THREE-DIMENSIONAL RECONSTRUCTIONS OF HOLOCENE AND NEOPROTEROZOIC OOLITES TO MEASURE POROSITY, PERMEABILITY, AND VOLUME-SHAPE EVOLUTION OF OOIDS

The presence and shape of ooids are used as environmental indicators of water depth and mode of transport. As an ooid grows the mode of transport transitions from saltation to traction causing a change in shape from spherical to elliptical. Identifying the size at which this transition occurs allows us to reconstruct paleohydraulic conditions in the ooid-forming environment. In addition to their usefulness as environmental indicators, the porosity and permeability of oolites make them among the most economically important reservoir rocks.

We use the Grinding, Imaging and Reconstruction Instrument (GIRI) in conjunction with a convolutional neural network image classification routine to build three-dimensional models of a Holocene oolite from the Bahamas. We use the three-dimensional models to directly measure the size and shape of the ooids, as well as the porosity and permeability of the oolites. By comparing the porosity and permeability measurements from the reconstruction to estimates of porosity and permeability derived from random two-dimensional surfaces in the reconstruction, we demonstrate that estimates of porosity and permeability from two-dimensional surfaces are inaccurate. We also reconstruct Neoproterozoic giant oolites and demonstrate that the volume-shape history preserved in the lamellae of an ooid provides a quantitative history of ooid transport processes and estimates of flow velocity, providing new paleohydraulic constraints on the exceptional conditions that led to the formation of giant ooids.