The transverse relaxation time (T2) cut-off value plays a crucial role in nuclear magnetic resonance for identifying movable and immovable boundaries, evaluating permeability, and determining fluid saturation in petrophysical characterization of petroleum reservoirs. This study focuses on the systematic analysis of T2 spectra and T2 cut-off values in low-permeability reservoir rocks. Analysis of 36 low-permeability cores revealed a wide distribution of T2 cut-off values, ranging from 7 to 50 ms. Additionally, the T2 spectra exhibited multimodal characteristics, predominantly displaying unimodal and bimodal morphologies, with a few trimodal morphologies, which are inherently influenced by different pore types. Fractal characteristics of pore structure in fully water-saturated cores were captured through the T2 spectra, which were calculated using generalized fractal and multifractal theories. To augment the limited dataset of 36 cores, the synthetic minority oversampling technique was employed. Models for evaluating the T2 cut-off value were separately developed based on the classified T2 spectra, considering the number of peaks, and utilizing generalized fractal dimensions at the weight <0 and the singular intensity range. The underlying mechanism is that the singular intensity and generalized fractal dimensions at the weight <0 can detect the T2 spectral shift. However, the T2 spectral shift has negligible effects on multifractal spectrum function difference and generalized fractal dimensions at the weight >0. The primary objective of this work is to gain insights into the relationship between the kurtosis of the T2 spectrum and pore types, as well as to predict the T2 cut-off value of low-permeability rocks using machine learning and data augmentation techniques.

A high-temperature and high-pressure tight gas sandstone reservoir, located in the Ledong area of the Yinggehai Basin, shows considerable resource potential. However, the diagenetic characteristics of this reservoir remain unclear. In this study, reservoir petrology and diagenetic characteristics were systematically investigated through different experimental methods. Further, the porosity and diagenetic evolution models were constructed. The methods employed included petrophysical analysis, observation of core and thin sections, and grain-size distribution analysis. Furthermore, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy, cathodoluminescence, carbon and oxygen isotope analysis, digital image analysis, mercury injection capillary pressure tests, and micro-CT imaging were also employed. In the study area, gravity-flow channel sediments composed of feldspathic litharenite primarily developed in a restricted submarine canyon. And tight sandstones in target formation have an average porosity of 9.34% and permeability of 1.79 mD. Mechanical compaction and cementation (mainly illite and ferrocalcite) were regarded as crucial roles in the densification process. Further, the diagenetic evolution model indicated that the reservoir underwent a high degree of thermal diagenetic evolution. Five diagenetic facies types were classified and characterized at multiple scales. Moreover, a vertical distribution model of different diagenetic facies was also developed considering the overpressure, depth of formation, thickness and frequency of mudstones, and injection concentration of the CO2. This study helps to improve the understanding of the diagenetic characteristics and diagenetic facies types in high-temperature and high-pressure clastic reservoirs.

莺歌海盆地LD10区中深层黄流组-梅山组重力流水道、海底扇储层已被证实具有重大的天然气资源潜力.但是前期测试结果显示气藏中混有高含量CO2气体.因此,精细厘定天然气充注期次,明确CH4、CO2等时空分布规律对规避高CO2风险至关重要.本研究在对不同产状流体包裹体岩相学特征精细观察的基础上,综合激光拉曼光谱分析和包裹体显微测温技术识别出3幕不同成分天然气充注,时间分别为:4.0～2.9 Ma、2.0～1.2 Ma和0.8～0.3 Ma.其中,第一幕充注以烃类气为主,伴有少量有机CO2和N2;第二幕和第三幕充注以大量无机CO2、烃类气为主,伴有少量N2.结合天然气及烃源岩地化特征、天然气组分分析及输导体系识别,总结了LD10区的成藏模式,以期为研究区下一步天然气勘探开发和规避高含量CO2风险提供依据.

The Ledong 10 area of the famous HTHP Yinggehai Basin, South China Sea hosts abundant natural gas resources, which is expected to be a significant gas production base in China. The formation overpressure is a key factor that impacts geophysical responses, rock quality, diagenesis evolution, drilling safety and other geological engineering problems during the exploration and development. However, overpressure mechanism is poorly understood in this area. Using acoustic compaction curve, Eaton method, Bowers method and the density - sonic interval transit time method to reveal the two-stage overpressure mechanism of Ledong 10 area. The overpressure stage I was related to disequilibrium compaction in the formations below 2,300 m, with a pressure coefficient (PC) of approximately 1.6, while the overpressure stage II was caused by fluid expansion, which caused PC of formations below 4,000 m to increase to 2.2. Specifically, the early tight carbonate cementation layers with calcium provided by biological debris, mudstones and the barrier of canyon channels provided a closed system and protected primary pores to resist against further compaction. The overpressure stage II was caused by the charging of deep inorganic-origin CO2 and hydrocarbons, which did not physically improve rock quality, but the carried CO2 differentially dissolved carbonate cements in the aquifers to produce secondary pores. Besides, the effects of different overpressure types on rock quality were quantitatively analyzed through petrophysical properties, 2D digital image analysis, and 3D pore network modeling. Compared with other 2D parameters, average pore radius has a better correlation with permeability. When permeability is less than 0.5 mD, spatial pore distribution becomes more heterogeneous. Finally, considering pore structure and minerals, a comprehensive overpressure evolution model and an overpressure-induced differential dissolution model were developed, explaining two unique rock structure phenomena in deep tight gas sandstones: "similar burial depths with different compaction degrees" and the overpressure-induced "differentiated dissolution with similar burial depths". In this study, it provides some insights into the developmental mechanisms of overpressure in the Yinggehai Basin and differential dissolution mechanism at pore scale, which contributes to overpressure mechanism analysis of other deep basins in the world.