Journal of Natural Gas Geoscience最新文献

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.

Tight oil is a great potential resource for unconventional oil exploration and development. Determining the content of adsorbed tight oil in sandstone reservoirs and the factors controlling its content is crucial for the evaluation and development of tight oil resources. Using fluorescence petrography, environmental scanning electron microscopy, porosity as well as permeability analyses, high-pressure mercury intrusion, and nuclear magnetic resonance imaging, the present study analyzed the occurrence form, location, and relative content of free and adsorbed tight oils and effect of controlling factors on the content of adsorbed tight oil in sandstone reservoirs of the Chang 8 member in the Huaqing area of the Ordos Basin. The free tight oil mainly exists in residual intergranular, intergranular dissolved, intragranular dissolved, and dissolution-enlarged pores as well as some throats in the form of flakes with various areas and slender strips. The adsorbed tight oil mainly occurs on the surfaces of the mineral particles and mica fragments as well as in fractures, some throats or the intragranular as well as intergranular dissolved pores in the form of films with variable thicknesses and long strips. The lower limit of the pore radius at which free tight oil occurs in sandstone is approximately 0.1 μm and the adsorbed tight oil occurs on the surface of mineral particles and in spaces with pore radii of 0.02–0.3 μm. The contribution of free tight oil to total tight oil in the present study has highest and lowest content of 60.95% and 32.48%, respectively, with an average content of 50.14%. The contribution of adsorbed tight oil to total tight oil has highest and lowest content of 67.52% and 39.05%, respectively, with an average content of 49.86%. The content of adsorbed tight oil is generally proportional to the median pressure and inversely proportional to the porosity as well as permeability of the reservoir; particularly, porosity is relevant. The content of adsorbed tight oil is inversely proportional to the quartz content and directly proportional to the feldspar, carbonate cement, and clay mineral content of the sandstone.

Microfractures at the Ziliujing lacustrine shale at Fuling in eastern Sichuan Basin were investigated through core observation, SEM image observation and high-pressure mercury injection. and quantifies the impact of micro fracture development to reservoir capacity, leads to the following understanding: (1) fracture types in lacustrine shale reservoir are defined; (2) the contribution of fractures to lacustrine shale reservoir is determined. Results show that the microfractures in the Lower Jurassic Ziliujing lacustrine shale reservoirs in eastern Sichuan Basin can be divided into three types, each with four groups: micro fractures related to sedimentation (bedding fractures), microfractures related to diagenesis (clay–mineral shrinkage fractures), and micro fractures related to hydrocarbon generation (organic-matter shrinkage fractures and organic-matter-cracking fractures). High-pressure mercury injection and plane porosity determination confirm that microfractures in lacustrine shale reservoirs exhibit good reservoir performance, whose contribution to shale reservoir capacity is about 25%.

China is rich in shale gas resources, which are mainly distributed in densely populated southern marine carbonate areas. The geological and surface conditions are complex, the ecological environment is fragile, and water resources are scarce or unevenly distributed. Therefore, large-scale fracturing mining has high pressure of water resources utilization and serious risk of water environment pollution. In this paper, the geochemical characteristics of traditional and non-traditional stable isotopes such as hydrogen, oxygen, boron, lithium, strontium in shale gas hydraulic fracturing flowback/produced water (FP water) were comprehensively analyzed. The results show that, the hydrogen and oxygen isotopic composition of the FP water in Sichuan Basin, China has similar evolution trend with the produced water of conventional wells from Cambrian, Permian and Triassic Xujiahe, Jialingjiang and Leikoupo formations, but different from that of the Sinian conventional produced water. It indicates that the FP water in Sichuan Basin is a mixture of fracturing injected fluid and formation brine retained in Silurian shale. The saline end member is close to the formation water of Cambrian, but with higher δ¹¹B values. The FP water in Sichuan Basin has δ¹¹B values close to that of the Marcellus FP water, and both are derived from the evaporated seawater. The δ¹¹B values of FP water in Sichuan Basin have overlap with that of the conventional produced water from different strata, so it can not be precisely distinguished. However, the δ¹¹B and B/Cl values of the FP water in Sichuan Basin can be clearly distinguished from the river and the FP water from non-marine facies shale in Qaidam Basin. The FP water in Sichuan Basin has slightly higher δ⁷Li values than that of the Marcellus FP water, but has overlap with that of the Yangtze River. The average value of ⁸⁷Sr/⁸⁶Sr of FP water is 0.7197 in Weiyuan and 0.7193 in Changning, which is much higher than that of the produced water from conventional wells in different formations. This is because the Silurian shale is affected by terrestrial siliceous sediments and the underlying Sr-rich fluids. As a result, ⁸⁷Sr/⁸⁶Sr values measured in the Silurian strata in southern Sichuan Basin are high, which makes ⁸⁷Sr/⁸⁶Sr values become an effective index to distinguish shale gas fracturing FP water from conventional gas well produced water and shallow groundwater in Sichuan Basin.

In recent years, fiber-optic distributed temperature sensors (DTS) have been widely used for the dynamic monitoring of horizontal well during fracturing treatments. It is proposed to address the frequent issues of unknown artificial fracture initiation location, unclear crack extension pattern, unknown fracturing fluid injection success, and difficult fracturing effect evaluation in the hydraulic fracturing of horizontal wells. The temperature prediction model is the basis for fracturing diagnosis based on DTS monitoring, however, the quantitative prediction of temperature distribution throughout the horizontal well fracturing process remains a challenge. In this paper, a set of temperature-distribution prediction model for multi-stage fracturing in horizontal wells is established, with various micro-heat effects taken into account, and the models are solved through coupling. The temperature distribution during multi-cluster fracturing, and the temperature distribution characteristics during staged fracturing of horizontal wells have been analyzed. Moreover, the influence of fracturing fluid displacement, formation filtration coefficient, fracture width, and height on temperature distribution has also been clarified. The findings of this study provide theoretical support for fracturing performance diagnosis, fracture initiation identification, and analysis of the whereabouts of fracturing fluids using DTS monitoring. It has a great significance on fracturing effect evaluation and fracturing design optimization for horizontal wells.