Ahr W1 H. Geologyof Carbonate Reservoirs

Ahr W1 H. Geologyof Carbonate Reservoirs

(Parte 2 de 8)

6.1 Diagenesis and Diagenetic Processes / 144 6.1.1 Defi nition of Diagenesis / 145 6.1.2 Diagenetic Processes / 146 viii CONTENTS

6.2 Diagenetic Porosity / 150 6.3 Diagenetic Environments and Facies / 153 6.3.1 Diagenetic Facies / 155 6.4 Diagenetically Enhanced Porosity / 156 6.4.1 Enhancement by Recrystallization / 158 6.4.2 Enhancement by Solution Enlargement / 160 6.4.3 Large-Scale Dissolution-Related Porosity / 161 6.4.4 Porosity Enhancement by Replacement / 163 6.4.5 Recognizing Enhanced Porosity / 163 6.5 Porosity Reduction by Diagenesis / 164 6.5.1 Pore Reduction by Compaction / 165 6.5.2 Pore Reduction by Recrystallization / 165 6.5.3 Pore Reduction by Replacement / 166 6.5.4 Pore Reduction by Cementation / 167 6.5.5 Recognizing Diagenetically Reduced Porosity / 170 6.6 Diagnosing and Mapping Diagenetic Reservoirs / 171

Suggestions for Further Reading / 174 Review Questions / 175


7.1 Fractures and Fractured Reservoirs / 176 7.1.1 Defi nition of Fractures / 177 7.1.2 Types of Fractures / 177 7.1.3 Genetic Classifi cation of Fractures / 178 7.1.4 Fracture Morphology / 181 7.1.5 Where Do Fractures Occur? / 184

7.2 Fracture Permeability, Porosity, and Sw / 186 7.2.1 Fracture Permeability / 187

7.2.2 Fracture Porosity / 188

7.2.3 Sw in Fractured Reservoirs / 189 7.3 Classifi cation of Fractured Reservoirs / 190

7.4 Detecting Fractured Reservoirs / 191 7.4.1 Direct Observation of Fractures in the Borehole / 192

7.4.2 Indirect Methods to Detect Fractures in the Borehole / 192

7.5 Predicting Reservoir Fracture Spacing and Intensity / 195

7.5.1 Factors that Infl uence Fracture Spacing and Intensity / 195

7.6 Identifying and Developing Fractured Reservoirs / 195

Suggestions for Further Reading / 198 Review Questions / 198



8.1 Rock Properties and Diagnostic Methods / 201

8.1.1 Fundamental Rock Properties and Depositional Reservoirs / 202

8.1.2 Reservoir Morphology / 203 8.1.3 Derived Properties: Porosity and Permeability / 204

8.1.4 Tertiary Properties and Petrophysical Characteristics / 204

8.2 Data Requirements / 206 8.2.1 Regional Scale Investigations / 207 8.2.2 Field Scale Studies / 207 8.2.3 Quality Ranking of Flow Units / 208 8.2.4 Pore Scale Features / 209 8.3 Depositional Reservoirs / 209 8.3.1 Finding and Interpreting Depositional Reservoirs / 210 8.3.2 Selected Examples of Depositional Reservoirs / 213 Haynesville Field / 214 Field / 219 8.4 Diagenetic Reservoirs / 224 8.4.1 Finding and Interpreting Diagenetic Reservoirs / 224 8.4.2 Field Examples of Diagenetic Reservoirs / 226 Overton FIeld / 227 Happy Field / 231 8.5 Fractured Reservoirs / 239 8.5.1 Finding and Interpreting Fractured Reservoirs / 239 8.5.2 Field Examples of Fractured Reservoirs / 240 City Field / 241 Field / 244 8.6 Conclusions / 249 Review Questions / 254


This is a book on the geology of hydrocarbon reservoirs in carbonate rocks. Although it is written for petroleum geologists, geophysicists, and engineers, it can be useful as a reference for hydrogeologists and environmental geologists because reservoirs and aquifers differ only in the fl uids they contain. Environmental geoscientists interested in contaminant transport or hazardous waste disposal also need to know about porosity (capacity to store) and permeability (capacity to fl ow) of subsurface formations. The fi rst two chapters focus on defi nitions and on rock properties that infl uence fl uid movement. The third chapter focuses on reservoir properties — the interaction between rocks and fl uids — and how rock properties infl uence saturation, wettability, capillarity, capillary pressure, and reservoir “ quality. ” Although carbonate rocks differ in many ways from siliciclastic rocks, the laws of physics that govern fl uid movement in terrigenous sandstones also govern fl uid behavior in carbonates; therefore many of the principles discussed in this text are applicable to reservoirs and aquifers in any porous and permeable rock. There are fundamental differences between carbonates and siliciclastic rocks that will be emphasized thoughout, and knowing those differences can be used to advantage in exploration, development, and management of reservoirs and aquifers.

This book evolved from my graduate course on carbonate reservoirs at Texas

A & M University. It is written as a textbook for geologists, engineers, and geophysicists in graduate and upper - level undergraduate courses. I hope it may also be useful for continuing education courses and as a reference book for industry professionals, especially for those who are not experts on carbonate rocks and reservoirs. It is not easy to write a survey of this subject in about 300 pages with a limited number of illustrations; consequently, this book emphasizes only fundamental principles. The vast literature on carbonate sedimentology, stratigraphy, geochemistry, and petrography makes it impractical if not impossible to include an extensive bibliography on all of those subjects. I did not include much material on borehole logging and seismology because they require lengthy explanations with examples that exceed xii PREFACE the scope, purpose, and size limits of this book. I have tried to address these potential shortcomings by including suggestions for additional reading at the end of each chapter. This is a book for students — not for experts. Having taught university classes and continuing education courses for the past 38 years, I have learned that there are limits to what can be taught effectively in one university term or in a continuing education short course. I limited the material in the book to that which I believe can be taught in one university term or an intense, week - long short course. Clearly, I had to choose subjects and reference material carefully, focusing on the subjects I have found most helpful in understanding carbonate rocks and reservoirs. Other texts on carbonate reservoirs, including those by Chilingar et al. (1992) , Lucia (1999) , and Moore (2001) , concentrate on engineering aspects of carbonate reservoirs (Chilingar and Lucia) or on sequence stratigraphy as it relates to carbonates (Moore), but they are not textbooks on the general geology of carbonate reservoirs. I have written this book to help university students and industry professionals learn more about how, when, why, and where carbonate reservoirs form, and about how to recognize, analyze, and map the end - member reservoir types in carbonates — reservoirs with depositional, diagenetic, or fracture porosity systems. Special emphasis is given to relationships between genetic pore types and carbonate reservoir properties. To that end, a new classifi cation of carbonate porosity that focuses on the genetic pore types is presented. Two themes are repeated throughout the book: (1) it is not possible to understand carbonate reservoirs without looking at the rocks; and (2) one cannot accurately predict the spatial distribution of rock and reservoir properties that are linked by cause – effect mechanisms without using a genetic classifi cation of carbonate porosity.

Development geologists and engineers will fi nd the book useful, as will exploration geophysicists and geologists. Development geologists and engineers will fi nd the book helpful because it emphasizes the relationships between rock and reservoir characteristics. Explorationists should fi nd the distinction between genetic pore types in carbonate reservoirs helpful because exploration strategies need to be built around geological concepts that are in turn based on knowledge of how and where porosity and permeability may occur together in depositional and diagenetic facies or in fractured rocks.

There is a tradition among petroleum geologists to search for analogs or “ look - alikes ” for exploration or production prospects. This noncritical application of geological form over critically analyzed substance presumes that reservoir models can be exported from one geological age and setting to another with little concern about possible differences in reservoir characteristics. All too often, geologists fi nd themselves having to explain why the “ look - alike ” failed to predict depositional or diagenetic porosity loss, or why structural and stratigraphic models for exploration prospects did not turn out to be realistic after the drill reached target depth. Analogs offer comfortable “ sameness ” but they provide no help to explain the unexpected. They lack information to fi nd hydrocarbon reservoirs in the wide variety of geological situations that typify carbonates and they lack information needed to develop carbonate reservoirs in the most effi cient and profi table ways.

This book emphasizes ways to formulate geological concepts rather than “ look - alikes ” to predict the spatial distribution of porosity and permeability. Optimum combinations of porosity and permeability and least resistance to fl uid fl ow are called fl ow units (the origins of this term are discussed later). When fl ow units are


estimated 891 billion barrels or more (Ahlbrandt et al., 2005 )If unconventional

identifi ed and ranked on their rock and reservoir properties, accurate maps, volumetric calculations, and economic forecasts can be made. Primary recovery methods have produced only about one - third of the world ’ s original oil in place, leaving an sources of oil and natural gas are included, the fi gure will be even larger. If reservoir fl ow units could be mapped with a higher degree of precision than was available previously, then a signifi cant percentage of those 1 trillion barrels of remaining oil could be within reach with novel methods of improved recovery. Knowing the size, shape, and connectivity of fl ow units, secondary and tertiary recovery methods are economically attractive, especially at current oil prices. This rings especially true when one considers the extreme cost of deep water drilling and production, the risk of geopolitical confl icts, and the risk of drilling dry holes as compared to extracting bypassed hydrocarbons from proven fi elds. Also importantly, if hydrogeologists have accurate maps of aquifer connectivity, their models for groundwater fl ow or contaminant transport pathways will be greatly improved. If fl ow barriers were more accurately mapped, site evaluation for dangerous waste disposal could be improved signifi cantly. These are only a few of the exciting reasons to learn more about carbonate reservoirs and aquifers.

I would not have embarked on this project without the encouragement of the graduate students who have taken my course on carbonate reservoirs over past years and who have continually asked me to write a book for the course. Old friend Robert Stanton read some of the early chapters and offered helpful comments. Rick Major and P. M. (Mitch) Harris read early versions of the entire manuscript and gave encouragement, guidance that kept me on track, and criticisms that greatly improved the book.

Wayne M. Ahr

College Station, Texas December 2007

To understand carbonate rocks at reservoir scale, one fi rst has to understand them

ABOUT THIS BOOK at pore scale. Carbonate reservoirs are porous and permeable rocks that contain hydrocarbons. Carbonate porosity includes three end - member genetic categories: purely depositional pores, purely diagenetic pores, and purely fracture pores. Intermediate types exist, of course, but the point is that there are three main types of carbonate porosity that represent distinctly different geological processes. Before one can fully appreciate these differences and be profi cient at distinguishing between the varieties of carbonate reservoir types, one must understand what carbonates are, how and where they form, and how they become reservoirs. One must understand the differences between reservoirs, traps, and seals and learn to appreciate that reservoir characterization is the study of rocksplus the fl uids they contain. The operative word is rocks. Carbonate rocks consist of component particles and maybe some lime mud matrix and cement. The skeletal and nonskeletal particles, along with mud and cement, hold an enormous amount of information about the depositional and diagenetic environments that produced the reservoir rock. This book begins with defi nitions, with discussions about how, where, and why carbonates are formed and about how fundamental rock properties are used to create a language for communicating information about the rocks — carbonate rock classifi cations. Reservoir porosity and permeability are variables that depend on fundamental rock properties. The book explores how rock classifi cations do or do not correspond with conventional porosity classifi cations. Reservoirs contain fl uids; therefore we explore reservoir properties such as saturation, wettability, capillarity, and capillary pressure.

Geophysical (borehole) logs are briefl y mentioned because they provide information about third - order rock properties. Logs provide important information to develop static and dynamic reservoir models, to calculate fl uid properties such as saturation and movable oil volumes, to make stratigraphic correlations, and to interpret lithological characteristics in boreholes where no rock samples are available.


Logs are only briefl y mentioned because an extensive literature on logs and log interpretation already exists. Today ’ s digital technology and sophisticated computer software have expanded the need for petrophysicists who specialize in computer - assisted log interpretation. Even with the modern computer - assisted log evaluation software available in almost every company and university laboratories, the working geoscientists still must be familiar with the types of logs that are useful in studying carbonate reservoirs.

Seismic methods for exploration and development are mentioned only briefl y because a satisfactory treatment of seismological methods in exploration and reservoir analysis is beyond the scope of this book. Selected references are given at the end of each chapter to help the reader fi nd more information.

Following the discussions on the hierarchical order of rock properties and the different reservoir characteristics, basic sedimentological and stratigraphic principles are reviewed to explain carbonate platform characteristics, stratigraphic relationships, and depositional facies. This background is intended to guide the reader into depositional models and greatly simplifi ed, “ standard depositional successions ” that characterize different platform types. The standard depositional successions will become models for depositional reservoirs — reservoir rock bodies with depositional porosity. Following the discussions of depositional models and depositional reservoir types, diagenetic environments and diagenetic processes are introduced to illustrate how carbonate reservoir porosity is enhanced, reduced, or created by the chemical and mechanical processes that typify each diagenetic environment. Finally, fractured reservoirs are reviewed after the reader has a thorough grasp of rock and reservoir properties, along with of depositional and diagenetic processes and attributes. Checklists for the diagnosis and interpretation of depositional, diagenetic, and fractured reservoirs are given at the end of each of the respective chapters. A summary of the topics covered in the book and selected fi eld examples of depositional, diagenetic, and fractured reservoirs round out the fi nal chapter.

Geology of Carbonate Reservoirs: The Identifi cation, Description, and Characterization of Hydrocarbon Reservoirs in Carbonate Rocks By Wayne M. Ahr Copyright © 2008 John Wiley & Sons, Inc.

The goal of this book is to explain in plain language for the nonspecialist how and where carbonate rocks form, how they do, or do not, become reservoirs, how to explore for carbonate reservoirs or aquifers in the subsurface, and how to develop them once they have been found. The book is organized around a genetic classifi cation of carbonate porosity and ways it can be employed in exploration and development. The genetic categories include three end members — depositional pores, diagenetic pores, and fractures. Genetic pore categories are linked with geological processes that created, reduced, or enlarged pores during lithifi cation and burial. In the end, a chronology of pore origin and evolution is developed to put in the larger stratigraphic context for identifi cation of reservoir fl ow units, baffl es, and barriers. Connectivity can be evaluated by determining the range of porosity and permeability values for the different pore categories within reservoirs. Connected pore systems can be correlated stratigraphically to identify reservoir zones that have the highest combined porosity and permeability and the least resistance to the passage of fl uids. Such zones are defi ned in this book as reservoir fl ow units somewhat similar to the defi nition of Ebanks ( 1987 ; Ebanks et al., 1992 ) but different in that rock units that impede fl ow are defi ned as baffl es and units that prevent fl ow are defi ned as barriers. Each end - member reservoir type generally has characteristic pore - scale features (porosity and permeability) that correspond to petrologic and stratigraphic properties (borehole - scale features). When the zones with good, fair, and poor connectivity are identifi ed, the characteristic petrologic and stratigraphic features that correspond with them can becomeproxies for connectivity . The larger scale features, or proxies, are generally easier to identify in borehole cores, on wireline log traces, and in some sequence stratigraphic “ stacking patterns. ” When mode and time of origin of the proxies are known, geological concepts can be formulated to predict the

2 INTRODUCTION spatial distribution of reservoir fl ow units at fi eld scale. In other words, the fundamental rock properties that correspond to good, fair, and poor combined values of porosity and permeability can be identifi ed and put in larger stratigraphic context, or “ scaled - up. ” Then the temporal and genetic characteristics of the large - scale petrologic and stratigraphic properties (proxies) are used for reservoir prediction and fl ow unit mapping.

Carbonate reservoir porosity usually represents the combined effects of more than one geological process. Sometimes it refl ects multiple episodes of change during burial history; therefore particular care must be given to identifi cation of the sequence of events that led to the fi nal array of rock properties and pore characteristics. Usually it is possible to identifycross - cutting relationships between rock properties so that their relative times of origin are distinguishable. Reservoir porosity governed only by depositional rock properties, a rather uncommon occurrence, will not exhibit cross - cutting relationships because rock texture, fabric, porosity, and permeability share a single mode and time of origin. In that case, reservoir architecture and spatial distribution conform to depositional facies boundaries. These reservoirs are referred to asstratabound , and porosity is facies - selective, fabric - selective , or both. Diagenesis and fracturing do not always follow depositional unit boundaries. Although carbonate reservoirs exist in which diagenetic porosity corresponds with depositional rock properties (fabric - selective or facies - selective diagenesis), in many instances it does not. In the latter case, it is especially important to identify the type of alteration, how it was formed, when it was formed, and what cross - cutting relationships it shares with other diagenetic and fracture attributes. Fractures cut across most rock boundaries but there are some fundamental rock properties that dictate how and where fractures will form. Fractures happen as a result of brittle failure under differential stress, usually in conjunction with faulting or folding. Fault and fold geometry can be determined; therefore it follows that associated fracture patterns can also be determined. In short, there are many rock and petrophysical characteristics in carbonates that expose a wealth of information about the origin and architecture of carbonate reservoirs.


1.1.1 Carbonates

Carbonates are anionic complexes of (CO 3 ) 2− and divalent metallic cations such as Ca, Mg, Fe, Mn, Zn, Ba, Sr, and Cu, along with a few less common others. The bond between the metallic cation and the carbonate group is not as strong as the internal bonds in the CO 3 structure, which in turn are not as strong as the covalent bond in carbon dioxide (CO 2 ). In the presence of hydrogen ions, the carbonate group breaks down to produce CO 2 and water. This breakdown reaction, commonly experienced when acid is placed on limestone, is the chemical basis for the fi z test that distin- guishes carbonates from noncarbonates. It is also used to distinguish dolostones, which fi z slowly, from limestones, which fi z rapidly. Carbonates occur naturally as sediments and reefs in modern tropical and temperate oceans, as ancient rocks, and as economically important mineral deposits. The common carbonates are grouped into families on the basis of their crystal lattice structure, or the internal arrange- ment of atoms. The families are known by the crystal systems in which they form, namely, the hexagonal, orthorhombic, and monoclinic crystallographic systems. The most common carbonate minerals are in the hexagonal system, notably calcite

(CaCO 3 ) and dolomite (Ca,Mg(CO 3 ) 2 ) (Figures 1.1 and 1.2 ). Aragonite has the same composition as calcite, CaCO 3 , but it crystallizes in the orthorhombic system. The monoclinic system is characterized by the beautiful blue and green copper carbonates — azurite and malachite, respectively. Calcite and aragonite are polymorphs of calcium carbonate because they share the same composition but have different crystal structures. Dolomite, like calcite, crystallizes in the hexagonal

Figure 1.1Internal atomic (lattice) structure of calcite. The ball - and - stick model at the top

of the fi gure shows the position and orientation of calcium and carbonate ions in layers, or sheets, within the lattice. Note that the orientation of the triangular carbonate ions changes in alternate layers from top to bottom. The bottom drawing shows the hexagonal crystal structure of calcite, the scalenohedral calcite unit cell, and the position of cleavage rhombs with respect to the c crystallographic axes. (Adapted from illustrations in Hurlbut and Klein

Carbon Calcium Oxygen α = 46˚ 07’ α = 101˚ 5’ c c


(Parte 2 de 8)