natural or induced fractures ?

How to identify natural fractures in rock core.

Here’s some rules I follow when identifying natural fractures (i.e. not induced by coring or core handling) in the core store.

1. Top indicator is fracture cement i.e., minerals that have been precipitated out from fluids travelling through the fracture - Fig. 1.

2. Deformation bands which may look like veins (cement) but are made up of crushed sand grains - Fig 2a. This includes other fault rock processes like clay smear.

   http://www.ogilviegeoscience.co.uk/blog/2022/11/24/deformation-bands-bad-news-for-your-reservoir-

3. Displacement /cross-cutting of beds or older fractures - Fig 2b.

4. Slickenlines/polished surfaces on fracture faces - these are scratches from the shear movement of opposing walls and are therefore indicators of natural shear fractures.

   https://www.ogilviegeoscience.co.uk/blog/2025/2/3/shear-fractures

5. Fractures that are confined to beds/layers or certain lithologies i.e., mechanical stratigraphy.

   https://www.ogilviegeoscience.co.uk/new-page-15

6. Stylolites form as a result of pressure solution which dissolves the rock - Fig. 3.

7. Enclosed within the core - both ends of the fracture terminate within the core stick - Fig. 4.

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. 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 (Fig. 5a, Fig 6). The example in Fig 5a is close to a casing shoe point in a North sea well.

• Petal fractures [1,2] - due to frictional stick-slip between core and core bit giving 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.

• 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]

• Disc fractures are generally horizontal in a vertical core and they form as a result of in-situ stresses, unloading and bit pressure assisted by torsional stresses and drilling vibrations.

• 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.

  BUT bedding planes can become reactivated as fractures in which case look out for slickenlines, polished surfaces and cements - for evidence of natural fractures.

• 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

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 Reservoirs. Determining Natural Vs Induced Fractures.

[3] Reiss, L.H. 1980. The Reservoir Engineering Aspects of Fractured Formations. Gulf Publ. Co. 107 pp.