Pore Structure and Petrophysical Characterization of Hamelin Pool Stromatolites, Shark Bay, Western Australia

Abstract:

Stromatolites are organic-sedimentary structures that form by trapping and binding of sediments and calcium carbonate precipitation through microbial activity. The largest modern stromatolite province is the hypersaline Hamelin Pool, Western Australia. Microbial precipitation generates a rigid framework with a wide range of porosities and pore sizes that influence the ultrasonic velocity permeability and resistivity in stromatolites.

Stromatolites generally have simple and large pore structures and an impressive high permeability values. In the 55 core plugs, permeability varies from 0.5 D to 9 D, while porosity ranges from 17% to 46%. Ultrasonic velocity, measured under dry and saturated conditions, is generally high with a large scatter at any given porosity. Likewise large variations of porosity exist at any given velocity. For example, at 29% porosity, (dry) velocity ranges from 3611m/s to 5384m/s. Similarly at a velocity of 4048m/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 faster velocities at equal porosity. In saturated plugs compressional velocities increase up to 365m/s. In contrast, shear velocities show both a decrease (up to 578m/s) and an increase (up to 391m/s) in shear velocity (v_s) with saturation. These changes in v_sindicate that the stromatolites do change the shear modulus with saturation, thus violating the assumption by Gassmann.

The cementation factor “m” (from Archie’s equation, F = φ^-m) determined from electrical resistivity varies in a narrow range from 2.1 to 2.6. This narrow range reduces the uncertainty in predicting the hydrocarbon/water saturation in stromatolites. 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.