Volcanic Gas Reservoir Research Articles

Accumulation mechanism of multi-type unconventional oil and gas reservoirs in Northern China: Taking Hari Sag of the Yin’e Basin as an example

Abstract Many multi-types of unconventional oil and gas reservoirs have been found in some faulted basins in northern China, showing good exploration potential. However, the hydrocarbon accumulation mechanism in these areas is still unclear, which limits the understanding of the distribution of oil and gas. In this study, we took Hari Sag in Yin’e Basin as an example, conducted a systematic analysis on various types unconventional oil and gas reservoirs, and revealed its characteristics and accumulation mechanisms. The study showed that there were many types of unconventional oil and gas reservoirs in Hari Sag, such as biogas reservoirs, shale gas reservoirs, shale oil reservoirs, tight sandstone oil reservoirs, tight sandstone gas reservoirs, and volcanic gas reservoirs. These reservoirs generally had characteristics of “near/within source rocks accumulation,” “coexistence of oil reservoirs and gas reservoirs,” “shallow oil and deep gas,” and so on. Research on the mechanism of hydrocarbon accumulation showed that: the lack of effective hydrocarbon migration pathway was the main reason for “near/within source rocks accumulation” of oil and gas reservoirs; the differences in the thermal evolution degree of the main source rocks at different structural positions in the sag made the distribution characteristics of hydrocarbon as “coexistence of oil reservoirs and gas reservoirs” and “shallow oil and deep gas”; and the joint development of multi-type effective unconventional reservoirs created the situation of “coexistence of multi-type unconventional oil and gas reservoirs.” It is predicted that six types of unconventional oil and gas reservoirs have a cumulative area of 381 km2, indicating that the Hari Sag has great potential for unconventional oil and gas exploration. The research results can not only guide the unconventional oil and gas exploration in Hari Sag but also provide a theoretical basis for exploration research in similar blocks.

Formation of large- and medium-sized Cretaceous volcanic reservoirs in the offshore Bohai Bay Basin, East China

Based on the geological and geophysical data of Mesozoic oil-gas exploration in the sea area of Bohai Bay Basin and the discovered high-yield volcanic oil and gas wells since 2019, this paper methodically summarizes the formation conditions of large- and medium-sized Cretaceous volcanic oil and gas reservoirs in the Bohai Sea. Research shows that the Mesozoic large intermediate-felsic lava and intermediate-felsic composite volcanic edifices in the Bohai Sea are the material basis for the formation of large-scale volcanic reservoirs. The upper subfacies of effusive facies and cryptoexplosive breccia subfacies of volcanic conduit facies of volcanic vent-proximal facies belts are favorable for large-scale volcanic reservoir formation. Two types of efficient reservoirs, characterized by high porosity and medium to low permeability, as well as medium porosity and medium to low permeability, are the core of the formation of large- and medium-sized volcanic reservoirs. The reservoir with high porosity and medium to low permeability is formed by intermediate-felsic vesicular lava or the cryptoexplosive breccia superimposed by intensive dissolution. The reservoir with medium porosity and medium to low permeability is formed by intense tectonism superimposed by fluid dissolution. Weathering and tectonic transformation are main formation mechanisms for large and medium-sized volcanic reservoirs in the study area. The low-source “source-reservoir draping type” is the optimum source-reservoir configuration relationship for large- and medium-sized volcanic reservoirs. There exists favorable volcanic facies, efficient reservoirs and source-reservoir draping configuration relationship on the periphery of Bozhong Sag, and the large intermediate-felsic lava and intermediate-felsic composite volcanic edifices close to strike-slip faults and their branch faults are the main directions of future exploration.

Reservoir Characteristics and Main Factors Controlling Carboniferous Volcanic Rocks in the Well CH471 Area of the Hongche Fault Zone: Northwest Margin of Junggar Basin, China

Nearly 100 million tons of reserves have been explored in the Well 471 area of the Hongche Fault zone. The Carboniferous volcanic rock reservoir is the main oil-bearing reservoir in the well CH471 area and is the main target of exploration and development. The characteristics of the Carboniferous volcanic rock reservoir are studied through core, thin section, physical property, logging, and other data, and its main controlling factors are analyzed in combination with actual means of production. The lithologies of the volcanic reservoir in the study area are mainly volcanic breccia, andesite, and basalt. The matrix physical properties of volcanic rock reservoirs are medium-porosity and ultralow-permeability, among which volcanic breccia has the best physical properties. The reservoir space mainly comprises primary pores, secondary dissolution pores, and fractures, resulting in a dual medium pore-fracture-type reservoir. Combined with production data analysis, the lateral distribution of oil and gas is controlled by lithology and lithofacies, with explosive volcanic breccia being the best, followed by the basalt and andesite of overflow facies, which are vertically affected by weathering and leaching and distributed within 50~300 m from the top of the Carboniferous system. The area with densely developed fractures was conducive to developing high-quality reservoirs. The tectonic movement promoted the formation of weathering and controlled the development of faults. Based on a comprehensive analysis, it is believed that the formation of Carboniferous volcanic oil and gas reservoirs in the study area was controlled and influenced by the lithology, lithofacies, weathering, leaching, faults (fractures), and tectonics.

The Water Problems and Treatment Strategies of Volcanic Gas Reservoir in Daqing

In order to realize the development strategy of “stabilizing oil and increasing gas and simultaneously boosting oil and gas”, Daqing Oilfield has continuously strengthened the exploration and development of deep natural gas resources since the discovery of Xushen gas Field. The volcanic gas reservoir is the main gas producing area of Songliao Basin. Through “internal excavation and external expansion”, natural gas production has maintained a trend of growth for 12 consecutive years. However, due to the development of fractures and bottom water in volcanic gas reservoir, the development contradiction caused by gas well water is increasingly prominent as the gas field development enters the end of stable production stage. Therefore, on the basis of the understanding of four types of effluent water in volcanic rock reservoir, this paper carries out the analysis of the production dynamic characteristics of typical gas wells, and identifies the difficulties faced by gas wells with different types of effluent water. Through field tests, economic and feasible treatment measures are explored, including continuous circulation gas lift of strong water breakthrough gas well in fracture vertical direction, intelligent foam drainage gas production of weak water breakthrough gas well in fracture vertical direction, and deep water unlocking of gas well in pore vertical water cone and gas reservoir with high water saturation gas reservoir effluent water, which provides technical support for the sustainable and efficient development of volcanic gas reservoir.

Core String Parallel Experiment Study for Volcanic Rock in Xushen Gas Field

Volcanic gas reservoir in Xushen gas field has complex microscopic pore throat structure and dense physical property, which is quite different from that of conventional high-permeability sandstone gas reservoir. It is of great significance to know its seepage mechanism for scientific and efficient development technology countermeasures. Through the parallel and series experiments of volcanic fracture cores and matrix cores, the gas flow changes of fractures and matrix cores with different permeability levels under different pressure gradients were measured, and the percolation mechanism of volcanic gas reservoirs with different pressure gradients was revealed when fractures and matrix cores were in parallel or series. The experimental results show that the fracture is the dominant channel relative to the matrix, and the supply and exhaust capacity of the fracture is much larger than that of the matrix, and the permeability of the matrix determines the supply and exhaust capacity of the series system composed of fracture and matrix

Assessment of CO2 storage potential in high water-cut fractured volcanic gas reservoirs—Case study of China’s SN gas field

CO2 storage is an important technique for achieving both carbon peak and carbon neutralization, and depleted gas reservoirs are primary potential candidates for geological CO2 storage. After 15 years of development, the recovery rate of the SN gas field in China has reached 70 % and entered the middle and late stages. Most importantly, it has entered the stage of high water cut, rendering the stabilization of production challenging. As the gas produced in this gas reservoir contains a considerable amount of CO2, high water-bearing wells can be selected for CO2 storage in volcanic depleted gas reservoirs on site. Therefore, in this study, combined with the salinity characteristics of the formation water of the target block, a buried-potential evaluation study on typical wells in volcanic gas reservoirs is conducted, the gas–water distribution characteristics of the gas reservoir after CO2 injection are evaluated, and the CO2 injection timing and volume are optimized. The results show that the different ion content in the formation water leads to two reaction processes: dissolution and precipitation of reservoir minerals. When the CO2 in the formation water is low, the reservoir is dominated by dissolution reaction and the CO2 mineralization buried amount is negative; gradually decreasing with the age of the reservoir. As more CO2 is injected into the formation, a considerable amount of HCO3– is produced. In this context, the formation water is dominated by a precipitation reaction, and the CO2 mineralization buried amount is positive. In 2026, compared with the dissolved buried capacity of 1.0 × 105 m3/d, the injection rate of 2.5 × 105 m3/d will increase by 6.38 × 108 mol. When the injection rate is 1.5 × 105 m3/d, the mineralization buried capacity is the highest (2.02 × 105 mol.). Combining the total amount of CO2 dissolution and mineralization storage, the total amount of storage is the highest (1.38 × 108mol.) when the injection rate is 2.5 × 105 m3/d, indicating that the SN volcanic gas reservoir has significant CO2-storage potential. Guided by the research results of this study, typical wells have been selected to inject CO2 for storage purposes.

Natural Gas Accumulation and exploration potential of permian Volcanic Rocks in Southwest Sichuan Basin

To figure out gas enrichment laws in Permian volcanic rocks, Sichuan Basin, an association among reservoir-forming elements was explored to analyze forming modes and point out exploration prospects in accordance with reservoir characteristics, gas source conditions, and fault elongation in Zhougongshan and Jianyang areas. Results show that there mostly exist porous volcaniclastic rock and lava reservoirs of explosive facies. In Jianyang with Lower Cambrian Qiongzhusi Formation as the principal source rock, early faults offered pathways to assemble ample initial oil and gas. Following an increase of burial depth, oil started cracking vastly to create oil-cracked gas. These elements constitute one reservoir-forming mode of Cambrian hydrocarbon supply–fault transporting–in-situ cracking in Jianyang. While in Zhougongshan with Lower Permian Maokou 1 Member as the main source, active faults during hydrocarbon generation and expulsion acted on trap preservation. Late kerogen-cracked gas gathered at structural highs far from giant faults. Thus, the other mode exhibited in Zhougongshan is Permian hydrocarbon supply–fault adjustment–effective trap accumulation. Reservoir-forming conditions in volcanic rocks present certain disparity among all kinds of tectonic settings inside the basin, giving rise to various volcanic gas reservoirs developed. The Permian volcanic rocks are expected to be a major domain for reserve and production increase in Sichuan Basin.

Petroleum Geology and Exploration of Deep-Seated Volcanic Condensate Gas Reservoir around the Penyijingxi Sag in the Junggar Basin

Many types of volcanic rock oil and gas reservoirs have been found in China, showing great petroleum exploration potential. Volcanic reservoir also is one of the key fields of exploration in the Junggar Basin and mainly concentrated in the middle and shallow layers, while the deep volcanic rock and natural gas fields have not been broken through. Based on comprehensive analysis of core observation, single well analysis, reservoir description, source rocks evaluation, combined with seismic data and time-frequency electromagnetic technology, multiple volcanic rock exploration targets were identified, and industrial oil and gas flow was obtained in the well SX 16 of the Penyijingxi Sag, western Junggar Basin. It is believed that the deep Permian source rocks have relatively higher natural gas generation potential and volcanic breccia usually have large reservoir space. And the mudstone of the Upper Wuerhe Formation played as the role of caprock. The success of exploration well SX16 has achieved a major breakthrough in natural gas exploration in the Penyijingxi Sag, which has essential guiding significance for the exploration of deep volcanic rocks and large-scale gas exploration in the Junggar Basin.

Pore-scale simulation of gas-water two-phase flow in volcanic gas reservoir based on Volume of Fluid method

Understanding the transport process of gas-water flow in volcanic gas reservoir is of key importance for natural gas production. However, there is still limited evidence on the precise influence of volcanic reservoir type, capillary number and wettability. We thus performed gas-water flow simulations (using Volume of Fluid method) at different capillary numbers under different wettability conditions directly on microcomputed tomography (μ-CT) images. The simulation results demonstrated that the eddy current in dead-end corners is the main mechanism for the formation of residual gas. The gas phase near the wall of fractured porous medium was mainly dominated by drag force, resulting in lower residual gas saturation. Moreover, it is generally believed that a low capillary number facilitates the displacement of residual gas in dead-end corners. However, we found that under high temperature (>100 °C) and high pressure (>100 MPa), less residual gas distributed in dead-end corners at higher capillary number. This showed that the conventional percolation law was unlikely to provide reliable predictions in fluid distribution under high temperature and high pressure. The wettability of rock affected the shape of displacement front. The water-gas flow dynamics under water-wet condition was piston like. However, fingering flow occurred under non-hydrophilic condition, and snap-off trapping was more likely to occur, resulting in higher residual gas saturation. This work provides fundamental data on the influence of pore structure, capillary number and wettability on gas-water flow and aids in the further advancements of improved nature gas recovery in volcanic reservoirs.

Classification and Evaluation of Volcanic Rock Reservoirs Based on the Constraints of Energy Storage Coefficient

The current classification and evaluation methods of volcanic rock reservoirs have low accuracy and cannot effectively provide guidance for the selection of volcanic rock gas reservoirs, which have efficient properties for gas production. In this research, we have analyzed the lithology, lithofacies, reservoir space type, pore combination mode, and reservoir microscopic characteristics of volcanic reservoirs using the energy storage coefficient as a constraint. Then, the method of reservoir classification was proposed. The results showed the following: 1) The energy storage coefficient can better characterize the single-layer productivity of gas wells. The volcanic rock reservoirs in the Wangfu gas field can be subdivided into three categories by considering the energy storage coefficient. 2) Type I reservoirs mainly develop structural fractures–matrix dissolution pores, structural fractures–intercrystalline micropores, and matrix dissolution pores. Type II reservoirs mainly develop matrix dissolution pores and residual intergranular pores, and Type III reservoirs are dominated by structural fractures. From Type I to Type III reservoirs, the skewness of the mercury intrusion curve and the sorting coefficient deteriorated, and the physical properties of the reservoir and the maximum mercury saturation and other parameters also decreased, whereas the displacement pressure and the median saturation pressure increased. 3) The characteristics of conventional and special logging curves of typical reservoirs were comprehensively analyzed, and the combination of sensitivity parameters reflecting gas-bearing properties and logging curves was optimized. Furthermore, a reservoir classification chart was established, and the results enabled to confirm the choice of the reservoir and demonstrated that the standard classification has high accuracy. 4) The diagenesis processes such as weathering, leaching, and dissolution improved the physical properties of the reservoir. The research results can effectively provide guidance for the evaluation of the “sweet spot” of volcanic rock reservoirs.

Lithologic classification of pyroclastic rocks: A case study for the third member of the Huoshiling Formation, Dehui fault depression, Songliao Basin, NE China

With the ongoing discovery of volcanic oil and gas reservoirs, pyroclastic rocks have begun to attract widespread attention from both the academic and industrial sectors. Because there is more information about the cementation mode and the grain size in the Chinese classification system for pyroclastic rocks than there is in the system used throughout the rest of the world, we use the Chinese classification system to investigate the lithologies of the pyroclastic rocks in our study area. The three cementation modes dictate the three classes of pyroclastic rock: pyroclastic lava, normal pyroclastic rock, and sedimentary pyroclastic rock. Within these three modes, pyroclastic rocks are further classified by their clastic material grain size into agglomerate, breccia, and tuff. If the pyroclastic material is moved by aeolian transport, it tends to follow the positive sedimentation cycle sequence. In this study, we classify the pyroclastic rocks from the third member of the Huoshiling Formation (Fm) in the Dehui fault depression in the southern Songliao Basin. Draw the cross plots of pyroclastic lava, normal pyroclastic rock and sedimentary pyroclastic rock in the study area. Based on core date, fullbore micro-resistivity images (FMIs), and thin section analyses, we determine the mineral composition, particle size, and cementation mode of breccia bearing tuff lava, tuff lava, breccia bearing welded tuff, volcanic breccia, breccia bearing tuff, tuff, sedimentary volcanic breccia, and sedimentary tuff. Our results indicate that the decision tree method does not require all of the input curves to identify a given sample's lithology. We also found that adding more data to a support vector machine (SVM) does not necessarily translate into more accurate classification results. This classification is undertaken using two different techniques: a combination of elemental capture spectroscopy (ECS) cross plots and FMI and a SVM learning algorithm with information derived from node information in a decision tree. Ultimately, the results produced by the SVM algorithm are more accurate (recognition accuracy of 98.6%) than those generated by the ECS/FMI approach. Furthermore, the SVM algorithm requires less data and operates very efficiently. Using data from multiple wells allows for better and more rapid identification of the lithology type in the target layer; large observational or training datasets provide a strong foundation for further reservoir evaluation.

Quantitative characterization of fracture-pore distribution and effects on production capacity of weathered volcanic crust reservoirs: Insights from volcanic gas reservoirs of the Dixi area, Junggar Basin, Western China

Igneous hydrocarbon reservoirs worldwide are of significant importance to the petroleum industry for oil and gas exploration. The Carboniferous weathered volcanic crust reservoirs of the Dinan salient in the northern Junggar Basin are major exploration targets of PetroChina. The present study analyzes the geological, well logging, and production data to formulate development and distribution methods for quantitative characterization of weathered volcanic crust fractures and examine their fracture-pore distributions. The effects of the fracture-pore distribution on the productivity of various lithologies of weathered volcanic crust reservoirs were investigated. Various volcanic lithofacies demonstrate the lithological differences in their physical properties, fracture development, and distributions due to the conspicuous variations in weathering resistance among different volcanic rock types. The present study revealed the level of physical properties of volcanic rocks, which are ranked as follows: volcanic breccia > mafic lava > tuff > felsic intrusive rock. However, the degree of fracture development ranked differently from most developed to least developed as follows: felsic intrusive rock > mafic lava > volcanic breccia > tuff. The fracture distribution was primarily controlled by lithology, rock stratum thickness, degree of weathering and leaching, along with tectonism. Compared to other rock types, felsic intrusive rock and mafic lava were more prone to fracturing owing to different petrophysical properties. Additionally, fractures typically developed in single lithological bodies with a thickness of less than 15 m and were characterized by the “single-section” developing mode. For lithological bodies of thickness greater than 15 m, fracture development was more limited, primarily concentrated at the top and bottom of the lithological body and characterized by the “bi-section” or “tri-section” developing modes. These differences can be quantitatively characterized by the fracture development thickness, fracture layer density, and fracture development thickness ratio. Moreover, the influence of the degree of fracture development on the productivity of mafic lava and felsic intrusive gas reservoirs was higher than that of the porosity; however, volcanic breccia and tuff exhibited the opposite effect. The production capacity of weathered volcanic crust reservoirs is affected by the following factors in the order of degree of fracture development > porosity > lithology > thickness of a single lithological body > distance from the top surface of the weathered crust. These quantitative characterization methods can provide valuable technical support for optimizing favorable exploration targets.

New Opinion of Development Potential of Carboniferous Volcanic Gas Reservoir: Taking the Kelameili Gas Filed as an Example

OPINION article Front. Energy Res., 15 March 2022Sec.Advanced Clean Fuel Technologies https://doi.org/10.3389/fenrg.2022.861901

Fracturing Optimization Design of Fractured Volcanic Rock in Songliao Basin Based on Numerical Research and Orthogonal Test

Abstract The efficient development of fractured volcanic reservoir relies on fracturing, while the existence of natural fractures has an important impact on hydraulic fracturing in this reservoir. There are few reports on fracturing optimization of deep volcanic gas reservoirs in Songliao Basin in China; thus, the parameter optimization under single-stage action is studied. On the basis of logging interpretation, coring observation, and seismic interpretation, the fracture development degree is obtained. By employing numerical simulation and orthogonal tests, hydraulic fracturing optimization design is carried out. The results indicate that the pumping rate of 0.01 m3/s/m and perforation direction parallel to the maximum principal stress are the optimum parameters for the studied volcanic reservoirs. In addition, multibranch seam expansion will occur when the displacement reaches 0.01 m3/s/m according to fracture morphology results after fracturing. And the research method has certain significance for improving the fracturing design of fractured reservoir in the research block.

An Improved Scheme of Azimuthally Anisotropic Seismic Inversion for Fracture Prediction in Volcanic Gas Reservoirs

Seismic prediction of natural fractures is essential for the characterization of unconventional reservoirs because fractures provide seepage paths for fluid migration and storage space for hydrocarbon accumulation. However, it remains challenging to robustly estimate anisotropic parameters for fracture characterization based on seismic inversion methods. We propose an improved inversion scheme for the robust and accurate estimation of anisotropic parameters using PP-wave azimuthal amplitude differences incorporated with a hybrid optimization algorithm. Modeling analysis indicates that by removing the effect of isotropic terms, azimuthal amplitude differences are more sensitive to anisotropic parameters than the traditional azimuthal reflection coefficient; this can avoid possible instabilities in anisotropic parameter estimates that may occur in conventional methods owing to unbalanced weighting coefficients between isotropic and anisotropic terms. Meanwhile, the proposed hybrid algorithm takes advantage of the global optimization ability of the simulated annealing algorithm and the fast convergence characteristics of the particle swarm optimization algorithm. Synthetic data tests indicate that the hybrid algorithm achieves reliable and stable estimates of anisotropic parameters with higher computational accuracy and efficiency than traditional methods. The improved inversion method is applied to characterize fractures in volcanic gas reservoirs. Based on azimuthal amplitude differences obtained after estimating fracture orientation using the Fourier series method, anisotropic parameters are computed and converted to weakness properties, which is more meaningful in terms of the fracture properties. Our results indicate that the obtained fracture tangential weakness exhibits an apparent correspondence with well-log permeability, justifying the applicability of the proposed method for fracture prediction.

Assessment of Co2 Storage Potential in High Water-Cut Fractured Volcanic Gas Reservoirs——Case Study of China Sn Gas Field

Assessment of Co2 Storage Potential in High Water-Cut Fractured Volcanic Gas Reservoirs——Case Study of China Sn Gas Field

Characterization of a Volcanic Gas Reservoir Using Seismic Dispersion and Fluid Mobility Attributes

Abstract Seismic dispersion and fluid mobility attributes are used to characterize a volcanic gas reservoir in the Songliao Basin of China. A rock physics model is constructed to describe poroelastic behaviors associated with heterogeneous fluids saturation within the volcanic gas reservoirs, where velocity dispersion and attenuation of propagating waves are attributed to the wave-induced fluid flow described by the patchy saturation theory. Modeling results indicate that the frequency-dependent bulk modulus at the seismic frequency is more sensitive to gas saturation than the P-wave velocity dispersion. Accordingly, a new inversion method is developed to compute bulk-modulus-related dispersion attribute DK for improved characterization of volcanic gas reservoirs. Synthetic tests indicate that DK is more sensitive than traditional P-wave dispersion attribute DP to the variations of reservoir properties. The high value of dispersion attribute DK indicates the volcanic gas reservoirs with high porosity and gas saturation. At the same time, fluid mobility attribute FM can discriminate the volcanic gas reservoir as DK. Field data applications illustrate that DK and FM exhibit anomalies to the gas zones in the volcanic gas reservoir on the cross-well section. However, DK is more robust than FM to identify favorable zones on horizontal slices for specific target layers. Overall, rock physical modeling provides insights into the poroelastic behaviors of volcanic gas reservoirs, and inversion for seismic dispersion attribute DK improves hydrocarbon detection in the volcanic gas reservoir.

Accumulation and Distribution of Natural Gas Reservoir in Volcanic Active Area: A Case Study of the Cretaceous Yingcheng Formation in the Dehui Fault Depression, Songliao Basin, NE China

Tight gas sandstone and volcanic gas reservoirs have received global attention in the energy arena for further exploration and exploitation attempts. Considering the Yingcheng Formation of Dehui fault depression in the Songliao Basin as an example, this study focused on the accumulation and distribution of natural gas reservoirs in volcanic area in a fault depression basin. Volcanic activities occurred in the Yingcheng Formation, which is distributed centrally in the northwest of the study area. During the sedimentation of the Yingcheng Formation, fan-delta, lacustrine, and nearshore subaqueous fan facies were deposited. The source rocks of the Yingcheng Formation have high abundance of organic matter mainly in type III at high-overmature stages, indicating favorable conditions for gas production. The porosity of volcanic reservoir is 3.0%-14.8%, the permeability is 0.0004 mD-2.52 mD, and the pore types are mainly secondary dissolved pores and fractures. Besides, the porosity of the tight sandstone reservoir is 0.5%-11.2%, and the permeability is 0.0008 mD-3.17 mD. The pore types are mainly interparticle pores, with a small proportion of intraparticle pores and microfractures. The intrusion of late volcanic magma provided sufficient heat for the thermal maturity progression of organic matter in Yingcheng Formation and promoted the generation of natural gas in large quantities. Volcanic rocks formed at the early and middle stages of volcanic activities occupied the sedimentary space and hindered the development of sedimentary sand bodies to a certain extent. However, volcanic rocks can become the seal to promote the formation of tight sandstone gas traps. Comparing tight sandstone reservoirs with volcanic ones, the latter are less affected by compaction; thus, their petrophysical properties do not vary much with depth, showing more homogeneous characteristics. The pyroclastic rocks influenced by volcanic activity and the secondary pores formed by dissolution in the later stages also provide reservoir space for gas accumulation. Ultimately, the tight sandstone and volcanic rocks in the study area form a complex gas reservoir system, which can become a reference for exploration and exploitation of natural gas in other petroliferous fault depressions that are affected by volcanisms.

Experimental Study on the Saturation Model of Volcanic Rock Based on Fluid Distribution

A saturation evaluation model suitable for Nanpu volcanic rock formation is established based on the experiment of acoustic velocity changing with saturation during the water drainage process of volcanic rock in the Nanpu area. The experimental data show that in the early stage of water drainage, the fluid distribution in the pores of rock samples satisfies the patchy formula. With the decrease of the sample saturation, the fluid distribution in the pores is more similar to the uniform fluid distribution model. In this paper, combined with the Gassmann-Brie and patchy formula, the calculation equation of Gassmann-Brie-Patchy (G-B-P) saturation is established, and the effect of contact softening is considered. The model can be used to calculate water saturation based on acoustic velocity, which provides a new idea for the quantitative evaluation of volcanic oil and gas reservoirs using seismic and acoustic logging data.

Formation Mechanisms and Distribution Law of Effective Volcanic Reservoirs -- Taking Carboniferous Volcanic Reservoirs in Malang Depression of Santanghu Basin as an Example

The understanding of the formation mechanisms of volcanic reservoirs can provide a reference for the distribution law of effective volcanic reservoirs and oil and gas reservoirs, and has an important guiding role in accelerating the pace of the exploration and development of volcanic reservoirs. Using the data of three-dimensional seismic interpretation, logging, core description and thin section identification, it is concluded that the formation of volcanic reservoirs in Santanghu Basin was controlled by many factors such as tectonics, lithology, lithofacies, fluids, volcanic apparatus, weathering and leaching, including fault rebuilt, unconformity, alkali metasomatism, dissolution, weathering and leaching and other formation mechanisms, 5 types of reservoir formation models are obtained according to the different superimposition modes of the depth mechanism for the formation of volcanic reservoirs in different periods, and five types of distribution law of effective volcanic oil and gas reservoirs are summarized from the understanding of reservoir types, accumulation models, and accumulation mechanisms. The study provides a reference for the prediction of effective volcanic reservoirs, and further defines the exploration direction of volcanic oil and gas reservoirs.