology-engineering integration is the only way to achieve large-scale benefit development of tight oil in China. The tight oil discussed in this paper - oil in shale formations - refers to unconventional continental oil resources that can be economically developed on a large scale using existing technologies such as horizontal well volume fracturing. There are 2 aspects to the concept of “exploring oil inside source kitchen” of tight oil: identifying “sweet spot areas”, and producing from “sweet spot bodies”. According to differences in lithology of the reservoir and source-reservoir assemblages in shale formations, onshore tight oil in China can be divided into 3 main types: clastic rocks; diamictite-sedimentary tuff, and carbonate rocks. Geology-engineering integration for tight oil is a systematic industrial process, based on evaluation and identification of “sweet spot areas (intervals)”, with high and stable production from “sweet spot bodies” as the goal, and “design by reverse thinking and operation by forward construction” as the operating methodology. The focus is on the effective organization, management and integration of geological design and engineering operations, the effective evaluation and description of “sweet spot areas (intervals)”, stimulation and development of “artificial reservoirs”, and, finally, transforming blueprint designs into engineering operations and benefit production. The examples in this paper include clastic rocks of Fuyu reservoir in the Songliao Basin, Zhahaquan clastic rocks in the southwest Qaidam Basin, diamictite of Lucaogou Formation in the Jimsar Sag, sedimentary tuff of Tiaohu Formation in the Malang Sag, carbonate rocks of Lower Ganchaigou Formation in the Yingxi area of the southwest Qaidam Basin, and shales in the Da’anzhai member in central-northern Sichuan basin. This paper expounds the research progress of tight oil geology-engineering integration in PetroChina’s study and exploration areas. Three aspects of this work are described: geological evaluation and prediction; key engineering technologies, and major management measures. In the first three years of the “13th Five-Year plan”, newly-added three-level reserves were 9.97×108 t, and newly-established productivity was 225×104 t, achieving a “double harvest” of the oil and gas discovery and the construction of productivity. Finally, some preliminary suggestions for the future are proposed, which include: relying on the development of tight oil geology-engineering integration; strengthening multi-layer stereoscopic development of “artificial reservoirs”; establishing a “reference template” for different oil types in small blocks, and paying close attention to source rock oil and gas in shale formations.

Geology-engineering integration emphasizes the interaction between geological research and engineering operations. In China, the geological conditions of unconventional oil and gas fields are complex. As a result, geological modeling faces unique challenges in terms of data foundation and application requirements. On the one hand, the data are mainly from horizontal wells, so data volumes are large and data types are miscellaneous. On the other hand, geological models are required to iterate rapidly, or even perform “timely” modeling during operational processes. Therefore, it is critical to make full use of the various data to quickly establish high-quality geological models. This paper focuses on the unique nature of unconventional reservoir modeling, taking shale gas reservoirs as an example, and proposes specific workflows and methods. First, the geological modeling workflow of horizontal wells is systematically described. Through cycle correlation in true thickness domain and 2D geo-steering section and integration of seismic and well data, the difficulties of structure and attribute modeling of horizontal wells have been solved from one- to three-dimensions. Second, taking ant tracking technology as an example, natural fracture prediction and modeling methods are summarized. Fractures are widely developed in unconventional reservoirs. Based on understandings of the geological settings of fracture development, cross-validation from multidisciplinary data, such as FMI logging, drilling and micro-seismic data, is helpful in achieving reasonable fracture modeling. Third, geological modeling workflows and their application under different requirements are described, such as well deployment optimization, timely modeling to support geo-steering and fracturing operations by multi-disciplinary integration.

Compared with North America, the exploration and development of unconventional oil and gas resources in China began relatively late. Shale gas exploration and development started at the beginning of the “12th Five Year Plan” period. The exploration and development of tight oil, tight gas and shale oil started at the beginning of the “13th Five Year Plan”, and developed rapidly during the “13th Five Year Plan” period. In particular, development of shale gas in Sichuan and Chongqing, conglomerate oil field in the Mahu sag, and shale oil reservoir in the Jimsar sag in the Junggar Basin, have entered the stage of large-scale development, becoming the most important field for oil and gas production increase at the end of the “13th Five Year Plan” and into the period of the “14th Five Year Plan” of CNPC. Compared with conventional oil and gas, unconventional oil and gas resources have poor reservoir conditions, which require large-scale “horizontal well + volume fracturing” to achieve effective development. However, due to limitations of rig equipment and performance of horizontal well drilling tools, and especially shortcomings in concepts and methods, issues of low drilling efficiency and long well construction periods are common in unconventional horizontal wells in China. Generally, the drilling period of a horizontal well is 60-90 days, with a vertical depth of 2,000-3,000 m and a horizontal section of 1,500-2,000 m. In North America, the drilling period of a horizontal well can basically be controlled within 15-25 days, with a vertical depth of 2,000- 3,000 m and a horizontal section length of 2,000-3,000 m. A drilling rig in China can only drill 2-3 horizontal wells a year, while in North America a drilling rig can complete 15-20 horizontal wells a year. With the large-scale development of unconventional oil and gas resources in China, the challenge of drilling rig shortage is increasing. Drilling efficiency has become a bottleneck problem which seriously restricts the rapid and large-scale production of unconventional oil and gas resources in China. There are two advantages which support the great improvement of drilling efficiency of horizontal wells in North America: management and technology. In technology, there are five main aspects: the first is the continuous upgrading of supporting capacity of drilling rig; the second is improvement of the reliability and stability of downhole tools; the third is large-platform factory operation; the fourth is remote real-time support and decision-making systems, and the fifth is the application of the concepts and methods of systematic drilling optimization. In 2018, Strait Energy cooperated with K&M drilling engineering consulting company of the United States to introduce internationally-advanced concepts and methods for systematic drilling optimization. Pilot testing of drilling speed increase in horizontal wells was first carried out by Changning Shale Gas Co., Ltd. in the Sichuan and Chongqing area, and in the conglomerate oil field in the Mahu sag in Xinjiang, which greatly shortened the well construction period and verified the feasibility of systematic drilling optimization methods in the development of unconventional oil and gas resources in China. In order to further promote this, testing of speed increase in the whole block was carried out in the conglomerate oil field in the Mahu sag in 2019, which also achieved remarkable results. This further proves that the concepts and methods of systematic optimized drilling speed increase are scientific, universal and replicable, which is an important shift in practice from experience-based drilling to scientific drilling, and shows very promising prospects for popularization and general application.

With the expansion of global oil and gas exploration towards deep, subtle, and unconventional reservoirs, new breakthroughs have been made in oil and gas exploration in China. The Mahu tight conglomerate reservoirs in the Junggar Basin, Xinjiang, are rich in oil resources and have great potential for the establishment of large-scale productivity. However, the factors that affect oil flow capacity in the reservoir are complex and diverse. Also, there are a number of challenges during development, for example; reservoir heterogeneity is strong, determination and classification evaluation of “sweet spots” are difficult, the difference in horizontal principal stress between the two directions is large, the formation and extension mechanisms of hydraulic fractures is unclear, costs are high, and geological conditions are complex. Guided by geology-engineering integration, combining the geological characteristics of the study area with production practice, a “sweet spot” classification evaluation standard for this type of reservoir has been established. Intensive volume fracturing, stereoscopic development technologies and tests of “long horizontal section, intensive segments, small well spacing, large well clusters and well factory operation” have been explored, forming a set of technical systems for efficient development of tight conglomerate reservoirs and have achieved good results. The improvements of accuracy in “sweet spot” description and 3D geological modeling have significantly increased the efficiency of drilling and completion engineering. The average number of adjustment times of horizontal section trajectory has decreased from 5 times to 2. Penetration rate of the oil layer has reached more than 90%. The average cluster spacing in the Ma-131 well block decreased from 67 m to 35 m. Average cumulative production increased by 37.5% in 300 days.

In the Kelasu structural belt of the Kuqa Depression in the Tarim Basin, the burial depth of reservoir is deep and the geological conditions are complex, which leads to great challenges in drilling and completion operations, such as well control safety, well construction time efficiency, engineering quality, single well production increase and so on. In view of these key technical issues, which restrict the exploration and development process, a set of technical processes of geology-engineering integration with geo-mechanics as a bridge has been established, forming a series of methods, such as well location selection, well trajectory optimization, pre-drilling formation pressure prediction, borehole stability prediction, completion and stimulation optimization, and sand and water production prediction in development. Geological research, engineering design, and on-site operation, are organized and integrated into a collaborative system to solve engineering issues and achieve “3 improvements” in the Kelasu structural belt: improvements in drilling speed, improvements in quality, and improvements in production. The drilling success rate of development wells is 100%, complex drilling issues in single wells are reduced by 40% on average, and well control safety is effectively guaranteed. After stimulation, the average open gas flow of a single well increased from 68×104 m3/d to 279×104 m3/d. Gas productivity of 150×108 m3/a in the eastern Kelasu area has been successfully established and contributed to the major discovery of a trillion square meters of gas reservoirs in the western Kelasu area, which has provided technologies and accumulated experience for efficient development of ultra-deep complex reservoirs.

The Permian Lucaogou Formation in the Jimsar sag, in the Junggar Basin, Xinjiang is rich in continental shale oil resources, with an estimated resource of more than 15.8×108 t. In recent years, in order to realize the effective development of shale oil, and based on detailed analysis of the geological characteristics, engineering technologies, production characteristics and laws of continental shale oil in the Lucaogou Formation, combined with development practice, the following achievements and understandings have been obtained: ① The source rocks of the Lucaogou shale oil in the Jimsar sag are fine-grained continental shale deposited in saline lakes. They are high quality source rocks with large thickness, and are characterized by integrated sources and reservoirs. Oil accumulated in-situ in the shale intervals and, in the sweet spot intervals, hydrocarbons were mainly supplied from adjacent source rocks with self-generation as auxiliary. All of these characteristics indicate typical continental shale oil. ② It is clear that the free porosity and free oil reserves abundance in the fracture controlling areas created by volume fracturing from horizontal wells are the basis for high production of shale oil. ③ With increase in burial depth, the horizontal stress difference between the two directions increases, and the complexity of hydraulic fractures decreases, which is the main reason for the low production of some horizontal wells with good oil-bearing properties in the “upper sweet spot interval”. ④ Oil viscosity is the key control factor for production from horizontal wells in the “lower sweet spot interval”. ⑤ The penetration length of high quality “sweet spots” shale and the proppant volume per meter during hydraulic fracturing are the key engineering factors for high production from horizontal shale oil wells in the Jimsar sag. ⑥ Due to imbibition and replacement between fracturing fluid and shale oil in the matrix pores, production can be improved by shutting in wells for some time after fracturing. ⑦ Due to the limited length of hydraulic fractures, the appropriate well spacing should not be larger than 200 m. Through continuous research and development practice, shale oil development has obtained good results. The maximum annual production of single horizontal well has been more than 1.3×104 m3. Since 2019, the area has basically entered the stage of scale productivity construction test, accumulating rich experiences for the industrial development of continental shale oil in China.

The high temperature and high pressure area in the western South China Sea is rich in oil and gas resources. The buried depth of the main target layer in this area is deeper than 4000 m, the formation pressure coefficient is greater than 2.2, and the temperature is about 200 ?C. Therefore, exploration and development operations in this area are extremely difficult. The conventional operation mode cannot meet the operating requirements for high temperature and high pressure wells. Based on this situation, according to the geological and engineering characteristics of high temperature and high pressure wells, the concept and technical system of geology-engineering integration of high temperature and high pressure wells in the South China Sea is proposed, and described from the aspects of management innovation, technology optimization and big data. Research and practice have shown that the geological and engineering issues which affect high temperature and high pressure wells in the South China Sea can be effectively solved by optimizing the drilling process and data acquisition scheme through management and design of geology-engineering integration, establishing large databases for geology-engineering integration to achieve multi-source information sharing, and studying the prediction, monitoring and effective control technologies of geology-engineering integration. A series of integrated special operation systems have been formed, such as “combination technology of Acoustic-Logging-While- Drilling (ALWD), midway-VSP and look-ahead-while-drilling”, “the six-in-one pre-monitoring technology of seismic, drilling, logging-while-drilling, mud logging, midway-VSP, and look-ahead-while-drilling combination”, “geology-engineering intelligent monitoring and safety early-warning system”, etc. These operation systems have achieved good application results in fine prediction, monitoring and control of formation depth, abnormal high pressure and drilling conditions. In the future, further big data platforms and intelligent exploration – development – drilling and completion schemes will be established, the application of geology- engineering integration will be deepened, and the exploration and development of similar complex oil and gas resources supported.

The tight oil reservoirs of the Chang-7 member in the Longdong area of the Ordos Basin are primarily gravity flow sand bodies. The distribution of sand bodies is complex in both vertical and lateral directions, and the thickness of single sand bodies is thin, which poses a challenge to efficient drilling and production. In this paper, a set of geology-engineering integration methods are proposed, which combine multi-disciplinary knowledge such as near-bit ‘Measurement While Drilling’ (MWD) technology. On the basis of comprehensive geological research, 3D fine geology, reservoir, and geo-mechanics, models are established for selection of well locations, design of factory-like platforms, drilling operations, and optimization of geo-steering schemes. The aim is to design well trajectories scientifically and reasonably, enhance the penetration rate of sand bodies during drilling, and ensure higher single well production in later production stages as well as the ultimate long-term cumulative production of the well block. Results show that high-quality reservoirs in the study area are mainly massive clastic-flow sandstones. The key to improving penetration rates is to use real-time transmission MWD data, and comprehensively analyze drilling, well logging and mud logging data to determine accurate bit location in the sedimentary cycle in order to determine the geo-steering operation scheme. Guided by this method, two horizontal wells have been drilled, with penetration rates of oil layers 5?10% higher than those of the surrounding wells. According to early-stage numerical simulation based on geology and geo-mechanics models, combined with scientific well spacing during drilling and production practice, the optimum horizontal well spacing in the study area was finally determined to be 400 m.