Maurice E. Tucker

Upper Permian (Zechstein) microbialites: Supratidal through deep subtidal deposition, source rock, and reservoir potential

Zechstein 2 (Z2) carbonate microbialites flourished under arid paleoclimatic conditions in the Late Permian. Microbial carbonates from the Roker Formation outcrop in northeast England, with its subsurface equivalent being the Main Dolomite from northwest–central Poland. The Z2 carbonate deposits developed in supratidal through deep subtidal zones and consist of various stromatolites and thrombolites. Planar stromatolites and thrombolites characterize intertidal and supratidal facies, and biohermal stromatolites with oolitic grainstone and crinkled stromatolites typify shallow subtidal facies. The Z2 subtidal and/or intertidal microbialites with oolites form complexes more than 10 m (33 ft) thick and are important reservoir facies for hydrocarbons. Subtidal (slope) and intertidal (lagoonal) microbial mudstone and wackestone have poor reservoir properties but contain total organic carbon as much as 2 wt. % and are considered as potential source rocks. The thermal maturity assessed from C27 17-trisnorhopane (Tm) and C27 18-trisnorhopane (Ts) as the Ts/(Ts + Tm) ratio, C30 moretane/hopane ratio, sterane ratio expressed as 20S/(20S + 20R), and /( + ) ratio shows to indicates a mature character of organic matter with respect to oil generation.

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.