In China, the South China Sea Block has great potential of natural gas hydrate resources. At present, the pilot production of natural gas hydrate has been successfully conducted in the Shenhu area, the northern part of the South China Sea, indicating a major breakthrough in hydrate exploration. However, the geological conditions, accumulation factors and hydrate distribution are still unclear, so it is necessary to further study the formation conditions and distribution of natural gas hydrate. Based on the oil and gas exploration data and the achievements of oil, gas and natural gas hydrate in the South China Sea, the accumulation conditions and distribution of natural gas hydrate in the South China Sea have been discussed in the aspects of sedimentation, structure, oil and gas conditions and hydrate enrichment factors. The results show that: ① The thin crustal thickness and high heat flow value of the South China Sea Block are conducive to the thermal evolution of organic matter; ② The sedimentary facies of delta, alluvial fan, turbidite fan and slump fan are developed in the meso-Cenozoic, and the sand and mud are interbedded, which provides favourable conditions for gas hydrate accumulation; ③ The diverse and complex structure elements such as tensional or transtensional fault system can form migration channel for natural gas hydrate; ④ Four types of gas resources such as biogas, shallow-layer pyrolysis gas, middle-deep pyrolysis gas and gas field gas are developed, which provide a good material basis for gas hydrate accumulation; ⑤ Uplifts and depressions are favorable locations for gas hydrate accumulation. The results not only provide the guidance for the natural gas hydrate exploration and development, but also provide scientific evidence for the natural gas hydrate exploration in other sea areas.

As a hot area in global gas hydrate research field, the hydrate exploration in the Gulf of Mexico has been carried out for decades. Especially in the selected key exploration targets, study area WR313 and GC955, the hydrate accumulation and enrichment features have been studied. Based on the gas hydrate exploration results of these two study areas, the vertical distribution, the lithofacies features of favorable reservoirs are summarized, and the hydrate accumulation mechanisms have been analyzed to reflect the latest gas hydrate exploration and research progress in this gulf. The gas hydrate exploration results indicate that there are mainly two occurrence types of gas hydrates existing in these study areas, namely, the pore-filling type in sand and the fracture-filling type in mud. The pore-filling type occurs in the deeper sand-rich sediments with relatively coarse grains, and the hydrate saturation is normally greater than 80 %, and may be up to 95 %; whereas the fracture-filling type occurs in the shallower clay dominant sediments with the low gas hydrate saturation generally less than 5 %. In addition, the main problems existing in the research of the gas hydrate accumulation mechanisms also have b een discussed, emphasizing that the accumulation of gas hydrate is a complex process influenced by many factors. Meanwhile, it is also pointed out that the identification of local sand-rich bodies deserve special attention during gas hydrate accumulation selection.

H Oilfield in the Pearl River Mouth Basin mainly develops delta front deposits. Its reservoir thickness is thin, the lateral change is fast, and the understanding of lithologic boundary and sand body connectivity is not clear, which seriously restricts the efficient development of oil field. Seismic waveform indication inversion can fully use the lateral changes of seismic waveforms to carry out high-frequency component estimation, and establish the interpolation model which is more consistent with the sedimentary characteristics. This method has good inter-well prediction ability, and is particularly suitable for high-precision prediction of fast lateral changes, strong non-average, and thin interbedded sand and mudstone reservoirs. While for another method, seismic sedimentology, is to use the seismic attribute slice to describe the lateral distribution range and sedimentary characteristics of the reservoir under the guidance of isochronous stratigraphic framework. It has a good detection effect on the plane sedimentary characteristics of thin reservoir which cannot be resolved vertically. Combined with the above two methods, a set of technical processes for fine description and sedimentary characterization of marine thin sandstone reservoirs is formed and has achieved good application results in H Oilfield, meanwhile provides effective technical support for remaining oil prediction and potential tapping adjustment in the study area.

In order to solve the problems in Oilfield Q, such as unclear understanding of geological model, strong heterogeneity of reservoir development and complexity of production performance rule, the sedimentary characteristics and the fracture development of carbonate rocks in buried-hills have been summarized by comprehensive application of logging, drilling and coring, rock section, logging and 3D seismic data. The results show that, the Karst slope and highland are the main development areas of dolomite, where the karstification is intense and the dissolution pores are developed. The fractures are easy to appear so as to form the high-quality reservoirs. Karst depression mainly develops calcareous mudstone or argillaceous limestone, with poor reservoir physical property. The upper oil formation is mainly fractured reservoir with large thickness and developed fractures, while the lower oil formation mainly develops dissolution reservoir. On the basis of this achievements, the geological mode is summarized and the geological model of a dual porosity medium reservoir is established by combination of dynamic and static methods. This achievement has been well applied in the actual development of the oilfield, and has a certain guiding significance for the fine and efficient development of similar fractured reservoirs.

The geological conditions of marine fluvial sedimentary reservoir are very complex. In the process of water flooding development, the remaining oil distribution is dispersed and it is difficult to develop due to different water absorption in the same permeability reservoir. In order to guide the exploration direction of remaining oil in the later stage of each reservoir, the pore throat distribution characteristics and micro percolation characteristics of different permeability reservoirs are studied based on the micro pore throat scale by the mercury injection experiment and micro water drive visualization experiment. The results show that there is a good logarithmic correlation between the average pore throat radius and permeability, and the correlation coefficient is 0.963 5. With the increase of the reservoir permeability, the mean pore throat radius increases gradually, whereas the micro fingering phenomenon of water drive is reduced, and the water flooding process is gradually changing to finger-network-uniform displacement. Both the sweep efficiency and recovery efficiency of each model show an increasing tendency, and the proportion of continuous remaining oil is gradually decreasing. When the permeability increases to 2 228.7×10^-3 μm ², both the sweep efficiency and recovery efficiency reach their maximum value, the for is 82.97 %, the latter is 66.1%, and the proportion of continuous remaining oil reaches the minimum value of 45.82 %. For the reservoirs with permeability of （73.1 ~ 1 005.2）×10^-3 μm ², the main direction of potential tapping in the later period should be to increase the viscosity of displacement fluid and increase sweep. For reservoirs with permeability of （1 005.2~3 509.6）×10^-3 μm ², it turns to be improving oil displacement efficiency. For reservoirs with permeability of （3 509.6~ 4 040.6）×10^-3 μm ², it is appropriate to increase water injection pressure or block large water breakthrough channels to change the direction of injected water, so as to improve the conformance as the main direction of the potential tapping.

The acid sensitivity test has been carried out to evaluate the ultra-low permeability reservoir in structure-B of Bohai Bay. Combined with the analysis and test data such as casting thin sections, scanning electron microscope, nuclear magnetic resonance and X-ray diffraction, the reason of rock pore structure changes before and after the acid-rock reaction has been analyzed, and the applicability of the mud acid acidizing reservoir has been discussed. The results showed that the reservoirs of the glutenite and gneiss in the study area are basically unharmed after reacting with the soil acid（7.2 %HCl+3 %HF）. After the acid-rock reaction of the samples in the study area, the micropores and the dissolution pores of feldspar particles develop, no carbonate or other soluble minerals are found, the effective face rate and data of large and small pores increase, the cut-off value of T₂ spectrum and irreducible water saturation decrease, and the pore structure become better. Meanwhile, the clastic grains of the glutenite are still in linear-convexity contact, the mineral metamorphic grains of gneisses reservoir are still in granular close contact, and the reservoir framework is still stable. The acidifi cation and dissolution of soluble carbonate cement is the key factor to change the pore structure of reservoir in the study area. Secondary precipitation caused by acid-sensitive mineral chlorite and blockage of pore throat by insoluble particles have little damage to the reservoir. After the reaction of soil acid with the reservoir in the study area, the pore structure is improved, showing a constructive role, which has a certain applicability to improve the reservoir permeability.

The unclear understanding of the micro pore structure in the offshore low permeability reservoir affects the determination of the key parameter limits of water injection quality. Therefore, water injection quality evaluation experiment is carried out based on the characterization of reservoir micro pore structure in Liushagang Formation of WZ11-4N Oilfield studied by indoor constant velocity mercury injection experiment, then to evaluate the influence of the key parameters of water quality on the seepage capability. The results show that the throat size and distribution are the main factors that determine the seepage capability of reservoir. There is a good power function relation between permeability and mainstream throat radius. The throat radius of the first section in the Liushagang Formation is about 4 μm, while 1 μm for the third section, and the former has strong flow capacity. The increase of the particle size, particle concentration and bacterial content of the suspended solids in the injected water will greatly increase the injection pressure but decrease the core permeability. Taking the damage degree of water absorption index to reservoir, 20 %, as the lower limit, the key parameter limits of injected water quality of reservoirs with different permeability levels are established. For the first section of the Liushagang Formation, the water quality of the recommended injected seawater are of the concentration that the suspended solids particles is less than or equal to 2.0 mg/L, the median diameter of suspended solid particles is less than or equal to 3.0 μm, and the number of bacteria per milliliter is less than or equal to 100. According to the guidance of these above data, the effective development of the main formation in Weizhou 11-4N Oilfield is realized by the combination of water injection and seawater softening treatment technology.

The offshore oil and gas exploration is gradually advancing deeper. Meanwhile, the South China Sea has great potential for deep-water oil and gas resources. Generally, for the deep water exploration, the project scale is large, the requirements of the project management are high, and the investigation is difficult. Due to the factors such as the changeable marine climate environment, the relatively complex geological environment, the professional survey vessels or survey equipment, and the high requirement of underwater positioning accuracy, the commissioned technical requirements, project capital cost and operation safety risk in the deep water survey area are significantly higher than those in the shallow area. In a deep-water work area in the South China Sea, the seabed picking up, well pass section, maximum negative amplitu de and seismic slice of the 3D seismic data are used to evaluate the bathymetry, seabed topography, shallow strata, faults, amplitude anomalies and other investigation contents of the work area, so as to carry out the disaster prediction and safety assessment of the shallow geological conditions to guide the emplacement, installation and production of the underwater engineering facilities system in the target oil and gas mining area. Based on the 3D seismic data, a relatively simple and practical method for geological hazard assessment in shallow layer of the deep water is explored. As a part of the desktop research stage in the whole life cycle of EPCI （Engineering, Procurement, Construction and Installation）, this method provides necessary support for the smooth development of the project plan.

When the conventional binomial productivity equation is used to analyze the deliverability of the gas wells in a low permeability thick layer condensate gas reservoir with high condensate content in Bohai Sea, the negative deliverability equation coefficient B can be obtained, which is still negative after pressure correction. Therefore, this equation is not suitable for the gas well productivity analysis. In order to obtain the reasonable productivity equation of the gas well, the following measures are taken. Firstly, the judgment criterion of high-speed non Darcy seepage and the theoretical calculation of productivity equation coefficient B are used to confirm that the high-speed non Darcy of the gas well is negligible, and the formation pressure and bottom hole flow pressure also need to be corrected. Then, the modified productivity equation is obtained without high-speed non Darcy productivity equation. Finally, the exponential productivity equation is used for verification, and the calculated Q_AOF（open flow, the same below） is basically consistent with the Q_AOF the modified productivity equation. When this new method is applied to the actual gas wells in low permeability condensate gas fields, a reasonable gas well productivity equation can be obtained, which shows the effectiveness of the modified productivity equation.

At present, the methods such as numerical simulation and linear regression are mostly used to predict the oil recovery of sandstone reservoirs in the eastern South China Sea, but some of them are time-consuming or with low accuracy. In order to predict the oil recovery quickly and accurately, 50 developed reservoirs are selected as the data samples. Based on the feature extraction of the influencing factors of the principal component analysis on recovery, a recovery prediction model suitable for sandstone reservoirs in the eastern South China Sea is established by the neural network. Compared with that of two methods of support vector machine regression and linear regression, the prediction results of neural network regression model have high prediction accuracy, which can evaluate the development potential of the similar reservoirs quickly.

After the long-term water and polymer flooding, the remaining oil distribution in the late stage of high water cut is complex, and it is difficult to select fracturing wells and layers. In order to solve these problems, an improved calculation method of the dimensionless liquid production index for wells in polymer flooding oilfields is proposed to evaluate the production capability and changes, then the wells that the dimensionless liquid production index dropped more than 30 % are selected. At the same time, combined with the fine geology and reservoir analysis, the quantitative characterization of displacement multiple in the late stage of high water cut is studied and the remaining oil for each layer is evaluated. At last, the method of selecting fracturing well and layer in Bohai J Oil Field is established. Field applications showed that this method was reasonable and effective, the oil production incremental for each well after fracturing is about 14 to 42 tons. It can also provide reference for the selection of fracturing measures and the evaluation of well stimulation potential of similar oilfields in the late stage of high water cut.

For the reef carbonate reservoirs, the vxdarious lithology and pore types and the complex throat structure are the fundamental factors of complex pore-permeability relations, which greatly affect the accuracy of the reservoir parameters, especially the logging interpretation of permeability, and bring great difficulty to the reservoir evaluation. First, the reef-flat facies reservoir of Feixianguan Formation in X Gas Field is took as an example to define the amplitude difference of the logging curve by the change characteristics of density curve and acoustic time difference curve on the basis of clarifying the four property relationship of the reservoir and according to the data of core analysis, casting thin section, mercury injection analysis and logging. And than, the cross plot of the new parameters and the density curve is made to establish the pore type identification template by the multiple regression analysis method. At last, the reservoir parameter interpretation model based on the pore type identification is established to make the accurate interpretation of the parameters of the reef-flat facies poro us reservoir. The interpretation results show that the logging interpretation model with pore identification ability can significantly improve the logging interpretation accuracy of reef-flat facies reservoir, and can provide technical reference for the logging interpretation of the same reservoir type.

During the drilling process, the wellbore instability frequently occurres in Baxigai Formation in the Kelasu structural belt of the Tarim Basin, which seriously affects the drilling efficiency. In order to study the physic-chemical and mechanical mechanical properties properties, differences of physical and pechanical characteristics and influence of drilling fluid immersion on the mechanical properties of the sandstone and mudstone in Baxigai Formation, the physic-chemical characteristic tests （such as mineral composition test and cation exchange experiment）, and mechanical test （such as triaxial compression test） have been carried out. The research results show that the clay minerals content of mudstone in Baxigai Formation is high, and the cation exchange capacity is about three times that of sandstone. The mudstone here also has a large number of micro-cracks, and the hydration swelling is obviously stronger than that of sandstone. All these things make it m ore prone to be damaged. At the same time, the strength decreases under the immersion and lubricating effects of oil-based drilling fluid. The strength decrease of mudstone is about 1.5 times that of sandstone. Under the same confining pressure, the mudstone and sandstone strength, the corresponding axial and radial strain at the critical failure point, and the rock damage mode of mudstone and sandstone in Baxigai Formation are different. The damage to mudstone structure caused by drilling fluid immersion is greater than that of sandstone. The analysis of the mechanical properties of the rock in Baxigai Formation is of great significance to clarify the mechanism of wellbore instability in this formation and formulate prevention and control measures for wellbore instability.

In general, fracturing well pattern is used for the development of low-permeability reservoir. This development process relates to the oil-water two phase flow, while the current researches on fracturing well productivity consider more about single phase flow. Based on the consideration of the reservoir anisotropy, threshold pressure gradient and oil-water two phase flow, a prediction method of fracturing well productivity is proposed by numerical method and sweep theory. And then, a contrast between the calculated results of the proposed method and field data is performed to verify the accuracy of the proposed model. The results show that when the water cut is constant, the increase of fracture conductivity results in the increase of the oil production. However, as the conductivity increases, the increase rate of the oil production decreases. The increase of the fracture conductivity also results in the increase of the cumulative oil production. When the conductivity is greater than 0.4 μm ²·m, the cumulative oil production increases slowly. When the length of the fracture increases, the seepage resistance near the bottom decreases, so that the oil production increases. The increase of the permeability ratio indicates the planar heterogeneity becoming stronger, so that the productivity of the fractured well reduces showing a poorer water flooding effect. This study provides a theoretical basis for the optimization of the fracturing of the development well pattern by water flooding.

During the development of oil and gas field, the analogy method is often used to study the development technology policy and predict the development index for the undeveloped or early stage reservoir. The selection of analogy samples, i.e. similar oil and gas fields, determines the reliability of analogy results. Therefore, gray correlation and geological factor analysis are used to determine the analogy parameters and weights quantitatively on the basis of the essential correlation between analogy parameters and analogy objectives. According to the reservoir type of the studied object, the parameter thresholds are introduced to calculate the similarity of the samples, and the oilfields with high similarity are selected as the reference samples for the technical policy research and index prediction, so as to make the basic conditions of the objective basis of analogy analysis are more reasonable, and further improve the efficiency of screening and the reliability of analogy results.

In recent years, Sinopec has used the unconventional natural gas idea to strengthen the Permian natural gas exploration in eastern Sichuan and obtained industrial gas flow in the first member of Maokou Formation of several wells in Jiaoshiba of Fuling and its adjacent area, revealing a good prospect for natural gas exploration. In this paper, the sequence stratigraphy, sedimentary characteristics, hydrocarbon generation conditions, micro pore structure characteristics and gas bearing property of Mao-1 Member in Jiaoshiba area are systematically analyzed. The results show that: ① Mao-1 member can be divided into two fourth-order sedimentary cycles and eight small layers, which mainly develops four kinds of lithology such as Limestone, micritic limestone, nodular limestone and nodular micritic limestone. ② The average TOC of the Mao-1 Member ranges from 0.75 % to 0.87 %, which on the whole, belongs to carbonate source rock with medium organic matter abundance. The organic matter abundance of micritic limestone and nodular micritic limestone is relatively low, while the organic matter abundance of limestone and nodular limestone is relatively high. The type of organic matter is mainly type I and supplemented by type Ⅱ. R_o ranges from 1.71 % to 2.18 %, with an average of 1.97 %. ③ The Mao-1 Member is an ultra-low porosity and permeability reservoir, and the local layers show the characteristics of fractured reservoir. The reservoir space is mainly composed of organic pores, diagenetic contraction fissures of clay minerals, and grain margin fractures of mineral particles, followed by dissolution pores of carbonate minerals. Moreover, the pore size distribution is mainly mesoporous and macroporous. ④ The natural gas of Mao-1 member is mainly composed of methane with low heavy hydrocarbon content, which is typical dry gas; δ ¹³C₁ ranged from -31.25 ‰ to -31.12‰, δ¹³C₂ is between -35.28 ‰ and -31.23 ‰, δ¹³C₃ is between -34.87 ‰ and -34.66 ‰, which is a typical oil type gas. The carbon isotope of alkane series is reversed, showing a trend of change δ¹³C₁>δ¹³C₂>δ¹³C_3. According to the characteristics, the natural gas from Mao-1 Member is the mixture of kerogen cracking gas and liquid hydrocarbon cracking gas. ⑤ Mao-1 Member has the characteristics of self-generated and self-reservoired, which is a set of special carbonate reservoir between shale reservoir and fractured reservoir. It is necessary to learn from shale gas exploration experience, strengthen sweet spot interval evaluation and adaptive engineering technology research, and promote the large scale production and benefit development of natural gas.

Refracturing is currently one of the most used methods to rejuvenate productivity of existing wells in low-permeability reservoirs in China. Different refracturing strategies have been conducted for vertical wells in a tight conglomerate reservoir in the Xinjiang. However, the underlying mechanisms are still unclear. By establishing the geomechanical model, the mechanism model of pore pressure inducing stress change based on the finite element method and the numerical model of unstructured reservoir, the researches on the fracture geometry of refracturing and the effect of different refracturing methods have been carried out. The results are as follows. Compared with the initial fracturing and conventional refracturing, the fracture geometry changes significantly after the volume refracturing. The azimuth of the in-situ stress before and after production does not change significantly, but the changes in pore pressure and in-situ stresses have a great impact on the geometry of hydraulic fractures. For the layers that have been perforated and produced, due to the strong energy loss, the propagation of fracture is more difficult, resulting in short fracture length and wide fracture width. In the case of adding new perforation in un-developed layers, as the formation can provide enough energy, it is easier to propagate fractures. The wells which are at the higher place of the new perforation layers have better productivity after the volume refracturing. The timing of refracturing also has a significant impact on oil production. A proper choice of refracturing time can effectively improve the well productivity.