Well Log Response Character

(source: Emery, 1996)

There are three general trends or curve shapes that can be recognized when looking at well log curves. For instance the Gamma log tracks the upward change in clay-mineral content. NB: When possible, one should use core data to confirm or augment well log analysis.

1.     Cleaning-up trend (funnel shape); a gradual upward decrease in gamma response. In shallow marine settings, this trend reflects a change from shale-rich into sand-rich lithology and upward increase in depositional energy with shallowing-upward and coarsening. In deep marine settings, this trend reflects an increase in the sand contents of turbidite bodies. This trend also may indicate gradual change from clastic to carbonate deposition.

2.     Dirtying-up trend (bell shape); a gradual upward increase in gamma response: This trend may reflect upward fining (eg: a lithology change from sand to shale) or upward fining of sand beds in a thinly interbedded sand-shale unit. This trend usually implies a decrease in depositional energy. In a non-marine setting, fining upward is predominant within meandering or tidal channel deposits with an upward decrease in fluid velocity within a channel (coarser sediments at base of channel). In a shallow-marine setting, this trend usually reflects an upward deepening and a decrease in depositional energy (shoreline retreat). In deep-marine settings, this trend reflects waning of submarine fans (reducing of sand contents).

3.     Boxcar trend (cylindrical or block shape) with low gamma and sharp boundaries and no internal change: this trend is predominant in fluvial channel sands, turbidites (typically with greater range of thickness), and aeolian sands. Evaporites also can have a cylindrical gamma trend.

Additionally two in-between trends can be recognized:

1.     Bow trend (symmetrical or barrel) with gradual decrease then gradual increase in gamma response: this is usually the result of progradation and retrogration of clastic sediments.

2.     Irregular trend with lack of character: this trend represents aggradation of shales or silts and can occur in other settings.

Curve character can be smooth, complex or serrated (sawlike) with contacts can be sudden or gradual.

The figure below summarises the log response a variety of different clastic depositional systems that Malcolm Rider (1996) assembled in his excellent book on Well Logs. His diagram has been added to and slightly been modified for this web site.