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.

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.

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.

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.

Based on special exploitation mode of heavy oil-oil sand, this paper established a recoverable reserves calculation method for heavy oil-oil sand. First, calculate recoverable reserves of single exploitation mode according to oil layer selection criterion including four parameters of buried depth, single layer thickness, porosity and oil saturation for different exploitation models. Second, calculate recoverable reserves for different exploitation model combination and the max recoverable reserves of the block; Last, set up heavy oil-oil sand recoverable reserves database. The method has great correlation with PRMS reserves evaluation system. The method has been used in the heavy oil classification evaluation for larger M of the J block in Venezuela heavy oil belt, the recoverable reserves are 4.06′108 t, 15.23′108 t and 7.40′108 t respectively by cold production, steam drive and steam-assisted gravity drainage, and the max recoverable reserves are 15.97′108 t, which provides reserves basement for block development.

Through analyzing the development of large ultra-deep structural gas fields in China, strategies for the efficient development of such gas fields are proposed based on their geological characteristics and production performance. According to matrix properties, fracture development degree and configuration between matrix and fractures, the reservoirs are classified into three types: single porosity single permeability system, dual porosity dual permeability system, and dual porosity single permeability system. These three types of gas reservoirs show remarkable differences in different scales of permeability, the ratio of dynamic reserves to volumetric reserves and water invasion risk. It is pointed out that the key factors affecting development efficiency of these gas fields are determination of production scale and rapid identification of water invasion. Figuring out the characteristics of the gas fields and working out pertinent technical policies are the keys to achieve efficient development. The specific strategies include reinforcing early production appraisal before full scale production by deploying high precision development seismic survey, deploying development appraisal wells in batches and scale production test to get a clear understanding on the structure, reservoir type, distribution pattern of gas and water, and recoverable reserves, controlling production construction pace to ensure enough evaluation time and accurate evaluation results in the early stage, in line with the development program made according to the recoverable reserves, working out proper development strategies, optimizing pattern and proration of wells based on water invasion risk and gas supply capacity of matrix, and reinforcing research and development of key technologies.

To find out the seismic wave field propagation principles in loess plateau near surface of the Ordos Basin and the seismic response characteristics of tight oil reservoirs, this study established a geological-geophysical model under the real conditions of ground surface of loess plateau, and launched full elastic seismic wave equation forward modeling and pre-stack elastic seismic inversion study. Comparison of modeling and real seismic data shows that, the loose and wavy loess plateau surface is the main reason for causing the problems of seismic static correction and interference wave. Tomographic static correction method with the constraint of traces near shot point can effectively solve the problem of seismic static correction in the loess plateau and enhance seismic interpretation accuracy, S-wave impedance obtained from pre-stack seismic inversion can identify sandstone effectively, and Poisson's ratio can identify oil-bearing reservoirs. The seismic forward and inverse simulation and rock physical analysis provide a solid theoretical and experimental basis for the seismic prediction of tight oil reservoir, and have worked well in the oil exploration and development in the loess plateau of the Ordos Basin.

Based on the present situation and trend of underground coal gasification in China and overseas, this article puts forward the basic concept, mechanism and mode of underground coal gasification, and presents the challenges, development potential and development path now faced. In China, underground coal gasification which is in accord with the clean utilization of coal can produce "artificial gas", which provides a new strategic approach to supply methane and hydrogen with Chinese characteristics before new energy sources offer large-scale supply. Coal measure strata in oil-bearing basins are developed in China, with 3.77 trillion tons coal reserves for the buried depth of 1 000-3 000 m. It is initially expected that the amount of natural gas resources from underground coal gasification to be 272-332 trillion cubic meters, which are about triple the sum of conventional natural gas, or equivalent to the total unconventional natural gas resources. According to the differences of coal reaction mechanism and product composition of underground coal gasification, the underground coal gasification can be divided into three development modes, hydrogen-rich in shallow, methane-rich in medium and deep, supercritical hydrogen-rich in deep. Beyond the scope of underground mining of coal enterprises, petroleum and petrochemical enterprises can take their own integration advantages of technologies, pipeline, market and so on, to develop underground coal gasification business based on their different needs and technical maturity, to effectively exploit a large amount of coal resources cleanly and to alleviate the tight supply of natural gas. It can also be combined with using the produced hydrogen in nearby area and the CO₂ flooding and storage in adjacent oil areas to create a demonstration zone for net zero emissions of petroleum and petrochemical recycling economy. It is significant for reserving resources and technologies for the coming "hydrogen economy" era, and opening up a new path for China's "clean, low carbon, safe and efficient" modern energy system construction.

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.

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.

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.

The physical simulation of diagenesis was conducted for the Cretaceous Bashijiqike Formation deep sandstone reservoir in Kelasu structural belt of Kuqa Depression, Tarim Basin, and the dissolution rate and increased dissolution pores of feldspar matrix grains in such reservoirs were quantitatively calculated in the process from long-term shallow burial in early stage to quick deep burial in late stage. Through the field emission large-area SEM analysis, the dissolution rate and increased dissolution pores of feldspar matrix grains in core samples taken from Dabei and Keshen areas were quantitatively calculated. After the experimental data and the actual core data were cross-checked, the evolution model was established for increased feldspar dissolution pores in deep continental reservoirs with high content of feldspar matrix grains. According to the calculation results, the maximum increased feldspar dissolution pores in Keshen area during the process from long-term shallow burial in early stage to quick deep burial in late stage is by 0.86%-2.05%. The simulated sandstone reservoir with burial depth of more than 7 000 m reveals a larger quantity of increased feldspar dissolution pores, with the absolute error value of 0.23% after calibration. In Dabei area, calcite is the primary contributor to secondary dissolution pores, followed by feldspar. Quantitative calculation shows the maximum increased feldspar dissolution pores in Dabei area to be by 0.62%-1.48%. Similarly, the simulated sandstone reservoir with burial depth of more than 7 000 m reveals a larger quantity of increased feldspar dissolution pores, with the absolute error value of 0.15% after calibration. There are two causes of the experiment errors: One cause is that the simulation experiment uses ideal conditions and the simulation reservoirs are homogeneous; Another one is that deep reservoirs have strong heterogeneity and there are big differences in the dissolution within different areas.

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.

The mechanical properties such as Young’s modulus, hardness and fracture toughness of Lower Silurian Longmaxi shale samples from Youyang area in southeast Chongqing, China were investigated using dot matrix nanoindentation measurements. With the help of field emission scanning electron microscope (FESEM) and energy dispersive X-ray fluorescence spectroscopy (EDS), the indentation morphology and mineral composition in indentation area were quantitatively analyzed. According to mechanical strength classification, a micromechanical model with three components was introduced and the Mori-Tanaka model was used to upscale mechanical parameters from nano-scale to centimeter-size scale, which were further compared with uniaxial compression results. The experimental results show that there is a positive linear correlation between Young’s modulus and hardness and between the Young’s modulus and the fracture toughness under nano-scale; the Young’s modulus, hardness and fracture toughness perpendicular to the bedding are slightly lower than those parallel with the bedding. According to data statistics, the mechanical properties at the nano-scale follow Weibull distribution feature and the dispersion degree of hardness results is the highest, which is mainly due to shale anisotropy and nanoindentation projection uncertainty. Comparing the results from nanoindentation test, with those from upscaling model and uniaxial compression test shows that the mechanical parameters at the nano-scale are higher than those from upscaling model and uniaxial compression test, which proves mechanical parameters at different scales have differences. It’s because the larger the core, the more pores and internal weakness it contains, the less accurate the interpreted results of mechanical parameters will be.

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 systematic coring of 500 m of Kong 2 Member of the Paleogene Kongdian Formation in Cangdong sag of Bohai Bay Basin, identification and XRD (X-Ray Diffraction) analysis of over 1000 thin sections, a simplified method to quantitatively calculate contents of fine grained minerals with conventional logging data such as acoustic travel time (AC) and density log (DEN) has been proposed, and a quick lithologic identification "green mode" has been worked out in this study. By fitting the relationship between normalization of logging curves and mineral content measured by XRD, the mineral contents of sections or wells not cored can be calculated to identify lithology. With this method, several dolomite sweet spot intervals and one sandstone sweet spot interval have been found in the Kong 2 Member of Cangdong sag, where high production oil and gas flows have been tapped from drilled wells. The study shows that the dolomite is in band distribution and enriched in local parts of the study area. This method is applicable to lithologic identification of fine grained deposits in front delta-lake basin center, especially lithologic identification of mud and dolomite dominated fine grained deposits with low sand content of semi-deep, deep lake facies.

Classification, superimposed evolution and sedimentary filling of prototype basins are analyzed based on the Wilson cycle principle of plate theory, by dissecting the evolution history of 483 sedimentary basins around the world since the Precambrian, combined with the three stress environments of tension, compression and shear. It is found that plate tectonic evolution controls the superimposed development process and petroleum-bearing conditions of the prototype basins in three aspects: first, more than 85% of the sedimentary basins in the world are developed from the superimposed development of two or more prototype basins; second, the superposition evolution process of the prototype basin takes Wilson cycle as the cycle and cycles in a fixed trajectory repeatedly. In each stage of a cycle, a specific type of prototype basin can be formed; third, each prototype basin can form a unique tectonic-sedimentary system, which determines its unique source, reservoir, cap conditions etc. For hydrocarbon accumulation, the later superimposed prototype basin can change the oil and gas accumulation conditions of the earlier prototype basin, and may form new petroleum systems. Based on this, by defining the type of a current basin as its prototype basin formed by the latest plate tectonic movement, 14 types of prototype basins can be classified in the world, namely, intracontinental growth rift, intracontinental aborted rift, intercontinental rift, passive continental margin, interior craton, trench, fore-arc rift, back-arc rift, back-arc depression, back-arc small ocean, peripheral foreland, back-arc foreland, strike-slip pull-apart, and strike-slip flexural basins. The classification scheme can ensure the uniqueness of the types of individual sedimentary basin, and make it possible to predict their oil and gas potential scientifically through analogy.

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.

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 reservoir characteristics of the Upper Ordovician Wufeng-Lower Silurian Longmaxi Formations in southern Sichuan Basin were preliminarily revealed in this study by identifying and quantitatively evaluating the fracture pores of five appraisal wells in the central and northern parts of the southern Sichuan Depression by several methods. Four conclusions were reached as follows: (1) In the central zone of the Depression, the deep reservoir space of the Wufeng-Longmaxi producing pay is composed mainly of matrix pores and the microcracks are not common, whether on the local structural highs, flanks or lows. The physical properties are similar to that of the matrix pores in Changning, Weiyuan and Fuling gas fields. (2) In the northern zone of the Depression, the deep reservoir space of the Wufeng-Longmaxi black shale is composed mainly of matrix pores, and fracture pores mainly occur in local discrete intervals, with a total porosity range from 3.5% to 6.7%, on average 5.3%, and fracture porosity of 0-2.1%, on average 0.3%. (3) In the central and northern parts of the southern Sichuan Depression, the Wufeng-Longmaxi producing pays have undeveloped fracture pores and chiefly extensively distributed matrix pores, indirectly indicating relatively stable tectonic activities and corresponding weaker reservoir reworking there than Fuling field located in eastern Sichuan Basin. (4) The size and distribution of the gypsum-salt layer in the Cambrian are the key controlling factors of fracture pore development in the Wufeng-Longmaxi Formations. Therefore, the areas including Wellblocks L7, GS1, eastern Sichuan Basin and western Hubei province, where gypsum-salt layer in the Cambrian is thick and stable, and fracture intervals are likely to occur in the Wufeng-Longmaxi producing pay controlled by decollement structure above salt structure since the Yanshan Movement, are the potential favorable areas for fracture pore development.

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.

Factors affecting CO₂ flooding of shale oil reservoir were studied by nuclear magnetic resonance (NMR) experiments, the effects of time, pressure, temperature on the recovery of CO₂ flooding in shale oil reservoir were analyzed based on nuclear magnetic resonance T₂ spectrum, and the effect of fracture development degree on recovery of CO₂ flooding in shale oil reservoir was analyzed based on NMR images. In the process of CO₂ flooding, the recovery degree of the shale oil reservoir gradually increases with time. With the rise of pressure, the recovery degree of the shale oil reservoir goes up gradually. With the rise of temperature, the recovery degree of shale oil increases first and then decreases gradually. For CO₂ flooding in matrix core, the crude oil around the core surface is produced in the initial stage, with recovery degree going up rapidly; with the ongoing of CO₂ injection, the CO₂ gradually diffuses into the inside of core to produce the oil, and the increase of recovery degree slows down gradually. For CO₂ flooding in matrix core with fractures, in the initial stage, the oil in and around the fractures are produced first, and the recovery degree goes up fast; with the extension of CO₂ injection time, CO₂ diffuses into the inside of the core from the fractures and the core surface to produce the oil inside the core, and the increase of recovery degree gradually slows down. Fractures increase the contact area between injected CO₂and crude oil, and the more the fractures and the greater the evaluation index of fractures, the greater the recovery degree of shale oil will be.

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.

Natural fractures in the deep Cretaceous Bashijiqike Formation sandstone reservoirs of Kuqa foreland thrust belt, NW China, are classified according to fracture aperture based on the data of outcrops, cores, thin sections and imaging logging, using industrial CT scanning, laser scanning confocal microscope (LSCM), cathodoluminescence (CL), electron probe, and scanning electron microscopy (SEM). The types, characteristics, genesis ages and formation sequence as well as reservoir significance of such natural fractures are examined. Four categories of fractures are classified. Category I (aperture>100 μm) is macro structural fractures, which cut single sand body to form the dominant migration pathway, helping to increase the reservoir permeability. Category II (aperture=10-100 μm) is associated micro structural fractures, which cut matrix grains to connect large matrix pores and improve the seepage performance. Category III (aperture=1-10 μm) is micro digenetic fractures at grain edges, which connect medium and small pores to improve the pore network connectivity and the gas migration and charging efficiency. Category IV (aperture<1 μm) is nano-scale matrix fissures, which connect intragranular micro pores to expand the reservoir space, thereby increasing the reserves scale. Category I and Category II fractures were developed in three stages (early, middle and late). The early- and middle-stage fractures, predominantly half-filled-filled fractures, were formed before early Pliocene when extensive oil and gas charging had not occurred. The late-stage opened fractures were formed after the Late Pliocene, they were at the same time as or later slightly than extensive oil and gas charge. The fracture network has low contribution to porosity, but it can improve the permeability by 2-3 orders of magnitudes in the parallel direction of fractures.

Based on reviews and summaries of the naming schemes of fine-grained sedimentary rocks, and analysis of characteristics of fine-grained sedimentary rocks, the problems existing in the classification and naming of fine-grained sedimentary rocks are discussed. On this basis, following the principle of three-level nomenclature, a new scheme of rock classification and naming for fine-grained sedimentary rocks is determined from two perspectives: First, fine-grained sedimentary rocks are divided into 12 types in two major categories, mudstone and siltstone, according to particle size (sand, silt and mud). Second, fine-grained sedimentary rocks are divided into 18 types in four categories, carbonate rock, fine-grained felsic sedimentary rock, clay rock and mixed fine-grained sedimentary rock according to mineral composition (carbonate minerals, felsic detrital minerals and clay minerals as three end elements). Considering the importance of organic matter in unconventional oil and gas generation and evaluation, organic matter is taken as the fourth element in the scheme. Taking the organic matter contents of 0.5% and 2% as dividing points, fine grained sedimentary rocks are divided into three categories, organic-poor, organic-bearing, and organic-rich ones. The new scheme meets the requirement of unconventional oil and gas exploration and development today and solves the problem of conceptual confusion in fine-grained sedimentary rocks, providing a unified basic term system for the research of fine-grained sedimentology.

Based on 3-D seismic data and drill-hole data in Moxi-Gaoshiti area, the impression seismic thickness from the top of Longwangmiao Formation, which is easier traced, to the bottom of Cambrian System was selected to characterize the karst paleogeomorphology of the fourth Member of Dengying Formation. The paleogeomorphology of the member can be further divided into three geomorphic units: karst platform, slope and superimposed slope, which had different paleohydrologic conditions, and thus different karstification intensity and reservoir quality. Among them, the superimposed slope with the strongest dissolution, has larger solution groove and cave systems, where the reservoirs with mainly honeycombed dissolved pores and cavities as storage space, are best in quality; the platform slope with weaker dissolution, has smaller solution groove and cave systems, where the reservoirs are poorer than those in the superimposed slope; the karst platform with the weakest dissolution, has piebald karst systems mainly, where the reservoirs with mainly pinhole to smaller dissolved pores and cavities as storage space, are poorer in quality on the whole, but there are some good quality reservoir bodies in local areas. The results show that, besides the highly explored scarp belt, the karst platform with low exploration degree can also be favorable exploration area of the fourth member of Dengying Formation.

To solve the difficulties in exploration and development in Yuanba Great Ultra-deep Gas Feild in Sichuan Basin, SW China, the article studies the mechanism of development of quality reef reservoirs gas accumulation and innovates techniques for ultra-deep seismic exploration, drilling, completion and testing. Through the dynamic depositional evolution process of homoclinal ramp-fringed platform and reconstruction of regional depositional framework in the Permian, it is found that the reservoirs in the Changxing Formation of Yuanba area, Sichuan Basin are developed in a pattern of “early shoal and late reef, multiple stage stacking, in rows and belts”, dissolution in early exposure stage and dolomitization during shallow burial give rise to the pores in matrix, overpressure caused by cracking of liquid hydrocarbon during deep burial induces fractures, and coupling of pores and fractures controls the development of ultra-deep high quality reservoirs. From correlation of oil and source rock, it is concluded that the Wujiaping Formation and Dalong Formation of deep-water continental shelf are the major source rocks in the Permian of northern Sichuan Basin. The hydrocarbon accumulation mode in ultra-deep formations of low-deformation zones is characterized by “three-micro migration, near-source enrichment, and persistent preservation”. Through seismic inversion using the pore structure parameters of pore-fracture dual structure model, the high production gas enrichment area in Yuanba gas field is 98.5 km². Moreover, special well structure and unconventional well structure were used to deal with multiple pressure systems and sealing of complex formations. A kind of integral, high pressure resistant FF-level gas wellhead and ground safety linkage device was developed to accomplish safe and environmentally friendly gas production.

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, we clarified the progress and stage of tight oil and gas exploration and development in China, and envisaged the future development orientation of theory and technology, process methods and development policy. In nearly a decade, relying on the exploration and development practice, science and technology research and management innovation, huge breakthroughs have been made. 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 have been innovated and integrated. These technology series include enrichment regions selection, well pattern deployment, single well production and recovery factor enhancement, and low cost development. As a result, both of reserves and production of tight oil and gas increase rapidly. 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 stimulating technology between China and North America. In the future, 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 global situation of distribution, reserves and production of deep oil and gas are examined systematically, the progress and exploration achievement of onshore China deep geophysical technology are summarized, and the challenges and developing direction of deep geophysical technology are pointed out. Aiming at the exploration of deep onshore strata in China, the article analyzes the key geophysical problems, such as low noise-to-signal ratio, low resolution, low imaging accuracy and low amplitude-preservation, in the exploration of deep clastic, carbonate, and volcanic reservoirs, and presents the corresponding technical countermeasures such as wide-line large-array 2-D acquisition, wide-azimuth and high-density 3-D acquisition, anisotropic pre-stack depth migration (PSDM),reverse-time migration (RTM), complex structure modeling, quantitative reservoir prediction, and pre-stack fracture prediction. The analysis from two aspects, imaging of deep complicated structures and prediction of deep complicated reservoirs, shows that wide-frequency, amplitude-preservation, high-accuracy and information integration are the challenges to and key issues in the geophysical technology. It is pointed out that wide-frequency acquisition, rock physical modeling of complex reservoirs, high-accuracy prestack amplitude-preservation imaging, comprehensive evaluation of complex reservoirs, non-seismic techniques, and drilling steering with seismic data are the key geophysical techniques needed to be developed in the future.

True tri-axial test system was deployed for fracturing simulation of coal outcrops to investigate the initiation and propagation of hydraulic fractures in vertical and directional wells. The influences of in-situ stress and cleats on non-planar propagation of hydraulic fractures in directional wells under different relative azimuths were analyzed. The test results show that the general propagation pattern of hydraulic fractures is jointly controlled by azimuth, cleats and in-situ stress. As the relative azimuth increases, the hydraulic fractures become more complicated in geometry and subject to increasing pumping pressure and propagation pressure. If the hydraulic fractures are initiated along a direction skewed with wellbore, the effect of cleats would alter the extension path and appear distortion of hydraulic fractures, inducing more complicated fracture geometry near the wellbore, with many fractures at the initiation point. Compared with vertical wells where I-shaped or X-shaped hydraulic fractures are formed, directional wells often have twisty propagation of dominant fractures near the wellbore and presence of multi-level fractures, which impede the further extension of hydraulic fractures in coal seams.

Based on the study of natural gas resource, low buried hill trap formation mechanism, high quality reservoir control factors and natural gas preservation conditions, the formation conditions and reservoir accumulation characteristics of Bozhong 19-6 large condensate gas field were summarized. Large gas generation potential of multiple sets of thick humic-sapropelic source rocks in high maturity stage in Bozhong depression was the basis of large gas field formation. The multi-stage tectonic evolution since Indosinian period formed large-scale buried hill traps. The Tanlu fault activity formed multi-type reservoirs, and buried hill metamorphic rock of Archean and sand-conglomerate of Kongdian Formation were high-quality reservoirs. Thick overpressure lacustrine mudstone and weak neotectonic movement provided good preservation conditions. Bozhong 19-6 gas reservoir was a condensate gas reservoir with very high condensate oil content, and the gas origin was humic-sapropelic and kerogen-cracking gas, and the gas field had large gas thickness, high gas column characteristics and the accumulation process was first oil and then gas. The buried hill reservoir was a massive reservoir and the Kongdian reservoir was a stratified reservoir. The gas field had multi-channel hydrocarbon intense charge from overpressure source rocks, atmospheric-weak overpressure reservoir favorable for accumulation, thick overpressure mudstone caprock favorable for preservation, and natural gas ultra-late rapid accumulation model.

A large data bank of more than 700 gas-condensate samples collected from literature and experiments was established. On this basis, empirical correlations and equations of state commonly used to calculate dew-point pressure (DPP) were evaluated. A new model for estimating DPP was proposed. All the empirical correlations and the Peng-Robinson state equation were compared, and sensitivity of parameters was analyzed. The current standards used to identify gas condensate were evaluated and found to be not accurate enough. The Peng-Robinson state equation has no unique solution and is affected by multiple factors such as the characterization of C₇₊ components and the splitting scheme. The Nemeth-Kennedy correlation has the highest accuracy when applied to the data bank established in this study, followed by Elsharkawy correlation and Godwin correlation. While Shokir correlation cannot be used for samples without C₇₊ components, it is therefore the lowest in accuracy. The newly proposed model has an average absolute error, root mean square error and coefficient of determination of 7.5%, 588, and 0.87, respectively, and is better than the above four correlations statistically. The proposed model proved to be more accurate and valid when compared to experimental results and simulation with the Peng-Robinson state equation.

The gas kick characteristics during carbonate reservoir drilling were analyzed taking carbonate reservoirs in Shunnan area of Tazhong as an example, a quantitative evaluation method for gas kick risk was established, and then a case was simulated. Looking into reservoir space characteristics and gas invasion mechanisms of carbonate reservoirs in Shunnan area based on drilling geologic data, it is found that the reservoirs are rich in fractures, pores and caves, and the gas invades into wellbore through gas-liquid replacement and differential pressure. By fully considering the gas invasion mechanism, gas migration law and wellbore temperature-pressure field, and introducing the gas volume fraction density function, a quantitative evaluation method for gas kick risk and a method for ranking well control risk were established. The case study shows that the wellhead back pressure method can be used to tell the mode of gas invasion; pore-fracture-cave and acid gas characteristics in carbonate reservoirs have stronger effect on the gas volume fraction in wellbore, and may cause hidden and severe gas kick; the effects of well depth, well diameter, drilling fluid density, drilling fluid displacement, drilling fluid viscosity and drilling rate on gas kick risks weaken in that order.

Paleo-sedimentary environment of Chang 7 Member of Upper Triassic Yanchang Formation in Ordos Basin, including the paleoclimate, paleo-salinity and paleo-redox conditions were restored through geochemical elements analysis of 289 samples collected from the outcrop sections around and wells drilled in the basin and using a series of identification indexes of paleo-climate, paleo-salinity and paleo-redox conditions, such as CaO/MgO?Al₂O₃, Sr/Cu, Rb/Sr, Rb/K₂O, Th/U, V/(V+Ni), the content of element B tested from the mudstone. Comprehensive analysis shows that in sedimentary period of the Chang 7, the paleo-climate was warm temperate to subtropical climate with temperature higher than 15 °C, the water body was continental brackish water to freshwater, and the sediments were deposited under strong reduction conditions. Suitable temperature, extensively deep lake basin and strongly reductive paleo-sedimentary environment led to the blooming, enrichment and preservation of organic matter in the submember Chang 7₃. As a result, a set of high-quality source rock was formed, laying material foundation for large-scale accumulation of shale oil.

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.

Plane stain assumption is widely used in the simulation of the distribution of stress and displacement around hydraulic fractures. According to the comparison of the solutions of elliptical fracture and plane stain fracture, the applicable conditions of plane strain assumption were discussed and the correction method was put forward. On the basis of the past research, a classical stress and displacement solution around a flat elliptical fracture was deduced, fulfilled and corrected. In comparison with the plain strain fracture solution, simulating results show that if taking plane strain assumption into consideration in the fracture height profile, the difference between the elliptical fracture solution and plane strain solution is negligible when the ratio between fracture length and fracture height is larger than 10, and the above critical value could be relaxed to 5 in some simulations which normal stress perpendicular to fracture face plays a decisive role. The plane strain fracture solution is accurate enough when the above critical value is satisfied, the correction of the plane strain fracture solution is needed when the length-height ratio is small. The correction charts of the additional initial stress of the horizontal well with an open-hole and the width of a single fracture were drawn under different length-height ratios. The fracture width of pseudo three dimension propagation models is easier to be modified by the correction charts.

Based on comprehensive analysis of outcrops, cores, rock thin sections, mineral X-ray diffraction and Argon ion-milling - scanning electron microscopy, nine lithofacies types and five facies marks in the Lower Cambrian Meishucun Formation and Qiongzhusi Formation in the Upper Yangtze are identified, a classification scheme of the shale sedimentary facies is proposed, and the deposition and evolution model of the shale in the Lower Cambrian is figured out. This research shows there are difference in sedimentation, types and distribution of the sedimentary facies in the Meishucun Formation and Qiongzhusi Formation. The main sedimentation modes in the Meishucun Stage were mechanical-chemical and biological sedimentation, the sedimentary facies (from west to east) were carbonate ramp, shelf, and slope and bathyal basin. The main sedimentation of the Qiongzhusi Stage was clastic mechanical, argillaceous flocculation and biological deposition, and the sedimentary facies were shore, shelf, and ramp and bathyal basin. There are two depositional centers of rich organic shale in the Upper Yangtze which are the prospective area for shale gas exploration. The first one is in the Ziyang-Changning area in nearly north-south strike, which is characterized by multiple thin layers. The other one is in the Western Hubei-Eastern Chongqing-Middle Guizhou and Yichang-Jianshi-Fangxian, which is characterized by the thick-layer shale. The shale gas exploration in these areas in the future should take pertinent strategies in line with their differences.

Aiming at the disadvantages of existing shale reservoir evaluation methods, a new comprehensive index was proposed to accurately predict the distribution of high quality shale reservoirs and favorable fracturing intervals. The comprehensive index can be calculated using the physical properties index and fracturing index by the equivalent method. Computed by logging rock-electric parameters and mineral bulk physical model, the physical properties index characterizes reservoir property and gas-bearing property; the fracturing index characterizes reservoir fracability and is acquired by equivalent porous medium model considering mineral components. According to the comprehensive index, combined with the macro-micro characteristics of cores and logging data, the shale reservoirs in the Ordovician Wufeng Formation to Silurian Longmaxi Formation of Jiaoshiba area in the southeastern margin of Sichuan Basin are subdivided into four types, the high terrigenous siliceous and high authigenic siliceous types are the best in reservoir property and fracability, followed by the middle siliceous and then low siliceous. The comprehensive index can be used to interpret the logging data of horizontal well to figure out the proportion of reservoirs of different types, identify the spatial distribution of reservoirs with good physical properties and good fracability. The predicted results match well with actual production after fracturing.

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.

通过室内实验和理论分析,研究了不同温度、压力条件下添加碳酸钙前后海泡石水基钻井液对不同渗透性地层的封堵性能,论证将碳酸钙作为堵漏剂添加到海泡石钻井液中的可行性。在实验温度为27~204 ℃、实验压力为