Introduction
The world’s largest volcanic gas reservoirs have been discovered in China. Their rational development can potentially mitigate the energy supply and demand imbalance and accelerate the growth of the natural gas industry in China. Meanwhile, this development is of great significance for the advancement of natural gas production technology in general and for the development of similar gas reservoirs.
Due to their complex geological settings, the development of volcanic gas reservoirs has received little study up to now, with only limited experiences accumulated in this area. It is difficult for us, therefore, to put volcanic gas reservoirs into production with current technologies. To develop volcanic gas reservoirs effectively, a comprehensive knowledge base needs to be built up about this type of reservoir first, with reservoir characterization being the focal point. However, the complex petrogenic processes and internal architecture of volcanic gas reservoir rocks imply that the volcanic gas reservoir characterization is different from the characterization of conventional gas reservoirs in terms of concepts, methods, and techniques. Based on the production practice of large-scale volcanic gas reservoirs in the Daqing, Jilin, and Xinjiang regions, we propose in this book new concepts, methods, and techniques based on numerous experiments, modeling, and other scientific investigation for the characterization of volcanic gas reservoirs. Meanwhile, this book also summarizes the latest research results, and attempts to standardize volcanic gas reservoir characterization procedures and techniques, which have been applied with various degrees of success in the production of volcanic gas reservoirs and have accelerated the development of exploitation technology.
1.1 Current status and significance of volcanic gas reservoir development
1.1.1 Resources of volcanic gas reservoirs and development status in the world
Since the first discovery of volcanic hydrocarbon reservoirs in the San Joaquin Basin in California, United States, in 1887, a large number of volcanic hydrocarbon reservoirs have been found worldwide, such as the hydrocarbon-bearing basins in Japan, the United States, Venezuela, Cuba, the former Soviet Union, and China [1–3]. By the end of 2003, a total of 169 volcanic hydrocarbon reservoirs had been discovered, with 65 oil and gas shows and 102 oil seepages, and the proven hydrocarbon resources amount to more than 1.5 billion tons of oil equivalent [4].
Volcanic gas reservoirs are distributed mainly in China, Japan, the United States, Brazil, Namibia, Congo, and Indonesia. Most of the discovered volcanic gas reservoirs, however, have not been put into real development. Only the Higashi-Kashiwazaki Volcanic gas field (1968) and the Southern Nagaoka Volcanic gas field (1978) in Japan have produced for a sufficiently long duration with good results. Nonetheless, the geological research on volcanic gas reservoirs has been sporadic, and investigation on development technology remains at a preliminary level. The available research results could only meet the initial production demand [5,6].
1.1.2 Resources and development status of volcanic gas reservoirs in China
It has been over half a century since the first discovery of volcanic oil reservoirs in the northwestern margin of Junggar basin in 1957, although large-scale discoveries in the Songliao and Junggar basins were not made until 2004, with estimated natural gas resources up to several trillion cubic meters and 3P(proved, probable, and possible) reserves approaching 1 trillion cubic meters. Hitherto these are the largest volcanic gas reservoirs known in the world.
The exploration of volcanic gas reservoirs in China experienced a challenging period of probing, innovation, and development. In the absence of related experiences as well as theoretical and technical guidance, we had to start from scratch our theoretical and technological research as well as and production trials in the Xushen gas field of Daqing, the Changling gas field in Jilin, and the Kelameili gas field in Xinjiang Province. In the process, we solved various problems in volcanic gas reservoir development and in a short period established several billion cubic meters of annual natural gas deliverability.
1.1.3 Significance of volcanic gas reservoir development
As an important part of natural gas resources in China, the rational development of volcanic gas reservoirs is of great significance to the domestic economy, especially in the following aspects:
1. Mitigate energy supply and demand imbalance and optimize energy infrastructure in China. Energy is one of the fundamental resources for national economic growth. With the rapid development of economy, the energy demand in China is increasing swiftly. In 2009, annual crude oil import in China exceeded 0.2 billion tons for the first time, accounting for about 52% of domestic consumption and exceeding the currently acceptable international warning line of 50%. Thus, rational development of volcanic gas reservoirs will relieve the pressure of rapidly growing oil and gas demand and mitigate the energy supply and demand imbalance in China. Natural gas is also a type of clean and efficient energy and is of great significance for optimizing national energy infrastructure and promoting the rapid development of a “low-carbon” economy.
2. Increase gas resources and promote the development of natural gas industry in China. The exploration and development of natural gas in China is progressing rapidly, and the golden time for natural gas development is expected in the near future. We have the largest volcanic gas reservoirs in the world, with a huge exploration and development potential. As an important part of natural gas resources in China, the rational development of volcanic gas reservoirs is of great significance for the development of the natural gas industry.
3. Provide energy support for revitalizing economy in northeastern China and implement the major development program in western China. In the Xushen and Changling gas fields, billions of cubic meters of annual gas deliverability has been established to supply natural gas to Changchun and Harbin through the Northeast Pipeline Network, which provides important energy support for industrial revitalization and economic development in Northeast China. The Kelameili gas field will supply more than 1 billion cubic meters of natural gas annually to cities in western China, such as Urumqi, which plays an important role in promoting national harmony and maintaining social and economic stability.
1.2 Geological features of volcanic gas reservoirs and challenges in reservoir characterization
1.2.1 Geological features of volcanic gas reservoirs
Due to their fundamentally different petrogenic condition, internal architecture, lithology and lithofacies, pore-space types, and fracture features, volcanic rocks are very different from sedimentary rocks in terms of their reservoir distribution and gas-water relationships.
1 Special petrogenesis and complex internal architecture
Volcanic rocks are the products of eruption and deposition, and they are controlled by multiple factors including eruption mode, eruption energy, magma properties, eruption amount, eruption frequency, paleotopography, and subsequent alteration. Volcanic rocks have multilevel internal architectures and can be divided into various units, from large to small scales, as volcanic buildup, volcanic edifice, volcanic massif, volcanic lithofacies, and volcanic lithology. Each architectural unit has various shapes, different scales, and complex superposition relationships (e.g., cross-cutting and stacking patterns). As a result, volcanic reservoir rocks are characterized by unique petrogenesis and complex internal architecture compared to sedimentary reservoir rocks.
2 Various reservoir rocks and complex lithology and lithofacies
The components of volcanic rocks are complex and contain almost all the chemical elements and cation oxides in the earth’s crust. As a result of volcanic eruption and diagenetic processes, there are diverse volcanic rock structures and constructions with various rock types and complex lithology. In terms of the volcanic eruption mode, eruption energy, and rock assemblage features, six volcanic lithofacies are recognized: eruptive facies, effusive facies, extrusive facies, volcanic conduit facies, subvolcanic lithofacies, and volcanic sedimentary facies. In terms of the occurrence and petrographic characteristics, these can be subdivided into a dozen subfacies. As is typical of an event deposition, volcanic lithofacies can change rapidly in rock types, shapes, superposition relationships, and scales.
3 Development of reservoir pores, cavities and fractures, various pore-fracture assemblages, complex reservoir accumulation, and permeation patterns
The volcanic reservoir space comprises mainly vesicles, intergranular pores, dissolution pores, and various fractures. Due to the complex shapes of pores and fractures and highly variable pore sizes, volcanic reservoirs are multimedium reservoirs with developed pores, cavities, and fractures. Pores are major reservoir spaces in volcanic reservoirs, and throats and fractures are flow passages. Various pores and fractures lead to different pore-fracture assemblages. Meanwhile, different petrogenesis, shape, size, and the degree of matrix-pore growth lead to various throat types with complex shapes and large variations in throat radius. Therefore, the reservoir accumulation and permeation pattern in a volcanic reservoir is expected to be complex.
4 Vertical and lateral continuity and strong heterogeneity of pores
The complex shape, superposition relationships, and spatial distribution of each architectural unit in volcanic rocks lead to scattered distribution of reservoirs, poor vertical and lateral continuity, and limited horizontal distribution, controlled by internal architecture. In addition, volcanic reservoirs have strong interlayer heterogeneity, which, together with the great changeability of physical properties in effective reservoirs, results in strong intrastratal heterogeneity.
5 Gas-Water distribution controlled by structure, internal architecture, multi-medium porosity, and complex gas-water relationships
The gas-water distribution in volcanic gas reservoirs is controlled by structure, internal architecture, and multiple porous media, with a broad gas-over-water spatial relationship. However, the gas-water contacts for different volcanic edifices and different volcanic massifs are not the same. As multiple porous media reservoirs, the gas saturation thresholds in different volcanic matrices and fractures are different, also leading to different gas-water distribution patterns, resulting in complex gas-water relationships in volcanic gas reservoirs.
1.2.2 Challenges in volcanic gas reservoir characterization
The unique geological conditions and complex geological features require new concepts, approaches, and methods for volcanic gas reservoir characterization, which is drastically different from conventional sedimentary reservoir rocks. In the absence of existing theoretical guidance and technical references, our research on volcanic gas reservoir characterization started from scratch and met numerous challenges, especially in the following aspects (Table 1.1).
Table 1.1
Geological Features and Characterization Challenges for Volcanic Gas Reservoirs
| Research Contents | Geological Feature | Difficulties in Volcanic Gas Reservoir Characterization | ||
| Sedimentary Rock | Volcanic Rock | Characterization Content | Challenges | |
| Internal structure | Internal architectural features of “mud-in-sand” or “sand-in-mud” | Multi-level internal architectures composed of volcanic formation—volcanic edifice—volcanic massif—volcanic lithofacies—volcanic lithologies. | Shape, scale, superposition relationship and spatial distribution of volcanic formation, volcanic edifice, volcanic massif, volcanic lithofacies, and accumulation-permeation unit | From large- to small-scaled internal architecture, more complex shape and superposition relationships, smaller difference from surrounding rocks, and higher difficulty in identification and characterization |
| Microporous structure | 1. Dominated by intergranular pores and dissolved pores 2. Mainly pore-shrinking type throat 3. Simple pore-fracture assemblage 4. Simple reservoir accumulation and fluid-flow pattern | 1. Vesicles, intergranular pores, dissolved pores and fractures developed, with multimedium characteristics 2. Various throat types 3. Various pore-fracture assemblages 4. Complex reservoir accumulation and permeation pattern | 1. Reservoir spaces, genesis type, shape, size of throats, accumulation and permeation ability 2. Reservoir accumulation and fluid-flow pattern, accumulation and permeability, and distribution 3. Reservoir microscopic producibility | 1. Difficulty in characterization of accumulation capacity due to various reservoir spaces and complex pore-cavity-fracture assemblage relationships 2. Difficulty in characterization of fluid-flow capacity due to complex throat genesis and throat combination 3. Difficulty in evaluation of reservoir production capacity due to multiple pore-fracture assemblages and complex reservoir accumulation and fluid-flow pattern |
| Effective reservoir | 1. Simple lithology and less effective reservoir types 2. Fewer fracture types 3. Relatively simple gas-water relationships 4. Weak heterogeneity 5. Controlled by sand body and physical properties, with laminar distribution | 1. Complex lithology and diversified effective reservoir types 2. Diversified fracture types 3. Complex gas-water relationship 4. Strong heterogeneity 5. Controlled by complex internal structure, with irregular distribution | 1. Distribution of favorable lithology or lithofacies zones 2. Distribution of fracture zones 3. Gas-water distribution 4. Classification and identification of effective reservoirs 5. Spatial distribution of effective reservoirs | 1. Volcanic rocks with the same components have similar seismic response, and these rocks are difficult to predict from seismic data 2. Fractures are difficult to predict due to rapid changing in lithology, complex internal architecture, and boundary influence of lithology and internal architecture 3. Difficulty in identification of low-resistivity gas layers 4. Complex internal structure and irregular distribution of effective reservoirs make it difficult to realize body-controlled seismic inversion |
| Reservoir parameter | 1. Limited change in rock matrix parameters 2. Simple reservoir accumulation and permeation pattern 3. Relatively simple conduction mechanism 4. Mainly simple structural fractures | 1. Great change in rock matrix parameters 2. Complex reservoir accumulation and permeation pattern 3. Complex conduction mechanism 4. Various complex fractures | 1. Interpretation of matrix porosity, permeability; and gas saturation 2. Interpretation of fracture width, density, and length, as well as porosity, permeability, and gas saturation | 1. Complex reservoir spaces, throats, and accumulation and permeation pattern, leading to complex permeability characteristics make it difficult to interpret permeability2. Complex rock conduction mechanism; fluid distribution and accumulation state in various pores make it difficult to interpret fluid saturation |
| Geological model building | 1. Reservoir distribution controlled by sedimentary facies 2. Fluid distribution controlled by sand body and reservoir physical properties | 1. Reservoir distribution controlled by multilevel internal architecture 2. Fluid distribution controlled by structure, internal architecture, and multimedium | 1. Multilevel structural model 2. Multilevel reservoir framework mode; 3. Attribute model of multimedium reservoirs under framework model constraint 4. Gas-water distribution model in matrix and fractures under constraints of structure, internal architecture, and reservoir attributes | 1. Difficulty in building structural and reservoir framework models because of complex internal architecture and difficulty in occurrence control and surface trend control 2. Difficulty in building attribute model because multimedium reservoir distribution is controlled by internal architecture 3. Difficulty in building gas-water distribution model because gas-water distribution is controlled by structure, internal architecture, and reservoir attributes |
1 Difficulties in identification and characterization of complex internal architecture
Volcanic rock bodies are composed of complex multilevel internal architectural units, which have different sizes and geological features from one other, and the difficulties in their identification and characterization are also different. Large-scale architectural units with a simple geometry and superposition relationships are highly different from surrounding rocks and can be easily identified and characterized with readily detectable geological and geophysical logging and seismic characteristics. On the contrary, small-scale architectural units with complex geometric and superposition relationships are only slightly different from surrounding rocks and thus difficult to identify and characterize on the basis of geological and geophysical logging and seismic characteristics. Moreover, volcanic edifices with complex petrogenesis and various types are more difficult to identify and characterize because their diagnostic features are usually destroyed by external factors such as tectonic activities.
2 Challenges in characterization of microporous structures due to variability of reservoir space and complex pore-cavity-fracture assemblage
There are many varieties of volcanic reservoir space, each characterized by unique shapes and volumes, degree of development, and complexity of pore-cavity-fracture assemblage, which make it difficult to characterize the capacity of reservoir space accurately. Compared to sedimentary rocks, the pore throats in volcanic rocks have more complex origins, more variable shapes, a wider range of sizes, and more complex pore-throat assemblages. This provides challenges to categorize the shape, size, and permeability of throats in volcanic rocks. The complexity of pores, cavities, fractures, and throats leads to complex reservoir accumulation and fluid-flow patterns in volcanic gas reservoirs, and it is difficult to evaluate the characteristics pertinent to accumulation, permeability, and productivity of the reservoir patterns, respectively. Furthermore, volcanic reservoirs have multiple pore-fracture assemblages and extremely strong heterogeneity, making it difficult to evaluate accurately the effectiveness and producibility of volcanic reservoirs.
3 Challenges in identification and prediction of effective volcanic gas reservoirs due to diverse reservoir types and strong heterogeneity
Volcanic reservoirs have complex components and highly variable rock textures and structures that are difficult to distinguish using geophysical logging data. Volcanic reservoirs have various fracture types with different shapes, scales, and growth characteristics. It is difficult to identify blast fractures, contraction fractures, and dissolution fractures, respectively. There are various effective reservoir types, which are interbedded with various low-resistivity gas layers and high-resistivity aquifers, thus rendering effective reservoirs difficult to identify.
Volcanic rocks have a wide range of lithologies, but seismic responses for volcanic rocks with similar composition are nearly identical. This makes it difficult to predict volcanic lithology distribution using seismic data. In volcanic reservoirs, blast fractures, contraction fractures, weathered fractures, and sutured fractures with a small size and limited extension are also difficult to detect through seismic data. Moreover, the interference as a result of rapid changes in lithological boundaries and complex internal architectural boundaries seriously hinders fracture prediction using seismic data. For volcanic rocks with complex internal architecture, it is difficult to carry out reservoir parameter inversion under the control of these structures. Moreover, strong heterogeneity, nonregular distribution, thickness fluctuation, and low seismic resolution contribute to the challenges in predicting effective thin reservoirs, making it more difficult to achieve classified prediction in effective reservoirs.
4 Difficulties in interpreting parameters of multimedium volcanic gas reservoirs
Parameters of volcanic rock matrix vary greatly with various pore types, different pore sizes, and pore distribution, resulting in different geophysical logging responses. The lithology and reservoir space characteristics exert a strong influence on porosity interpretation. Various volcanic reservoir spaces have different shapes, sizes, distribution patterns, and connection modes. Similarly, various throat types are reflected by their different shapes, sizes, sinuosities, and pore-throat assemblages with complex pore structures. Volcanic reservoir accumulation and fluid-flow patterns are highly variable, leading to different fluid-flow characteristics and large differences in permeability. This, in combination with the interference of fractures, presents difficulties for interpreting volcanic reservoir permeability. Also, the highly variable electric conduction modes and combination types, together with complex conduction paths, give the resistivity logs complicated response characteristics. In various pore types, fluid distribution and accumulation state are complex, causing complicated nuclear magnetic logging responses and greatly hampering saturation interpretation.
5 Difficulties in building 3D geological models for multimedium volcanic gas reservoirs with complex internal architecture
Multilevel internal architectures, together with complex distribution and superposition relationships of individual structures, present difficulties for building an architectural reservoir framework model with occurrence and trend surface control. Volcanic reservoirs have a complex petrogenesis, and the reservoir distribution is controlled by internal architecture, with multiple media such as pores, cavities, and fractures. It is thus difficult to characterize reservoir parameters quantitatively and build attribute models. Volcanic gas reservoirs also have complex gas-water relationships, and the gas-water distribution pattern is controlled by structures, internal architecture, and reservoir characteristics. This contributes to the challenges of building gas-water distribution models.
1.3 Significance of volcanic gas reservoir characterization and its technical concept
1.3.1 Significance of volcanic gas reservoir characterization
The abundant volcanic natural gas resources with large reserves in China impart great significance to rational development of volcanic gas reservoirs. The key to volcanic gas reservoir development lies in its geological understanding, which depends on reservoir characterization. By taking into consideration the geological features, research difficulties, and production practice, volcanic gas reservoir characterization techniques have been developed recently through innovative research concepts and multidiscipline approaches, and they have contributed effective solutions to many key technological problems. For example, the characterization of internal architecture and microporous structures, identification and prediction of effective volcanic reservoirs, dual-medium parameter interpretation, and internal architecture helped constrain geological modeling of multimedium volcanic reservoirs. Based on reservoir characterization, the shape, scale, and superposition relationships of internal architectural units at each scale of volcanic reservoirs are clarified; the scale and relationships of volcanic stratigraphic sequences are revealed; the spatial distribution of effective volcanic reservoirs are predicted; and the three-dimensional (3D) geological model is built with the constraints of multimedium and complex internal architecture. Reservoir characterization provides the basis and guidance for drilling site optimization, horizontal well trajectory design, well-controlled dynamic reserves evaluation, and optimization of technology development strategies to establish a foundation for increasing drilling success rate, well production, and overall development level of gas fields.
1.3.2 Technical concept of volcanic gas reservoir characterization
1 Definition of conventional reservoir characterization
At the International Seminar for Reservoir Characterization first held in 1985, the reservoir characterization was defined as “a method to determine reservoir properties, identify geological information and spatial change quantitatively.” The ultimate goal is to understand the reservoir heterogeneity accurately and to improve reservoir management. Conventional reservoir characterization includes stratigraphic correlation, structural features and distribution, type and distribution of sedimentary facies, reservoir-caprock features and distribution, reservoir types and fluid distribution, hydrocarbon reserves calculation, hydrocarbon reservoir temperature and pressure system, and driving characteristics [7]. Present reservoir characterization is an integrated multidisciplinary approach, including seismology, geophysical logging, geology, computer science, outcrop study, modern sedimentology, high-density well pattern data study, production performance research, qualitative and quantitative methods of geostatistics, and neural network [8]. Wu believed that reservoir characterization was to study subsurface heterogeneous reservoirs quantitatively based on multidiscipline data. He divided reservoir characterization into four aspects as characteristics identification, pattern recognition, vertical interpretation, and lateral prediction [7].
2 Technical concept of volcanic gas reservoir characterization
Conventional reservoir characterization involves mainly stratigraphic correlation, structural interpretation, sedimentary facies analysis, reservoir evaluation, and gas reservoir type study. Due to their unique petrogenesis and complex geological settings, volcanic gas reservoir characterization is significantly different from conventional reservoir characterization in terms of the research subjects and technical concept. Volcanic gas reservoir characterization mainly aims to solve the following problems.
1) Dissection of volcanic internal architecture
Complex internal architecture is a fundamental geological feature of volcanic gas reservoirs and an important factor controlling spatial distribution of these reservoirs. Dissection of the volcanic internal architecture makes it possible to clearly determine the relationships of volcanic internal architectural units at each level and to characterize the shape, scale, superposition relationships, and spatial distribution of such internal architectural units. These steps enable the construction of a framework model to constrain volcanic reservoir seismic inversion and geological modeling and to establish a foundation for volcanic stratigraphic sequence division and correlation.
2) Volcanic stratigraphic sequence division and correlation
Stratigraphic sequence is the basis for the division of development layer series. On the basis of the constraint of internal architecture and the principle of “hierarchical control and stepwise correlation,” the volcanic stratigraphic sequence division and correlation are carried out, and the relationships among volcanic stratigraphic sequence, internal architecture, and development layer series are distilled to provide the basis for a logical division of development layer series.
3) Characterization of volcanic reservoir microporous structures
Reservoir microporous structures refer to the shape, size, distribution, and connectivity of pores, throats, and fractures. Characterization of volcanic reservoir microporous structures can reveal effectively the shape, scale, accumulation, and permeability of reservoir space and conduits in volcanic reservoirs. Investigations at this stage aim to characterize the accumulation and permeation capacity, as well as distribution features of various reservoir accumulation and permeation patterns, to evaluate reservoir effectiveness and fluid mobility quantitatively, and hence to provide a basis for evaluating reserve producibility and calibrating ultimate recovery efficiency.
4) Identification and prediction of effective volcanic reservoirs
In comparison with sedimentary reservoirs, the characteristics of effective volcanic reservoirs are much more complicated. Lithological and lithofacies analyses can help predict the spatial distribution of volcanic lithofacies and point to favorable lithological and lithofacies zones. Fracture identification and prediction can clarify the fracture growth level, fracture effectiveness, and fracture trend in volcanic rocks, predict the distribution of fracture zone, and guide the deployment of high-yielding wells. Prediction of effective reservoirs also helps determine the spatial distribution of various effective reservoirs in volcanic rocks and to optimize drilling locations and horizontal well trajectory.
5) Identification of gas layers and aquifers in volcanic gas reservoirs
Identification of gas layers and aquifers is a major step for determining gas and water distribution, which provides a basis for identification of the gas-water system in volcanic gas reservoirs and determination of gas reservoir types and gas-bearing area.
6) Interpretation of volcanic reservoir parameters
The interpretation of volcanic reservoir parameters aims to clarify the accumulation and fluid-flow capacity and gas-bearing characteristics of volcanic gas reservoirs, thus establishing a framework for the quantitative characterization of reservoir properties, evaluation of in situ gas reserves, and geological modeling of gas reservoirs.
7) 3D geological modeling of volcanic gas reservoirs
Three-dimensional geological modeling of complex internal architecture and dual-medium characteristics of volcanic rocks is the integrated representation derived from volcanic gas reservoir characterization. It is applied mainly to evaluation of in situ gas, to guide the planning of well location and horizontal well trajectory, and to provide models for numerical simulation.
1.3.3 Techniques of volcanic gas reservoir characterization
For different geological features and research subjects, the approaches for volcanic gas reservoir characterization are different from those used for conventional reservoir characterization. In view of the challenges in volcanic gas reservoir characterization, as discussed earlier, we follow a step-by-step approach to characterize volcanic gas reservoirs, progressing from qualitative to quantitative and from macroscopic and microscopic levels, using geological, geophysical logging, seismic, and dynamic data to carry out the study in the following steps: internal structure, stratigraphic sequence, microporous structure, reservoir identification, fluid identification, parameter interpretation, and finally construction of geological model (Figure 1.1).
1 Technique for dissecting volcanic internal architecture
Based on the division of multilevel internal architectures and the established identification criteria for each architectural unit, the internal architectures are described in the order of single well, cross-section profile, and two-dimensional and three-dimensional rendering, using geological, geophysical logging, and seismic data. The shape, scale, and superposition relationships of the architectural units are characterized quantitatively, and the distribution of each such unit is revealed, which will provide a basis for the division of development layer series, reservoir seismic inversion, accumulation-permeation unit characterization, and construction of geological model.
2 Techniques for volcanic stratigraphic sequence division and correlation
The identification of volcanic stratigraphic sequence is based on the determination of stratigraphic sequence markers with level-control principle; the correlation of each sequence can be determined by streamlining the relationships among stratigraphic sequence, internal architecture, and gas reservoir layer series. Within the architectural constraint, volcanic stratigraphic sequences can be divided and correlated, the spatial distribution of volcanic stratigraphic sequences delineated, and the volcanic stratigraphic framework ultimately established.
3 Technique for volcanic reservoir microporous structure characterization
Through integration of core logs, thin section analysis, experimental study, analyses of reservoir space type, throat features, reservoir accumulation and fluid-flow patterns, and microscopic producibility, the microporous structural characteristics in volcanic reservoirs are defined, and the shape, scale, assemblage features, and distribution are characterized quantitatively, which in turn provides a basis for the producible reserve evaluation of volcanic gas reservoirs.
4 Techniques for effective reservoir identification and prediction
The construction of a geophysical logging identification model of volcanic rock components, rock texture/structure, and rock type is based on image analysis, TAS (total alkaliversus silica) plot, and dual-variable cross-plot analysis. Volcanic lithology may be identified by ECS (Eliemental Capture Spectroscopy) logging, acoustic and electrical image logging, and conventional geophysical logging. On the basis of fracture study and logging response analysis, acoustic and electrical image logging and conventional geophysical logging may be used to identify volcanic rock fractures, calculate fracture parameters, evaluate fracture growth degree and effectiveness, and determine fracture occurrence. Combined with various static and dynamic data, the identification markers for various effective reservoirs are determined and used to identify effective reservoirs in volcanic rock qualitatively. The establishment of cutoff standard and classified evaluation standard of effective reservoirs will lead to the quantitative identification of effective volcanic reservoirs.
Volcanic lithologies can be predicted by seismic reflection profile analysis, waveform classification, and frequency-divided inversion, using the results from single well identification, calibration of well-seismic assemblage, and analysis of seismic response characteristics. The fracture distribution in volcanic reservoirs can be predicted by poststack seismic attribute analysis, prestack azimuth treatment, and fracture parameter inversion. The reservoirs can be predicted level by level using seismic inversion under the constraint of internal architecture. The distribution of effective lithology and lithofacies, fractures, and effective reservoir space can be ultimately defined.
5 Techniques for identification of gas layers and aquifers in volcanic gas reservoirs
On the basis of mud logs indication for gas layers, aquifers and dry layers, formation tests, and geophysical logging response characteristics, the gas layers and aquifers in volcanic rocks are identified qualitatively and quantitatively in combination with gas-bearing data from wells.
6 Technique for interpreting reservoir parameter logs
By combining core experiments with nuclear magnetic logging, acoustic and electrical image logging, and conventional logging data, the porosity, permeability, and gas saturation in volcanic rock matrix and fractures can be interpreted through integration of theoretical and statistical models to define the physical properties and gas-bearing features of volcanic gas reservoirs.
7 Technique for 3D geological modeling of volcanic gas reservoirs
Core data, geophysical logging, mud logs, formation test, and production performance data are used as constraints for well point calculation. Internal architectural features, such as volcanic eruption cycles, volcanic edifices, volcanic massif, volcanic lithofacies, and accumulation-permeation units, are used as constraints for interwell calculation. In addition, seismic attribute volumes and inverted parameter volumes are used as co-constraints. The volcanic gas reservoir structure model, reservoir framework model, reservoir attribute model, and fluid distribution model are constructed using Kriging interpolation, sequential indicator simulation, and stochastic simulation. In this process, geological modeling technique is also developed for dual-medium volcanic gas reservoirs with complex internal architectures.