Holocene Intertidal Microbial Mats of Qatar and Their Implications for Petroleum Source Rock Formation in Carbonate-Siliciclastic-Evaporite Systems

Organic-rich mesohaline microbial mats occur in the intertidal zone of a lagoonal area developed to the lee of a coastal spit in Mesaieed, Eastern Qatar. The mats grow on a substrate of seagrass-rich carbonate mud with cerithid and monachid gastropods and other small bioclasts, reaching a thickness >3.5 cm. The mats are well laminated with different microbial communities, from cyanobacteria to sulphur bacteria, reflected in the distinct colour changes from green to pink to brown. The mat layers contain spheroids of dolomite, the precipitation of which was plausibly mediated by bacteria. The lipids reflect the biomass of the principal mat-building phototrophic and heterotrophic microorganisms. A variety of hydrocarbons, including n-alkanes, diploptene, and isoprenoids such as phytane, phytene, phytadiene and squalene were detected, in varying concentrations amongst the particular mat layers. In particular, n-heptadecane, likely derived from cyanobacteria, dominated the n-alkane distribution at a depth of 0-0.1 mm. The concentration and abundance of n-alkanes increase with depth through the mat, likely representing the early diagenetic initiation of hydrocarbon generation. Therefore, understanding early diagenetic organic matter alteration and preservation in marine mixed carbonate-evaporite-siliciclastic systems, as well as the processes operating in the early stages of diagenesis, could improve understanding of the hydrocarbon potential of such systems. This will help considerably in the prediction of hydrocarbon occurrence in frontier, as well as mature, petroleum carbonate-evaporite basins.