Journal of Natural Gas Geoscience最新文献

Shale oil and gas resources mainly exist in the pore and fracture system. Quantitative characterization of pore development characteristics and gas-bearing properties is crucial for shale reservoir evaluation. The pore development of shale reservoir exhibits strong complexity and heterogeneity, and research on pore development characteristics of coal measure shale lags behind that of marine shale reservoir. Hence, it is urgent to investigate the pore heterogeneity characteristics of coal-bearing shale and its influence on gas bearing properties. Therefore, the coal-bearing Cretaceous Nenjiang shales from the Songyuan area of the Songliao Basin were selected as the research object in this study. Through total organic carbon (TOC) analysis, X-ray diffraction experiments, porosity analysis, nitrogen adsorption–desorption experiments, and methane isothermal adsorption experiments, the characteristics of pore structure, heterogeneity, and gas bearing properties of coal-bearing shale were analyzed. The influence of rock and mineral components on pore structure and heterogeneity characteristics, the relationship between pore structure characteristics and fractal characteristics, and the effects of pore structure and heterogeneity on gas bearing properties were also discussed. The results show that: (1) The organic matter abundance of the shale in the Nenjiang Formation does not change significantly (the average TOC content is 2.38%). Ink bottle-shaped pores are mostly developed, and the Nenjiang shale is rich in clay minerals (average content 55.6%), with slit-shaped pores mostly developed. The pore surface of shale exhibits obvious fractal characteristics, with average fractal dimensions D₁ and D₂ of 2.54 and 2.74, respectively, indicating that the internal structure is more complex than the surface structure. (2) The enrichment of organic carbon increases the specific surface area by affecting the development of micropores and pores, consequently increasing the fractal dimension of pores. Similarly, the development of clay minerals increases the number of mesopores and macropores, thereby increasing the fractal dimension of pores. (3) Small pores develop larger specific surface areas, which increases the complexity and heterogeneity of the pore structure. This promotes remarkable fractal characteristics, expands the adsorption sites, and improves the adsorption capacity. This work will provide a scientific theoretical basis for the comprehensive evaluation of coal-bearing shale reservoirs and research on shale gas reservoir formation theory.

This study aims to address the problem of porosity preservation in the Mangahewa Formation of five main hydrocarbon fields covering onshore and offshore of the Taranaki Basin. An integrated reservoir characterization of the Middle to Late Eocene Mangahewa Formation is achieved through petrophysical evaluation, sedimentological and petrographical descriptions, and well log analysis methods. Petrophysical parameters (porosity and permeability) were acquired from the available core analysis and using mathematical equations to obtain other petrophysical matrices such as normalized porosity index (NPI) and reservoir quality index (RQI). Factors that affected Mangahewa reservoir were studied through thin-section microscopy and well-log analysis helped to measure the reservoir and hydrocarbon potentiality in the Mangahewa Formation. The Mangahewa Formation is dominated by sandstone and a range of marginal to shallow marine facies with varying hydraulic flow units (HFU). The Mangahewa Formation has a high positive correlation in porosity-permeability relationship and has a maximum of 4.67 μm RQI and 20.08 μm FZI (Well Kapuni-14) which reflect potential reservoir. The Mangahewa Formation observed from Wells Kapuni-14, Maui-A1G, McKee-16A, and Mokau-1 are dominated with 59.6%, 56.4%, 79.3%, and 68% of macro- and megapores, respectively. The presence of authigenic clay and calcite cement has greatly reduced the reservoir quality; however, primary and secondary pores are still observed within the Mangahewa sands. Moreover, well log analysis was carried out on four wells in Taranaki Basin, to run a qualitative and quantitative analysis of the Mangahewa reservoir. Eight potential reservoir zones were examined, revealing that the Mangahewa Formation has a very low shale volume of less than 6%, good effective porosity ranging between 11.0% and 13.3%, up to 36.2% of average water saturation and maximum of 69.8% average hydrocarbon saturation. In conclusion, from this comprehensive study, it can be deduced that the Mangahewa Formation possesses fair to good reservoir quality and hydrocarbon potentiality.

Scholars have primarily focused on statistical analysis of exploration practices and simple physical simulation experiments when investigating the relationship between stratigraphic dip and hydrocarbon resources during the hydrocarbon accumulation period. However, there is a notable lack of research on the theoretical relationship between stratigraphic dip and hydrocarbon resources during this critical period. This study addresses this gap by exploring the principle of minimum energy dissipation rate governing oil and gas migration. Through this principle, the existence of a stratigraphic dip window for hydrocarbon migration and accumulation system is strictly proved during the hydrocarbon accumulation period. It is established that when the stratigraphic dip window coincides with the hydrocarbon accumulation period, the effective driving power for hydrocarbon migration is at its weakest, resulting in the lowest energy dissipation rate within the hydrocarbon system. Consequently, the hydrocarbon migration and accumulation yields the highest efficiency, leading to the greatest reserves of hydrocarbon resources. This study resolves the puzzle of why the amount of hydrocarbon resources in 44 natural gas-effective zones and 49 oil-effective zones, that have been put into commercial development in China, as reported by Hou et al. (2021), exhibit the statistical characteristic of “downward parabola of opening” in their average stratigraphic dip during the main hydrocarbon accumulation period. Furthermore, it explains the influence of the stratigraphic dip size during the hydrocarbon accumulation period on oil-gas reservoir formation. Moreover, the theory is used to investigate the evolutionary changes in the stratigraphic dip window of tight gas reservoirs in the Upper Paleozoic strata of the Ordos Basin. The study traces the transition of Upper Paleozoic high-pressure tight paleo-gas reservoirs, where the sum of net buoyancy gradient, excess pressure gradient, and discharge pressure gradient serve as the effective driving force of hydrocarbon migration, to modern low-pressure tight gas reservoirs, where the sum of net buoyancy gradient and discharge pressure gradient prevail. Correspondingly, during the hydrocarbon accumulation period, the stratigraphic dip window of the high-pressure tight paleo-gas reservoirs is relatively small (0.2°–0.3°), gradually evolving into the comparatively larger stratigraphic dip window (0.35°–0.45°) characteristic of the current low-pressure tight gas reservoirs.

Taking the deep-ultra deep glutenite reservoirs in the Upper Wuerhe Formation of Permian in Fukang Sag and Dongdaohaizi Sag as the research object and representative of the eastern Junggar Basin, its characteristics and main controlling factors are analyzed using various methods, including core analysis, thin section examination, and scanning electron microscope observation. High-pressure mercury injection and logging imaging were also employed to reveal the reservoir that Fukang and Dongdaohaizi sags harbor deep-ultra deep glutenite reservoirs, with Fukang Sag being a typical low porosity to ultra-low permeability reservoir, while Dongdaohaizi Sag is a low porosity to low permeability reservoir. Reservoir space types vary between the two sags, with Fukang Sag characterized by microfractures and corrosion pores, while a large number of corrosion pores with fewer fractures existing in Dongdaohaizi Sag. Both compaction and cementation exhibit a strong destructive effect on the reservoirs in the eastern Junggar Basin. However, the compaction effect in Fukang Sag is very strong, and the dissolution effect is weak. The large number of fractures generated by overpressure becomes an effective channel for ultra-deep oil and gas migration. The cracks in the Dongdaohaizi Sag are underdeveloped, and a large number of intra-particle corrosion pores generated by the dissolution of feldspar and turbidite improve reservoir properties. In addition, its rich turbidite also plays a compressive and pore retention role. There are two types of reservoir models developed in the Upper Wuerhe Formation of Permian in the eastern Junggar Basin: the deep fracture model represented by the Fukang Sag and the solution pore model rich in turbidite in Dongdaohaizi Sag. These models create favorable conditions for oil and gas accumulation in the deep-ultra deep reservoirs in the depression area.

The evaluation of potential helium source rocks’ effectiveness is a core issue in the field of helium resource exploration and development. However, previous studies predominantly rely on uranium (U) and thorium (T) contents and the age of the rock for evaluating the effectiveness of potential helium source rocks, which fails to fully characterize the key factors affecting their effectiveness. Therefore, this paper takes four typical potential helium source rocks—granite, mud shale, gneisses, and bauxite, as the research object. Through the establishment of a calculation model for accumulated dissolved helium in pore water, combined with gas reservoir examples, a quantitative analysis of dissolved helium accumulation and exsolution in the pores of each potential helium source rock is carried out. This analysis aims to discuss and summarize the effectiveness of each potential helium source rock and the evaluation method for identifying effective helium source rocks. It is believed that: (1) The exsolution of dissolved helium accumulated in the pores of potential helium source rocks into free helium on a large scale under suitable conditions is the key prerequisite for identifying their effectiveness; (2) In addition to the content of U and Th elements, parameters such as large volume, suitable porosity, and water saturation, the good matching relationship of “sedimentary and burial history, gas accumulation history and tectonic evolution history”, and relatively specific helium generation ability are also the key parameters for judging the effectiveness of potential helium source rocks; (3) By establishing the calculation method of dissolved helium enrichment efficiency (η_He), it is concluded that under similar conditions, free helium enrichment is most easily achieved in granite, followed by mud shale and bauxite, while gneiss poses the greatest difficulty.

This research presents a comprehensive investigation to enhance understanding of subsurface structural complexities within the Upper Cretaceous Pab Sandstone reservoir level in a gas-producing field within the Kirthar Fold Belt of the Southern Indus Basin, Pakistan. The seismic interpretation for accurate structural delineation and characterization of intricate fault systems is often challenging in Fold and Thrust Belt settings. The 3D structural maps of Upper Cretaceous Pab Sandstone at the reservoir level and the application of the ant-tracking attribute for fault extraction improve the structural understanding of the gas-producing field in the Kirthar Fold Belt in the Southern Indus Basin, Pakistan. Attribute-assisted 3D seismic interpretation has revealed several subsurface structure elements, including a large thrusted anticline extended towards the north-south, the pattern of north-south oblique ramp thrusts on the southeastern flank, and a combination of easterly vergent thrusts with a counter-back thrust creating a local pop-up structure in the area. A 3D structural model at the reservoir level was generated using horizon and fault framework volume-based modeling approach, serving as a structural model for Pab sandstone. The structural smoothing, variance and ant-tracking attribute were applied iteratively to get the best results and correlated with manual interpretation. 3D seismic interpretation and application of ant-tracking for fault extraction identifies discrete structural styles and revealed that all thrusting occurred due to compressional tectonics of the Plio-Pleistocene period. The apparent different styles result from the reactivation of earlier extensional fault systems and probably during phased periods of compression. The resulting 3D reservoir model can be used as an input to populate petrophysical properties and results can be extended for future field development plans. Integration of well and 3D seismic data interpretation and the application of automatic fault extraction techniques are highly recommended in structurally complex areas and have an equal implication for worldwide basins with similar conditions for reliable structural interpretation in Fold and Thrust Belts with structural complexity.

This research aims to address the ambiguity surrounding the extent of development of Changxing Formation bio-reef reservoirs in the Longhuichang-Tieshan area of the northeastern Sichuan Basin, China, and to elucidate the relationship between these reservoirs and gas distribution. Utilizing drilling and logging data, this study provides a comprehensive summary of the seismic geological characteristics of bio-reef reservoirs. Furthermore, it investigates the effects of reservoir parameters, structural location, and strike-slip faults on the development of bio-reef gas reservoirs. The research shows that bio-reef reservoirs in the area predominantly manifest vertical development in the middle and upper parts of the Changxing Formation, with horizontal expansion occurring along the platform margin and local highland areas. Notably, potential exploration areas are identified, particularly the western wing of the Longhuichang structure and the southwestern side of the Tieshannan structure. By comparing and analyzing the relationship between bio-reef gas reservoirs in the study area, the study aims to clarify the controlling effects of reservoir parameters, structural location, strike-slip faults, and other pertinent factors on the development of bio-reef gas reservoirs. It is observed that while these factors do not exhibit a clear strong linear relationship, they have a comprehensive effect on the development of gas reservoirs. The enrichment mode and failure mode of favorable gas reservoirs in the study area have been analyzed and established, providing crucial technical support to facilitate further exploration of Changxing Formation bio-reef gas reservoirs in the northeastern Sichuan Basin.

Basement buried hill reservoirs represent significant emerging prospects among the newly discovered growth poles in the deepwater areas of the northern South China Sea. Addressing the unclear key factors contributing to their formation, this study dissects successful global exploration cases of basement buried hill reservoirs and analyzes the common characteristics of basement reservoir accumulation under different basin types, structural backgrounds, basement lithologies, and oil and gas geological conditions. A three-element coupling relationship, termed “source-reservoir-cap”, is proposed as the dominant mechanism controlling basement buried hill reservoir formation. The genesis of these reservoirs requires adequate oil and gas supply, appropriately sized accumulation bodies, and effective sealing layers. The optimal configuration of the “source-reservoir-cap” relationship directly influences the efficient charging and preservation of oil and gas within basement buried hill reservoirs. Four configurations are identified, including circumstances such as the source-underlying low-positioned basement buried hill with a “source-reservoir cover docking migration type”, the source-border middle-positioned basement buried hill with a “source-reservoir lateral window docking migration type”, and the source-outside high-positioned basement buried hill with both “source-reservoir short-distance transport and migration type” and “source-reservoir long-distance transport and migration type”. The first to three models present favorable accumulation conditions. Based on the “source-reservoir-cap” three-element coupled model, this study identifies the Yunkai basement buried hill in the Pearl River Mouth Basin, the central depression in the Qiongdongnan Basin, and the northern and southern basement buried hills belts as crucial exploration targets in the deepwater areas of the northern South China Sea.

Baiyun Sag has become the primary focus of deepwater exploration in the Pearl River Mouth Basin. However, its complex and high-variate geothermal characteristics have severely constrained further oil and gas exploration and resource evaluation. In this study, the present-day geothermal field and tectono-thermal evolution histories of Baiyun Sag were systematically studied based on measured rock thermal conductivity and heat generation data, borehole temperatures, low-temperature thermochronometer analyses, and geodynamic methodologies. The thermal conductivity of 251 core samples ranges from 1.131 to 4.478 W/(m·K), with an average of 2.258 W/(m·K), while the heat generation rate of 106 samples ranges from 0.868 to 1.735 μW/m³, averaging 1.499 μW/m³. The thermal conductivity in Baiyun Sag exhibits a gradual decrease from the Wenchang Formation to the Hanjiang Formation, whereas the heat generation rate decreases with depth. The present-day heat flow in Baiyun Sag ranges from 66.6 to 139.1 mW/m², with an average of 89.7 ± 14.7 mW/m², showing a gradual increasing trend from northwest to southeast. Formation temperature at depths of 1–5 km increases proportionally with depth. Thermal inversion, as inferred from low-temperature thermochronological data of six basement samples, reveals distinct temperature paths for each tectonic unit in Baiyun Sag. These paths are primarily linked to regional tectonic uplift-subsidence and basement heat flow variation. Geodynamic simulations further indicate two extensional events in Baiyun Sag during the Eocene and Middle Miocene, leading to a rapid increase in basement heat flow. This study systematically elucidates the present-day geothermal field characteristics and tectono-thermal evolution history of Baiyun Sag, bearing significant implications for regional tectonic evolution and future deepwater oil and gas explorations.

The deepwater area is one of the frontiers of oil and gas exploration, with the understanding of distribution and genesis of deepwater reservoirs being crucial for reservoir formation research. Despite Baodao Sag in the Qiongdongnan Basin being proven to be rich in hydrocarbon generation, significant oil and gas fields have yet to be discovered in the ultra-deepwater areas, and the distribution patterns of reservoirs in deep water remain unclear. Taking the southern slope area of the Baodao Sag as an example, the distribution characteristics of submarine fans are studied by employing seismic prediction methods, including seismic reflection structure analysis, seismic facies geometry, seismic attribute analysis, and the source-to-sink theory. The results show distinct characteristics: the fan delta exhibits parallel oblique progradational reflection, the slumping submarine fan displays lenticular reflection, and the submarine fan of the Sanya Formation demonstrates subparallel sheet reflection. The provenance of sediment is traced back to the denudation area of the Songnan low uplift and the Southern Uplift area in the southwest. The sediments were primarily transported through two main incised channel systems in the north and south, ultimately flowing into the southern slope area of the Baodao Sag. The application of seismic sedimentological prediction methods and source-sink theory has laid a solid geological foundation for oil and gas exploration and the analysis of reservoir forming conditions in the deepwater area of the Baodao Sag.