Microbial-Mediated Dolomite from Coastal Sabkha Environments of Abu Dhabi and Qatar: Analogues to Subsurface Arid Climate Dolomitized Reservoir Rocks
Arid climate carbonate reservoirs like the Permian Zechstein, the Permo-Triassic Khuff, the Triassic Kurra Chine, and the Jurassic Arab formations show early diagenetic dolomite clearly linked to facies successions. They are dominated by intertidal, microbial-laminated carbonates and often encased by salina- and sabkha-type evaporite layers. SEM analysis of samples from the Permian Zechstein Formation reveals filaments around dolomite crystals interpreted as fossilized extracellular polymeric substances (EPS) of ancient biofilms. Similar, fine-crystalline dolomite displaying authigenic spheroids of subhedral to euhedral dolomite rhombohedra, embedded in an organic matrix of EPS have been found in Recent and sub-Recent microbial mats and lagoonal deposits of the Abu Dhabi and Qatar coastal sabkha environments. The main factor controlling the occurrence of significant amounts of dolomite (up to 50%) within these sediments is interpreted to be the presence of an organic matrix and not, as proposed in previous studies, a replacement process. In contrast to arid carbonate settings (e.g. the Arabian Gulf), where relatively thick polygonal microbial mats are dominating the intertidal environment, only relatively thin leathery and crinkly-laminated microbial mats are found in the supratidal environment under humid conditions (e.g. the Bahamas). Penecontemporaneous dolomite is relatively abundant in arid carbonate coastal environments, often associated with evaporites and thick, aerially extensive microbial mats. Humid carbonate coastal environments show less penecontemporaneous dolomite, and also microbial mats are less common. Dolomite is more common in carbonate reservoirs of the Arabian Peninsula, deposited under arid climatic conditions (e.g. Khuff and Arab formations), compared to those deposited under humid climatic conditions (e.g. Thamama Group). Microbial-mediated dolomite is therefore interpreted to play an important role in the dolomitization process observed in arid climate carbonate reservoirs. Dolomite is also found to form in small semi-closed pores or micro-niches (e.g. shell borings and micro-borings) within grain-dominated carbonate sediments. These micro-niches retain connate water and become anoxic due to microbial activity, thereby mediating the precipitation of dolomite. Microbial dolomite is therefore not exclusively restricted to sabkha settings, but can be found in any depositional environment.
AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014