Example of a Logged Natural Fracture Zone
1990. "Example of a Logged Natural Fracture Zone", Fractured Core Analysis: Interpretation, Logging, and Use of Natural and Induced Fractures in Core, B. R. Kulander, S. L. Dean, B. J. Ward, Jr.
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One particular 23-ft (7-m) section of core provides excellent examples of characteristics commonly used to distinguish natural fractures from those that are drilling- or coring-induced (Figure 56 and Enclosures 1 and 2). The continuous section penetrated a fracture zone containing at least eight parallel natural fractures, each with a discrete plume. One drilling-induced fracture was observed. The overlapping natural fractures within the zone are closely spaced (1 cm or less), and most fractures show upper or lower boundaries that overlap and hook into adjacent overlying or underlying fractures within the core. The connected fractures, each the result of a single fracture event, link together to form two large composite fracture faces within the core (Figure 56). The composite pattern is similar to that developed by overlapping cooling joints (DeGraff and Aydin, 1987). Finley and Lorenz (1988) classify a similar pattern at smaller scale in sandstones of the Mesaverde Formation as vertical extension fractures. However, they describe the fracture traces as en echelon because each seg-ment did not appear to hook into its neighbor in the two-dimensional view afforded by the core surface. Cored vertical fractures probably do hook into each other in the third dimension, as indicated by outcrop studies of the Mesaverde Formation (Lorenz et al., 1989). Lorenz et al. (1989) propose a vertical connection, whereas our example here documents a horizontal link.
Such fracture connection increases the vertical conductivity of the planar fracture openings. Additional fractures that were not cored may also exist within the zone. Enclosures 1
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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.