China's carbonate gas reservoirs are facing a strategic situation of enhanced gas recovery (EGR). Currently, there is no universally applicable theoretical and technical system for improving recovery rates in carbonate gas reservoirs both domestically and internationally. This article summarizes the theoretical research and field practice of EGR of carbonate gas reservoirs in China in the past 70 years, and summarizes five research paradigms from four levels of "Philosophy, Strategy, Tactics, Execution". Based on the connotation of EGR, a calculation method for gas reservoir recovery rate is established from the perspective of engineering philosophy, and the technical methods and key issues for evaluating gas reservoir recovery rate are discussed in detail. By comparing the connotation, technical methods, and practical status of enhanced gas recovery in five typical carbonate gas reservoirs, the following three suggestions are put forward: (1) Improve the systematic evaluation of gas reservoir recovery efficiency, integrate various research data and results from the perspective of the whole gas reservoir, and establish the workflow of understanding→new data supplement→re-understanding under the framework of "static and dynamic data integration"; (2) Quantitatively evaluate the contribution of different strategies to EGR, conduct field research and verify primary data, fully compare theoretical models with field data, improve the match of static and dynamic descriptions, and repeatedly adjust and establish reasonable prediction models; (3) Strengthen the comparative study of different strategies to enhance gas reservoir recovery, gradually upgrade the scattered different strategies from perceptual understanding to qualitative and quantitative understanding, and quantitatively evaluate the EGR values of different strategies in the research area, and quantify the role of different technologies in improving gas reservoir recovery. The research results can promote the concept and theoretical technology progress of EGR in China's carbonate gas reservoirs and ensure national energy security.

Qiongdongnan Basin, a passive continental deep-water basin in the north of the South China Sea, is an important natural gas base in China. The deep Oligocene Lingshui Formation in Qiongdongnan Basin represent a primary target for forthcoming exploration in the South China Sea, however, due to limited exploration efforts, the sedimentary system of the Lingshui Formation remains inadequately characterized. Based on core, logging and 3D seismic data, the sedimentary characteristics of Lingshui Formation in northern slope of Baodao Sag, Qiongdongnan Basin are systematically dissected. It is clear that Lingshui Formation can be subdivided into two third-order sequences. The sedimentation of the 3rd member of Lingshui Formation is a complete three-order sequence with low, transgressive and high system domains. Under the control of the fault co directional transition zone and transition slope formed by the differential activity of F12 fault, several large differentiated river delta groups of different scales in the west and east are developed in the 3rd member of Lingshui Formation. The fault step zone mainly developed the underwater distributary channel of braided river delta front in the late low and high system tracts, and it is a high-quality reservoir development interval with the thickness of sand body being large, the particle size being coarser, the separation being better, and the physical property being better. Therefore, the development model of multistage fault-controlled material source and transition fault-controlled sand body distribution of the 3rd member of Lingshui Formation in the northern fault step zone of Baodao Sag is established. The high quality reservoir of large-scale delta distributary channel of the 3rd member of Lingshui Formation directly covers the source rock of Yacheng Formation and forms a favorable transport system with source rock-linked fault. And it is covered by thick transgressive shallow-sea mudstone deposited in late depression period. High-quality reservoer-cap combination and transport system are the key to forming large gas fields in Baodao Sag, and are worth of more attention in the future exploration.

The Cenomanian-Lower Turonian in southeastern Iraq is characterized by thick bioclastic limestone, with complex stratigraphic structure and nested sequences of different levels. The sequence recognition is important for stratigraphic division and reservoir correlation. Thick bioclastic limestone reservoir is commonly developed by separated waterflooding, and the study of sequences can lay a geological foundation for the division of development units. Taking six giant oilfields as example, all of which developed thick bioclastic limestone as main pay formations in the Cenomanian-Lower Turonian, the sequence schemes of Cenomanian-Lower Turonian are summarized. This paper adopts the scheme of dividing the Cenomanian-Lower Turonian into four of a third-order sequence corresponding to the four maximum flooding surface (K120, K130, K135, and K140). Mechanisms and characteristics of key sequence boundary such as unconformities, leaching and dissolution, thin carbonaceous mudstone, facies transition, facies mutation, maximum flooding surface, and hardground are summarized. Southeastern Iraq is located in the northeastern margin of the Arabian Plate, and during the Cenomanian-Early Turonian it is in a stable passive continental margin environment. This paper uses the quadratic model of classical stratigraphy to study the sequences of thick bioclastic limestone. According to the sequence theory, the sequence model of slightly rimmed carbonate ramp is established based on the depositional setting of Cenomanian-Lower Turonian. Different paleogeographic locations have different water depths during the process of sea-level rise and fall, so carbonate deposition rates and petrological features have different sensitivities to sealevel changes at different location. The water background and paleogeography control the sedimentation of different oilfields in southeastern Iraq. The sequence and depositional evolution are therefore revealed. Through elucidating the mechanism of sequence, and the effects of sequence order and sequence boundary on bioclastic limestone reservoirs are clarified. The effects of sea-level fall magnitude, exposure span and climate on formation structure, lithology and physical properties are illustrated. Typically, the high-order sequences boundary exposes for a long time, and the stratigraphy is weathered to a high degree. In arid climates, soilization and breccia collapse occur, and an unconformable surface can be formed, which is not conducive to improve the physical properties of the reservoir. In humid climates, large-scale leaching and dissolution occur, or thin carbonaceous mudstone can be formed locally. The exposure time of the low-order sequences boundary is short, and the leaching and dissolution occurs without destroying the strata structure, which can form high-quality reservoirs. Finally, problems and development trend in the sequence study of Cenomanian-Early Turonian in southeastern Iraq are pointed out, providing a reference for the separated waterflooding development of thick bioclastic limestone reservoir.

Based on analysis of a large number of core samples, thin sections, scanning electron microscopy, X-ray diffraction of whole-rock minerals, and well logging data, a systematic study is conducted on the favorable geological factors and conditions for hydrocarbon accumulation in the bedrock (at depths exceeding 7 000 m) reservoirs in the northern Qaidam Basin. The specific findings are as follows: (1) Intermediate-acidic granitic bedrocks serve as highquality reservoir lithologies. Under the coupled effects of multiple phases of metamorphism, tectonic fracturing, weathering and erosion, and other favorable reservoir-forming factors, two genetically distinct effective reservoirs have formed: weathering crust and interior reservoirs, with fractures and dissolution pores as the main storage spaces and minimal impact from burial compaction. (2) Two effective sealing layers, namely Jurassic carbonaceous mudstones (which are also source rocks) and Paleogene Lulehe Formation gypsiferous mudstones, provide condition for longdistance hydrocarbon migration and effective accumulation. (3) The synchronization of hydrocarbon generation periods with conductive faults and trap formation periods offers excellent conditions for hydrocarbon enrichment and reservoir formation. (4) The Jurassic high-maturity coal measure gas source rocks, primarily distributing in the depression area and overlying the bedrocks, form a laterally adjacent source-reservoir association with the bedrock reservoirs, with faults and the unconformity at the top of the bedrock serving as hydrocarbon migration pathways, resulting in two types of hydrocarbon reservoirs: extra-source and intra-source. Controlled by differential or successive hydrocarbon migration mechanisms, the reservoirs exhibit characteristics of oil accumulation at structurally high positions with low fullness and abundance, and gas accumulation at structurally medium to low positions with high fullness and abundance. The research indicates that the ultra-deep bedrocks still hold significant potential for hydrocarbon exploration. The analysis of the favorable conditions for hydrocarbon accumulation and their spatiotemporal configuration in these ultra-deep bedrocks can provide target areas for hydrocarbon exploration in this region and also contribute to a deeper understanding of the reservoir-forming and hydrocarbon accumulation processes in deep to ultra-deep bedrocks.

The western slope zone of Xihu Sag is characterized by "small, faulted, poor and scattered". A comprehensive utilization of various data such as geological logging, well logging, seismic and production has been conducted to summarize the distribution patterns of oil and gas discovered in the western slope zone. It is believed that the western slope zone has the characteristics of oil and gas distribution of "north-south zoning, upper-lower stratification, near source enrichment". By dissecting typical oil and gas reservoirs and analyzing the geological reasons of the failed wells in the western slope zone, it is believed that traps and migration are the main controlling factors for oil and gas accumulation. Structural styles determine the type and effectiveness of the trap, and control the types of oil-gas reservoir and differential enrichment in different zones. The different migration of oil and gas, as well as the two-stage filling and evaporation fractionation, control the characteristics of near source accumulation and upper oil and lower gas in the study area.By analysis of reservoir forming condition, it is pointed out that buried hill of Pinghu slope, north section of Tiantai slope and Yingcuixuan fault belt of Hangzhou slope have better trap and migration conditions, which are new exploration directions of the western slope zone in Xihu Sag.

Rifted basins are rich in oil and gas resources，and play an important role in petroleum industry. This thesis， which focuses on the influencing factors of structure styles of a sedimentary cover，can provide more theoretical basis for oil and gas exploration and development in rifted basins. Many researchers have been studying the influencing factors such as geometric conditions of the basin boundary，pre-existing faults in the basement，basement properties (ductile or rigid)，extension rate，syndeposition and so on. However，fewer study have been carried out on the influence of viscosity of ductile basements on structure styles of a sedimentary cover. In this study，the effects of viscosity of ductile basements on the structure styles of a sedimentary cover are systematically investigated by using discrete element modeling. Model results show that in models with different viscosity of ductile basements，the morphology of basins is similar，all are homogeneously extended. The fault distance of different models is not much different. The center of settlement is always in the middle and does not migrate during extension. The greater the viscosity of the ductile substrate，the greater the number of faults generated. According to the experimental simulation results and the profile characteristics of Changchang Sag in Qiongdongnan Basin，when the terrestrial heat flow value is large，the basement viscosity is small and the number of faults generated is small; conversely，when the terrestrial heat flow value is small，the basement viscosity is large and the number of faults generated is large.

Seismic prediction of formation pressure is of great significance to the evaluation of powerful gas-producing areas and exploration and development of shale oil and gas reservoirs. The study area of western Chongqing is a favorable area for exploration and development of deep shale gas. The shale gas resources in the area are abundant, and formation overpressure is common in the reservoirs. However, due to complex geological conditions, it is difficult to accurately obtain the formation compaction background trend while using conventional formation pressure seismic prediction methods, making it difficult to properly solve the seismic prediction of formation pressure. Usually, the formation compaction background trend is obtained directly through well trend fitting. The established formation compaction background often has large errors, resulting in inaccurate formation pressure prediction. Therefore, this paper constructs the formation compaction background trend through actual logging analysis, based on rock physics theory and rock physics model, which effectively improves the modeling accuracy of formation compaction background trend. By substituting the normal compaction background trend and measured parameters constructed by petrophysical modeling into the bulk modulus equation, the seismic prediction of formation pressure of the study area of western Chongqing in the Sichuan Basin is achieved, and good application results are achieved. By comparing with the actual drilling measured results, it is found that the predicted formation pressure parameters are consistent with the measured formation pressure parameters of multiple wells, indicating the applicability and accuracy of the method.

The Lower Cambrian Yuertusi Formation is an important source rock layer in Tarim Basin. The research on the organic matter enrichment mechanism could provide valuable guidance for the exploration of ultra-deep marine oil and gas reserves. Based on data of mineral petrology, inorganic geochemistry, and elemental analysis of Well A, combined with data of Well LT1 cited from reference, this study explores the sedimentary paleoenvironment of the Yuertusi Formation source rock in Tarim Basin and its control on organic matter enrichment, with integrated element geochemical indicators such as paleoproductivity, redox conditions and others. The results show that: (1) Yuertusi Formation in the Tahe area has high organic matter content, and the organic matter quality of the lower section, with an average TOC of 6%, is superior to the upper section. (2) Paleoproductivity level (P/ Ti, Babio), water retention degree (Mo/TOC, Co·Mn), terrigenous detrital input (Ti/Al) and anoxia degree (Ni/Co, U/Th, V/Sc) show a gradual decreasing trend from early to late period of Yuertusi Formation. (3) Closely corresponded with variations in the sedimentary paleoenvironment induced by sea level fluctuations, the organic matter enrichment in the Yuertusi Formation is jointly influenced by preservation conditions, weak hydrothermal activity, and upwelling currents. In the early stage of Yuertusi, under a sea level of relatively low, a medium-high water retention developed, and the weak hydrothermal activity made the bottom seawater being in an anoxic state, and the long-term anoxic and sulfidation environment was conducive to the preservation of organic matter, thus the lower section of Yuertusi Formation with high TOC formed. In the late stage of Yuertusi, the sea level experienced fluctuating rise and gradual decline, the water retention degree reduced under the high sea level, and when the sea level decreased significantly, the environment gradually changed from anoxic to oxygen-poor, which may lead to the oxidation and decomposition of some organic matter, thus the upper section with low TOC formed.