• Energy Research

Based on the transitional background of the global energy structure, exploration and development of unconventional oil and gas, and investigation of key basins, the unconventional oil and gas resources are divided into three types: source rock oil and gas, tight oil and gas, and retention and accumulated oil and gas. Source rock oil and gas resources are the global strategic supplies of oil and gas, the key resource components in the second 150-year life cycle of the future petroleum industry, and the primary targets for “exploring petroleum inside source kitchen”. The geological connotation of source rock oil and gas was proposed, and the models of source rock oil and gas generation, expulsion and accumulation were built, and five source rock oil and gas generation sections were identified, which may determine the actual resource potential under available technical conditions. The formation mechanism of the “sweet sections” was investigated, that is, shale oil is mainly accumulated in the shale section that is close to the oil generation section and has higher porosity and permeability, while the “sweet sections” of coal-bed methane (CBM) and shale gas have self-contained source and reservoir and they are absorbed in coal seams or retained in the organic-rich black shale section, so evaluation and selection of good “sweet areas (sections)” is the key to “exploring petroleum inside source kitchen”. Source rock oil and gas resources have a great potential and will experience a substantial growth for over ten world-class large “coexistence basins” of conventional-unconventional oil and gas in the future following North America, and also will be the primary contributor to oil stable development and the growth point of natural gas production in China, with expected contribution of 15% and 30% to oil and gas, respectively, in 2030. Challenges in source rock oil and gas development should be paid more attention to, theoretical innovation is strongly recommended, and a development pilot zone can be established to strengthen technology and promote national support. The source rock oil and gas geology is the latest progress of the “source control theory” at the stage of unconventional oil and gas. It will provide a new theoretical basis for the new journey of the upstream business in the post-industry age. Key words: source rock oil and gas, shale gas, shale oil, coal-bed methane, sweet section, sweet area, source control theory, man-made oil and gas reservoir, unconventional oil and gas revolution, large “coexistence basins” of conventional-unconventional oil and gas

The widely distributed, thick Chang 7 Shale is the richest shale oil formation in China. A calculation method for the evaporative hydrocarbon recovery coefficient based on formation volume factor is proposed considering the correction of heterogeneity-based total organic carbon differences to improve the adsorbed oil calculation method, and light hydrocarbon evaporative sampling losses, which can make mobile and total oil calculations more accurate. The adsorbed oil, S1 evaporative loss, total oil yield, and movable oil yield of 200 shale samples from the Chang 7 Member were calculated using the new methods. Results show that S1 evaporative loss accounts for 29% of S1, total oil yield is 3.5 times S1, and movable oil yield accounts for 37% of total oil yield. Based on the calculated total oil yield and movable oil yield results, the relationships among total oil yield, movable oil yield, and total organic carbon of the Chang 7 were established yielding total oil yield and movable oil yield estimates of 11.12 × 109 t and 4.01 × 109 t, respectively, revealing its tremendous shale exploration potential.

The new century has witnessed a strategic breakthrough in unconventional oil & gas. Hydrocarbon accumulated in micro‐/nano‐scale pore throat shale systems has become an important domain that could replace current oil & gas resources. Unconventional oil & gas plays an increasingly important role in our energy demand. Tight gas, CBM, heavy oil and asphaltic sand have served as a key domain of exploration & development, with tight oil becoming a ‘bright spot’ domain and shale gas becoming a ‘hotspot’ domain. China has made great breakthroughs in unconventional oil & gas resources, such as tight gas, shale gas, tight oil and CBM, and great progress in oil shale, gas hydrate, heavy oil and oil sand. China has an estimated (223–263)×108t of unconventional oil resources and (890–1260)×1012 m3 of gas resources. China has made a breakthrough for progress in unconventional oil & gas study. New progress achieved in fine‐grained sedimentary studies related to continental open lacustrine basin large‐scale shallow‐water delta sand bodies, lacustrine basin central sandy clastic flow sediments and marine‐continental fine‐grained sediments provide a theoretical basis for the formation and distribution of basin central reservoir bodies. Great breakthroughs have been made in unconventional reservoir geology in respect of research methodology & technology, multi‐scale data merging and physical simulation of formation conditions. Overall characterization of unconventional reservoirs via multi‐method and multi‐scale becomes increasingly popular and facilitates the rapid development of unconventional oil & gas geological theory, method and technology. The formation of innovative, continuous hydrocarbon accumulation theory, the establishment of the framework of the unconventional oil & gas geological theory system, and the determination of the implications, geological feature, formation mechanism, distribution rule and core technology of unconventional oil & gas geological study lays a theoretical foundation for extensive unconventional oil & gas exploration and development. Theories and technologies of unconventional oil & gas exploration and development developed rapidly, including some key evaluation techniques such as ‘sweet spot zone’ integrated evaluation and a six‐property evaluation technique that uses hydrocarbon source, lithology, physical property, brittleness, hydrocarbon potential and stress anisotropy, and some key development & engineering technologies including micro‐seismic monitoring, horizontal drilling & completion and “factory‐like” operation pattern, “man‐made reservoir” development, which have facilitated the innovative development of unconventional oil & gas. These breakthroughs define a new understanding in four aspects: ① theoretical innovation; ② key technologies; ③ complete market mechanism and national policy support; and ④ well‐developed ground infrastructure, which are significant for prolonging the life cycle of petroleum industry, accelerating the upgrade and development of theories and technologies and altering the global traditional energy structure.

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. Key words: shale oil and gas, man-made reservoir, well cluster development, volume stimulation, imbibition displacement, enhanced recovery, intellectual development

Fluid mobility has been important topic for unconventional reservoir evaluation. The tight sandstones in Chang 7 Member of the Ordos Basin has been selected to investigate the fluid mobility based on the application of core flooding-NMR combined method and core centrifugation-NMR combined method, and the porous structure is studied using optical microscope, field emission scanning electron microscope (FE-SEM), CT and mercury injection. Our results include: (i) Feldsparrock fragments dissolution pores, calcite dissolution pores, clay mineral dissolution pores, intergranular dissolution expansion pores, inter-granular pores, intra-kaolinite pores, and intra-illite/smectite mixed layer pores are developed in Chang 7 tight sandstones; 3D CT pore structure shows that the pore connectivity is positively related to physical properties, and the overall storage space is connected by the throat with diameter between 0.2 and 0.3 µm. The percentage of storage space connected by throats with diameter less than 100 nm can reach more than 35%. (ii) Movable fluid saturation of Chang 7 tight sandstones is between 10% and 70%, and movable oil saturation is between 10% and 50%. Movable fluid saturation may cause misunderstanding when used to evaluate fluid mobility, so it is recommended to use movable fluid porosity in the evaluation of fluid mobility. The porosity ranging from 5% to 8% is the inflection point of the fluidity and pore structure. For samples with porosity less than 8%, the movable fluid porosity is generally less than 5%. Moreover, the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.1 to 1 µm. For samples with porosity greater than 8%, the porosity of the movable fluid is more than 5%, and the movable fluid is mainly concentrated in the storage space with a throat diameter of 0.2 to 2 µm. (iii) The movable fluid saturation measured by core flooding-NMR combined method is generally higher than that measured by core centrifugation-NMR combined method. The former can evaluate the mobility of the oil-water two-phase fluid in samples, while the latter can better reflect the pore structure and directly evaluate the movable fluid in the pore system controlled by different throat diameters. All these results will provide valuable reference for fluid mobility evaluation in tight reservoirs.

Petroleum geology is evolving into two branches, conventional petroleum geology and unconventional petroleum geology, with the latter becoming a new theoretical frontier in the petroleum industry. The core of conventional hydrocarbon geological study is based on identifying the match between source rock, reservoir, caprock, migration, trap, preservation and timing; the core of unconventional hydrocarbon geological study evaluates if the oil and gas is part of a continuous accumulation, where stress is placed on the evaluation of “lithology, physical properties, brittleness, oiliness, source rock features, stress anisotropy” and their configuration. The oil and gas accumulation mode and theoretical formula at various low limits of pore throat diameter have been established, as well as the “L” type production curve. Theoretical production prediction models for unconventional oil and gas, and formation mechanism and development patterns for unconventional oil and gas are being revealed. The connotation, characteristics, potential and technology for unconventional oil and gas have been observed, and two key marks to identify unconventional hydrocarbon have been put forward: (1) continuous distribution of hydrocarbon-bearing reservoirs over a large area, with no obvious trap boundary; and (2) no natural stable industrial production, and no obvious Darcy flow. Systematic research shows that the proportion of global unconventional to conventional hydrocarbon resources is 8:2, in which the unconventional oil is almost equal to conventional oil, and the unconventional gas is about 8 times that of conventional gas. In China, unconventional oil resources are about 240×108 t and unconventional gas resources are about 100×1012 m3. In recent years the development of tight gas and tight oil should be strengthened to realize industrial reserves and increase production. Construction of shale gas pilot plants and shale oil research should be strengthened. Unconventional oil and gas industrial systems and research should be set up, including unconventional hydrocarbon geology, fine particle sedimentology, unconventional reservoir geology, seismic reservoir prediction, massive fracturing of horizontal wells, “factory-like” operation, low cost management and subsidy policy and personnel training. Key words: unconventional petroleum geology, conventional hydrocarbon geology, unconventional oil and gas industrial system, tight oil and gas, shale oil and gas

The Anyue Sinian–Cambrian giant gas field was discovered in central paleo-uplift in the Sichuan Basin in 2013, which is a structural-lithological gas reservoir, with 779.9 km2 proven gas-bearing area and 4 403.8×108 m3 proven geological reserves in the Cambrian Longwangmiao Formation in Moxi Block, and the discovery implies it possesses trillion-cubic-meter reserves in the Sinian. Cambrian Formations in Sichuan Basin. The main understandings achieved are as follows: (1) Sinian–Cambrian sedimentary filling sequences and division evidence are redetermined; (2) During Late Sinian and Early Cambrian, “Deyang–Anyue” paleo-taphrogenic trough was successively developed and controlled the distribution of source rocks in the Lower-Cambrian, characterized by 20–160 m source rock thickness, TOC 1.7%–3.6% and Ro 2.0%–3.5%; (3) Carbonate edge platform occurred in the Sinian Dengying Formation, and carbonate gentle slope platform occurred in the Longwangmiao Formation, with large-scale grain beach near the synsedimentary paleo- uplift; (4) Two types of gas-bearing reservoir, i.e. carbonate fracture-vug type in the Sinian Dengying Formation and dolomite pore type in the Cambrian Longwangmiao Formation, and superposition transformation of penecontemporaneous dolomitization and supergene karst formed high porosity-permeability reservoirs, with 3%–4% porosity and (1–6)×10−3 μm2 permeability in the Sinian Dengying Formation, and 4%–5% porosity and (1–5)×10−3 μm2 permeability in the Cambrian Longwangmiao Formation; (5) Large paleo-oil pool occurred in the core of the paleo-uplift during late Hercynian—Indosinian, with over 5 000 km2 and (48–63)×108 t oil resources, and then in the Yanshanian period, in-situ crude oil cracked to generate gas and dispersive liquid hydrocarbons in deep slope cracked to generate gas, both of which provide sufficient gas for the giant gas field; (6) The formation and retention of the giant gas field is mainly controlled by paleo-taphrogenic trough, paleo-platform, paleo-oil pool cracking gas and paleo-uplift jointly; (7) Total gas resources of the Sinian–Cambrian giant gas field are preliminarily predicted to be about 5×1012 m3, and the paleo-uplift and its slope, southern Sichuan Basin depression and deep formations of the high and steep structure belt in east Sichuan, are key exploration plays. The discovery of deep Anyue Sinian–Cambrian giant primay oil-cracking gas field in the Sichuan Basin, is the first in global ancient strata exploration, which is of great inspiration for extension of oil & gas discoveries for global middle-deep formations from Lower Paleozoic to Middle–Upper Proterozoic strata. Key words: Sichuan Basin, Anyue gas field, Fuling shale gas field, paleo-taphrogenic trough, paleo-oil pool, paleo-uplift, carbonate platform, unconventional oil and gas, shale gas, Weiyuan shale gas field