Using Magnetic Resonance Imaging and Petrographic Techniques to Understand the Textural Attributes and Porosity Distribution in Macaronichnus-Burrowed Sandstone

Magnetic resonance images are paired with petrographic data to evaluate the textural characteristics of rocks dominated by Macaronichnus segregatis, a trace fossil that is commonly associated with rocks deposited in shal-low, marginal marine sedimentary environments. MRI techniques used revealed the three-dimensional geometry of the trace fossil. Burrows are typically horizontal and in plan view range between straight, sinuous, meandering, and spiral geometries. Changes in burrow morphology may be related to population den-sity and patchy resource distribution. The pairing of MRI and petrographic data helped map the distribution of porosity in the burrowed rock. Because MRI images represent complex composites of nuclear spin density and MR relaxation times, each of which is related to pore size, stronger MR signals must be calibrated to known porous zones by integrating petrographic data with MR data. The complex distribution of porosity and its relationship to the matrix show that this fabric represents a dual porosity–permeability system and may affect the resource (reservoir or aquifer) quality of similarly burrowed sedi-mentary rocks. Future research should elaborate upon the porosity–perme-ability model for this and similar fabrics.