Certain parts of this chapter have been taken directly from the publication Important geological properties of unconventional resource shales, by Roger M. Slatt, published in the fourth-quarter issue of the Central European Journal of Geosciences (2011). The journal’s permission to reproduce those parts of that paper here is gratefully acknowledged.
Reservoir characterization as a discipline grew out of the recognition that more oil and gas could be extracted from reservoirs if the geology of the reservoir was understood. Prior to that awakening, reservoir development and production were the realm of the petroleum engineer. In fact, geologists of that time would have felt slighted if asked by corporate management to move from an exciting exploration assignment to a more mundane assignment working with an engineer to improve a reservoir’s performance. Slowly, reservoir characterization came into its own as a quantitative, multidisciplinary endeavor requiring a vast array of skills and knowledge sets. Perhaps the biggest attractor to becoming a reservoir geologist was the advent of fast computing, followed by visualization programs and theaters, all of which allow young geoscientists to practice their computing skills in a highly technical work environment. Also, the discipline grew in parallel with the evolution of data integration and the advent of asset teams in the petroleum industry. Finally, reservoir characterization flourished with the quantum improvements that have occurred in geophysical acquisition and processing techniques and that allow geophysicists to image internal reservoir complexities. Practical resource describing different types of sandstone and shale reservoirs Case histories of reservoir studies for easy comparison Applications of standard, new, and emerging technologies
Shallow marine environments, from the shoreline to the shelf edge, are complex and result in complex deposits. In turn, complex deposits translate into complex reservoirs. To maximize reservoir performance, it is imperative that we understand the type of shallow marine deposit that makes up the reservoir. That is not an easy task, as is exemplified by the various interpretations that have been assigned to linear sandstones of the U.S. Cretaceous Western Interior Seaway. These sandstones, in both outcrop and subsurface reservoirs, have been interpreted to be offshore shelf bars or ridges, shoreface bodies, and incised valley fill. Interpreting the type of deposit is not merely an academic exercise, it is essential because each of these different types of sandstone bodies is characterized by different geometries and degrees of compartmentalization. There are numerous examples of shoreface deposits that are truncated by younger incised valley fill. Subtle variations in gamma-ray log response can be used to identify such strata. Barrier-island deposits provide a particularly challenging reservoir characterization problem. Because of the variety of sedimentary processes that can influence barrier-island formation, several different sandstone and shale geometries and trends can occur. That variation in geometries can lead to the potential for a high degree of compartmentalization that is difficult to predict. Again, depositional-geometry prediction and well placement are facilitated by an understanding of the nature of the deposit and how it was formed.
This chapter has summarized the concepts, techniques, and definitions of sequence stratigraphy. As in most subdivisions of geology, sequence stratigraphers have developed their own set of definitions and terminology, which have been outlined here for use in subsequent chapters. It is proposed that sequence stratigraphy form the basis for reservoir characterization, as will be expanded upon in subsequent chapters.
There are many tools and techniques for characterizing oil and gas reservoirs. Seismic-reflection techniques include conventional 2D and 3D seismic, 4D time-lapse seismic, multicomponent seismic, crosswell seismic, seismic inversion, and seismic attribute analysis, all designed to enhance stratigraphy/structure detection, resolution, and characterization. These techniques are constantly being improved. Drilling and coring a well provides the “ground truth” for seismic interpretation. Rock formations are directly sampled by cuttings and by core and indirectly characterized with a variety of conventional and specialized well logs. To maximize characterization and optimize production, many of these tools as possible should be employed. It is often less expensive to utilize a wide variety of tools that directly image or measure reservoir properties at different scales than to drill one or two dry holes.
Globally, deltas often contain major oil and gas reservoirs. The geometry, size, and internal architecture of deltas are functions of many variables related to the delta's mode of formation. A tripartite classification of deltas, into river-, wave-, and tide-dominated deltas, has been a standard for many years. However, even within each of these delta types, the distribution of properties can vary considerably depending on the delta’s depositional history and the relative influence of rivers, waves, and tides. With regard to reservoir performance and optimization, perhaps the most significant difference in delta properties is in orientation and continuity of sand (reservoir) and shale (barrier) trends. Reservoir quality also varies according to the facies within the delta. To maximize hydrocarbon production, it is not sufficient to merely classify the reservoir as a delta. A complete understanding of the characteristics and variations of an individual delta’s reservoir is required for proper well placement and reservoir management.
This chapter has summarized the important characteristics of deepwater deposits and reservoirs. These reservoirs are quite complex and variable. An understanding of the different architectural elements and their interrelations is critical to hydrocarbon recovery, because the elements exhibit different external geometries, sizes, spatial orientations, and internal sedimentary and stratigraphic features. Because of these differences, the volume of hydrocarbons and the anticipated recovery efficiency will vary by architectural element (). There are many new and awaiting opportunities for deepwater reservoirs both onshore and offshore. The US Gulf of Mexico and many other parts of the world are hot spots or emerging areas for exploration and development of vast resources of oil and gas (Fig. 11.93).
Reservoir quality controls the storage, distribution, and flow of fluids within a reservoir. Porosity and permeability are key parameters that are readily measured on rock samples and from well logs; with calibration, porosity can be mapped from 3D seismic surveys. If core material is obtained from a well and porosity and permeability measurements are made on the core, the values can be compared with porosity logs and a permeability log can be developed. Although “flow units” can be determined using a suite of geologic and petrophysical parameters, method uses only the three easily obtained wellbore parameters of porosity, permeability, and thickness to calculate flow units in terms of their capacity to store and transmit fluids within the reservoir. Three-dimensional flow-unit models of a reservoir can be used for reservoir fluid-flow and performance simulation. Flow units can be upscaled, as needed, to meet the requirements of computing time and capability. Capillary properties of a rock also affect the storage and flow of fluids through the rock. Capillary properties are routinely measured and used to determine fluid saturations, height of the oil column above the free water level, and maximum height of the column that can be retained by a reservoir topseal. These are very important parameters for characterizing a reservoir for development and management purposes. Values of porosity, permeability, and capillarity will vary not only according to the nature of rocks comprising a reservoir but also according to the way in which the values were obtained. Caution is the key to interpreting laboratory-derived data, and it is worth knowing just how and where on a rock sample the measurements were made prior to using them for reservoir characterization. Also, upscaling or averaging values such as Sw can provide misleading results, particularly in thin-bedded stratigraphic intervals. The greater the amount of upscaling, the less realistic the reservoir geologic model becomes!
The concept of long periods of time being required for reservoirs to assume their present form is difficult to grasp, particularly for those individuals who track daily oil and gas production from reservoirs. However, the lengthy formative processes for hydrocarbon reservoirs can be understood, and this understanding is important for proper knowledge of why a reservoir is configured the way it is. The geologic time scale is divided into a series of time intervals that are based on significant events in the geologic record. Various temporal names applied to rock units commonly are used and must be recognized by people studying reservoirs. For a simple example, a Cretaceous reservoir rock was not deposited at the same time as a Devonian reservoir rock. The time during which a rock formed is dated by two means: absolute dating and relative dating. Absolute dating refers to the analysis of radioactive components in a mineral (within a rock), which provides the age at which the mineral formed (solidified) in the rock. Such techniques are used mainly for igneous rocks that cool directly from magma, but some chemically precipitated minerals and cements in sedimentary rocks can be dated in this manner. More common to the study of sedimentary rocks is relative age dating, where the age of a particular rock is determined relative to its position within a stratigraphic succession. If sedimentary rocks are crosscut by datable igneous rocks, sometimes the absolute age range of deposition of the sedimentary rock can be determined. An analysis of microorganisms in sediments and sedimentary rocks can provide a useful means of establishing rock zonations (biozones) and sometimes for determining absolute age. Micropaleontology, biostratigraphy, and palynology are critical disciplines in the petroleum industry, for exploration and for reservoir characterization. In addition to providing a means for absolute dating of sedimentary rocks, high-resolution biostratigraphy and palynology can aid in (1) interpreting stratigraphic intervals and their ages on seismic reflection profiles, (2) correlating between-well stratigraphic and temporal relationships, (3) determining sedimentation rates, and (4) determining depositional environments and changes in environments over time. Walther’s law of succession of sedimentary facies is key to understanding the origin of sedimentary deposits and reservoirs. It is a fundamental principle that is the backbone of stratigraphy. Stratigraphic sequences, such as those that comprise reservoirs, exhibit systematic and somewhat predictable vertical stacking patterns that are explained by Walther’s law. By understanding the vertical stratigraphy of a reservoir, one can make improved interpretations of the lateral (dis)continuity of reservoir intervals.
In summary, physical, biogenic, and chemical sedimentary structures are important to many aspects of reservoir characterization and should be included in every characterization, whether the analyst is using cores, borehole-image logs, or an analog outcrop. Sedimentary structures provide important information about the depositional environment of the reservoir rock, and from that information, one can determine the extent and geometry of the reservoir, its trend, and any likely impediments to hydrocarbon production. Porosity and permeability and, in particular, fluid-flow paths are also affected and guided by how the sediment grains are arranged into specific structures. Finally, one should bear in mind that some sedimentary structures can produce misleading or erroneous well-log results.
There are different types of fluvial deposits and reservoirs. The two end-member depositional types are braided-river and fluvial-river deposits. A third type, incised valley fill, can contain either or both of these end members within the confines of the valley. In addition, fluvial deposits near the mouths of the valleys may become reworked by estuarine and tidal processes, which ultimately produce a different set of reservoir properties. The geometry, size, and reservoir characteristics of each fluvial type depend upon transportational, depositional, and postdepositional (diagenetic) processes that are controlled by several external variables, including geographic location, sediment source areas (provenance), climate, and degree of tectonic activity. Braided-river deposits tend to be relatively coarse-grained and consist of gravel and sand, with little to no mud. Because of this, the beds tend to be laterally continuous over much or all of the width of the braidplain, although the presence of some shale beds may disrupt the continuity locally. By contrast, meandering-river deposits tend to be finer-grained, more lenticular, and partially or completely encased in floodplain shales. Depending upon the deposit's degree and type of postdepositional compaction and cementation, its porosity and permeability can be quite variable. However, in general, braided-river facies are more porous and more permeable than are meandering-river facies. A typical sequence stratigraphic stacking pattern for fluvial deposits consists of a basal erosion surface, formed during a falling stage of relative sea level, upon which sits, from the base upward, a lower braided-river deposit (deposited during early turnaround in relative sea level), a floodplain–meandering-river system, and then lacustrine and/or estuarine/floodplain deposits of a transgressive systems tract, capped by highstand floodplain/meandering-river deposits. As a result of differences in properties, fluvial reservoirs can be expected to have quite varied performances. Any reservoir-management plan should include an evaluation of the type of fluvial reservoir and its characteristics. For example, waterflood sweep efficiency will be higher in a braided-river reservoir than in a meandering-river reservoir. Also, horizontal wells may be more efficient in a set of discontinuous meandering-river sandstones than in a more continuous and interconnected set of braided-river deposits. Seismic-reflection techniques, as well as well-log, core, and well-test analyses, all can be used to adequately define the type of fluvial reservoir and predict the recovery performance and efficiency of that reservoir.
The focus of this chapter has been on eolian reservoirs, with only a secondary emphasis on description of outcrops. That is because the unique, fine-scale stratification characteristics of eolian deposits that affect their reservoir performance have been very well documented from the reservoirs themselves. Because of the likelihood of stratigraphic compartmentalization and permeability anisotropy resulting from bounding surfaces, it is very important that eolian reservoirs be characterized in detail. In addition to the effects of bounding surfaces, variations in cementation within laminae of different grain sizes result in small-scale variations in porosity and permeability, which are difficult and expensive to measure and document. This fact further emphasizes the importance of detailed reservoir characterization.
A major reason for the lack of understanding of argillaceous rocks is that the key to understanding their history lies in their microfabric. Argillaceous Rock Atlas addresses this aspect by taking a systematic approach to the analysis of shale fabrics. This approach combines such techniques as scanning electron microscopy, x-radiography and thin-section petrography. Thus the book is amply illustrated with scanning electron and thin section photomicrographs and x-radiograms which show the salient features of shale at a variety of scales. A great portion of the book is devoted to case studies, supplemented by mineralogical and geochemical data, which demonstrate the utility of these techniques in the interpretation of depositional environments, diagenetic processes and possible economic significance of argillaceous rocks. Argillaceous Rock Atlas is a valuable and unique reference for students and researchers involved in the fields of sedimentology and stratigraphy, petrology, petroleum geology, hydrogeology and geochemistry.
Reservoir characterization as a discipline grew out of the recognition that more oil and gas could be extracted from reservoirs if the geology of the reservoir was understood. Prior to that awakening, reservoir development and production were the realm of the petroleum engineer. In fact, geologists of that time would have felt slighted if asked by corporate management to move from an exciting exploration assignment to a more mundane assignment working with an engineer to improve a reservoir’s performance. Slowly, reservoir characterization came into its own as a quantitative, multidisciplinary endeavor requiring a vast array of skills and knowledge sets. Perhaps the biggest attractor to becoming a reservoir geologist was the advent of fast computing, followed by visualization programs and theaters, all of which allow young geoscientists to practice their computing skills in a highly technical work environment. Also, the discipline grew in parallel with the evolution of data integration and the advent of asset teams in the petroleum industry. Finally, reservoir characterization flourished with the quantum improvements that have occurred in geophysical acquisition and processing techniques and that allow geophysicists to image internal reservoir complexities. Practical resource describing different types of sandstone and shale reservoirs Case histories of reservoir studies for easy comparison Applications of standard, new, and emerging technologies
Reservoir Engineering ebook Collection contains 7 of our best-selling titles, providing the ultimate reference for every reservoir engineer's library. Get access to over 5000 pages of reference material, at a fraction of the price of the hard-copy books. This CD contains the complete ebooks of the following 7 titles: Civan, Reservoir Formation Damage 2nd Edition, 9780750677387 FANCHI, Principles of Applied Reservoir Simulation 3rd Edition, 9780750679336 Chin, Quantitative Methods in Reservoir Engineering, 9780750675680 Dake, The Practice of Reservoir Engineering, 9780444506719 Ahmed, Reservoir Engineering Handbook 3rd Edition, 9780750679725 Ahmed, Advanced Reservoir Engineering, 9780750677332 Slatt , Stratigraphic reservoir characterization for petroleum geologists, geophysicists and engineers, 9780444528186 *Seven fully searchable titles on one CD providing instant access to the ULTIMATE library of engineering materials for professionals in the petroleum industry *5000 pages of practical and theoretical reservoir engineering information in one portable package. *Incredible value at a fraction of the cost of the print books
A major reason for the lack of understanding of argillaceous rocks is that the key to understanding their history lies in their microfabric. Argillaceous Rock Atlas addresses this aspect by taking a systematic approach to the analysis of shale fabrics. This approach combines such techniques as scanning electron microscopy, x-radiography and thin-section petrography. Thus the book is amply illustrated with scanning electron and thin section photomicrographs and x-radiograms which show the salient features of shale at a variety of scales. A great portion of the book is devoted to case studies, supplemented by mineralogical and geochemical data, which demonstrate the utility of these techniques in the interpretation of depositional environments, diagenetic processes and possible economic significance of argillaceous rocks. Argillaceous Rock Atlas is a valuable and unique reference for students and researchers involved in the fields of sedimentology and stratigraphy, petrology, petroleum geology, hydrogeology and geochemistry.
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