Borehole images, and to a lesser extent dipmeters, can provide very high quality and high-density maps of a borehole wall. These are often of a state that sedimentological interpretations can be completed as if the images were core, with the added advantage that features can be orientated. As in the field, images allow the recognition, classification and orientation of these different bedding surfaces. We know from field studies that sedimentological successions are bounded by surfaces, on a range of scales, that result from different processes and depositional situations. The orientation and significance of such surfaces vary. Examples would include cross-bedding and the orientation of the underlying bounding surface that they were traversing. For example, the recognition and analysis of the cross-bedding would lead to a knowledge of local flow conditions, but interpretation of the bounding surface may yield a migration direction for a larger scale sediment body. For these reasons it is important that manual dip picking of images is completed within a framework of geological consistency that is appropriate for the depositional system under study.
The recognition of a bedding hierarchy used in architectural element analysis of outcrops helps drive bedding schemes used when picking and classifying surfaces from the wellbore. A leading example is Miall’s work within fluvial systems, where he has established a ranking of bounding surfaces. Other schemes are available for use from other environments. So, dip-picking schemes can vary. Some are summarised below:
Turbiditic environment
- Bed tops,
- Bed bases,
- Bedding internal to sandstone bodies,
- Oversteep bedding,
- Slumps and slides,
- Nodule boundaries,
- Shale bedding.
Fluvial environment
- Cross-bedding,
- Range of different (set and co-set) bounding surfaces,
- Shale bedding,
- Oversteep and deformed bedding,
- Erosion surfaces,
- Bed tops and bases,
- Base of thick stacked sections of sandstones,
- Coal bed boundaries,
- Rooted surfaces,
- Nodule boundaries.
Aeolian environment
- Cross bedding, often at various scales,
- Range of different bounding surfaces,
- Grain flow strata,
- Grain fall strata,
- Wind-ripple laminae,
- Surfaces of deflation,
- Bedding in sabkha and lacustrine deposits,
- Deformed bedding,
- Reactivation surfaces,
- Bedding within interdune deposits,
- Super surfaces,
- Polygonal fractured surfaces,
- Nodule boundaries.
Shallow marine
- Cross-bedding,
- Range of different set and co-set bounding surfaces,
- Surfaces of bioturbation,
- Erosion surfaces,
- Deformed bedding,
- Sandstone-shale transitions,
- Pebble horizons,
- Nodule boundaries,
- Bed boundaries,
- Sequence and parasequence boundaries,
- Tidal bundles,
- Reactivation surfaces,
- Herring bone cross-bedding.
The above listings are not complete or always applicable. Image quality, knowledge of the section and local circumstances should drive any dip picking scheme. It is essential that cores, sidewall cores and ditch cutting, along with open hole logs, be allowed to constrain, verify and augment data derived from images and dipmeters. Images alone will not deliver the full answer.