An invaluable reference that helps geologists recognize and differentiate the many types of natural fractures, induced fractures and artefacts found in cores
Atlas of Natural and Induced Fractures in Core offers a reference for the interpretation of natural and induced fractures in cores. The natural and induced fracture data contained in cores provides a wealth of information once they are recognized and properly interpreted. Written by two experts in the field, this resource provides a much-needed tool to help with the accurate interpretation of these cores.
The authorsinclude the information needed to identify different fracture types as well as the criteria for distinguishing between the types of fractures. The atlas shows how to recognize non-fracture artefacts in a core since many of them provide other types of useful information. In addition, the text's illustrated structures combined with their basic interpretations are designed to be primary building blocks of a complete fracture assessment and analysis. The authors show how to recognize and correctly interpret these building blocks to ensure that subsequent analyses, interpretations, and modeling efforts regarding fracture-controlled reservoir permeability are valid.
Presented in full color throughout, this comprehensive reference is written for geologists charged with interpreting fracture-controlled permeability systems in reservoirs as well as for students or other scientists who need to develop the skills to accurately interpret the natural and induced fractures in cores.
John C. Lorenz and Scott P. Cooper are Senior Geologists with FractureStudies LLC in Edgewood, New Mexico.
Purpose of the Atlas
We were once emailed a long list of questions, arranged with paragraph-sized spaces below each question for our detailed answers and including photos of specific cored fractures, from a student starting to work on fractured cores. The questions were both basic and important, and included queries such as: Are both the slabs and butt of the cores used in fracture studies? How do you distinguish extension from shear fractures? Should I record the induced fractures? The list illustrated some of the problems and uncertainties in understanding natural fractures in core; it also indicated that people charged with assessing fractures in core do not always know enough about fractures, or cores, to make valid assessments.
This atlas is a tool, intended to help geologists recognize, differentiate, and interpret different types of natural fractures, induced fractures, and artifacts found in cores. We hope that this atlas will provide a reference for cored fractures for the industry, one that enables geologists to recognize the differences in fracture types as well as the significantly different effects that the different types have on a reservoir. Moreover, we hope that it fills what we perceive to be a gap in the literature, in that many fractured-reservoir textbooks start fracture analyses with the assumption that a geologist can already recognize and differentiate the various fracture types in a data set. We sincerely hope that this volume complements the seminal works of Nelson (1985, 2001) and Kulander et al. (1990).
The default concept of fractures is that they are planar, open cracks in a formation, when in fact there are many types of fractures and the different types can have significant differences in planarity, roughness, aperture, length, spacing, interconnectedness, and height, all affecting permeability. Knowledge of whether a fracture system formed in extension or shear, whether the fractures are open or mineralized, whether they are dissolution-enhanced slots or slickensided shear planes, all play into understanding the effects of fractures on a reservoir.
Significant information on a fracture network and the associated in situ stress system can often be found in a core even though core is a relatively miniscule and one-dimensional sampling of a reservoir. Since core is expensive and the sample is small, it is incumbent on the geologist to maximize the amount of fracture information recovered from a core, through knowledgeable examination of the core and informed analysis of the data collected from it.
We hope that this book provides a means for identification of many of the different fracture types found in cores as well as criteria for distinguishing between them. Some fracture types are widespread and control the basic plumbing of a reservoir, others are local and have minimal effect on formation permeability. A few fracture types provide orientation references or useful information on the in situ stress system. We have also illustrated some of the non-fracture artifacts found in cores since many of them provide context as well as important information that can be used for natural fracture analyses.
Scale of Interest
This book illustrates fractures at the scale of four-inch diameter cores. The illustrations are clarified where necessary with close-up photos, but for the most part we have not illustrated fractures at either the millimeter scale that is important to fracture mechanics, or at the meter/outcrop scale that is important to the construction of fracture-controlled permeability networks. This atlas is restricted to illustrating individual fractures as viewed when logging core, and describes their potential as individual permeability pathways. These structures and their basic interpretations are the primary building blocks for a complete fracture assessment and analysis, so they must be properly identified and correctly interpreted if subsequent analyses, interpretations, and modeling efforts are to be valid. For example, shear fractures commonly form intersecting conjugate pairs whereas extension fractures commonly form as single, parallel sets, and the difference greatly influences drainage and well-to-well interference patterns in a reservoir.
Industry geologists have recently had fewer opportunities to participate in the on-site coring process due to changing techniques, liability issues, and the increasing use of service companies to retrieve and process cores. Fewer company geologists have the opportunity to be familiar with drilling operations or with coring and core processing procedures; thus they are often unfamiliar with the important ways in which such operations affect a core. Geologists rarely get to look at a core any more until it has been cut, cleaned, marked, plugged, slabbed, boxed, sampled, and laid out in the lab, by which time significant natural fracture information has been lost and additional fractures have been created in the core.
Once the geologist gains access to a core, a big gap looms between counting and understanding fractures. It is easy to count fractures and measure their dips and strikes, which provides a data base that can be readily analyzed statistically. But such analyses are meaningless if fractures are not fully understood and fully characterized before they are analyzed, since fractures are so much more than planar breaks in the rock.
Core samples typically consist of fresh exposures of the rock and therefore provide unweathered detail compared to outcrops. However, the ability to extrapolate beyond the core into the other two dimensions in a reservoir is limited; for example, it is difficult to derive the lateral spacing of vertical fractures from the data provided by a vertical core unless the restrictive assumptions of Narr's (1996) analysis are met. Likewise, fracture heights are difficult to assess in horizontal core. Nevertheless, with experience and carefully acquired data, one can construct conceptual and often even semi-quantitative models of the three-dimensional fracture distributions, dimensions, spacings, and interconnectivities from cores.
Several systems have been used in classifying natural fractures. Some systems are based on fracture geometry, some on fracture origin, some on their electrical properties, and some on the potential effects of a fracture on a reservoir. For example, Nelson (2001) offers several classification schemes based on origin (extension, tension, or shear), on a fracture's potential permeability (open fractures vs. filled fractures), or the structural associations of the fracture system (fault related, fold related, regional, etc.). In contrast, petrophysicists commonly classify fractures in image logs by their electric or acoustic properties (i.e., "conductive" or "resistive").
For this atlas, natural fractures are divided into two main categories based on origin, i.e., extension fractures vs. shear fractures, with subcategories and modifiers for postfracture alterations.
It is human nature to categorize and classify, but as often as not, we are artificially compartmentalizing samples that form parts of a spectrum rather than discovering and documenting natural divisions. Fracture categorization serves a purpose but in fact, fractures may grade from one category into another. For example, "hybrid shears" (Hancock, 1986; Hancock and Bevan, 1987) offer a bridge between extension and conjugate-shear fracture categories, and induced petal fractures morph into and blend with centerline fractures. Natural fractures can also be reactivated over geologic time intervals, leading to ambiguities in classification, i.e., fractures that formed in extension are sometimes reactivated in shear. Similarly, the distinction between faults and shear fractures would seem to be self-explanatory, but if the distinction is based on offset magnitude it is arbitrary since shear offsets occur within a continuous range.
Organization of the Atlas
This atlas is organized into three parts.
Part 1: Natural Fractures
This section describes the characteristics of extension and shear fractures in core, which is not without complications. Most extension fractures are vertical, but intermediate-angle and horizontal fractures are also found in some cores. Shear fractures for the most part can be subdivided into Anderson's (1951) three dip-angle categories, corresponding to high-angle strike-slip shears, intermediate-angle dip-slip shears, and low-angle reverse dip-slip shears, but shear fractures with oblique slip and bedding-parallel slip are common in cores cut from some structural settings. We have also included short descriptions of other, less common types of cored natural fractures such as ptygmatically folded fractures and deformation bands.
Part 2: Induced Fractures
The two most important units in this second section on fractures created by coring and handling processes describe petal fractures, which can take many different forms, and centerline fractures. These two induced fracture types are important because they can be used to orient both a core and the natural fractures it contains relative to the in situ stress field, and sometimes even relative to north. Other induced fracture sections include descriptions of fractures created by twisting the core, by bending the core, and...