EARLY MICROBIAL CEMENTS – IMPLICATIONS FOR MARINE DIAGENESIS AND ROCK PHYSICS (Invited Presentation)

Modern marine cemented carbonates, such as beach rocks, stromatolites, hardgrounds and coquina pavements, often display strong rock strength despite their high porosity. We document how microbially mediated precipitates or precipitates from endolithic borers at grain contacts form micritic meniscus cements prior to typical marine cements. These microbial cements dramatically increase the stiffness of the grain aggregate, producing a high impedance rock with excellent reservoir qualities.

Samples for this study consist of intraclasts from active portions of the ooid shoals of Schooners Cay, Joulters Cay and hardground samples from the oolitic tidal bar belt in the Tongue of the Ocean as well as stromatolites and cemented seafloor pavements from Hamelin Pool in Australia. In all samples, grain to grain contacts are colonized by diverse microbial forms and biofilm-EPS. EPS is subsequently replaced by nanograins precipitates that act as nucleation centers for micrite and fine mesh of aragonite needle-like crystals. A second path of producing micritic-bridging cement is through passive mineralization along the negatively charge microbial cell walls, which act as mineralizing templates for crystal growth. The thereby produced micritic cements fuse the grains together to form the template for subsequent marine cements like acicular aragonite needles. These findings imply that early cementation in the marine realm appears to be initially governed by a biological driven mechanisms and questions the assumption that the assumption that early cements are non-biologically mediated.

The micritic cements, although small in extent, produce a very strong fusing of the grains and dramatically alter the petrophysical behavior of the young rock. First, they make the rock resistant to compaction and thereby preserve primary porosity. The strong fusion of grains also increases the bulk and shear modulus, enabling the fast passage of acoustic waves. As a result, these rocks have a high velocity despite their high porosity. Because the porosity in these rocks are mostly intergranular, they also have a high permeability. If subsequent diagenesis does not fill the pore space with aragonite or calcite spar, these rocks have excellent reservoir quality to great burial depth.