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 gastropods and other small bioclasts, these mats reach 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 plausibly mediated by bacteria. The lipids reflect the biomass of the principal mat-building phototrophic and heterotrophic microorganisms. Moderate amounts of hydrocarbons, n-alkanes, cyanobacterial n-heptadecane and diploptene, isoprenoids including phytane, phytene, phytadiene and squalene were detected in the particular mat layers. The concentration and abundance of n-alkanes and hopanoids increase with depth at the early stage of diagenesis. Such high accumulation of oil-typical compounds may lead to the formation of source rocks for hydrocarbons. Therefore, understanding of hydrocarbon-generating processes will improve our knowledge of organic matter alteration and preservation in marine mixed carbonate-evaporite-siliciclastic systems, as well as the processes operating in the early stages of diagenesis. This will help considerably in the prediction of hydrocarbon occurrence in frontier, as well as mature, petroleum carbonate-evaporite basins.