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Pore-Structure and Petrophysical Characteristics of Hamelin Pool Stromatolites and Associated Cemented Coquina Beds


The stromatolites at Hamelin Pool, Shark Bay area, Australia, are impressive modern microbial buildups in a hypersaline environment. The largest but often ignored microbial facies in Hamelin pool, however, is what is called “pavement”. It comprises 67% of all micobialites in the pool and consists of mircobially bound and cemented coquina layers. This association of coquina beds and overlying stromatolites is reminiscent to the facies association in the largest pre-salt discovery of Brazil. We measured the pore structure and the petrophysical properties of Hamelin Pool stromatolites and associated pavements and explain why both microbial facies potentially are excellent reservoir at great burial depth. The measured petrophysical properties include, porosity, permeability, acoustic velocity and resistivity. In order to explain the rock properties, the pore structure of each sample was investigated with digital image analysis and compared to marine and continental carbonates. Despite their microbial origin and/or binding the Hamelin Pool microbialites have simple pores of variable sizes. Porosity is high, ranging from 16 – 46% and permeability is generally very high reaching up to 9000 mD with no correlation to porosity. In these high-porosity and high permeability rocks acoustic velocity is also high, ranging from 3120and 5383 m/s, because microbial fusing of grains produces frame stiffness and preserves primary porosity. In addition, velocity does not increase significantly with increasing confining pressure, indicating that these Hamelin Pool microbialites are pressure resistant, making them potentially excellent reservoir rocks. As in non-microbial marine carbonates, the wide range of velocity at any given porosity and the erratic behavior in regard to saturation make seismic inversion and AVO analysis in these rocks a challenge. Resistivity is low in the Hamelin Pool microbialites compared to continental carbonates such as travertine and, thus rocks with travertine-like rocks. The Hamelin Pool microbialites display a narrow range of cementation factor m (2- 3.7) that is welcomed for an accurate calculation of oil saturation. Hamelin Pool stromatolites and pavements are an interesting porous media that partly differs from non-microbial carbonates. These differences need to be taken into account when applying rock physics models to rocks with similar pore structures.