Natural Fractures
Here’s some rules I follow when identifying natural fractures (i.e. not induced by coring or core handling) in the core store.
Top of the list is fracture cement i.e., minerals that have been precipitated out from fluids travelling through the fracture - Fig. 1.
Deformation bands which may look like veins (cement) but are made up of crushed sand grains - Fig 2. This includes other fault rock processes like clay smear.
Displacement /cross-cutting of beds or older fractures - Fig 2.
Slickenlines/polished surfaces on fracture faces - these are scratches from the shear movement of opposing walls and are therefore indicators of natural shear fractures - Fig. 3.
http://www.ogilviegeoscience.co.uk/blog/2018/6/9/importance-of-slickenlines
Fractures are confined to beds/layers or certain lithologies i.e., mechanical stratigraphy.
Stylolites form separations within the matrix - consisting of chemical residues due to P/T reactions - Fig. 4.
Enclosed within the core - both ends of the fracture terminate within the core.
Figure 1. Pyrite cemented fractures in sandstone, West of Shetland, UK.
Figure 2. Deformation bands in Rotliegende sandstones from Southern North Sea. These small fault-like structures are particularly pervasive on the right hand core stick which is close to a seismic scale fault. Note bedding displacement as shear indicator throughout the core.
Figure 3. Slickenlines/polished surface in mudstone core, North Sea, UK. http://www.ogilviegeoscience.co.uk/blog/2018/6/9/importance-of-slickenlines
Figure 4. Stylolites in limestones, Zagros, Iran.
Induced Fractures
Induced fractures are often called hydraulic fractures (Fig. 5a) if the intention is to create fractures to stimulate production - otherwise they are just induced fractures e.g., through careless core handling. Please see refs [1 - 2] for a detailed discussion.
In addition to not having the above features, induced fractures [1] tend to be
Irregular or conchoidal [2] particularly in fine grained rocks.
Centerline fractures - during drilling - an extension fracture propagated in front of the coring/drilling bit due to the weight of the drill string. The fracture often appears in the centre of the core hence the name [1]
Petal fractures [1,2] - due to frictional stick-slip between core and core bit - well spaced fractures either side of the core - they can intersect from opposite sides of the core creating unusual shapes - the fractures often propagate down the core as centerline fractures - Fig 5b.
Bedding parallel fractures - more of these get created with increased handling [3]. Flexing of core boxes/rough transportation also a factor especially in shale core [2] - Fig 5c
Helical/spiral fractures due to core breakage during coring - due to twisting or torque applied to the core from the core bit or core barrel [2].
Desiccation fractures due to core drying, in shale cores - Fig 5c (right)
Rubble - often through poor core handling/transportation - BUT often rock gets broken up because the well has been drilled through a geological fault zone - those in Fig 6 are natural (hairline) fractures (from a fault zone) that surround pods of mudstone- the pods have become dislodged during coring.
Figure 5. Induced fractures in core (a) hydraulic fractures with large apertures, parallel to core axis (b) petal fractures, (c) bed parallel fractures in shale with further fractures created due to drying in right hand stick.
Figure 6. Rubble zone in mudstone (N Sea) where well has penetrated a seismic scale fault. The rubble has been created during drilling which has broken up pods of mudstone surrounded by natural fractures.
References
[1] Kulander, B.R., Dean, S.J., Ward, B.J. 1990. Fractured core analysis. Interpretation, Logging, and Use of Natural and Induced Fractures in Core. AAPG Methods in Exploration Series, No. 8.
[2] Determining Natural Vs Induced Fractures (p230) in - Nelson, R.A. Geological Analysis of Naturally Fractured Resevoirs. Determining Natural Vs Induced Fractures (p230) in Nelson,
[3] Reiss, L.H. 1980. The Reservoir Engineering Aspects of Fractured Formations. Gulf Publishing Co. 107 pp.