Online Journal for E&P Geoscientists

Sadooni, Fadhil N.^*1
(1) Environmental Studies Center, Qatar University, Doha, Qatar.

Microbial dolomites have been fully documented in lagoonal and sabkha sediments from different parts of the world. Detailed textural characterization, using SEM coupled with Energy Dispersive X-ray (EDX), of three deep cores (down to six meters) and fifty surface samples from the intertidal and supratidal zones of the coastal sabkha of Abu Dhabi indicate that dolomite constitutes up to 50% of the bulk sediments. This dolomite is found to form in small semi-closed pores or “micro-niches” within the carbonate sediments. Typical “micro-niches” include skeletal grains chambers, angles between mineral plates and local dissolution depressions. These “micro-niches” retains connate water for longer periods of time compared to the rest of the sediments; the water becomes anoxic due to the decay of organic materials by microbial activity and hence leads to the precipitation of dolomite in such pores. The later spread of dolomite would depend on the presence of well-connected networks of such pores. This concept means that microbial dolomite is not restricted to the lagoonal-sabkha settings and can be found in any depositional environment of any scale because most of carbonate sediments have high porosities upon deposition and these pores are usually filled with marine water. The presence of skeletal components will provide the organic materials needed for oxygen depletion. The abundance of dolomite in shallow water carbonates rich in organisms may be attributed to the presence of high number of relatively large “micro-niches” and abundant organic matter resulted from the decay of such organisms.

The Upper Triassic reservoirs of the northern parts of the Arabian Plate are formed of thick dolomites of wide distribution and usually associated with microbial mats and evaporites. Thin sections of such dolomites show that many of these may have formed in “micro-niches” and then spread out to other parts of the rocks. Furthermore, dolomite reservoirs have characteristically high wettability values which support the notion of microbial intervention. Therefore, many of these porous dolomite horizons were formed of microbial mats rather than limestone as thought earlier.

Such findings suggest that microbial dolomite may be found across a wider range of depositional environments. If proven correct, this concept will have significant impact on the exploration of dolomite reservoirs.