Journal of Natural Gas Geoscience最新文献

There is a significant relationship between in-situ stress and reservoir quality, to reinvent the theoretical understanding and technical methods of reservoir evaluation from the perspective of in-situ stress. Based on rock mechanical test and logging data, combined with regional evolution and structural deformation, this study carried out systematic geomechanics research, proposing the quantitative characterization of stress concentration parameters to realize the optimization of favorable zones. The results show that in-situ stress and fracture permeability under its control are important factors in determining the productivity in the ultra-deep reservoir. It is preferable to choose a location with low stress and good fracture activity, rather than an adverse location such as a local stress concentration area characterized by frequent borehole breakouts. The imbalance between in-situ stress and rock strength causes high stress. Moreover, the accuracy of reservoir classification is improved by incorporating the relevant parameters under the influence of in-situ stress into the evaluation method of reservoir quality. It is vital to fully utilize the multiple advantages of highly deviated wells with various favorable areas and multiple vertical fractures to address complex problems and improve the efficiency of exploration and development.

In-situ conversion technology has been more popular lately as an effective way to realize the industrial development of extensive medium- and low-mature shale oil/oil shale resources in China. The Ordos Basin has been recognized as the main basin containing the largest in-situ conversion recoverable resources of China. Researchers have revealed the hydrocarbon generation kinetics of low mature oil shales in the 7th Member of the Yanchang Formation (Chang 7 Member). However, the kinetics of medium mature shales may be different from those of low mature shales, and they can also be used as potential targets for in-situ conversion. At present, there is a lack of research on the in-situ hydrocarbon generation kinetics of medium- and low-mature shale oil/oil shale. In this paper, open system pyrolysis experiments were carried out on natural shales and shale samples derived from semi-open system pyrolysis with different maturities in the Chang 7 Member of the Ordos Basin respectively. Using the parallel first-order reaction theory, the frequency factor of low-mature shale was calculated to be 5.47 × 10¹⁰ s⁻¹, and the distribution of activation energy ranged from 38 kcal/mol to 61 kcal/mol. The main peak of activation energy was 49 kcal/mol, and accounted for 66.91% of all shales. With the increase in maturity, the average activation energy becomes higher, and therefore more in-situ conversion energy is needed. At the same time, the hydrocarbon generation potential characterized by S₂ peak of pyrolysis decreases during the maturation process. The activation energy is divided into three groups according to its distribution characteristics: low, main peak and high activation energy groups, representing <47 kcal/mol, ranging from 47 kcal/mol to 52 kcal/mol and >52 kcal/mol respectively. The proportion of low and high activation energy groups increased with the increase in maturity, while the proportion of main peak activation energy groups decreased. When the kinetic parameters are extrapolated to the condition of in-situ conversion, it is better to choose shales with low maturity (R_O <1 .0%) and fully transform them by rapid heating to the main hydrocarbon generation stage, and different conversion temperature ranges should be set for different maturity samples.

Deep shale gas (3500–4500 m) is the important replacement field of shale gas production growth in the future China. The research on key parameters of deep shale-gas reservoir is critical to determine its basic geological characteristics and establish a suitable development mode. In order to clarify the adsorption characteristics and controlling factors of deep shale gas in Longmaxi Formation, the main tests such as high-pressure methane adsorption, low-temperature nitrogen and carbon dioxide adsorption coupled with the adsorption fitting model and comparative analysis were conducted. The results show that the adsorption isotherms of deep shale gas also have a downward trend in spite of the higher pressure, and there is no obvious difference in adsorption characteristics, which is mainly due to the similar characteristics of microscopic pore-structure between deep shale and shallower shale. It is found that different adsorption models can well fit the experimental adsorption curve of deep shale gas, but the absolute adsorption capacity converted from excess adsorption capacity shows the same fitting result, i.e., DA-LF model > DR model > Langmuir model. Furthermore, DR model based on micropore filling theory is more suitable for characterizing the adsorption law of deep shale gas combined with the correlation analysis between pore structure and adsorbed-gas capacity. In addition, TOC is the key material factor controlling the adsorption capacity, and specific surface area of micropore is the key spatial factor. Compared to shallower shale, the deep shale shows higher siliceous content, lower calcite content, lower TOC content and lower adsorbed-gas content (the proportion of adsorbed-gas is about 30%).

Through a large number of rock thin section microscopic observations, scanning electron microscopy analysis, and fluid inclusion homogenization temperature measurement, the lithological composition of tight reservoirs in the Shanxi Formation of the Yan'an Gas Field in the southeast of the Ordos Basin was studied, lithofacies types were classified, and the diagenetic evolution of different types of rocks was combined with the burial history, thermal history, and hydrocarbon charging process to analyze the time matching relationship between the key oil and gas charging period and reservoir densification. Studies have shown that pure quartz sandstone and quartz-rich low-plasticity lithic sandstone mainly develop mechanical compaction, secondary dissolution, and kaolinite precipitation. Before the two essential hydrocarbons are charged, the porosity is 15.8%–31.5%, which is typical of medium and high permeability reservoir rocks. The porosity of the high tuffaceous heterogeneous quartz sandstone, plastic-rich granular lithic sandstone, and carbonate tight cemented sandstone ranges from 4.6% to 10.8%, indicating that the reservoir is ultra-low porosity-low porosity reservoirs before the first key hydrocarbon charge. It is difficult to charge the hydrocarbons in the later stage. Therefore, the diagenesis of pure quartz sandstone and quartz-rich low-plastic granular lithic sandstone that maintained high porosity and permeability during early oil and gas charging has been inhibited, and the physical properties of the reservoir are relatively good. They are the primary migration channels and accumulation spaces for late-natural gas. This constitutes a sweet spot in tight sandstone gas reservoirs. The research findings are critical for understanding the coupling relationship between reservoir densification and accumulation, clarifying the formation mechanism of effective reservoir rocks, and predicting the distribution of sweet spots.

The Shenhu area of the South China Sea is one of the leading areas for natural gas hydrate investigation and exploitation in China. In this study, total organic carbon (TOC) content, total nitrogen (TN) content, and gas chromatographic-mass spectrometric (GC–MS) analysis of the soluble organic matter of sediments from sites W01B, W02B, and W03B were completed to understand the characteristics of organic matter in the sedimentary system with gas hydrates and provide theoretical support for further exploration in this area. Source indicators such as δ¹³C_org, TAR, and C_21-/C₂₁₊ revealed that sites W01B and W02B have similar provenance characteristics in vertical profiles, with an increase of terrestrial source in the hydrate occurrence layers, while site W03B shows a gradual increase in marine source contribution as depth decreases. The asymmetric erosion and lateral deposition of the channel-levee depositional system are considered the controlling factors for the discrepancies in provenance indexes among the three sites. Analysis of the different source indicators revealed that the different kinds of soluble organic matter were subjected to varying degrees of biodegradation, and the distribution characteristics of n-alkanes were preferentially altered. Furthermore, it is found that biodegradations at sites W01B and W02B were more pronounced, particularly in hydrate occurrence layers. All of the microbial degradation indexes increase simultaneously and are consistent with the vertical distribution of hydrates, showing a similar triangular distribution trend. The increase in squalane, a characteristic compound representing methanogens, indicates that microbial degradation and methanogenesis are synchronized in the hydrate occurrence horizon, providing sufficient microbial gases for gas hydrates formation.

The 7th Member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin contains rich shale oil resources in which the exploration and development of the Chang 7 Member sandstone interbedded shale oil has made substantial breakthroughs. The exploration and development potential of the shale oil, an inevitableoil and gas replacement for Changqing Oilfield Company in the future, is immense. Its related geological basic research, however, is relatively weak. This paper provides elaborations and analyzation on basic science issue, including the characteristics of mud shale, the significance of shale oil exploration, the restoration of the formation of paleoenvironment, the organic matter enrichment effect of tuff, the catalysis of hydrocarbon generation, and more. It is presumed that enduring geological basic research, such as pure mud shale-type shale oil and modern lacustrine semi-deep lake-deep lake facies sedimentation investigations can provide valuable ideas and methods for the study of mud shale formation, paleoenvironment restoration, and formation mechanism. Meanwhile, we should pay close attention to research on the catalysis of tuff on the hydrocarbon generation of source rocks, and conduct a series of thermal simulation experiments on the catalysis of enriched elements in the tuff, which will provide important basic geological parameters for the exploration and development of Chang 7 Member pure mud shale-type shale oil.