The following sets of conditions indicate that associated fractures in core may have been induced naturally.
Polished and slickenlined fracture faces in well-lithified rock usually are a clear indicator of natural fracture origin (Figure 6a). However, polished surfaces, perpendicular to the core axis and accompanied by curved slickenlines, can be induced during the coring process (Figure 6b).
Marker bed offsets along faults that have not lost cohesion indicate a natural fracture even though the fault surface in this case is not visible (Figure 5). Such faults may have developed in soft sediments prior to lithification.
Secondary mineral growths on the walls of fractures clearly indicate that the fractures were naturally induced (Figures 1 and 2).
Scribe-grooved fractures and conchoidal chips that originate at the scribe knife and that curve (hook) abruptly into a nearby fracture indicate a natural origin. However, drilling-induced fractures that originate before the bit, or any fracture that developed before the scribe, can also show this scribe-chip relationship (Figure 24b).
Natural fracture faces that are parallel or subparallel to the core axis may be cut by small conchoidal chips at the fracture-core margin intersection. These chips are located preferentially along the right-hand edge of the fracture face when the face is held toward the observer with the downcore direction pointing downward. The chips are formed by the plucking action of the drill bit rotating clockwise. Drilling-induced fractures originating before the bit may also show chipped right-hand margins (Figure 29).
Figures & Tables
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.