Volcanic Gas Reservoir Research Articles

Numerical simulation study on the volcanic gas reservoirs in block C of Xushen Gasfield

Block C of Xushen Field is one kind of the volcanic reservoirs with the bottom water. Since the development, the dynamic characteristics of the block are characterized by large differences in gas well productivity, formation water production, and low effective utilization of reserves. Using the dual-medium numerical model, through historical simulation and combination with gas well dynamic, the block pressure and, water body changes and remaining reserves distribution characteristics are clarified. The research shows that the remaining reserves of the block are mainly concentrated in the vicinity of xs901, xs903-p1 and the northwest periphery of xs902. With the increase of production allocation, the water production of six old wells such as XS9-7 increased rapidly and significantly, while the bottom water coning was obvious. Through six sets of gas production rate development plans with different magnification, consider whether the gas distribution can stabilize the output, water production, bottom hole flow pressure, and fluid accumulation. The optimal development plan of the old well in the block is determined comprehensively.

Sealing mechanisms in volcanic faulted reservoirs in Xujiaweizi extension, northern Songliao Basin, northeastern China

Distribution of volcanic reservoirs in Xujiaweizi half graben is controlled by both faults and sealing layers where their integrity could have been compromised. This paper documents the distribution of sealing units of the Denglouku and Yingcheng Formations based on seismic and well-log data to delineate all of the reservoirs with proper cap rocks within the volcanic Yincheng Formation via the relationship between the fault system and reservoir rocks. Based on the regional sealing effect of Denglouku Formation, two hydrocarbon traps are identified: the lower primary and upper secondary gas reservoirs where the formation with a mudstone percentage of more than 50% within the second member seals the primary reservoirs. The upper secondary trap is controlled by faults that have been activated during the structural reversal phase, causing the regional sealing layers in the Denglouku Formation to get displaced with a cap rock juxtaposition thickness of less than 35 m. This has created a series of fault-seal, dual-control reservoirs. The local seals within the Yingcheng Formation consist of mudstone, tight volcanic rock, and clayey breccia covering each volcanic eruption cycle. These local seals separate the volcanic gas reservoirs with a minimum thickness of 20 m. The local top seals in this tectonically active zone were placed on the hanging wall via the juxtaposition of the reservoir and overlying mudstone and/or clayey breccia. It was concluded that gas has migrated vertically through the faults and accumulated in the fault-controlled traps where sharp changes in the lithology (juxtaposition) form the seal, whereas the gas-water contact is controlled by the depth of the reservoir rock. Finally, this study concludes that the primary reservoirs are distributed in a sinusoidal configuration around the fault zone because of the dolphin effect of the Xuzhong strike-slip fault system that has connected the source and reservoir rocks.

Fracture propagation simulation and construction parameter optimization of eruptive phase volcanic rock

A large number of volcanic gas reservoirs are developed in the Sichuan Basin. Up to now, 92 wells have been drilled into Permian volcanic rock, which have good prospects for exploration. Among them, large areas of volcanic facies are distributed in the Sichuan Basin. The uneven distribution of pores in volcanic rocks of eruptive facies and the large difference in rock mechanics combination between different lithofacies make it difficult to develop volcanic gas reservoirs of eruptive facies effectively. Therefore, studying the characteristics of eruptive facies volcanic rocks and reservoir reconstruction schemes is very important for the later development of volcanic rocks Significantly, this paper builds a finite element model of a volcanic rock reservoir of the effusion facies based on the geological characteristics of the volcanic rock of the effusion facies, and optimizes the construction parameters in terms of construction displacement, fracturing fluid volume, and proppant. The research results show that, the injection pressure of the spouting phase under different stress difference conditions exhibits oscillating characteristics. The stress difference between the barrier and the reservoir is positively related to the injection point pressure and fracture fracturing. The recommended construction displacement is 4.5-5.5m3/min and the fracturing fluid dosage is 400-500m3, and the amount of proppant is between 30-40m3. The research content of this article is of great significance for the effective development of volcanic gas reservoirs in eruptive facies.

Research on Classification Management Method of Xushen Gas Field

Xushen gas field not only exploits volcanic gas reservoir, but also develops glutenite reservoir. In the production process, there are great differences in gas well productivity, formation pressure and single good reserves. By establishing the fine management mode of four types of wells, the “one type, one method, one well, one policy” has been realized. It mainly forms the multi-factor comprehensive production allocation technology with “two optimizations and three control” as the means, improves the winter peak regulation mechanism of withdrawal, supplement and shortterm emergency peak shaving, determines the drainage gas production technology to meet the current development needs, explores the treatment method of easy freezing plugging well with “three-step” as the main method, and effectively governs Low-yield and low-efficiency wells.

Gas–water two-phase productivity model for fractured horizontal wells in volcanic gas reservoirs with natural fractures

ABSTRACT Volcanic gas reservoirs, as new exploration and development fields, are rich in resources and have broad exploration and development prospects. However, compared with the conventional sandstone gas reservoirs, volcanic gas reservoirs have more complex storage and permeability mode, with multiple media including pores, caves and fractures, which determines the strong heterogeneity, complicated seepage mechanisms, and difficult development of volcanic gas reservoirs. To further improve the prediction accuracy of fractured horizontal well productivity in volcanic gas reservoirs, Xushen low-permeability volcanic gas reservoirs are presented as examples herein using the equation of motion based on the principle of equal flow. Considering the stress-sensitive effect, starting pressure gradient, high-speed turbulence effect, and dual porous medium structure, as well as combining the nature of gas and real gas state equations, a gas–water two-phase productivity model is established for a matrix-natural fracture-artificial fracture-wellbore volcanic gas reservoir fractured horizontal well. In addition, factors affecting production are studied. Results indicate that stress-sensitivity coefficient of different media has different effects on the production. The natural-fracture stress-sensitive coefficient has the greatest influence on production. With the increase of the natural-fracture stress-sensitive coefficient, the gas production decreases and the reduction amplitude changes little, which is almost uniform. When natural-fracture stress-sensitive coefficient is 0.08, gas well production decreases by 14.24%. The second is the artificial main fracture stress-sensitivity coefficient. With the increase of the artificial main fracture stress-sensitivity coefficient, the decrease of gas production becomes larger. When artificial main fracture stress-sensitivity coefficient is 0.1, gas well production decreases by 6.6%. The matrix stress-sensitivity coefficient on production is the least, almost zero. The high-speed nonlinear coefficient significantly affected the production. With the increase of the high-speed nonlinear coefficient, the gas well production decreases, and the decrease rate is faster and faster. When the high-speed nonlinear coefficient increases to 10000 m−1, gas well production decreases by 45.65%. However, the starting pressure gradient had little effect on production. Finally, the model is verified by the actual production data of Xushen1-H2 well. The average errors of the gas production, water production, and pressure calculations are 9.65%, 9.63%, and 9.8%, respectively. This study contributes positively to the productivity prediction of fractured horizontal wells in low-permeability volcanic gas reservoirs.

Geochemical characteristics, origin, and mechanism of differential accumulation of natural gas in the carboniferous kelameili gas field in Junggar basin, China

Since the discovery of the Kelameili gas field, the exploration of volcanic gas reservoirs in the Junggar Basin has attracted considerable attention. However, owing to the limitations involved in studying the characteristics of the origin and distribution of the volcanic gas reservoirs in this area, the study of gas reservoirs in the South fault terrace area of the Kelameili gas field as a typical peripheral area is greatly restricted, and the exploration process is relatively slow. Therefore, it is necessary to analyse and compare the differences in the geochemical characteristics of the natural gas between the Kelameili gas field and the fault terrace area of the south belt by analysing the natural gas geochemistry, hydrocarbon generation evolution, and structural evolution. In addition, geochemical experiments involving the natural gas components, carbon isotopes, light hydrocarbons, hydrocarbon generation, and evolutionary history of the source rocks and the interpretation of seismic profiles reflecting the structural evolution, help clarifying the genetic type of the volcanic gas reservoirs and the source of the natural gas in this area. The results indicate the following. (1) The natural gas in the study area has undergone thermal genesis and is generated by kerogen cracking. It is a typical humic gas with high maturity. (2) The origin of the natural gas is the Carboniferous source rock in the depression, which is significantly different from the natural gas from the typical Permian and Jurassic. (3) The primary reason for the difference in natural gas accumulation is that the Carboniferous source rock entered the high-maturity stage in the Early Jurassic. The formation time of the trap is earlier than that of the high-maturity stage of the source rock evolution, which can capture multi-stage source rock products. (4) Based on the hydrocarbon source conditions that the hydrocarbon supply horizon of the Kelameili gas field has a high maturity of organic matter, and insufficient hydrocarbon supply, it is considered that traps capturing the multi-stage source rock products represent a larger scale of natural gas accumulation. The results of this study can deepen the understanding of volcanic gas reservoirs in the Junggar Basin and guidethe exploration and development of global volcanic gas reservoirs and the surrounding large-scale gas fields.

New progress and prospect of Middle Permian natural gas exploration in the Sichuan Basin

In recent years, natural gas exploration in the deep marine carbonates of the Sichuan Basin has been strengthened continuously. And based on the research results in the regional geological setting, a great number of new geological understandings are obtained in terms of lithofacies palaogeography, reservoir genesis and hydrocarbon accumulation evolution. What's more, new progress of natural gas exploration is achieved in many strata and domains in the Sichuan Basin, e.g. Qixia Fm of Middle Permian, Maokou Fm and Emeishan basalt, and the prospect of natural gas exploration is promising. In this paper, the recent exploration achievements of Middle Permian natural gas in the Sichuan Basin were summarized. First, under the control of palaeogeomorphology of late Caledonian, marginal platform shoal and girdle intra-platform shoal that are distributed in a large scale were developed at the edge of western Sichuan sea basin and in the periphery of central Sichuan paleo-uplift during the sedimentation of Qixia Fm, and they are large in scale and extensive in distribution range. Dolomite reservoirs are developed with good gas bearing property. Second, two kinds of reservoir bodies are mainly developed in the Maokou Fm of the Sichuan Basin. One is the shoal body which is developed along the marginal platform and the intra-platform high belt under the control of facies belt superimposed with late dolomitization, and the other is the effective karst reservoir which is distributed at the karst slope belt under the action of large-scale erosion in the late Maokou period. Third, volcanic gas reservoirs are discovered in the areas of Chengdu and Jianyang, and their quality reservoirs are of volcanic eruption facies and characterized by great thickness, good physical properties and extensive distribution. The discovery of this new type of gas reservoir expands the exploration domain of Middle Permian natural gas in the Sichuan Basin. In conclusion, the improvement of the large-scale natural gas exploration potential in many strata and domains of Middle Permian in the Sichuan Basin promotes it to be one important replacement domain of the further increase of natural gas production and reserves in the Sichuan Basin.

Quantitative Characterization, Classification, and Influencing Factors of the Full Range of Pores in Weathering Crust Volcanic Reservoirs: Case Study in Bohai Bay Basin, China

Weathering crust volcanic oil and gas reservoirs were revealed in the Bohai Bay Basin, and the physical properties changed considerably in different volcanic reservoirs. To understand better the pore structure of weathering crust volcanic reservoirs, it is important to characterize quantitatively the full-range pore size distribution (PSD). In this study, six Lower Cretaceous volcanic samples from the Bohai Bay Basin were measured by scanning electron microscopy, low-temperature nitrogen adsorption (LTNA), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR) to uncover the full-range PSD of volcanic rocks. The results showed the following. Firstly, four types of pores were observed in the volcanic samples, and in ascending order of the pore size, these pores were devitrification pores, inter-crystalline pores, dissolution pores, and amygdales. Secondly, ink-bottle and slit-type nano-sized pores measured by LTNA generally accounted for 25% of the volcanic porosity, and in tight volcanic rocks with porosity lower than 5%, nano-sized pores accounted for more than 95%. Thirdly, LTNA was combined with NMR corrected by MIP to characterize the full-range PSD, and the pores were divided into three ranges according to boundaries of 0.05 and 1 µm, respectively. Fourthly, the clay-related inter-crystalline pores dominant in dacitic tuff and basalt reservoirs and the dissolution pores dominant in dacite and trachyte reservoir were characterized by “larger pores connected by narrow throats” and “honeycomb-like type,” respectively. Lastly, mafic minerals were prone to alteration under the weathering crust geological conditions, and the iron and magnesium ions promoted the spontaneous generation of clay minerals in the pores, thereby worsening the physical properties of the reservoir. Therefore, we propose that alkali-rich acidic volcanic lava is a favorable reservoir for oil and gas exploration.

Thin section and X-ray computed tomography for characterizing volcanic reservoirs in Chagan Sag, Yin-E Basin, China

We have synthesized macroscopic data (cores) and microscopic data (thin sections and computed tomography images) to characterize volcanic oil and gas reservoir spaces in the Chagan Sag, Yin-E Basin, China. We also have assessed the controlling factors affecting the formation of these reservoirs. Primary pores significantly improve the reservoir quality because they enable the formation of secondary pores (dissolution pores) by allowing migration, alteration, and filling by inorganic and organic fluids. Consequently, the development of secondary pores modifies the permeability of the volcanic rocks. The formation of volcanic reservoirs is influenced by lithology, diagenic minerals, faults, and fractures. The size and abundance of pores, especially gas pores, varies with the lithology. Gas pores are consistently larger and more abundant in basalts than in andesites and tuffs. Moreover, the carbonates and chlorites readily dissolve in the vesicles and fractures, thus resulting in the formation of secondary pores. However, quartzes are not susceptible to dissolution in this study area. Further, faults and fractures are necessary for the formation of reservoir spaces and for oil and gas migration. The tomographic images provide 3D distributions of fractures, vesicles, and matrix dissolution pores. The results suggest that fractures connecting isolated gas pores provide favorable spaces for oil accumulation.

Productivity Improvement by Reperforation of Multistage-Fractured Wells in High-Pressure/High-Temperature Tight Gas Reservoirs: A Case History

SummaryProductivity of multistage-fractured gas wells is possibly degraded by conductivity impairments and non-Darcy flow during long-term production. Such degradations are pronounced by flow convergence to short perforated intervals, while it is challenging to identify degraded stages for remediation. Moreover, remedial actions can be expensive under a high-pressure/high-temperature (HP/HT) environment. A field case demonstrates successful application of reperforation as a cost-effective way to mitigate the flow convergence by prioritizing targets with multirate production-logging (PL) results.This work presents theoretical investigations using numerical simulations and field execution of reperforation for a well with six-stage fracturing treatments in a HP/HT volcanic gas reservoir onshore Japan. Apparent conductivity reduction was suspected during more than 15 years of production, and it was pronounced by non-Darcy flow effects associated with flow convergence to short perforated intervals. Multirate PL was used to identify impaired stages by quantifying the inflow-performance relationship (IPR) of each stage under transient flow-after-flow (FAF) testing. The impaired stages were reperforated, adding perforation intervals with wireline-conveyed perforators. Pressure-buildup (PBU) tests before and after the job and post-job PL were used to validate productivity improvements.Target zones for reperforations were identified and prioritized with results of the multirate PL conducted. The stage IPRs were drawn, and relatively large non-Darcy effects were identified in three stages by shapes of the IPRs and/or decreasing inflow contributions as the surface rate increased. Also, a temperature log showed steep temperature change at the bottom of the fourth stage; the fracture might propagate below the perforated interval. Ranges of production increment were estimated using a numerical model calibrated against the estimated stage IPRs. The estimated increment was in the range of 15 to 30% with the planned reperforation program, while its magnitude depended on the connection between new perforations and existing fractures. Afterward, the reperforation job was performed and the gas rate was confirmed to be increased by 26% with the same wellhead pressure after 1 month of production. The post-job PL was conducted 3 months after the reperforation. The well IPR was improved, implying reduction of the non-Darcy effects. Results of PBU tests also indicated reduction of skin factor. The stage IPRs were redrawn with the post-job PL, and they suggested clear improvements in two stages where screenout occurred during fracturing treatments and a stage where significant non-Darcy effect was suspected.The workflow and strategy in this paper can be applied for productivity restoration in a cost-effective way to multistage-fractured gas wells with short perforated intervals and impaired apparent conductivity during long-term production. Especially, the interpreted results suggested effectiveness of the proposed approach for productivity improvement in stages where screenout occurs during fracturing treatments. Moreover, lessons learned on the importance of careful test designs for PL were discussed because they are keys for success.

Analysis on Original Gas Saturation of Volcanic Gas Reservoir Based on NMR Experiment

Analysis on Original Gas Saturation of Volcanic Gas Reservoir Based on NMR Experiment

Study on Failure Behavior of Gas Well Tubing Under CO2 Corrosion After Erosion

The volcanic gas reservoir in Xushen gas field in the Northeast of China has the characteristic of low single-well production. Before gas wells are put into production, most of wells require fracturing transformation of the large sand amount and large displacement. Hence, an integrated fracturing and production string was developed. In the process of natural gas production, the integrated fracturing and production string is faced with the erosion caused by fracturing and CO2 corrosion. In order to ensure the safety of production, analysis and assessment of the dual effects of erosion and corrosion on the pipe string are required. The influence of the erosion angle, liquid velocity and sand ratio on the erosion rate of the integrated fracturing and production string was studied by using a self-made erosion experimental device and finally established the erosion rate prediction model; the samples of the tubing material after the erosion test were used in a high-temperature autoclave to carry out corrosion evaluation experiments to study the CO2 corrosion resistance of the tubing after erosion. The test results showed that severe corrosion occurred in the sample of P110 tubing after erosion, while slight corrosion occurred in the Super 13Cr tubing. On the basis of theoretical analysis results, the design and field construction of the integrated fracturing and production string are guided and ensured the safety of string in the whole life cycle.

Research on optimization design method of horizontal well of volcanic gas reservoir in Daqing Xushen deep natural gas field

The volcanic gas reservoir in Daqing Xushen deep natural gas field has low porosity, low permeability, strong heterogeneity, more bottom water layer, low production of vertical wells in most blocks and other characteristics. The development of horizontal well can better improve the development effect of gas reservoir. The optimization design method of “One choice, two determination and three optimization” horizontal well is established based on the gas reservoir geology and dynamic new achievements on the fine identification of volcanic bodies, prediction of well control range of gas wells and so on according to the characteristics of volcanic bodies controlling reservoir development. This method shall be applied to the optimization design of horizontal wells, which makes the production capacity of the well increase significantly and better ensures the continuous and stable exploitation of gas reservoir.

Discovery and exploration inspiration of deep volcanic gas reservoirs in eastern sag of Liaohe depression

Discovery and exploration inspiration of deep volcanic gas reservoirs in eastern sag of Liaohe depression

Lithofacies and reservoir characteristics of Permian volcanic rocks in the Sichuan Basin

In order to provide basis for further exploration and deployment of volcanic gas reservoirs in the Sichuan Basin, we classified Permian volcanic rocks in this basin using drilling, mud logging, assay, logging and seismic data comprehensively. Then, the lithological characteristics, lithofacies characteristics, lithofacies distribution and reservoir characteristics of different types of volcanic rocks were studied. Besides, the main factors controlling the development of volcanic reservoirs were determined. And the following research results were obtained. First, volcanic rocks in the Sichuan Basin can be comprehensively classified into 4 categories (i.e., volcanic lava, volcaniclastic lava, volcaniclastic rocks and sedimentary volcaniclastic rocks), and basalt, breccia lava, tuffaceous breccia lava, tuff, sedimentary tuff are mainly developed. Second, 3 types of volcanic lithofacies are extensively developed in the Sichuan Basin, including overflow facies, effusive facies and volcanic sedimentary facies. Overflow facies is mainly dominated by basalt, effusive facies is mainly composed of volcaniclastic lava, and volcanic sedimentary facies is mainly composed of tuff and sedimentary tuff. Third, volcanic reservoirs are lithologically composed of breccia lava and basalt. Breccia lava reservoirs are pore-type reservoirs of moderate porosity and low permeability with devitrified micropores and dissolution pores as main reservoir space. In basalt, fractures and dissolution pores are mainly developed, and they are porous-fractured reservoirs with lower matrix porosity. Fourth, the factors controlling volcanic reservoirs mainly include lithology, lithofacies, later fluid alteration and tectonism. Volcaniclastic lava is devitrified in the process of rapid condensation, and a great number of dispersion-shape micropores are formed to provide favorable conditions for further dissolution and pore enhancement in the later stage. Post-diagenesis effect (e.g. dissolution) and fracture development make the reservoir properties better. In conclusion, the areal distribution of volcanic reservoirs in the Sichuan Basin is mainly controlled by the lithofacies. Porous-fractured basalt reservoirs are mainly developed in the Ya'an–Leshan–Pingshan area in the southwestern Sichuan Basin, and pore-type volcaniclastic lava reservoirs are mainly developed in the Chengdu–Jianyang–Santai area.

Characteristics of Permian volcanism in the western Sichuan Basin and its natural gas exploration potential

It was previously deemed that volcanic rocks in the western Sichuan Basin were not developed, which restricted the exploration of volcanic gas reservoirs in this area. Recently, however, Sinopec's wildcat well, Well YS1 encounters Upper Permian volcanic rocks about 300 m thick in the western Sichuan Basin, the reservoir physical properties of which are good with strong gas shows. In order to evaluate the exploration prospect of volcanic gas reservoirs in western Sichuan Basin, we studied the Permian volcanic rocks in this area from the aspects of formation and development characteristics, reservoir characteristics, gas genesis and plays based on the new understandings on exploration wells, combined with the analysis on tectonic evolution and its controlling effect on volcanism. Then, the volcanic eruption mechanism, the distribution mode of volcanic edifice and the hydrocarbon accumulation pattern of volcanic gas reservoir were figured out. Finally, the exploration potential of natural gas was evaluated. The following research results were obtained: (1) the Permian volcanic rocks in western Sichuan Basin are well developed and its tectonic setting is consistent with that of Emeishan basalt, which is the product of the Emeishan mantle plume. (2) Under the control of vertical faults, a great number of volcanic edifices are developed and there are many eruption cycles. Volcanic rocks are superimposed vertically and connected laterally. (3) The volcanic rocks in western Sichuan Basin are different to some extent from those in southwestern Sichuan Basin in terms of lithology, lithofacies and reservoir characteristics. In the western Sichuan Basin, the tuff of explosive facies is dominant with some basalt of effusive facies, and the reservoir space of volcanic rocks is mainly acted by inter-breccia skeletal pores, dissolved pores and matrix micro-pores. In conclusion, multiple sets of effective source rocks are developed around the volcanic intervals in western Sichuan Basin, and together they constitute effective natural gas plays. Therefore, it has a better natural gas exploration prospect, and it is a new field worthy of attention.

Fracture Study and Its Applied in Oil and gas Field Development

Fracture, as an important factor affecting the quality of reservoirs, has always been one of the most crucial research contents in the process of oil and gas field development. The basis of research on fault systems in oil and gas field development was introduced from two aspects, namely, data foundation and research methods. The volcanic reservoir of the Yingcheng Formation in the Xudong Area of the Songliao Basin, the glutenite reservoir of the Lower Karamay Formation in a block in the northwestern margin of the Junggar Basin, and the heavy oil reservoir developed by thermal recovery in the Yulou Oil Bearing Sets in an experimental area in the western slope of the western depression of the Liaohe Basin, were taken as examples to elaborate the application of fault research in oil and gas field development from three aspects: design of volcanic gas reservoir development program, architecture analysis of glutenite reservoir, and fault systems in heavy oil reservoirs developed by thermal recovery. Based on these different Chinese examples, the difficulties and hot issues in the research of fault systems in oil and gas field development were discussed. The results show that the fine characterization of location, scale, and distribution law of faults and fractures, analysis of fault architecture of reservoirs, fine interpretation and modeling of faults, and evaluation of fault sealing property are closely related to the effective development of oil and gas fields; therefore, they should be given adequate attention in the process of oil and gas field development.

A major discovery in Permian volcanic rock gas reservoir exploration in the Sichuan Basin and its implications

Recently, a high-yield gas flow of 22.5 × 104 m3/d was achieved in the wildcat well Yongtan 1, deployed by PetroChina Southwest Oil and Gasfield Company, and an extrusive facies volcaniclastic gas reservoir was first discovered in the Sichuan Basin, which is a major breakthrough in the exploration of volcanic rock gas reservoirs. In order to further evaluate the exploration potential in Permian volcanic rock gas reservoirs in this basin, we systematically reviewed and summarized the exploration process of volcanic rocks in this study area, as well as the characteristics of gas reservoirs of well Yongtan 1, the controlling factors and modes of hydrocarbon accumulation. The following findings were obtained. (1) Over the past five decades of gas exploration of volcanic rocks, the change of exploration thought and progress in seismic prediction contribute to a major discovery in this study area, i.e., the well location deployment of Yongtan 1 was determined in the Jianyan area, where the volcanic anomalies of the Permian developed outbursts are well developed and the gas reservoir conditions are favorable. This also proves that another new gas exploration area in this basin was found. (2) From the data of Yongtan 1, we found high-quality volcanic porous reservoirs with a thickness of over 100 m developed in the Permian strata, the effusive volcanic rock types dominated by breccia and tuffaceous breccia, reservoir space composed of dissolution pores, interbreccia dissolution pores, and other pores, the porosity of which is 6.68%–13.22% and 10.26% on average, the permeability 0.01–4.43 mD and 2.35 mD on average. (3) This abnormally high pressure gas reservoir was buried 4500–6000 m deep underground with the middle formation pressure being 125.625 MPa and the pressure coefficient being up to 2.22. Methane content in the minor H2S-bearing produced gas flow reached up to 99.03%. (4) Volcanic rocks in the Jianyang–Zhongjiang–Santai area are developed, and they are adjacent to the Cambrian high-quality source rocks in the Deyang–Anyue rifting. The early extensional deep faults play a role in connecting source and storage. There are many sets of caprocks, such as the Upper Permian Longtan Formation and Lower Triassic layered thick paste rocks, providing conditions for forming large-scale tectonic–lithologic composite trap gas reservoirs, and the favorable exploration area reaches up to 6000 km2. In conclusion, this major discovery not only proves that Permian volcanic rocks in the Sichuan Basin has large potential for oil and gas exploration, but reaffirms the confidence in changing our exploration thought, speeding up new exploration zones and areas, and discovering more and bigger gas fields.

Reservoir-forming conditions and key exploration & development techniques for Xushen gas field in Northeast China

Exploration and development of volcanic gas reservoirs is a world-class problem, and less experience can be learned at home and abroad. Through study of reservoir-forming conditions of Xushen gas field, four sets of hydrocarbon source rocks, mainly mudstone and coal seam of Shahezi Formation, are identified in this gas field; its cap rocks are mudstone in Member 2 of Denglouku Formation, and Member 1 and Member 2 of Quantou Formation. By simulating reservoir-forming history, four filling periods of the Xushen gas field are determined, and the most favorable exploration targets are crater areas with structural highs, large thickness and good physical properties. On the basis of diagenetic mechanism study, the criteria of sequence series, lithology identification and reservoir types of gas reservoirs have been established; the volcanic rock bodies have been carefully depicted and the distribution laws of effective reservoirs of gas pools have been figured out. Based on evaluating reservoir-permeability characteristics of reservoirs, the gas well productivity characteristics (mainly type II, III and IV wells) have been identified, and the optimum design technology for volcanic gas reservoir development has been developed. The gas reservoirs in all blocks of Xushen gas field are classified into three types: single-body gas reservoir model, disconnected multi-body gas reservoir model and interconnected multi-body gas reservoir model. Stimulation technologies, including multi-layer fracturing of vertical well, staged fracturing of open-hole horizontal well and volume fracturing, have been developed. According to gas well production performance, pressure change, well-controlled reserves and spatial distribution of volcanic bodies, combined with numerical simulation and prediction of gas reservoirs, the evaluation method of residual potential of volcanic gas reservoirs has been formed. Key technologies for effective exploration and development of complex volcanic gas reservoirs have been established and perfected, and bring good development effect. The research results can provide guidance and reference for exploration and development of similar gas reservoirs.

Evaluation of volcanic gas reservoir in Yingcheng formation of Block Shengshen2-1 of Northern Part of Songliao Basin

According to the thin sections of volcanic rocks, chemical composition analysis and log interpretation, and well data, it is determined that the volcanic rocks of Yingcheng Formation in Shengshen 2-1 block are mainly intermediate-acid volcanic lava and pyroclastic rocks. Through physical analysis of core samples, it is considered that Shengshen 2-1 block belongs to low-porosity and low-permeability reservoir. Microfracture-fracture is relatively developed in the compact section, and fracture is not developed in the porous section. Microfracture and reticular fracture can be seen in the thin section. Statistical analysis of porosity and density of volcanic reservoir shows that volcanic rock density below 2.53 can be used as reservoir. According to the relationship between reservoir space and gas content, the characteristics of volcanic lithology, lithofacies and reservoir space, the reservoir in this block can be divided into four types.