Migration Of Natural Gas Research Articles

Quantitative prediction of ultra-deep tight sandstone fractures based on the theory of minimum energy dissipation

Structural fractures are key factors that influence natural gas migration and accumulation within ultra-deep tight reservoirs. To obtain a quantitative forecast of the development and distribution of reservoir fractures in the Keshen gas reservoir, we analyzed the characteristics of the region's structural evolution and paleo-tectonic stress field settings. A reasonable geological model of the research area was built based on an interpretation of the geological structure, a test for rock mechanics, and an experiment on acoustic emission. Thereafter, a 3-D paleo-tectonic stress field during the mid-late Himalayan movement was simulated using the finite element method. By introducing the principle of minimum energy dissipation, which to our knowledge, was done for the first time in the field, mechanical constitutive models, rock failure criteria, and fracture parameter calculation models were derived and used to determine the quantitative development of fractures and predict zones prone to fracture development. The fracture distribution in the ultra-deep Keshen gas reservoir is mainly controlled by folds, buried depth, faults, and lithology. Interestingly, even in the strong tectonic compression movement, the stress field was not continuously transmitted forward but showed fluctuation or cyclicity, indicating that fractures might have occurred at each time stage. The predicted areas with developed fractures were consistent with wells having high fracture linear density (i.e., measured results) and in locations with high-producing gas wells. We believe that the novel method in this study can be of great significance to fracture research in other ultra-deep petroliferous basins.

Models of helium enrichment in helium-rich gas reservoirs of petroliferous basins in China

Owing to the characteristics of chemical inertness, low molecular weight, low boiling point, strong permeability and high heat conductivity, helium is extensively used in the fields of aerospace, low-temperature superconductivity, medicine and high tech, and it is a kind of irreplaceable strategic rare resource. So far, however, less research has been conducted on the enrichment of helium in natural gas reservoirs. To solve this problem, this paper discusses the classification standard of helium-rich gas reservoirs. Based on this, the origin, source and enrichment process of helium in typical helium-rich gas reservoirs are studied, highlighting the contribution of main helium sources to helium-rich gas reservoirs. In addition, helium enrichment models of helium-rich gas reservoir in petroliferous basins are established. And the following research results are obtained. First, 6 helium enrichment models of two categories (crust-source helium enrichment and crust-mantle mixing helium enrichment) are established for different petroliferous basins in China. Second, helium-rich gas reservoirs are characterized by using the multiple helium supply and diverse helium enrichment model. And no matter what enrichment model it is, the contribution of the main helium source to gas reservoirs is indispensable. Third, the helium in the gas reservoirs of central and western basins is mainly crust-source helium, and its enrichment models include ancient formation water moving upward along the fault to dilute helium, helium enrichment in the process of natural gas migration along ancient reservoirs, and helium enrichment of shale gas. Fourth, the helium in the helium-rich gas reservoirs of eastern basins is of crust-mantle mixing origin and its enrichment models are mainly hydrocarbon gas based, carbon dioxide based and nitrogen based. In conclusion, the establishment of the helium enrichment models of helium-rich gas reservoirs in petroliferous basins provides reference and guidance for studying helium enrichment laws and main controlling factors and is of great significance to the evaluation of favorable helium exploration areas.

Genesis Types and Migration of Middle and Lower Assemblages of Natural Gas in the Eastern Belt around the Penyijingxi Sag of the Junggar Basin, NW China

This study analyzes the geochemical characteristics of natural gas composition, carbon isotope, and light hydrocarbon in the eastern belt around the Penyijingxi sag of the Junggar Basin. The result shows the that natural gas content is dominated by alkane gas, with low contents of heavy hydrocarbon and non-hydrocarbon components. The overall carbon isotopic composition of the alkanes shows a trend as δ13methane (C1) < δ13ethane (C2) < δ13propane (C3) < δ13butane (C4), and all δ13C1 values are <−30‰, which are typical of gases of organic origin. The natural gas is mainly coal-derived gas from the Lower Urho formation, mixed with a small amount of oil-associated gas from the Fengcheng formation. The vertical migration of natural gas resulted in the mixing of oil-associated gas and coal-derived gas and the mixing of alkane gas at different stages of the same origin, which should be the origin of carbon isotope inversion. The diffusion migration of carboniferous oil and gas reservoirs has led to differences in gas geochemical characteristics among gas wells. These migration characteristics of natural gas may indicate that the shallow layers are a favorable stratum for the next step of oil and gas exploration in the eastern belt around the Penyijingxi sag.

CT-based 3D pore-fracture network analysis of volcanic reservoirs of Lower Cretaceous Yingcheng formation in southern Songliao Basin, China: Impact on natural gas migration

Volcanic rocks as an important part of basins are found in almost all basins in the world. Although volcanic rock reservoirs are denser and more heterogeneous than conventional reservoirs, they still have good storage capacity in the deeper hydrocarbon-bearing basins. Due to the presence of a large number of irregular and random pores, throats and fractures in volcanic reservoirs at micron-nano scale, its permeability and reservoir capacity are complex. A series of experiments show an abnormal negative correlation between porosity, permeability and natural gas diffusion coefficient in volcanic reservoir rocks of the Yingcheng Formation in the southern Songliao Basin. The dacite sample from CS105 well in Changling Fault Depression and the andesite sample from DS9 well in Dehui Fault Depression with this negative correlation phenomenon were selected, the micron-nano CT scanning technology and Avizo software are applied to quantitively evaluate the 3D pore-fracture network system of the volcanic reservoir samples at multiscale. By comparing the statistical results from CT scanning of the two samples, the dacite sample of CS105 well with higher diffusion coefficient has higher porosity, more connected pores, larger average pore volume and higher average coordination numbers, in addition, the micro-fractures in the CS105 sample promotes the development of secondary pore-throats through later dissolution and further improve the physical properties of volcanic rock reservoirs. In the meantime, the gas seepage simulation results also prove the positive correlation between porosity, permeability and diffusion coefficient of volcanic rock reservoir. The micron-nano CT scanning can more accurately evaluate the physical properties of volcanic rock reservoirs with a large number of pore-throats and fractures at the micron-nano scale, providing theoretical basis for the study of natural gas migration mechanism in highly heterogeneous tight volcanic rock reservoirs.

Research progress on isotopic fractionation in the process of shale gas/coalbed methane migration

The research progress of isotopic fractionation in the process of shale gas/coalbed methane migration has been reviewed from three aspects: characteristics and influencing factors, mechanism and quantitative characterization model, and geological application. It is found that the isotopic fractionation during the complete production of shale gas/coalbed methane shows a four-stage characteristic of “stable-lighter-heavier-lighter again”, which is related to the complex gas migration modes in the pores of shale/coal. The gas migration mechanisms in shale/coal include seepage, diffusion, and adsorption/desorption. Among them, seepage driven by pressure difference does not induce isotopic fractionation, while diffusion and adsorption/desorption lead to significant isotope fractionation. The existing characterization models of isotopic fractionation include diffusion fractionation model, diffusion-adsorption/desorption coupled model, and multi-scale and multi-mechanism coupled model. Results of model calculations show that the isotopic fractionation during natural gas migration is mainly controlled by pore structure, adsorption capacity, and initial/boundary conditions of the reservoir rock. So far, the isotope fractionation model has been successfully used to evaluate critical parameters, such as gas-in-place content and ratio of adsorbed/free gas in shale/coal etc. Furthermore, it has shown promising application potential in production status identification and decline trend prediction of gas well. Future research should focus on: (1) the co-evolution of carbon and hydrogen isotopes of different components during natural gas migration, (2) the characterization of isotopic fractionation during the whole process of gas generation-expulsion-migration-accumulation-dispersion, and (3) quantitative characterization of isotopic fractionation during natural gas migration in complex pore-fracture systems and its application.

A closer look at underground natural gas pipeline leaks across the United States

Underground natural gas (NG) pipeline leakage can result in methane (CH4) buildup and migration through the soil. What is not well understood in such scenarios is how the soil conditions affect the gas migration behavior, particularly in regard to the relative contributions of specific soil properties such as soil moisture content. The objective of this study was to investigate the effects of soil properties on CH4 concentration and migration from leaking underground NG pipelines. Site characteristics such as surface cover and spatial dimensions, soil samples, and gas concentration data were collected from over 70 gas leakage sites across the United States using a standardized sampling method. Soil samples were collected from excavation sites that were 1.5′–5′ in depth. The collected soil samples were analyzed in the laboratory to measure the soil texture, permeability, and moisture. Statistical analysis was performed to evaluate the effects of soil properties on CH4 migration distance and concentration. Soil texture was consistent across geographic locations due to standardized pipeline backfill protocols, allowing for the analysis of gas concentration and transport data with respect to soil conditions. Soil moisture content was the dominant influence on the gas concentration and spreading distance. High soil moisture content was associated with reduced lateral diffusion and elevated concentrations near the leak point, whereas dry conditions were associated with reduced concentrations and greater spreading distance. Increasing soil moisture content reduced the lateral diffusion of CH4 into the soil due to water-induced tortuosity, resulting in elevated concentrations close to the leak point. Lateral migration of CH4 was suspected to be by diffusion, starting at 5 m from the leaks, while transport within the immediate vicinity of the leak was controlled by advection. These findings demonstrate a pronounced effect of soil moisture content and permeability on CH4 migration distance and concentration, providing key insight into the effects of soil conditions on NG migration and how to account for such effects in leak detection surveys.

Methods and benefits of measuring non-hydrocarbon gases from surface casing vents

Surface casing vents divert natural gas migration along oil and gas boreholes to bypass groundwater, with the gas venting to the atmosphere. While this strategy is designed to protect groundwater, it constitutes a source of greenhouse gases to the atmosphere. In instances where gas leakage occurs, the characterization of the molecular and isotopic composition of natural gas emitted from surface casing vent flows can be used to assist in identifying the gas source. We compare concentration measurements of non-hydrocarbon gases (within natural gas) of samples analyzed by laboratory-based gas chromatography (N2, Ar, CO2 and O2) and magnetic sector noble gas mass spectrometry (He, Ar and Kr) with field measurements conducted using a field portable quadrupole mass spectrometer (miniRUEDI). The standard deviation of miniRUEDI concentration results was within plus/minus one standard deviation of samples measured using laboratory-based GC (N2, O2, Ar and He) and magnetic sector noble gas mass spectrometry (He, Ar). Additional laboratory-based determination of isotope ratios of methane and argon (δ13CCH4, δ2HCH4, and 40Ar/36Ar) enabled a comparison between information provided by the analysis of reactive gases compared with noble gas isotopes. Gases from different sources displayed quantifiable differences in δ13CCH4 and δ2HCH4, but these changes may or may not be distinguished if only one sampling event is conducted. By comparison, 40Ar/36Ar further enabled the differentiation of various gas sources. The objective of this paper is to discuss the advantages and trade-offs of the three different analysis methods considered, and the feasibility of their application in different environmental monitoring scenarios.

Natural Gas Geochemistry of Jurassic Shaximiao Formation in the Western Sichuan Basin, China: Fault-Controlled Differentiation in Accumulation Process

The Jurassic Shaximiao Formation in western Sichuan has made significant contributions to the natural gas reserves and production of China, with many gas fields successively discovered. The accumulation evolution progress and discrepant gathering law are continuously improved with the constantly deepened exploration and development. In order to find evidence for the difference in gas accumulation and the method to reshape the migration and accumulation of natural gas and their dynamic processes based on time and space, this study analyzes the dynamic accumulation process of the Middle Jurassic Shaximiao reservoir in the Zhongjiang gas field using the geochemical parameters, including the conventional composition of natural gas, light hydrocarbon components, carbon isotope, and formation water, of more than 160 natural gas samples from 155 wells. The results show that the natural gas-filling period, migration patterns, and phase state, etc., of the Shaximiao Formation are closely related to the distance between the hydrocarbon source faults. The geochemistry tracer parameters of natural gas and formation water reveal abundant information about the accumulation process. The Sha-3 gas reservoir, featuring a two-period hydrocarbon supply, has two migration patterns, i.e., seepage and diffusion, dominated by the water-soluble phase. Similarly, the Sha-2 gas reservoir, featuring a late single hydrocarbon supply, has the same migration patterns, dominated by the miscible phase of the water-soluble phase and free phase. The main migration pattern of the Sha-1 reservoir, which features a late single hydrocarbon supply, is seepage, dominated by the free phase. From the lower part of the Sand-3 gas reservoir to the upper part of the Sha-1 gas reservoir, the natural gas exhibits a trend where the early–late continuous filling turns into the late single filling, the lateral migration distance of oil and gas becomes shorter gradually, and the gas-rich range of the gas reservoir decreases gradually. The results of this study can provide a valuable reference for gas reservoirs controlled by faults and river sand bodies.

Characterization Method of Tight Sandstone Reservoir Heterogeneity and Tight Gas Accumulation Mechanism, Jurassic Formation, Sichuan Basin, China

Although significant progress has been made in tight gas exploration and development, there is still a limited understanding of the gas accumulation mechanism in tight formation. The description of heterogeneity in the tight reservoir is still an obstacle during tight gas exploration. In this work, we will develop a 3D gas accumulation facility that can reflect the high temperature and pressure under the geological condition to simulate the gas accumulation process in the tight formation of the Jurassic Formation, Sichuan Basin. Both the physical experiment simulation and numerical simulation methods will be combined to reveal the mechanism of gas accumulation. The results show that (1) the permeability ratio can characterize the heterogeneity of tight reservoirs. Based on this parameter, petroleum geologists can understand the interlayer heterogeneity during fluvial deposition and predict the sweet spots of tight gas; (2) the permeability ratio relationship between hydrocarbon accumulation of the tight sandstone reservoir mainly manifests as long as there is the existence of the differential permeability and there will be differences in natural gas migration dynamics, so that the natural gas accumulates in pores and depends on the extent of source rock; and (3) although the difference between capillary force curves is at the core of the influence of the permeability ratio on the gas-bearing capacity of the “sweet spot” sand body, the permeability ratio is a parameter of practical engineering significance. This work adds new methods to tight gas formation heterogeneity characterization and sheds light on the mechanisms of hydrocarbon accumulation in tight formation.

Investigation of the Law of Natural Gas Phase Behavior during the Migration and Reservoir Formation

It is of great significance to understand the phase regime of natural gas migration under high temperature and high pressure in the DF1-1 structure of Yinggehai Basin, South China Sea, to predict natural gas accumulation and evaluate resource reserves. However, the existing methane solubility models are not suitable for the higher temperature and pressure range required under current geological conditions. Through a self-designed experimental equipment with high precision, the solubility of CH4 in deionized water can be measured in a wide range of temperatures (313.15-473.15 K) and pressures (5-200 MPa), and a reliable pressure-temperature-solubility segmented model is obtained by fitting. Using this model, we found that if the free-phase natural gas accumulated in the DF1-1 structural reservoir is completely dissolved in the water-soluble phase, the required formation water volume is much higher than the actual calculated formation water storage. Therefore, it can be inferred that the natural gas generated from the source rock of the DF13-1 structure mainly migrated and accumulated in the form of the free phase.

Constraining well integrity and propensity for fugitive gas migration in surficial soils at onshore decommissioned oil and gas well sites in England

Petroleum resource development generates a legacy of energy wells that must be decommissioned effectively as we transition towards NetZero. Unfortunately, some decommissioned wells (DWs) can suffer integrity failure resulting in release of fugitive natural gas into the surrounding soils and atmosphere. After decommissioning there are typically no ongoing assessments to confirm well integrity, meaning integrity status remains uncertain into the future. Furthermore, factors affecting fugitive natural gas migration in surficial soils around DWs are poorly recognized, inferring integrity assessment and monitoring strategies are lacking. To better understand the integrity status of DWs, identify soil properties controlling fugitive gas migration and help develop more effective monitoring and detection methodologies, we undertook field investigations at six DWs in England involving surficial CH4 measurements, sub-surface soil-gas and sediment sampling and dynamic flux-chamber measurements. We found no evidence of integrity failure at any site. However, the composition and structure of soils in which examined DWs are embedded suggest fugitive gas migration to surface may be severely limited; potentially mitigating methane emissions while making integrity assessment at the surface challenging. Overall, the integrity status of DWs in England is poorly constrained and we show surficial soil properties must be characterised and considered to effectively constrain the fate of fugitive gas and in order to design effective field assessment and monitoring methods.

Natural Gas Migration Pathways and Their Influence on Gas Hydrate Enrichment in the Qiongdongnan Basin, South China Sea

2D and 3D seismic data and basin simulation were used to investigate the gas hydrate distribution and natural gas migration pathways in the Qiongdongnan Basin (QDNB). Hydrate-related amplitude anomalies and extensive bottom simulating reflectors (BSRs) were mapped within the uppermost part. Based on their seismic reflection characteristics, the three main types of natural gas migration pathways and their distributions in the QDNB were identified through high-resolution seismic data. Basin modeling was carried out to document the migration efficiency of different migration pathways and their effects on hydrate enrichment. The basin modeling results show the following: (1) Diapirs, fault structures, and fractures constitute the three main types of natural gas migration pathways that transport the thermogenic gas from the deep to shallow layers in the QDNB. (2) The three migration pathways impact hydrate enrichment in different ways. Diapirs and faults contribute significantly to hydrate enrichment due to their higher migration efficiency. In comparison, the migration efficiency of the fracture systems is lower, with minimal benefit to hydrate enrichment. (3) The natural gas hydrate in the QDNB is mainly distributed along the diapirs and deep faults and generally scattered around the fracture system. These conclusions indicate that the migration pathways in the QDNB are regionally distributed and are closely related to hydrate accumulation.

Geochemistry and origins of natural gas in the Hong-Che fault zone of the Junggar Basin, NW China

The uplift zone adjacent to the sag is the dominant direction of natural gas migration and accumulation, and the multi-layer source rocks developed in the sag have laid a material foundation for natural gas accumulation. However, owing to the deep burial and high drilling cost of the sag, drilling the source rocks in the sag is challenging, which severely restricts the fine gas-source correlation study of natural gas in the uplift zone. The Hong-Che fault zone is adjacent to the Shawan sag, the natural gas is of various types, and the origin and source of natural gas are unclear. The referential contrast research methods on multi-source and multi-period natural gas are absent, which hinders their subsequent exploration and development. In view of these problems, we analysed the pyrolysis gas of source rocks, along with its composition, including the light hydrocarbon and the carbon isotope of natural gas. The experimental results show that: (1) There are significant differences in the carbon isotopes of ethane in pyrolysis gas from different source rocks, which is one of the research bases of natural gas source analyses. (2) The natural gas in the region is mainly present as kerogen cracking gas. (3) The natural gas in the study area is divided into five types. Type-Ⅰ and type-Ⅱ gases derived from the humic source rocks of the Carboniferous stratum and the Jiamuhe Formation, respectively. Type-Ⅲgas was sourced from the sapropelic source rocks of the Fengcheng Formation. Type-Ⅳ gas was originated from mixed source rocks of the Wuerhe Formation. Type-Ⅴ gas was derived from the mixing of multiple sets of source rocks. (4) The south section of the Hong-Che fault zone has high gas production, and is a favourable exploration target for humic pure gas reservoirs. The slope area near the Shawan sag has better preservation conditions and is a favourable target area for subsequent natural gas exploration activities. These results lay the theoretical foundation for the evaluation and identification of the origins of multi-source and multi-period natural gas in complex areas.

Influence of three-dimensional tectonic stress field on natural gas migration and accumulation within Tianhuan Depression, Ordos Basin, China

Influence of three-dimensional tectonic stress field on natural gas migration and accumulation within Tianhuan Depression, Ordos Basin, China

Origin, accumulation and secondary alteration of natural gas around Qingshui Sub-sag, Liaohe Depression, China: Insights from molecular and isotopic composition

Fourteen gases and their associated oils were collected at the surface from production wells in the Shahejie Formation, Dongying Formation, and Archean in six zones around the Qingshui Sub-sag of the Liaohe Depression, Bohai Bay Basin. The molecular compositions and carbon and hydrogen isotopic compositions of natural gas were measured. The origin, migration, accumulation, and secondary alteration of natural gas were studied based on the lateral and vertical comparison of natural gas geochemical characteristics. The natural gases include humic-type (δ13C2 > −29.0‰, δ13C1 > −43.0‰), sapropelic-type (δ13C2 < −29.0‰, δ13C1 > −43.0‰), and secondary microbial (δ13C1 of −48.5‰, C1/(C2+C3) of 46.3, δ13CO2 of −1.5‰) gases. The humic-type and sapropelic-type gases were derived from the source rocks in the third and fourth members of the Shahejie Formation, respectively. The humic-type gases dominate. The biodegradation level of natural gas in the study area varies, and biodegraded gases are characterized by high Δ13C(C3–C2), low C3/C2, and low C3/iC4 and are associated with biodegraded oils. The vertical migration of natural gas in the west and east of the Qingshui Sub-sag is limited. There are good additional exploration prospects for humic-type gas, derived from high-maturity source rocks of the third member of the Shahejie Formation in the deep layers (depth >4.5 km) of the Qingshui Sub-sag. Secondary microbial gas is a potential exploration target in the shallow layers (depth <2.5 km).

Accumulation characteristics and large-medium gas reservoir-forming mechanism of tight sandstone gas reservoir in Sichuan Basin, China: Case study of the Upper Triassic Xujiahe Formation gas reservoir

Tight sandstone gas reservoirs of Xujiahe Formation in central Sichuan Basin have generally high water saturation. The characteristics of natural gas migration and accumulation, as well as gas reservoirs with high water saturation formation mechanism, were investigated using test data from natural gas geochemistry analysis and nuclear magnetic resonance (NMR) high pressure filling simulation experiment. The findings reveal that: (1)The Xujiahe Formation established the foundation for oil and gas near-source accumulation by interacting superimposed large areas of source rock and reservoir. As the maturity of source rocks increases, the wetness becomes smaller and the δ13C1 value becomes heavier of natural gas, indicating a near-source accumulation. (2)The gas saturation of Xujiahe Formation gas reservoirs ranges from 50% to 65%, with a pore radius of the reservoir space 0.1–10 μm. The larger the proportion of relatively large pore size, the higher the gas saturation will be. (3)As charging pressure is increased, natural gas accumulates in tight sandstone by progressively aggregating from large pore diameter to small pore diameter space, culminating “three-stage” charging characteristics of rapid increase, moderate increase and basically stable charge. The coupling between low-pressure drive and relatively large pore reservoir is the fundamental reason why the Xujiahe Formation of natural gas can form large-medium gas reservoirs but high water saturation in central Sichuan Basin. The findings will provide theoretical and technical support for the exploration of tight sandstone.

Characteristics of natural gas accumulation in the Cambrian weathered crust in southwestern Ordos Basin

The Cambrian weathered crust formed in the Early Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration, and important breakthroughs in oil and gas exploration have been made there. In order to further accelerate the process of natural gas exploration in this area, this paper based on systematic analysis of source-reservoir assemblage and trap type in the Cambrian weathered crust in southwestern Ordos Basin, and then evaluates its natural gas exploration potentials and favorable exploration zones. The following research results are obtained. First, the Upper Paleozoic coal-measure source rocks in the basin is the main gas source rocks, forming a source-reservoir assemblage of “upper source and lower reservoir” with Cambrian reservoir. Second, the types of the reservoir in the Cambrian weathered crust are mainly dolomite intercrystalline (dissolution) pores and karst fractures and vugs, whose development is controlled by grain shoal microfacies and karstification in the weathered crust stage, respectively. Third, the traps in the Cambrian weathered crust are mainly lithologic-stratigraphic traps, which were formed in the tectonic uplifting setting of the eastern Ordos Basin during the Yanshanian. There are geological conditions of lithologic and stratigraphic barriers in the updip direction, which is beneficial to the short-distance natural gas migration and accumulation. In conclusion, Huanxian-Zhengning area on the east margin of the central paleouplift in southwestern Ordos Basin is a favorable exploration zone of Cambrian weathered crust gas reservoirs, and the Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.

The Accumulation Characteristics of the Paleozoic Reservoir in the Central-Southern Ordos Basin Recorded by Organic Inclusions

The Permian tight clastic reservoir and Ordovician carbonate reservoir were developed in the central-southern Ordos Basin. This study investigated the fluid inclusion petrography, diagenetic fluid characteristics, formation process of natural gas reservoir, source rock characteristics, and reservoir accumulation characteristics of these Paleozoic strata by petrographic observations, scanning electron microscope imaging, fluid inclusion homogenization temperature, salinity, laser Raman spectrum, and gas chromatograph analyses. The results have suggested two phases of fluid inclusions in both the Permian sandstone and the Ordovician Majiagou Formation dolomite reservoirs, and the fluid inclusions recorded the history from the early thermal evolution of hydrocarbon generation to the formation, migration, and accumulation of natural gas. The early-phase inclusions show weak yellow fluorescence and recorded the early formation of liquid hydrocarbons, while the late-phase inclusions are nonfluorescent natural gas inclusions distributed in the late tectonic fractures and recorded the late accumulation of natural gas. The brine systems of the Permian and Ordovician fluid inclusions are, respectively, dominated by CaCl2-H2O and MgCl2-NaCl-H2O. The diagenetic fluids were in the ranges of medium-low temperature and moderate-low salinity. The natural gas hydrocarbon source rocks in the Ordos Basin include both the Permian coal-bearing rocks and the Ordovician carbonates. The process of the early-phase liquid hydrocarbon formation and migration into the reservoir corresponded to 220 Ma (Late Triassic). The late large-scale migration and accumulation of natural gas occurred at 100 Ma (early Late Cretaceous), which was close to the inclusion Rb/Sr isochron age of 89.18 Ma, indicating that the natural gas accumulation was related to the Yanshanian tectonic movement.

Effects of micro-fracture and micro-coal line on tight gas accumulation, Triassic Xujiahe Formation, Sichuan Basin, China

The study of natural gas migration and accumulation of tight gas has become the focus of the energy industry in recent years. One of the priorities is the research of hydrocarbon migration carrier system of the formation. This study reveals two types of hydrocarbon carrier systems (micro-fractures and micro-coal lines) in the tight formation through a large amount of geological observations, data statistics, and simulations. This study also first put forward the view that the coal lines can be a type of hydrocarbon carrier system in tight sandstone formation. During the research process, an improved permeability testing method and apparatus are used to test the core permeability widely. Based on the real core, three models are established to describe the micro-fracture/micro-coal line characteristics. According to the three typical models, the Voronoi method is used to rebuild the network models. The migration and accumulation process in tight formation is simulated with the Multiple Relaxation Time lattice Boltzmann method in numerical simulation and with displacement apparatus in physical simulation. The results indicate that the micro-fracture/micro-coal line will significantly improve the percolation capacity of sandstone as the hydrocarbon migration carrier system in tight formation, the mechanism is revealed by numerical simulations. The micro-fractures/micro-coal lines are favorable for natural gas accumulation in tight formation. Unlike the conventional formation, in tight formation, micro-fractures/micro-coal lines tend to be the main migration carrier systems during the gas accumulation process.

P-047. Elevated placental histone H3K4 methylation via upregulated histone methyltransferases in preeclampsia and its possible involvement in hypoxia-induced pathophysiology

The Cambrian weathered crust formed in the Early Paleozoic central paleouplift of the Ordos Basin and its periphery has always been a key target for oil and gas exploration, and important breakthroughs in oil and gas exploration have been made there. In order to further accelerate the process of natural gas exploration in this area, this paper based on systematic analysis of source-reservoir assemblage and trap type in the Cambrian weathered crust in southwestern Ordos Basin, and then evaluates its natural gas exploration potentials and favorable exploration zones. The following research results are obtained. First, the Upper Paleozoic coal-measure source rocks in the basin is the main gas source rocks, forming a source-reservoir assemblage of “upper source and lower reservoir” with Cambrian reservoir. Second, the types of the reservoir in the Cambrian weathered crust are mainly dolomite intercrystalline (dissolution) pores and karst fractures and vugs, whose development is controlled by grain shoal microfacies and karstification in the weathered crust stage, respectively. Third, the traps in the Cambrian weathered crust are mainly lithologic-stratigraphic traps, which were formed in the tectonic uplifting setting of the eastern Ordos Basin during the Yanshanian. There are geological conditions of lithologic and stratigraphic barriers in the updip direction, which is beneficial to the short-distance natural gas migration and accumulation. In conclusion, Huanxian-Zhengning area on the east margin of the central paleouplift in southwestern Ordos Basin is a favorable exploration zone of Cambrian weathered crust gas reservoirs, and the Middle Cambrian Zhangxia Formation and Upper Cambrian Sanshanzi Formation are favorable intervals of oil and gas exploration.