Abstract

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.

Introduction

Carbonate rocks are an important source of petroleum deposits, both as reservoir rocks and possibly also as source rocks. Evaporitic environments are one of the most productive sedimentary settings for organic matter, as evidenced by prolific hydrocarbon production from many evaporite-carbonate formations (Warren 2006 with references). This arises not only from the unique reservoir properties of plaform carbonates but also from early diagenetic transformations of organic matter and mineral diagenesis (e.g. Gautier et al., 1985; Wakeham & Ertel 1988).

Previously Emery (1956) and Evans (1966) showed that organic carbon contents in modern evaporitic deposits in the Persian Gulf range from 0.5–1.5 wt%. Later, Vita-Finzi & Phethean (1980) reported higher values between 0.15–3.34 wt%, of muds in the inlets of the Musandam Peninsula near the entrance of the Persian Gulf. Shearman & Skipwith (1965) suggested that the mucilaginous jackets which surround most shallow-water carbonate grains may indeed be the source of much of the kerogen in carbonate rocks. Kenig et al. (1989, 1991), together with Baltzer et al. (1994), analysed the organic matter in shallow-water lagoonal and intertidal sediments of Abu Dhabi. Three different organo-sedimentary facies were distinguished, with distinct organic signatures that persist after burial and which are related to the rate of change of sea-level, substrate morphology and rates of sedimentation. Both Kenig et al. (1991), and later Kendall et al. (2002), pointed out the source potential of these microbial mats.

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