Journal of Natural Gas Geoscience最新文献

Owing to the high hydrocarbon content and photosynthetic efficiency of phytoplankton, planktonic microalgae account for the main source of petroleum hydrocarbons in marine and continental petroliferous basins. Therefore, the presence of algal residues in such basins is considered an important indicator of high-quality hydrocarbon source rocks. The southwestern region of the Qaidam Basin, characterized by lacustrine deposits from the Paleogene, has substantial petroleum resources. However, studies on the development of source rocks and the mechanisms involved in their organic matter enrichment are insufficient, thus hampering ongoing shale oil exploration efforts. Herein, abundant Botryococcus fossils were discovered in the main source rocks of the upper member of Xiaganchaigou Formation of Paleogene in Southwest Qaidam, accounting for >50 % of the total organic matter content. The palynofacies assemblages reflected relatively distal and oxygen-rich sedimentary environments. The algae-rich rocks usually exhibited distinctive laminated structures alternately composed of quartz-feldspathic, clay and carbonate laminae, indicating periodic climate fluctuations. The Botryococcus, which was mainly preserved in the coarse-grain quartz-feldspathic laminae, probably reflected heavy precipitation conditions and subsequently high nutrient inputs. Finally, the oxygen-rich, low salinity and eutrophic water was likely taking form which benefit the growth of these hydrobiontic algae. These algal-rich shales exhibited typically high hydrogen index (I_H) and total organic carbon (TOC) values, indicating their high hydrocarbon-generation potential. Thus, they are important marker beds for high-quality source rocks in petroliferous basins. Concurrently, the coarse-grained detrital mineral laminae displayed excellent reservoir physical properties, probably providing sufficient reservoir space for planktonic algae-derived liquid hydrocarbon. Therefore, these Botryococcus-rich source rocks might represent important targets for ongoing shale oil exploration.

Tight limestone reservoirs in the Permian Taiyuan Formation of the Ordos Basin have garnered increasing attention in recent years, emerging as a pivotal domain in the quest for natural gas exploration. This research delves into the comprehensive investigation of petrology, reservoir space, reservoir physical properties, and microscopic characteristics through the examination of field outcrops, core observations, thin section identification, scanning electron microscopy, stable carbon and oxygen isotope analysis, and testing. The systematic analysis focused on elucidating the development characteristics of these reservoirs and identifying the controlling factors governing favorable reservoir conditions. Based on our research analysis, the specific rock types with the potential to serve as excellent reservoirs, includes bioclastic silty limestone, bioclastic micrite limestone, and algal-rich limestone. The predominant reservoir spaces within these formations were found to consist of dissolved pores, residual bioclastic cavity pores, intercrystalline pores, and microfractures. These reservoirs exhibit an average porosity of 2.1%, and an average permeability of 0.22 × 10⁻³ μm², indicating their classification as low porosity and low permeability reservoirs. The formation of favorable reservoirs in the Taiyuan Formation limestone was determined to be influenced by many factors; notably, the favorable sedimentary microfacies associated with bioclastic shoals and bioherms provided the foundational material basis for the formation of reservoirs, influencing the type of reservoir space and its extensive planar distribution. Penecontemporaneous karstification, guided by high-frequency cycles, was favorable for the formation of dissolution holes, effectively improving reservoir performance and facilitating the development of thick limestone reservoirs. Furthermore, fractures were identified as crucial agents in improving the seepage capacity of these tight limestone reservoirs. Drawing from our research results, this study offers valuable guidance for the future exploration of limestone formations in the Taiyuan Formation in the Ordos Basin. Additionally, these findings hold considerable significance as a reference point for research and exploration endeavors focused on bioclastic limestone reservoirs in the North China Platform.

The surface conditions in the northern slope area of Zhahaquan in the Qaidam Basin are complex, and the underground is affected by tectonic extrusion movements, resulting in the development of faults and fractures. The existing six blocks of 3D seismic data in this area have a significant time span in acquisition and varying data quality. The existing single-block processing results indicate a low signal-to-noise ratio in the 3D seismic data connection area, along with substantial differences infrequency, phase, and energy. The fracture imaging is poor, making it challenging to accurately identify and track layer positions and fault planes in space, thereby restricting further exploration in this region. Based on a detailed analysis of the characteristics and existing problems of the original data, we conducted key technical research on continuous static correction, pre-stack noise purification, consistency processing, data regularization, and anisotropic pre-stack time migration for continuous processing of six blocks of 3D seismic data in this area. The processing results demonstrate good consistency in frequency, phase, energy, and other aspects, highlighting prominent reflection characteristics and clear imaging of complex structures in the middle and deep layers. Clear breakpoints and fault planes are also evident, solving the inconsistency of frequency, phase, energy, and incomplete coverage in the block connection section. Additionally, this processing has resolved the problem of inaccurate migration positioning caused by inconsistent migration velocity fields, providing high-quality data for subsequent structural interpretation and reservoir prediction.

Solubility models for CH₄, CO₂, and noble gases are widely used in Earth Sciences, playing pivotal roles in the study of homogenization pressure of inclusions, paleoclimate variation, gas migration and accumulation, formation of helium-rich gas plays, and the volumetric ratio of gas to water in reservoirs. This paper reviews solubility models of CH₄, CO₂, and noble gases in pure water and aqueous NaCl solutions. Specifically, the models with high accuracy and wide applicability are introduced in detail: (1) CH₄ solubility model in aqueous solutions within the range of 0–250 °C, 0.1–200 MPa, and 0–6.0 mol/kg NaCl; (2) CO₂ solubility model in aqueous solutions within the range of 0–450 °C, 0.1–150 MPa, and 0–4.5 mol/kg NaCl; (3) Models for calculating the solubility and Henry's constant of atmospheric noble gases within 0–80 °C range; (4) Models for calculating the Henry's constant of noble gases in pure water; (5) Solubility models of noble gases in aqueous solutions within the range of 0–65 °C, 0.1 MPa, and 0–5.8 mol/kg NaCl. The paper also presents some calculated results obtained using these models. The solubility models of CH₄ and CO₂ are complex yet highly accurate, with a broad range of applications. In contrast, the solubility models of noble gases exhibit relatively lower accuracy and a narrower application range, necessitating corrections. In the noble gases-CO₂-H₂O system, low-density CO₂ has little effect on the solubility of noble gases, whereas high-density CO₂ significantly influences their solubilities. Currently, accurately evaluating the solubility of CH₄, CO₂, and noble gases in their mixtures proves challenging, warranting further research into solubility models for gas mixtures.

In the context of tight sandstone reservoirs in the Ordos Basin characterized by compact and heterogeneous rock formations, conventional fracturing techniques yield monolithic fracture shapes, rendering 3D reservoir reconstruction unattainable. This study investigates the principles governing balanced initiation and propagation of fractures in multi-cluster fracturing within unconventional fracturing technology. Employing a large-scale true triaxial simulation experiment system, we utilize the dimensional analysis method (π theorem) to design a physical simulation experiment similarity criterion. Through various experimental adjustments involving proportioning, curing, and mechanical testing, we generate an artificially cured rock mass with mechanical parameters akin to the target layer. The rock mass is maintained at a size of 30 cm × 30 cm × 30 cm. Systematic physical simulation experiments on unconventional volume fracturing are carried out using the 30 cm-sized dense sandstone outcrop rock mass. Taking the conventional fracturing technology as a reference and manipulating experimental conditions and design parameters, we simulate the non-equilibrium initiation and extension behaviors of fracturing fractures under five unconventional volume fracturing technologies, namely hydraulic pulse pretreatment, temporary plugging between clusters, flow-limiting method, cyclic loading and unloading, and pulse intermittent fracturing. Through this, we elucidate the non-equilibrium initiation and extension laws governing multi-cluster fractures. Comparative analysis with conventional fracturing, known for inducing stress interference on fractures and inhibiting their expansion, revels that the five unconventional volume fracturing techniques mitigate stress interference in multi-cluster fracturing. This promotes uniform fracture initiation and expansion, facilitating the creation of complex fractures and larger reconstructed volumes. Among these techniques, inter-cluster block fracturing stands out for its exceptional ability to generate complex fracture networks. The research culminates in the development and refinement of a balanced fracture and extension control technique tailored for multiple cluster fractures in bulk fracturing. This technique significantly contributes to enhancing the degree of 3D reconstruction achievable in unconventional tight oil and gas reservoirs.

To clarify the influence of organic matter and mineral composition on the pore structure of shale reservoirs in the 7th member of the Yanchang Formation (Chang 7 Member) in the Ordos Basin, we characterized the pore structure of Chang 7 shale reservoirs using argon ion polishing-field emission scanning electron microscopy (FE-SEM) and low-pressure nitrogen adsorption (LP-N₂A). This characterization was combined with whole-rock mineral composition and organic geochemical experiments to analyze the main controlling factors of the pore structure of Chang 7 Member shale. The results reveal the presence of various pore types in shale, including organic matter pores, intergranular pores, intercrystalline pores, dissolved pores, and micro-cracks. The LP-N₂A isotherms of shale consistently exhibit type Ⅱ isotherms with H₃ and H₄ hysteresis loop characteristics, indicating the relatively developed nature of mesopores and a pore morphology characterized by parallel lamellar and “ink bottle” shapes. The primary determinants of shale pore structure are identified as organic matter, clay minerals, quartz, feldspar, and pyrite. Among these factors, clay mineral phase transformation generates a substantial number of micropores and mesopores within the mineral crystal layers, serving as the main source of shale pores in the study area. Additionally, liquid hydrocarbons generated, solid bitumen, and euhedral pyrite fill inorganic mineral pores, thereby reducing the pore space of Chang 7 shale to a certain extent. These results provide a new cognition into understanding the pore structure characteristics and controlling factors of Chang 7 shale.

Bozhong Sag, the largest hydrocarbon generation sag in the Bohai Bay Basin, is characterized by presence of the third member of the Shahejie Formation (E₂s³), which serves as a significant source rock. While previous studies have provided insights into the source material, sedimentary environment, and thermal maturity of the E₂s³ source rocks, further investigation is required to deepen our understanding of sedimentary events and biological sources. In this research, nine mudstone core samples from the southwestern Bozhong Sag were thoroughly analyzed using organic-inorganic geochemistry and organic petrology. The results reveal the following key findings: (1) The middle and lower sections of the third member of the Shahejie Formation are characterized by high-quality source rocks, with the lower section exhibiting superior quality. The increase in water salinity during warm and humid climates, and the detection of 24-n-propylcholestane compounds reflected the occurrence of transgression events in the E₂s³. These transgressions gradually increased from the lower sections to the middle sections of the E₂s³. (2) The studied samples exhibit abundant presence of 4α-methyl-24-ethylcholestanes, 24-n-propylcholestanes, 2α-methylhopanes, oleananeabundant algainite, and a small amount of vitrinite. This diversity of hydrocarbon-forming organisms in the E₂s³ source rocks is evident. (3) The moderate marine incursion in the lower part of the E₂s³ results in nutrient elements that promote the flourishing of bacteria and algae, providing an abundant material basis for the formation of high-quality source rocks. However, the large-scale marine incursion in the middle of the E₂s³ weakens water eutrophication, resulting in a decrease in the quality of source rocks compared to the lower part of the E₂s³.

Often petroleum system modelers tend to build complex and time-consuming basin models to understand detailed aspects of hydrocarbon generation and migration within a sedimentary basin. This study built a simple basin model focused on the middle part of the organic-rich, Eocene-Oligocene Shahejie Formation which is the predominant lacustrine source rock in the Bohai Bay Basin. This map-based basin model was created using published regional cross sections, subsurface maps, geochemical data, geothermal gradient data, and structural elements. The integration of these datasets allowed for basin-wide, average TOC and thermal maturity maps to be modeled much more quickly than a geocellular model. These maps show that the majority of the known hydrocarbon fields are found above the Shahejie source rock kitchens. Some fields, however, are found slightly outside of the source rock kitchens indicating either contribution from deeper known source rocks intervals (e.g., the Kongdian Formation) or lateral migration from the middle Shahejie Formation.

The Bohai Bay Basin is a structurally complex rift basin with several strike-slip fault systems being present in the basin with the Tanlu fault system being the most prominent. Using potential field data, published cross sections, isopach maps, hydrocarbon fields, and the presence of Eocene evaporites, structural elements (lineaments) were interpreted. These structural elements influence depositional trends at various stratigraphic levels and the regional thermal maturity trends. Many of these structural elements likely represent thick-skinned faults and subtly influence the TOC trends within the Eocene. Within the middle Shahejie Formation, the average TOC is higher within the depocenters, which may show that lacustrine source rocks are driven more by accommodation space because of the inherent restricted nature associated with lacustrine depositional environments.

The distribution and mineral composition of the Wufeng-Longmaxi shale on the Yangtze platform are directly influenced by the geomorphology of the Late Ordovician-Early Silurian period. This research aimed to clarify the geomorphology of the Yangtze platform during this time period and its implication on the Wufeng-Longmaxi shale. Geophysical interpretation, chronostratigraphic division, and correlation, isometric map compilation, and mineral composition analysis were utilized to achieve this objective. The results of the study showed the following findings: (1) The Wufeng-Longmaxi shale was deposited on the southeastern slope of the Leshan-Longnüsi paleo-uplift on the Yangtze platform; (2) The southeastern slope exhibited three significant slope breaks, which allowed for the division of the slope into four geomorphology units: subaqueous high, subaqueous slope, subaqueous plain, and subaqueous sag; (3) The overlying Wufeng–Longmaxi shale was fully developed in the subaqueous plain and subaqueous sag, but lacked graptolite zones LM1-4 in the subaqueous high and subaqueous slope, with the shale onlapping the southeastern slope from southeast to northwest; (4) The southeastern slope significantly affects the grain size, mineral composition, and TOC content of the overlying shale. Specifically, as the slope transitioned from the subaqueous high to the subaqueous sag, the grain size becomes finer, the contents of TOC and silica increased, and the contents of carbonate and clay minerals decreased.

This research paper focuses on the laminated shale oil reservoir in the third sub-member of the seventh member of Yanchang Formation (Chang 7₃ sub-member) in the Ordos Basin. The study aims to comprehensively analyze the lithofacies type, micro qualitative and quantitative pore structure parameters, and the main controlling factors of the pore structure in laminated shale. The analysis involves various techniques, including a comprehensive analysis of TOC, rock-eval, X-ray diffraction, polarized light and fluorescence microscope observation, field emission scanning electron microscope observation, and low-pressure N₂ adsorption analysis. Based on the sedimentation characteristics, geochemistry, and mineral composition differences, the Chang 7₃ laminated shale can be classified into three lithofacies types: tuffaceous-organic matter binary laminated shale, clayey-organic matter binary laminated shale, and felsic-clayey binary laminated shale. The pore network consists primarily of organic hydrocarbon generation pressurization fractures, clay mineral felsic intergranular composite pores, and felsic plasmid intergranular pore fracture systems. Mesopores are the most developed pore type. The pore volume and specific surface area increase in the order of “tuffaceous-organic matter”, “clayey-organic matter”, and “felsic-clay”, while the heterogeneity of the pore network gradually weakens, and the roughness of the pore surface enhances. The overall development of organic matter pores is limited, with organic matter-pyrite-clay mineral composite pores being the main components of micropores. The primary intergranular pore system, associated with rigid quartz particles, dominates the mesopores and macropores, and acts as the main framework of the entire pore network. The development of feldspar dissolution pores is limited and contributes minimally to the pore network.