Development of Volcanic Gas Reservoirs: The Theory, Key Technologies and Practice of Hydrocarbon Development

By Qiquan Ran, Dong Ren and Yongjun Wang

Development of Volcanic Gas Reservoirs: The Theory, Key Technologies and Practice of Hydrocarbon Development introduces the geological and dynamic characteristics of development in volcanic gas reservoirs, using examples drawn from the practical experience in China of honing volcanic gas reservoir development. The book gives guidance on how to effectively develop volcanic gas reservoirs and similar complex types of gas reservoir. It introduces basic theories, key technologies and uses practical examples. It is the first book to systematically cover the theories and key technologies of volcanic gas reservoir development.

As volcanic gas reservoirs constitute a new research area, the distribution and rules for development still being studied. Difficulties in well deployment and supportive development technology engender further challenges to development. However, in the past decade, research and development in the Songliao and Junggar Basins has led to marked achievements in volcanic gas reservoir development.

• Introduces the theory, key technologies and practice of volcanic gas reservoir development
• Provides links between theory and practice, highlighting key technologies for targeted development
• Offers guidance on complex issues in volcanic gas reservoir development
• Presents practical evidence from effective development and exploitation of gas reservoirs

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 29, 2018
• ISBN: 9780128163061

Qiquan Ran

Dr. Ran Qiquan, director of the oil & gas development & strategy planning department at RIPED, a professorship senior engineer, doctoral supervisor and senior technical expert of CNPC. He has significant experience in oil & gas field development technology research, especially in volcanic gas reservoirs and unconventional oil & gas reservoirs. The research subjects covered oil & gas field development geology, development program, reservoir engineering, simulation software R&D, and strategic planning. He has won 24 awards for scientific and technological achievements, published 94 papers, 7 monographs, and owns 34 software copyrights.

Book preview

Development of Volcanic Gas Reservoirs

The Theory, Key Technologies and Practice of Hydrocarbon Development

First Edition

Qiquan Ran

Dong Ren

Yongjun Wang

Min Tong

Yuanhui Sun

Lin Yan

Jiaxin Dong

Zhiping Wang

Mengya Xu

Ning Li

Hui Peng

Fuli Chen

Dawei Yuan

Qing Xu

Shaojun Wang

Qiang Wang

Table of Contents

Cover image

Title page

Copyright

Preface

Chapter 1: Introduction

Abstract

1.1 Volcanic Gas Reservoir Development Before the 21st Century

1.2 Geological Features and Development Difficulties of Volcanic Gas Reservoirs

1.3 Theories and Key Techniques for Volcanic Gas Reservoir Development

1.4 Prospect of Volcanic Gas Reservoir Development

Chapter 2: Volcanic Rocks Architecture and Sequence

Abstract

2.1 Volcanic Architecture

2.2 Volcanic Stratigraphic Sequence

2.3 Volcanic Stratigraphic Sequence Correlation and Layer Series Classification

Chapter 3: Volcanic Reservoir Mode

Abstract

3.1 Types of Volcanic Reservoir and the Definitions of Reservoir Mode

3.2 Volcanic Reservoirs Evolution Mode

3.3 Distribution Mode of Volcanic Reservoirs

3.4 Storage and Seepage Mode of Volcanic Reservoirs

3.5 Characteristics of Effective Volcanic Reservoirs

Chapter 4: Volcanic Gas Reservoir Characteristics and Models

Abstract

4.1 Volcanic Gas Reservoir Classification and Characteristics

4.2 Structural Features and Structure Models in Volcanic Gas Reservoirs

4.3 Internal Structural Features and Framework Models in Volcanic Gas Reservoirs

4.4 Storage-Seepage Features and Property Models in Volcanic Gas Reservoirs

4.5 Gas-Water Distribution and Fluid Model in Volcanic Gas Reservoirs

4.6 Applications of Volcanic Gas Reservoir Models

Chapter 5: Flowing Mechanism and Development Performance in Volcanic Gas Reservoirs

Abstract

5.1 Micro-fluidity

5.2 Nonlinear Flowing Mechanism

5.3 Volcanic Gas Reservoir Development Performance

Chapter 6: Development Dynamic Description and Forecast Model of Volcanic Gas Reservoirs

Abstract

6.1 Flowing Mechanisms and Basic Mathematical Models

6.2 Dynamic Description Models and Methods

6.3 Productivity Forecasting Models and Methods

6.4 Numerical Simulation Models and Methods of Volcanic Gas Reservoirs

Chapter 7: Volcanic Gas Reservoir Development Technologies

Abstract

7.1 Effective Reservoir Prediction and Well-Location Optimization Technology

7.2 Technologies for Improving Single-Well Production

7.3 Productivity Evaluation and Optimized Production Allocation Technology of Volcanic Gas Reservoirs

7.4 Dynamic Description Techniques in Volcanic Gas Reservoirs

7.5 Numerical Simulation Technology of Volcanic Gas Reservoirs

7.6 Optimization of Development Modes and Strategies for Volcanic Gas Reservoirs

Chapter 8: Development Practices of Volcanic Gas Fields

Abstract

8.1 Types and Development Procedures of Volcanic Gas Reservoirs

8.2 Development of the CC/YT Volcanic Gas Fields

8.3 Development of the SS/XX Volcanic Gas Fields

8.4 Development of the DD Volcanic Gas Fields

8.5 Development of Volcanic Gas Fields in Japan

Index

Copyright

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Preface

Han Dakuang

The conventional natural gas industry in China is developing rapidly. New advances have been made in major gas fields, and breakthroughs have been achieved in unconventional gas field development, both resulted in continuous increase in production. In 2014, the national natural gas production has reached 1329 × 10⁸ m³, which was an increase of 10.7% over 2013. Currently, a number of medium- and large-sized gas fields are under evaluation or capacity-construction stage, such as Kumshan and Dabei gas fields in the Tarim Basin, Sulige and Daniudi gas fields in the Ordos Basin, Moxi-Gaoshiti, Puguang, and the Luojiazhai gas fields in Sichuan Basin. As projected, the natural gas production will maintain rapid growth in the coming years.

In recent years, the development of gas fields in China has entered a new stage. Economic development and environmental protection reinforcement have promoted the development of gas fields in China, especially to meet the urgent need in mitigating air pollution caused by fossil fuel consumption. Therefore, it is extremely important to summarize previous experience and technologies so as to bring China's gas field development to a higher level. The book compilation of China's Gas Field Development Series has successfully made the effort.

China’s Gas field Development Series is compiled according to different types of gas reservoirs. It systematically summarizes the experience and achievements of gas field development in China and the corresponding development theories and policies. This series of books is divided into eight volumes, including General Theory, Volcanic Gas Field Development, Low-permeability Tight Sandstone Gas Field Development, Multilayer Loose Sandstone Gas Field Development, Condensate Gas Field Development, Sour Gas Field Development, Carbonates Gas Field Development, and Abnormal High-pressure Gas Field Development. The majority of the authors are experts and scholars who have been actively involved in field production and scientific research for extended years. This series represent a set of scientific insights and best practice with large amount of information, strong practicability, and significant theoretical contribution.

The publication of China's Gas Field Development Series will play an important guiding role in further promoting China's gas field development in larger scale and with higher efficiency. They also provide references for teachers and students in related academic disciplines. Therefore, I'd like to extend my warmest congratulations on the publication and distribution of this series, and I sincerely thank people from all walks of life for their strong support and help during the preparation and publication of this series.

Chapter 1

Introduction

Abstract

The geological characteristics, development theories, and development technologies of volcanic gas reservoirs with special genesis and strong concealment are far different from those in sedimentary reservoirs. In lack of theoretical guidance, technical support, and development practice, it is extremely difficult to achieve large-scale developmental benefits. In this chapter, we will focus on the development of volcanic gas reservoirs before the 21st century and summarize the geological characteristics, developmental characteristics, and developmental difficulties. The chapter briefly introduces developmental geological theories such as genesis mechanism, architectural model, and reservoir evolution model of effective volcanic gas reservoirs, effective development theories such as nonlinear flowing mechanism, development law and development models of volcanic gas reservoirs, as well as effective development techniques such as effective reservoir identification and well location optimization, single-well production improvement technology, and development optimization. Finally, it clearly points out that volcanic gas reservoirs have broad development prospects.

Keywords

Volcanic gas reservoir; Geological characteristics; Development difficulties; Development theories; Development technologies; Development prospect

Chapter Outline

1.1Volcanic Gas Reservoir Development Before the 21st Century

1.2Geological Features and Development Difficulties of Volcanic Gas Reservoirs

1.2.1Geological Features of Volcanic Gas Reservoirs

1.2.2Development Features of Volcanic Gas Reservoirs

1.2.3Challenges in Volcanic Gas Reservoir Development

1.3Theories and Key Techniques for Volcanic Gas Reservoir Development

1.3.1Major Theories

1.3.2Key Techniques

1.4Prospect of Volcanic Gas Reservoir Development

References

Volcanic rocks have been known as hydrocarbon reservoirs for nearly a century. However, they are always kept outside of oil and gas exploration and development because these rocks with special genesis are highly subtle and cannot generate hydrocarbons themselves according to the organic origin theory [1]. Initially, volcanic reservoirs were discovered accidentally or in the process of drilling other target formations, with far fewer hydrocarbon reserves and less production than those in sedimentary reservoirs. Therefore, they were rarely investigated for petroleum geology, and even less for development. Since the beginning of the 21st century, several large volcanic gas reservoirs have been successively discovered in the Songliao Basin and the Junggar Basin in China, with individual reserves up to several hundred billion cubic meters and resources up to several trillion cubic meters. Clearly, the volcanic reservoirs have huge potential for development.

In the absence of theoretical and technical guidance, it is extremely challenging to effectively develop volcanic natural gas reservoirs (or volcanic gas reservoirs), which are characterized by special petrogenesis, complicated reservoir conditions, and fluid-flow mechanism. Over the past decade, the volcanic gas reservoir development in China went through several stages from initial evaluation to development adjustment. Under the guidance of the practice—recognition—re-practice principle, Chinese scientists made a variety of research into volcanic gas reservoirs, and accumulated a lot of experience. As a result, systematic theories and techniques for volcanic gas reservoir development were established, which have propelled the effective development and rapid productivity construction of volcanic gas reservoirs in China.

This book presents the theories, techniques, and modes of volcanic gas reservoir development in China, which were formed on the basis of development practices in the past decade and exhibit the updated summarization of reservoir characteristics, fluid-flow mechanism and production performance of volcanic gas reservoirs, and the supporting techniques for volcanic gas reservoir characterization and development. This book will hopefully be used as an introduction to catch the attention of other scientists all over the world, ignite new thoughts and more research on volcanic reservoirs, and improve the development techniques. This book can be used as a reference for effective development of subtle gas reservoirs and complex lithologic gas reservoirs.

1.1 Volcanic Gas Reservoir Development Before the 21st Century

As a special type of gas reservoir, the volcanic gas reservoir is widespread in many petroliferous basins in the world. Several hundreds of reservoirs related to volcanic rocks have been discovered in sedimentary basins in more than 100 countries, including China, Japan, the United States, Australia, Indonesia, Congo, and Brazil [2,3] (Fig. 1.1). According to the available data, the reserves of such discovered volcanic reservoirs are generally small, with original gas in place (OGIP) of less than 300 × 10⁸ m³; some volcanic reservoirs are large, such as Scott Reef (with OGIP of over 3000 × 10⁸ m³) in the Browse Basin, Australia. The reserve abundance of these reservoirs is generally low—less than 2.5 × 10⁸ m³/km², but as high as 20 × 10⁸ m³/km in some local areas. For example, the Yoshii-Kashiwazaki volcanic gas reservoir in the Niigata Basin, Japan, has a reserve abundance of 22 × 10⁸ m³/km² in the northwestern structural height.

Fig. 1.1 Distribution of representative volcanic gas reservoirs in the world.

Japan was the pioneer in volcanic gas reservoir development. As a typical example, the Yoshii-East Kashiwazaki gas field was discovered in 1968 with a long, narrow, anticlinal trap, which is a strongly water-flooded green tuff gas reservoir with a buried depth of 2310–2720 m. This gas reservoir has an accommodation space consisting of secondary dissolution pores and fractures, with porosity of 7%–32% and permeability of 5–150 mD. The net pay is 54–57 m, the superimposed gas-bearing area is 27.8 km², and the original recoverable gas reserves are 118 × 10⁸ m³. To develop this reservoir, 46 wells were drilled, of which, 15 wells have been put into production, with the maximum single-well daily gas production of 50 × 10⁴ m³, and the cumulative gas production of 88 × 10⁸ m³. Similar to laminar structural gas reservoirs, this gas reservoir usually distributes around conic or shield volcanic edifices resulting from a centered eruption, with small reserves. It was only developed in simple mode to meet production demand, and no research into theory and techniques have been made for its development. Later, volcanic gas reservoirs were successively discovered in Ghana, Brazil, Australia, the United States and other countries, but their development effects were not satisfactory. In China, the volcanic gas reservoirs first developed, include the Paleogene diabase volcanic gas reservoir (discovered in 1985) in Caojiawu in the Jizhong Depression, the Jiamuhe Fm volcanic gas reservoir (discovered in 1986) in Xinjiang, and the Zhougongshan Fm basalt volcanic gas reservoir (discovered in 1992) in southwestern Sichuan Basin [4,5]. These gas reservoirs, small in scale, were also developed in simple mode. In 1993, Well DD5 in the Luliang Uplift, the Junggar Basin, tapped the Carboniferous volcanic gas reservoir, which had limited contribution to gas reserves and production for its small scale [6].

To sum up, before the 21st century, rare systematic research was made in volcanic gas reservoir development, and no development theories and technologies were established.

In general, the development of volcanic gas reservoirs in the world can be divided into four stages.

(1)Ignoring stage (before the 1950s). According to conventional petroleum geology theory, volcanic rocks were incompatible with hydrocarbon resources. Therefore, volcanic rocks were always kept outside of hydrocarbon exploration and development.

(2)Discovery stage (from the 1950s to the 1960s). Some volcanic reservoirs were discovered accidentally in this stage, and even some wells in these reservoirs revealed daily oil production as high as over 1000 tons, making petroleum geologists realize that hydrocarbon accumulation in volcanic rocks was not an abnormality. Thus, geologists started to pay attention to volcanic rocks, and conducted purposeful exploration targeting volcanic rocks in some areas.

(3)Exploratory stage (from the 1970s to the end of 20th century). With the successive discovery of volcanic reservoirs, geologists initiated their exploratory works on the geology and development features of these reservoirs. However, their works were relatively superficial and not systematic [7]. The main research includes: (a) field studies of volcanic outcrops; (b) analytical studies of volcanic rock types and facies; (c) establishment of volcanic reservoir evaluation methods; (d) laboratory studies of volcanic reservoir properties; and (e) preliminary studies on development performance features.

(4)Growing stage (from the beginning of the 21st century to the present). In China, with the discovery of large-scale volcanic gas reservoirs in the Songliao Basin and the Junggar Basin, volcanic gas reservoirs have become the important areas for exploration and development. Under the guidance of the practice—recognition—re-practice principle, systematic research has been conducted over the past decade, and a series of mature and feasible development theories and techniques for volcanic gas reservoirs have been established. The commercial application of these theories and techniques has contributed successfully to the large-scale and effective development of volcanic gas reservoirs [8,9].

1.2 Geological Features and Development Difficulties of Volcanic Gas Reservoirs

1.2.1 Geological Features of Volcanic Gas Reservoirs

Special petrogenesis and complex geological settings make volcanic gas reservoirs obviously different from conventional sedimentary gas reservoirs in architecture, physical properties, gas-water distribution, and other aspects.

(1)Special petrogenesis and complex geological settings. Volcanic rocks cannot generate oil and gas themselves; the formation and distribution of volcanic gas reservoirs are decided by gas sources, reservoirs, source-reservoir assemblage, and other factors, and these reservoirs are subtle reservoirs. Meanwhile, volcanic reservoirs, with special petrogenesis, are controlled by multiple factors, including eruption environments (aquatic eruption, subaquatic eruption, etc.), eruption modes (centered eruption, fissured eruption, etc.), eruption energy (strong, weak, etc.), eruption stages (single-stage, multistage), magma properties (acidic, neutral and basic), eruption amount (large, small), eruption frequency (steady and continuous eruption, and intermittent eruption), paleotopography (gentle, steep), and later transformation (weathering, structural fragmentation, dissolution, etc.). All these result in complex geological settings for volcanic reservoirs [5,10–14].

(2)Nonlaminar superposition architecture. As a result of multistage eruption with multiple craters, volcanic rocks have multilevel architectures, and can be divided into such architectural units as volcanic formations, volcanic edifices, volcanic massifs, volcanic facies, and storage-seepage units (in a descending order of magnitude). The architectural units vary in shape and scale, and present superposition and nonlaminar features [15–18].

(3)Multiple porous media (e.g., pores, cavities, and fractures), and complicated fluid-flow mechanism. The accommodation space in a volcanic reservoir comprises pores, cavities, and fractures, represented by vesicles, intergranular pores, dissolution pores, and cracks. The pores, cavities, and fractures have variable shapes with big variations in diameter; and the pore throats have diverse types, complex shapes and a wide range of sizes. Furthermore, the combinations of pores, cavities, fractures and pore throats are very complicated, which lead to complex storage-seepage patterns and fluid-flow mechanisms.

(4)Rapid variation of reservoirs with strong heterogeneity. Volcanic reservoirs are significantly different in shape and scale, and change rapidly in lateral and vertical directions. Reservoir distribution is scattered with poor continuity. Pores in the reservoir are much different and variable in shape, size, and space-distribution. These features result in strong heterogeneity, horizontally and vertically (in and between layers).

(5)Multiple gas-water systems with complex gas-water relationships. The gas-water distribution (generally, gas over water), in volcanic gas reservoirs, is controlled by structures, internal architectures and multiple porous media. For volcanic edifices or massifs, the gas-water contacts are different, and several gas-water systems exist. In a single gas-water system, because of differences in reservoir physical properties, heterogeneity and fractures, the gas saturation of matrix and fractures is diverse, and the gas-water contact varies with physical properties, leading to complicated gas-water relationship in a volcanic gas reservoir.

1.2.2 Development Features of Volcanic Gas Reservoirs

Complex architectures, reservoir characteristics, gas-water distribution, storage-seepage patterns, and fluid-flow mechanisms of volcanic gas reservoirs make them quite different from conventional sedimentary reservoirs in development characteristics.

(1)Productivity varies greatly for wells targeting different reservoirs or different parts of a reservoir, since volcanic reservoirs have various lithologies, variable lithofacies, distinct physical properties, and strong heterogeneity. According to physical properties, volcanic reservoirs can be divided into four types, that is, high-porosity and high-permeability reservoirs, medium-porosity and medium-permeability reservoirs, low-porosity and low-permeability reservoirs, and tight reservoirs. Volcanic reservoirs have been found in various volcanic rocks, such as lavas, volcaniclastic rocks, and detrital lavas. For reservoirs with good physical properties, high natural productivity can be gained by conventional techniques. For reservoirs with poor physical properties, natural productivity is low, and thus hydraulic fracturing is needed to gain industrial gas flow. For tight volcanic gas reservoirs, natural productivity is almost zero, and volume hydraulic fracturing is required to get industrial production from wells. For example, in Block XX21 of the XX gas field, Daqing, wells have an initial stable daily production from 20 × 10⁴ m³ by natural energy to less than 1 × 10⁴ m³ after hydraulic fracturing, and adjacent wells with well spacing of less than 500 m may have initial stable productions differing from each other by more than five times. Clearly, wells in different volcanic gas reservoirs or in one volcanic gas reservoir may have great discrepancy in productivity.

(2)Production declines rapidly and stable production is hard to maintain. There are multiple types of volcanic reservoirs, including porous, fractured, fractured-porous, and porous-fractured reservoirs, with different fluid-flow mechanisms and production performance. For fractured and porous-fractured reservoirs, gas is mainly supplied through a fracture system; well production is high initially, but then declines quickly due to rapid energy depletion, limited gas supply of the fracture system and insufficient gas supply from the matrix to the fracture system. For fractured-porous reservoirs, gas is mainly supplied through a fracture system and macropores, resulting in a relatively higher initial gas production; subsequently, however, gas production declines sharply due to rapid energy depletion of the fracture system and macropores and insufficient gas supply from the matrix and micropores to the fracture system. For porous reservoirs, gas is mainly supplied through a pore system; well production is relatively stable, but it is susceptible to strong heterogeneity and poor connectivity and continuity of volcanic reservoirs. According to the actual production performance, volcanic gas reservoirs generally have an annual overall decline rate of 30% or more, and up to 80%. Obviously, volcanic gas reservoirs have the characteristics of rapid production decline and difficulty in maintaining stable production on the whole.

(3)The controlled reserves and cumulative gas production vary greatly among wells. The well-controlled reserves and the maximum cumulative gas production of wells are dependent on storage-seepage capacity, scales, and internal connectivity of reservoirs. Volcanic reservoirs have poor connectivity and great variations in shape, scale and storage-seepage capacity, leading to significantly different well-controlled reserves, with a wide range from 0.001 × 10⁸ m³ to 20 × 10⁸ m³. According to statistics of developed volcanic gas reservoirs in China, the efficiently developed volcanic gas reservoirs have large vertical well-controlled reserves, being 4–6 × 10⁸ m³ or even more than 10 × 10⁸ m³, but this kind of volcanic gas reservoir only accounts for about 20%; the inefficiently developed volcanic gas reservoirs have generally small vertical well-controlled reserves, being about 1–2 × 10⁸ m³, and this kind of volcanic gas reservoir accounts for about 30%; tight volcanic gas reservoirs have the lowest vertical well-controlled reserves, being less than 1 × 10⁸ m³, and this kind of volcanic gas reservoir is predominant, for about 50%. Great variations in well-controlled reserves and strong reservoir heterogeneity result in a wide range of cumulative gas production—from 0.001 × 10⁸ m³ to 10 × 10⁸ m³ per well. There are more than 50% of wells that demonstrate a cumulative gas production of less than 0.5 × 10⁸ m³ per well.

(4)Gas layers and water layers are connected by fractures, and volcanic reservoirs are easily flooded with several types of water. Water in volcanic gas reservoirs exists in various forms, such as edge/bottom water, interlayer movable water, intralayer irreducible water, and condensed water in a gas layer. Gas layers and water layers are communicated by direct contact, indirect contact, fractures, or other ways. Moreover, a well-developed fracture system is one of the key characteristics of volcanic gas reservoirs, and the fracture network systems, consisting of natural fractures and artificial fractures, may serve as the pathways for edge/bottom water channeling. Therefore, water breakthrough and water coning frequently appear in gas wells in volcanic gas reservoirs, leading to water production.

1.2.3 Challenges in Volcanic Gas Reservoir Development

The development of volcanic gas reservoirs is a new subject in research. However, the special and complex geology of volcanic gas reservoirs and the unavailability of mature theories and techniques in the early 21st century, when the development of volcanic gas reservoirs was just initiated in China, bring great challenges to this task.

(1)Volcanic reservoirs are difficult to identify for their complex genesis and distribution. Conventional theories of hydrocarbon development geology are usually based on laminar reservoirs of sedimentary genesis, but they are inapplicable to nonlaminar reservoirs of volcanic-eruption genesis. As a result of multicenter and multistage eruptions and rapid accumulation of volcanic rocks, volcanic reservoirs present multilevel nonlaminar superposition architectures. Such architectures, especially low-level architectures, are difficult to identify. Meanwhile, various accommodation spaces of volcanic reservoirs, such as vesicular pores, intergranular pores, dissolution pores, and fractures, are different in genesis and complex in evolution, and there is no appropriate guidance available for identifying the distribution modes of volcanic reservoirs and accommodation spaces. Therefore, it is necessary to set up geological modes and theories of volcanic gas reservoir development geology, to help geologists to identify the genesis and distribution of these reservoirs.

(2)Volcanic reservoirs are difficult to characterize and predict, and well-location optimization is challenging in these reservoirs. Great variations in parameters of volcanic rock matrix, diverse pore types, complex fluid distribution and accumulation state, and multiple electric conduction modes and combination types, result in complicated geophysical logging response mechanisms. Moreover, volcanic reservoirs have various rock types, rapidly variable physical properties, and complex accommodation spaces and fluid properties, and reservoir information is difficult to detect and distinguish with seismic data, making it challenging to effectively predict reservoirs. Volcanic reservoirs are characterized by multilevel complex architectures, strong heterogeneity and complex fluid distribution, so there are difficulties in structural controlled attribute modeling of these reservoirs. Also, challenges in reservoir characterization and prediction bring more troubles to evaluation and prediction of storage-seepage units and more risks to well deployment; thereby optimization of well locations is very difficult in volcanic reservoirs. In summary, it is of great necessity to develop volcanic gas reservoir characterization and prediction techniques to provide technical support for well-location optimization and well pattern deployment.

(3)There are challenges in the research on development features and patterns, due to complicated fluid-flow mechanism. Volcanic reservoirs are multiple-porous media with cavities, pores, and fractures in complex patterns and structures. Therefore, the fluid-flow mechanisms are very complicated in these reservoirs, which cannot be described according to conventional theories. Unavailability of nonlinear seepage theories and models for volcanic gas reservoirs make it very difficult to understand fluid-flow characteristics of different media at various development stages, and to do productivity forecasting, dynamic reserve evaluation, dynamic interpretation of reservoir parameters, and development index forecasting. Therefore, it is imperative to develop applicable seepage theories and models to get a better understanding of development features and patterns of volcanic gas reservoirs.

(4)Lack of effective development modes and supporting techniques makes it difficult to accomplish large-scale and economic development of volcanic gas reservoirs. The volcanic gas reservoirs distribute in scattered mode with poor reservoir continuity and strong heterogeneity, and they are greatly variable in quantity of reserves and reservoir quality, so it is very difficult to work out applicable modes of well-location and well pattern deployment, production planning, and exploitation for volcanic gas reservoirs with complicated geological conditions. In addition, the lack of supporting techniques, such as production performance analysis, productivity evaluation, and development optimization, for volcanic gas reservoirs, have caused poor production performance, low coincidence rate of development programming, and poor development benefit. Therefore, it is imperative to innovate applicable development modes and supporting techniques for volcanic gas reservoirs, to achieve large-scale and economic development.

1.3 Theories and Key Techniques for Volcanic Gas Reservoir Development

With consideration to the complicated geological conditions and special development features and challenges of volcanic gas reservoirs, some suitable development theories and techniques have been worked out, such as effective reservoir prediction and well-location optimization techniques, well production enhancement, and development optimization techniques. These theories and techniques can help to solve the problems in reservoir characterization, well-location optimization, production performance analysis, and development mode optimization, so as to guide the large-scale effective development of volcanic gas reservoirs. The research route is shown in Fig. 1.2.

Fig. 1.2 Research route of volcanic gas reservoir development.

1.3.1 Major Theories

Theories of volcanic gas reservoir development include development geology theories and effective development theories. The development geology theories focus on the genetic mechanism of effective reservoirs, architecture modes, reservoir evolution modes, reservoir distribution modes, storage-seepage patterns, volcanic gas reservoir models, and three-dimensional (3D) geological models, which help us to understand the complicated reservoir genesis, distribution modes and models of volcanic reservoirs. The effective development theories cover nonlinear fluid-flow mechanism, production performance, and development models of volcanic reservoirs, which help us to understand the fluid-flow mechanism, production performance, and development models of volcanic reservoirs.

1.3.1.1 Development Geology Theories—Genetic Mechanism, Reservoir Modes and Gas Reservoir Models

(1)Genetic mechanism

The formation of volcanic gas reservoirs is jointly controlled by the escape of volatile constituents in lava, tephra free-falling deposition, differential corrosion, condensation contraction, gas-liquid explosion, and structural fragmentation, etc., resulting in volcanic reservoirs with different geneses and development features. Vesicular reservoirs are products of high-temperature magma condensation with various diageneses, mainly occurring in volcanic lava, and their geneses mainly depend on volatile constituent content, lava inner-outer pressure difference, and condensation rate, etc. Intergranular-porous reservoirs, mainly occurring in volcaniclastic rock, are resulted from the accumulation and multiple diageneses of blasted pieces of volcanic magma or surrounding rocks by gas-liquid explosion, and their formation mainly depends on volcanic eruption energy, volcaniclastic size, and accumulation pattern, etc. Dissolved-porous reservoirs are generated from the constant dissolution of volcanic rocks in late diagenesis stages with a large number of dissolution pores; these reservoirs can be found in various volcanic rocks, and their genesis mainly depends on the time and intensity of weathering-leaching and burial dissolution, CO2 content, organic acid types, and soluble component content, etc. Fractured volcanic reservoirs, with few matrix pores and abundant fractures of various geneses, can be found in various types of volcanic rocks, and their formation mainly depends on tectonic movement intensity, gas-liquid explosion intensity, uneven condensation contraction, and condensation rate, etc.

(2)Reservoir modes

Volcanic reservoirs with a special genesis differ greatly from conventional sedimentary reservoirs in architecture modes, evolution modes, distribution modes, and storage-seepage patterns.

①Architecture modes. Based on the volcanic rock genesis, texture, and paragenetic associations, volcanic rock architectures can be divided into five levels in descending order, namely volcanic formations, volcanic edifices, volcanic massifs, volcanic lithofacies and storage-seepage units. These architectures are different in shape, scale, and superposition relationship, and have different effects on reservoir distribution, fluid distribution and reservoir connectivity.

②Reservoir evolution modes. Primary pores and fractures in volcanic rocks including vesicles, intergranular pores, shrinkage joints, and blasted fractures and have different formations, loss and preservation mechanisms, from that of secondary pores and fractures including dissolution pores, devitrification micropores, tectonic fractures, and weathered fractures. Therefore, the establishment of reservoir evolution modes can provide theoretical guidance for volcanic reservoir prediction.

③Reservoir distribution modes. Since effusive vesicular, burst intergranular-porous, fractured, and hybrid volcanic reservoirs are different from each other in genesis, have different accommodation space distribution modes, and different continuity/connectivity features. The research on volcanic reservoir distribution modes can provide important guidance for classified effective volcanic gas reservoir prediction, hydrocarbon-rich area selection, and well-location optimization [19].

④Storage-seepage patterns. Volcanic reservoirs are unitary or hybrid in development types, with eight kinds of storage-seepage patterns, such as vesicles, intergranular-pores, micropores, fractures, fractures-vesicles, fractures-intergranular-pores, fractures-micropores, and fractures-dissolution-pores. Their different storage and seepage capacities result in different productivity and stable production capacity of gas wells. The establishment of storage-seepage patterns of volcanic reservoirs can provide guidance for research on fluid-flow mechanisms and factors affecting productivity.

(3)Gas reservoir models

With multilevel architectures, including volcanic eruption cycles, volcanic edifices, and volcanic massifs, volcanic gas reservoirs have complex internal structures, multiporous media properties, and great differences in fluid properties and gas-water distribution. Their gas reservoir models and 3D geological models differ greatly from conventional sandstone and carbonated gas reservoir models.

①Model diversity. Volcanic gas reservoirs are diverse in genesis (e.g., in situ, ectopic, and hybrid) and in trap types (e.g., structural, internal structural, lithologic, and composite). They can be classified as high-quality, low-quality, or tight reservoirs in view of quality, and CO2-containing, condensation-containing or dry-gas reservoirs in view of fluid properties. All these volcanic gas reservoirs correspond to specific development techniques and strategies.

②3D geological models are used to exactly characterize multilevel internal structures, multiporous media reservoirs and complex fluid distribution of volcanic gas reservoirs. Structural models are used to characterize the multiple structural configurations and variations of volcanic eruption cycles, volcanic edifices, and volcanic massifs, etc. Framework models are used to characterize the morphology, scale, superposition, and spatial distribution of multilevel structures in volcanic gas reservoirs. Attribute models are used to characterize the effects of gas reservoir structures and frameworks on reservoir properties, storage and seepage capacity of the matrix and fracture systems. Fluid models are used to characterize the effects of gas reservoir structures, frameworks and reservoir properties on fluids and variations of gas-water contacts, fluid composition, and saturation within gas reservoirs [20,21].

1.3.1.2 Effective Development Theories—Nonlinear Flow Mechanisms, Development Patterns and Development Models

(1)Nonlinear flow mechanisms

With pores, cavities and fractures of various scales, volcanic reservoirs are multiporous media reservoirs with complicated pore structures and widely different reservoir properties, so they are different from the conventional mono-porous medium in flow mechanisms. The nonlinear flow mechanisms in a volcanic reservoir mainly manifest in three aspects: first, complex fluid-flow regimes in different scales of pore-fracture systems lead to nonlinear flow features; second, pore-throat deformation and fracture closure, resulting from stress sensitivity, cause nonlinear flow features; last, the relay gas supply-drainage in different scales of multiporous media lead to nonlinear flow features.

(2)Development patterns

Development patterns of volcanic gas reservoirs are dependent on both internal and external causes. Internal causes include complex architectures, multiporous reservoir properties, and complex fluid distribution, etc. External causes include development modes, technical policies, and technologies, etc. These internal and external causes work jointly to make various types of volcanic gas reservoirs, different in development patterns, well-controlled OGIP and reserve variation patterns, and water production patterns.

①Volcanic gas reservoir production performance. Volcanic gas reservoirs have (1) fractural storage-seepage pattern, (2) porous storage-seepage patterns, and (3) fractural-porous storage-seepage patterns. Gas wells in reservoirs with different storage-seepage patterns vary in production and pressure in initial high-production, production decline, and stable low-production stages.

②Variation of well-controlled dynamic reserves. In low-permeability porous reservoirs, due to the relatively poor reservoir properties and low-pressure wave propagation velocity, the pressure propagation range, gas drainage radius, and well-controlled dynamic reserves increase over time. For multiporous reservoirs, fluids are produced from macropores, mesopores and micropores, or macrofractures, mesofractures and microfractures successively, under a relay gas supply-drainage effect, and the corresponding well-controlled dynamic reserves gradually increase. A volcanic gas reservoir has edge and bottom water, so its development is commonly influenced by edge water and bottom water. Especially when a water block occurs due to water breakthrough in fractures, well-controlled dynamic reserves decrease.

③Water production patterns. Water produced in the development of volcanic gas reservoirs is condensed water, interlayer water, movable water, and edge/bottom water, which are different in production mechanism, timing and pattern, as well as their effect on gas well production.

(3)Development models

Gas wells in volcanic reservoirs usually have high initial gas production, rapid production decline, and different and nonuniform productivity, and most of them can only produce after the reservoirs are hydraulically fractured. Based on the nonlinear flow mechanism of volcanic gas reservoirs, together with type, morphology and superposition of storage-seepage units, and the characteristics of pores, cavities and fractures, the development models have been established, including the productivity forecast model for naturally producing and hydraulically fractured vertical and horizontal wells in volcanic gas reservoirs, the reservoir dynamic parameter characterization model of vertical and horizontal wells with different porous media and complex boundary geometry, and the numerical simulation model of nonlinear fluid-flow in multiporous media of volcanic gas reservoirs.

1.3.2 Key Techniques

Key techniques for volcanic gas reservoir development include effective reservoir prediction, well-location optimization techniques and well production enhancement, and development optimization techniques. The effective reservoir prediction and well-location optimization techniques focus on reservoir identification/prediction, and well-location optimization, covering gas-water layer identification, reservoir properties interpretation, architecture dissection, reservoir classification and prediction, and well-location optimization; these techniques can help to solve the problems in volcanic reservoir identification, well-location optimization and horizontal well trajectory design, and provide technical parameters for reserve estimation. The well production enhancement and development optimization techniques involve horizontal well development, hydraulic fracturing, productivity evaluation and production allocation, dynamic parameters characterization, numerical simulation of gas reservoirs, and development modes and technical measures; these techniques can help to enhance well production and well-controlled dynamic reserves and improve gas reservoir development performance and benefits.

1.3.2.1 Effective Volcanic Reservoir Prediction and Well-Location Optimization

(1)Effective reservoir identification

Volcanic gas reservoirs are complex in lithology with pores, cavities, and fractures, rapidly variable reservoir properties and complex gas-water distribution. Their logging responses are often interpreted vaguely. Accordingly, effective volcanic reservoirs should be identified progressively in respect of lithology, accommodation space (pore, cavity, and fracture), fluids, and reservoir parameters.

①Lithology. Based on rock classification and nomenclature, together with the data from rock chemistry analysis, Elemental Capture Spectroscopy (ECS) logging, core description, Formation MicroScanner Image (FMI) imaging logging, and conventional logging, the rock composition, structures and genesis types are identified using the methods like TAS chart interpretation, FMI image analysis, intersection of sensitive parameters, and cluster analysis, so as to evaluate comprehensively the lithological characteristics of volcanic rocks.

②Accommodation space (pore, cavity, and fracture). Core description, casted thin-section analysis, imaging logging analysis, nuclear magnetic resonance T2 spectrum analysis, and semiquantitative interpretation of conventional logging are used to identify pores, cavities, and fractures and evaluate their characteristics.

③Fluids or gas/water layer. Based on geological log, formation test and logging data, following the comprehensive interpretation approach, gas-layer, and water-layer interpretation models, by rock types, are established to effectively identify gas layers and water layers in volcanic gas reservoirs.

④Reservoir parameters. By integrating qualitative analysis with quantitative calculation and theoretical calculation with statistical models, and considering the influence of complex fluid variation, the reservoir parameter interpretation models are improved according to lithology and storage-seepage types of volcanic reservoirs, so as to increase the calculation accuracy of porosity, permeability, saturation, and other parameters of volcanic gas reservoirs.

(2)Effective reservoir prediction

Effective volcanic reservoirs contain various pores, cavities, and fractures, and distribute under the control of complex architectures and lithofacies, making them difficult to be predicted. Accordingly, effective volcanic reservoirs should be predicted using the rock mass-controlled seismic inversion techniques constrained by architectures and reservoir distribution and the classified prediction techniques constrained by effective reservoir classification criteria and gas-water relationship, after architecture dissection and lithology, lithofacies, and fracture prediction.

①Architecture dissection. Guided by volcanic reservoir architectures and reservoir modes, and according to geological, logging, and seismic responses of architecture units, the internal architectures of volcanic gas reservoirs are dissected by levels on the basis of both logging and seismic data, so as to reveal the morphology, scale, distribution, and superposition of architecture units.

②Lithology prediction. Seismic frequencies, amplitudes, phase features, and reflection behaviors of volcanic rocks vary, depending on their composition and internal architectures. Frequency-division attribute-inversion is used to predict lithology.

③Fracture prediction. Fracture belts in volcanic reservoirs reflect low energy, low frequency, poor continuity, and low impedance on seismic profiles. Seismic attribute analysis and fracture parameter inversion are used to predict fractures in volcanic rocks.

④Classified reservoir prediction. Based on the results of architecture dissection and lithology prediction, the rock mass-controlled seismic inversion is used to improve prediction accuracy and get 3D impedance and reservoir property data volumes. Then, seismic classification criteria of volcanic reservoirs are established, the classified net pays are extracted under the constraint of gas-water distribution, and accurate prediction of effective reservoirs is achieved.

(3)Well-location optimization

Volcanic gas reservoirs are characterized by different and variable reserve qualities, complex internal architectures, complicated gas-water distribution and nonuniform productivity distribution, which bring difficulties and challenges to well-location optimization. The well location should be selected according to the results of internal architecture characterization and effective reservoir prediction, the goals of well deployment in development stages, and the existing well pattern and available data.

①Appraisal wells. In the initial evaluation stage, only a few wells have been drilled and less information is known about the geology and productivity of volcanic gas reservoirs. The deployment of appraisal wells should be designed with consideration to structural parts, lithologies, lithofacies, fractures, reservoir types, and storage-seepage units, in order to effectively control and define the gas reservoirs.

②Development wells. In the development stage, the basic well pattern consisting of exploration wells, appraisal wells and development wells has been established, and the geology and productivity of reservoirs have been clearly known. The deployment of development wells should follow the principle of overall assessment, rolling development, and gradual improvement of well pattern under the given well pattern and well spacing. To be specific, development wells should be preferentially deployed in structural highs, favorable microstructures, favorable lithology/lithofacies belts, fracture-developed zones, and areas with high-quality reservoirs, large-scale storage-seepage units, and large gas layer thickness, in order to achieve a higher drilling success rate, a higher proportion of high-production wells, and build production capacity in time.

1.3.2.2 Well Production Enhancement and Development Optimization Techniques

(1)Well production enhancement techniques

All high-quality, low-quality, and tight volcanic gas reservoirs have strong heterogeneity, variable deliverability, and diverse reservoir types. Enhancing well production is the key to achieve efficient development of high-quality gas reservoirs, economic development of low-quality gas reservoirs, and effective development of tight gas reservoirs. There are two key techniques to enhance well production in volcanic gas reservoirs.

①Horizontal well drilling. The morphology and distribution of volcanic massifs, structure characteristics, distribution of storage-seepage units and fractures in volcanic gas reservoirs should, comprehensively be taken into consideration, to optimize horizontal well deployment, direction of horizontal interval, and spatial trajectory, so as to minimize relevant risks. In addition, analogy methods, gas reservoir engineering, numerical simulation, and other methods can be integrated to optimize horizontal interval length, perforation parameters, and other factors to make the horizontal wells fit into the distribution, morphology, superposition, and scale of storage-seepage units, so as to ultimately improve the reservoir drilling-ratio and reserve producing level [22].

②Hydraulic fracturing. Volcanic gas reservoirs are generally buried deeply underground with high-temperature, high-pressure, edge/bottom water, and variable reservoir properties, and they are difficult to be fractured hydraulically. Considering the morphology, scales and gas-water distribution of storage-seepage units and the fracture-initiation and propagation mechanisms, appropriate hydraulic fracturing techniques are used to enhance the production of wells in low-quality volcanic gas reservoirs with low permeability. In order to effectively produce tight volcanic gas reservoirs, horizontal wells with long horizontal sections and volume hydraulic fracturing techniques are used to realize the transformation from no production to commercial gas production and from low production to high production.

(2)Development optimization

Volcanic gas reservoirs are multiporous media with vesicles, dissolution pores, intergranular pores, and fractures of various sizes, rapidly variable lithology and reservoir properties, and strong heterogeneity. The well-controlled dynamic reserves and production are much different depending on the wells. Therefore, well productivity, well-controlled dynamic reserves, and development indexes/strategies should be optimized to improve development effects. For this purpose, four techniques can be applied.

①Productivity evaluation and production allocation techniques. Considering the characteristics of volcanic gas reservoirs, the production performance data from systematic well testing, isochronal well testing, modified isochronal well testing, and single steady point flowing tests in gas testing and pilot producing wells, are integrated to evaluate the productivity of wells. The well-productivity forecast models considering well types and storage-seepage patterns are used to forecast the productivity of new wells or drilled wells without gas testing. Then, according to the principles of production allocation optimization, well production is allocated by considering appropriate formation energy utilization, properly producing pressure difference, and stable production period, etc. Gas recovery rate is optimized to reasonably match with the seepage velocity of media, realize balance producing of different media, minimize residual gas saturation, maximize gas well productivity and improve development performance [23].

②Dynamic reservoir parameter characterization and well-controlled dynamic reserve evaluation. Depending on the media (e.g., pores, cavities, and fractures) in volcanic reservoirs, the dynamic descriptive models considering fluid-flow mechanisms (e.g., high-velocity non-Darcy flow, slippage effect, and stress sensitivity), and boundary conditions are adopted, together with analytical solutions, numerical solutions and typical chart methods, to calculate volcanic reservoir parameters, such as permeability, investigation radius, skin factor, cross flow coefficient, storage ratio, mobility ratio, storage factor ratio, fracture half-length, and other parameters. Meanwhile, based on the production data of gas wells, the well-controlled dynamic reserves are estimated for high-porosity and high-permeability volcanic gas reservoirs by pressure drawdown, pressure buildup and other conventional analysis methods, and for low-porosity and low-permeability volcanic gas reservoirs by Blasingame, A.G., Normalized Pressure Integral (NPI) and other modern production analysis methods [24].

③Volcanic gas reservoir numerical simulation. The structure, framework, property, and fluid models are used to reflect multilevel internal architectures, multiporous media reservoirs, and complex fluid distribution. Fluid flow models, suitable for the pore-cavity-fracture distribution in volcanic gas reservoirs, are chosen from dual-porosity and dual-permeability models, or dual-porosity and single-permeability models, or single-porosity and single-permeability models. Geological models are further modified on the basis of the production history match. Based on mechanism analysis and development strategy optimization, development plans are designed and compared, and finally the corresponding indexes of recommended plan are predicted [25].

④Development modes and development strategy optimization. Development modes are established depending on reservoir types with consideration to economic and technical limits. For high-quality reservoirs, the development mode of horizontal well, large well spacing, integral deployment with fewer wells and high well production is used. For low-quality reservoirs, the development mode of irregular well pattern and small well spacing, rolling deployment, vertical/horizontal well with hydraulic fracturing is adopted. For tight reservoirs, the unconventional technology of the horizontal well with a long horizontal section and a volume hydraulic fracturing mode is employed. The development strategies, in respect of gas recovery rate, reasonable production allocation, well-pattern and well-spacing optimization, productivity, and stable production period are defined for development modes. All of these are of great significance for the scientific and effective development of high-quality, low-quality, and tight volcanic gas reservoirs.

Volcanic gas reservoir development theories and techniques have developed rapidly and fruitfully in China since the beginning of the 21st century, but some aspects need to be improved.

(1)High-quality volcanic gas reservoirs have been developed satisfactorily, but they are facing major challenges such as water breakthrough and further enhancement of gas recovery. It is imperative to work out efficient theories and techniques in relation to four-dimensional dynamic description, gas reservoir development performance adjustment, comprehensive water control, and gas recovery enhancement.

(2)Low-quality and tight volcanic gas reservoirs, with low reserve producing levels and huge development potential, are confronting major challenges like low production, rapid decline, and poor development benefit. It is necessary to find out enrichment modes of micropores, fractures, and sweet-spots in low-permeability and tight gas reservoirs, develop microscopic reservoir characterization technology of microscale and nanoscale pores, fractures and throats, figure out gas reservoir nonlinear flow mechanisms, and develop lifecycle productivity forecast technologies.

(3)Innovative development modes should be figured out. Guided by the concepts of unconventional hydrocarbon development, the transformation of vertical well → conventional horizontal well → horizontal well with long horizontal section and no fracturing → conventional fracturing → volume fracturing is followed in order to realize the economic development of high-quality → low-quality → tight volcanic gas reservoirs.

1.4 Prospect of Volcanic Gas Reservoir Development

As a special kind of gas reservoir, the volcanic gas reservoir is a new field in natural gas development. Currently, volcanic gas reservoirs have been discovered in more than 300 sedimentary basins of more than 100 countries, suggesting wide distribution and good development prospects.

(1)Abundant volcanic gas resources in the world. Volcanic rocks spread extensively around the world, including the Pacific Rim, the Mediterranean and Central Asia, and in multiple systems such as Archean, Carboniferous, Permian, Cretaceous, and Paleogene. Moreover, volcanic rocks are important parts deposited earlier in sedimentary basins, accounting for 1/4 of the total basin volume [26]. These indicate the solid material foundation for volcanic gas reservoir development. So, volcanic gas reservoirs can be discovered and utilized with a big potential.

(2)Great development potential of volcanic gas reservoirs. As conventional natural gas resources decrease, volcanic gas reservoirs will become more attractive and will be effectively developed around the world with the promotion of relevant theories and techniques. Natural gas produced from volcanic rocks will certainly play a more important role in the world gas industry.

(3)Considerable development potential of the proved undeveloped volcanic gas reservoirs. Volcanic gas reservoirs contain abundant natural gas resources, but they are variable in quality (including high-quality, low-quality, and tight reservoirs). By far, high-quality volcanic gas reservoirs with medium-high permeability have been effectively developed, a small amount of low-quality volcanic gas reservoirs with relatively low-permeability have also been developed, but tight volcanic gas reservoirs have not been developed yet. Low-quality reservoirs and tight reservoirs have a considerable development potential, since they make up the majority in the total quantity of volcanic gas reserves.

References

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Chapter 2

Volcanic Rocks Architecture and Sequence

Abstract

Volcanic rocks originated from volcanism have special architecture, sequence stratigraphy characteristics, and thereby special internal framework and spatial distribution which are different from typical sedimentary rocks. This chapter first proposed the concept of volcanic architecture and volcanic stratigraphic sequence, established five levels of volcanic architectural model and stratigraphic sequence model, and further divided different levels of architecture. This chapter also clarified the logging and seismic response characteristics of volcanic architecture and stratigraphic sequence with different levels and types, brought out the correlation method of volcanic stratigraphic sequence, and established the concept and classification principles and methods for development strata in volcanic gas reservoirs which laid theoretical foundation for improving development results through the combination of strata development.

Keywords

Volcanic architecture; Volcanic stratigraphic sequence; Volcanic facies model; Seepage-storage unit; Volcanic sequence comparison; Sequence series classification in volcanic reservoirs

Chapter Outline

2.1Volcanic Architecture

2.1.1Volcanic Architecture Concepts and Hierarchies

2.1.2Volcanic Formation

2.1.3Volcanic Edifice

2.1.4Volcanic Massif

2.1.5Volcanic Facies

2.1.6Volcanic Storage and Seepage Unit

2.2Volcanic Stratigraphic Sequence

2.2.1Volcanic Stratigraphic Sequence and Hierarchy Classification Concepts

2.2.2Volcanic Eruption Cycles

2.2.3Volcanic Eruption Periods

2.2.4Volcanic Eruption Rhythms

2.2.5Condensation Units

2.3Volcanic Stratigraphic Sequence Correlation and Layer Series Classification

2.3.1Methods of Volcanic Stratigraphic Sequence Correlation

2.3.2Volcanic Layer Series Classification

References

Further Reading

Volcanic architecture indicates the spatiotemporal distribution and the overlapping relationship of the genetic units of volcanic rocks. It controls the inner structure and whole framework of volcanic rocks. Volcanic architecture is characterized by multiple hierarchies, which can be classified into five levels, which are volcanic formation, volcanic edifice, volcanic massif, volcanic facies, and storage-seepage unit (from largest scale to smallest scale, respectively). The different hierarchical structural units have differences on type, shape, scale, distribution, and overlapping relationship. The volcanic stratigraphic sequence indicates the spatial distribution and volcanic time units overlapping relationship, which reflects the periodicity, cyclicity, and rhythmicity of volcanic eruption. The volcanic rocks sequence can be classified into five levels, which are volcanic formation, eruption cycle, eruption period, eruption rhythm, and condensing unit (from largest scale to smallest scale, respectively). The different hierarchies of volcanic sequences have differences on eruption laws, volcanic rock thickness, intermittent symbols, and contact relationship. By adopting the cycle correlation principles and hierarchical control, a method, which is different from the method used on sedimentary rocks, is developed.

2.1 Volcanic Architecture

The different hierarchy architectures reflect different morphologies, distribution, and the genetic unit scales. Controlled by orogenesis and regional tectonic movement, volcanic formation is a volcanic combination of similar origin and spatial continuity, which is distributed mainly near deep faults, especially at a two faults group junction. The volcanic edifice is formed through the same-source magma migrating through the same main volcanic conduit and arising volcanism. Single-center and single-origin eruptive volcanic massif in a volcanic area is a volcanic formation. The multicenter scale and multiorigin volcanic edifice is smaller than the volcanic formation scale. Volcanic massif is a volcanic rocks combination formed from a single continuous eruption inside of a volcanic edifice, which the scale is smaller than, or equal to, the volcanic massif scale. The storage-seepage unit is a storage and seepage body formed under a favorable volcanism and the effective diagenesis at the later stage. The primary storage-seepage unit is controlled by the volcanic facies and a secondary storage-seepage unit is controlled by the later-stage diagenesis.

2.1.1 Volcanic Architecture Concepts and Hierarchies

2.1.1.1 Volcanic Rocks Concepts

Volcanic rocks are the rocks formed by erupted magma in volcanism through condensation and consolidation, compaction, diagenesis, etc. Narrowly, volcanic rocks are also known as effusive rocks [1], whose generalized conception includes magmatic deposits in an effusive channel and at the near surface.

Igneous rocks are formed from the cooling and magma freezing, which is derived from fusion or rock spatial fusion from the mantle or earth crust. Volcanic rocks belong to igneous rocks. Formed from cooling and magma freezing, both volcanic rocks and igneous rocks have the same material source and a similar forming mechanism. However, there are some differences in the rocks and the lithology distribution. Igneous rocks are distributed near a volcanic conduit, under the near-surface area and above the ground. The rock types include effusive rock, subvolcanic rock, and intrusive rock; volcanic rocks are mainly distributed in a volcanic conduit and above the ground. The rock types include volcanic lava and volcanic clastic rock [2–8].

Magmatic rocks are formed from deep crust lava condensation as the lava ascends along the crust’s fractures. As a kind of magmatic rock [2,3], igneous rocks have the same material source and a similar forming mechanism with magmatic rocks. However, magmatic rocks are distributed in the space from the magma chamber to the volcanic conduit and above the ground. The magmatic rock type can be divided into effusive rock and intrusive rock. Further, intrusive rock can be divided into plutonite and hypabyssal rock. The plutonite coming out in lava is not included into igneous rock.

In the distant area from volcanic eruption, the volcanic rocks are transitionally or alternatively developed with sedimentary rocks. The dominating lithology is a variety of sedimentary volcanic rocks or volcanic sedimentary rocks.

2.1.1.2 Volcanic Architecture Concepts

The geological architecture or internal structure concept reflects the spatiotemporal relationship of different geological unit types. It has been pointed out that reservoir architecture [9], reflecting the reservoir hierarchical separability and heterogeneity, indicates reservoir hierarchical interfaces and hierarchical entities, in which hierarchical interfaces are those between hierarchical entities, and hierarchical entities are structural composing units. Stow et al. [9] defined reservoir inner structure as reflecting geological units and their spatiotemporal relationship; and Wu et al. [10] defined the Pearl River deep-water fan internal structure in the South China Sea as the geological features interrelation (three-dimensional (3D) geometry, lithology, physical properties, etc.), the spatial distribution and the forming sequence of the sedimentary bodies and inner units in the system.

Volcanic architecture is complex. Volcanic geologists, in the early days, made comprehensive and useful research in volcanic intergrowth and combination relationship. Based on previous research and to make sense of the volcanic structure heterogeneity, volcanic architecture is defined as the genetic relationship, the spatiotemporal relationship, geometry, and the different hierarchical scale of volcanic structures.

2.1.1.3 Volcanic Architecture Characteristics

Formed from volcanism, volcanic rocks are affected by the crater distribution, magmatic properties, eruption patterns, duration, the eruption intermittent time, etc. Thus, they have the architectural characteristics, which are different from the sedimentary rocks characteristics. They are:

(1)Multihierarchical characteristics. Regional, distributed volcanic rocks generally consist of single or multiple volcanic edifices. The volcanic edifice interior is formed by different origins the rock unit’s superimposition and time-space relationships. Referring to the volcanism cyclicity and rhythmicity, those rock units are further divided into different lithofacies units, condensing units, and storage-seepage units (S-S unit). Therefore, volcanic rocks exhibit obvious multihierarchical characteristics from lithofacies units, condensing units, S-S units to lithofacies, and then to the volcanic edifice and regional volcanic formation.

(2)Complexity of morphology and superimposition relationship, huge scale difference. The volcanic rocks formation is affected by the magma source, passage type, eruption pattern, paleo-topography, paleo-geomorphology, and transportation medium. The different positions and different periods volcanic rocks have affect the diverse rock-type characteristics, huge rock morphologies, and scale variation; the architectural structural units of different hierarchies are combined in terms of different superimposition patterns by basic structure units of different positions and different eruption periods. Therefore, they exhibit the complex characteristics of morphology and superimposition relationship.

(3)The destruction and transformation of various degrees complicate volcanic architecture. Reformative geological processes occurring after the volcanic rocks contain forming: ① Volcanic cone partial collapse near crater at the end of eruption. ② The earlier stage volcanic rock destruction by later volcanic activities. The effect will be bigger as the volcanic edifice migrates gradually. ③ The exposure of volcanic rocks to weathering and eroding. ④ Cleavage of tectonic movement. ⑤ Allochthonous deposit, etc. Those geological processes changed the early-stage volcanic rocks’ internal structure, destroyed their integrity, and thus complicated volcanic architecture.

2.1.1.4 Volcanic Architecture Hierarchy

Considering global volcanic rocks of different periods as objects, especially volcanic origin, components and temporal relationship, earlier volcanic geologists proposed some concepts to describe the volcanic architecture hierarchy, which are the combination, type, petrographic province, series, rock series, rock suite, formation, composite pluton, rock province, etc. Li et al. [1] classified volcanic rocks into four levels: (1) Volcanic rock combination, indicates volcanic rocks’ group which are separated by some sort of gap, with relevant origin, similar age, associated space, and similar properties. (2) Volcanic formation, indicates the volcanic combination which have relevant origins and similar properties but in a similar developing stage of different orogenic cycles, and probably in different spaces. (3) Volcanic series, indicates the volcanic