Abstract
Stromatolites are organosedimentary, lithified structures showing layered growth via trapping and binding of sediments with calcium carbonate precipitation through microbial activity. The age of the oldest stromatolites from the Pilbara Block of Western Australia has been determined to be 3.4 – 3.5 Byr. As such they are the most ancient record of life and they provide 80% of the fossil record of Earth history. Hamelin Pool, Shark Bay, Western Australia is an internationally distinguished area known for hosting the most abundant and diverse modern marine stromatolites in the world. This study focuses on the petrophysical properties of Hamelin Pool stromatolites and associated pavement facies by interrelating porosity, permeability, sonic velocity, resistivity and pore geometry through Digital Image Analysis (DIA). The microcrystalline carbonate precipitation ‘micrite’ generates a rigid framework with a wide range of porosities and pore sizes that influence the ultrasonic velocity, permeability and resistivity in stromatolites. A helium pycnometer which measures the true volume and calculates the density was used for porosity determination; and permeability was measured using steady-state air permeability. Digital Image Analysis results show that stromatolites generally have simple and large pore structures and impressive high permeability values. In the 84 core plugs that were used in this study, the permeability values of the Hamelin Pool stromatolites and pavements range from 8 mD to 9000 mD, while porosity ranges from 16% to 46%. Ultrasonic velocity, measured under dry and saturated conditions, is generally high with a large scatter at any given porosity. For example, at 29% porosity, velocity ranges from 3611 m/s to 5384 m/s. Similarly at a velocity of 4403 m/s the porosity ranges from 23% to 46%. Digital image analysis indicates that the main control on the variations is the pore complexity and size. Larger pores produce higher velocities at equal porosity. In brine (35 ppt) saturated core plugs compressional velocities increase up to 686m/s. In contrast, shear velocities show both a decrease (up to 578 m/s) and an increase (up to 597 m/s) in shear velocity (Vs) with saturation. These changes in Vs indicate that the shear modulus of stromatolites changes with saturation resulting in both shear weakening and shear strengthening and thus violating the assumption by Gassmann. The large range of porosities at a given velocity, however, makes porosity estimates from seismic inversion a challenge, and similarly, the shear moduli changes and the resultant shear strengthening and weakening add uncertainties to AVO analysis in microbialite strata. The presence of quartz, which is diagenetically inert, precludes significant dissolution and reprecipitation, and results in lower seismic velocities of these carbonate rocks. The quartz percentage in each specimen was related to velocities and pore structures and confirms the effect of mineral variability on ultrasonic velocities. The cementation factor “m” (from Archie’s equation, F = φ-m) determined from electrical resistivity varies from 2.0 to 3.7 in stromatolites and from 2.0 to 3.1 in pavements. The cementation factors are much narrower with respect to other carbonates, which can decrease the uncertainties when making the hydrocarbon/water saturation estimations in stromatolites and pavements. Porosity, permeability, ultrasonic velocity and resistivity were investigated in different meso/microfabrics and showed that different internal structures of stromatolites do not have a distinctive effect on petrophysical properties.