Wednesday, April 1, 2015

Carbonates in Seismic Reflections

Normally, reflections from the top boundary of a carbonate unit have a large positive reflection coefficient because carbonates usually have high velocity and density compared to other common sedimentary rocks.

Only in cases where the carbonates are very porous or fractured are reflection coefficients of upper boundaries likely to be negative.

The usually high interval velocity introduces a potential resolution problem. Not only do thick sequences appear thin in time on seismic sections due to high velocities but the minimum thickness required for adequate vertical resolution can be quite high. Interval velocities between 4500 and 6000 m/s are common for older carbonate units.

A carbonate 100 m thick, with an interval velocity of 5500 m/s, would be represented by only 36 ms two-way time. Alternatively, for a seismic frequency of 25 Hz and interval velocity of 5500 m/s, the half wavelength thickness for no interference between reflections from top and base is 110m and the quarter wavelength tuning thickness is 55 m.

From a seismic viewpoint, carbonates can be conveniently divided into three groups :

  1. sheet-like deposits : These are often extremely extensive laterally and consist of fine-grained carbonate particles or calcareous microfossils deposited from suspension (e.g, , micritic limestones, chalk, calcareous claystone, etc). These deposits show characteristics similiar to those of other fine-grained deposits but can usually be recognized by their high amplitudes, good continuity, and if thick enough -by high interval velocities, which are rarely less than 3500 ms/s. Mistaking volcanic ash or tuff beds for carbonates is a potential interpretation pitfall. Tuff beds have high interval velocities and are laterally extensive, producing a seismic response similiar to that of bedded carbonates.
  2. Bioclastic deposits : Consisting of sand-sized carbonate grains transported and deposited by high-energy currents, these will have the same form and depositional setting as other noncarbonate clastics. Bioclastics may possibly be identified by their expected higher interval velocity and higher reflection amplitudes. Other considerations, such as the paleogeography and other recognizable associated lithlogies may aid an identification. In many cases, however, it may be impossible to differentiate between bioclastics and noncarbonate clastics.
  3. Buildups, reefs, bioherms, banks, mounds, etc. This type of deposit has a large biological element comprising the skeletal remains of living organism. These deposits are usually characterized by shape and high interval velocity. 

Bubb and Hatledid (1977) subdivided carbonate buildups into four major types 
  1. Barrier buildups, tending to be linear with relatively deep water on both sides during deposition.
  2. Pinnacle buildups-roughly equidimensional features surrounded by deep water during deposition.
  3. Shelf margin buildups- linear features with water on one side and shallow water on the other.
  4. Patch buildups-usually formed in shallow water, eiter in close proximity to shelf margins, or over broad, shallow seas.
Reference : Michael E. Badley, Practical Seismic Interpretation.

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