Natural Fractures in Core
A natural fracture is defined as any cored fracture that existed in a volume of rock prior to initiation of drilling. The natural fracture geometry in a given core can be simple or complex. A simple geometry may consist of only one pervasive fracture set of regularly spaced, open fractures of similar trend with no evidence of slip or pressure solution. This common fracture trend reflects a fracturing stress field that remained essentially unchanged through a given period of time. Conversely, a complex fracture signature composed of several such fracture sets may reflect the cumulative effect of different stress events that occurred during specific spans of geological time (Nickelsen, 1976; Nickelsen and Hough, 1967; Engelder and Geiser, 1980). For example, these diverse stress events may be related to initial basin development, superposition, or orogenic stresses and subsequent epeirogenic uplift
The geometrical relationships and relative growth chronology between the fracture sets developed during one or several regional stress episodes are made even more complex by the effect of previously existing fractures an do ther regional structures such as depositional basins, buried fault zones, folds, and regionally persistent anisotropies inherited from earlier depositional and tectonic events. These preexisting structures at a large scale can affect the regional stress field, giving rise to diverse fracture sets that commonly can be grouped into regional fracture domains, each with a unique fracture signature (Kulander et al., 1980;Pollard et al., 1988). Similarly, primary or secondary structures at a local scale can affect later stress states to
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The characterization of naturally fractured reservoirs should include core analyses that encompass interpretation of natural and induced fractures. Unfortunately, to date, the differentiation of induced fractures from natural ones in core has been somewhat speculative and often is based on improper techniques. Consequently, bad interpretations have been made and useful information contained in both natural and induced fractures is overlooked. This book addresses the problem of distinguishing natural fractures from induced fractures in both oriented and unoriented core. Natural fractures include any cored fracture that existed in a volume of rock prior to initiation of drilling or coring-related stresses. Induced fractures in core are those that develop during drilling, coring, and subsequent handling. Many of the procedures for distinguishing between the two are based primarily on recognition of fracture surface structures and fracture traces that differ between natural fractures and induced fractures.