Using X-ray Microtomography (Micro-CT) in Characterizing Biogenically Enhanced, Low-Permeability Reservoirs: A Case Study from the Upper Cretaceous Nise Formation, Møre Basin, Norwegian Sea
Recent research demonstrates the influences of bioturbation on reservoir petrophysics. Conventional reservoir characterization is often restricted to porosity and permeability assessments. Commonly, visualization of the reservoir properties is limited to two dimensional analyses by using destructive methods (e.g. thin sections). Due to intricate textural heterogeneities, assessing spatial variability in three dimensions (3D) is often overlooked. X-ray Microtomography (Micro-CT) is a non-destructive technique that allows visualization of density-associated petrophysical properties. In bioturbated, low-permeability reservoirs three dimensional visualization contributes to the understanding of burrow spatial distributions (e.g. burrow density and interconnectivity) and more accurate reservoir characterization.
Within the Upper Cretaceous Nise Formation (Møre Basin, Norwegian Sea) a biogenic rock fabric contributes substantially to reservoir storativity and permeability. Four cores are studied in order to assess the relationship between bioturbation and permeability distribution. Overall, Nise Formation strata comprise unburrowed to completely bioturbated fine- to very-fine sandstones and mudstones containing a highly-diverse trace fossil assemblage that represent parts of the proximal through distal Cruziana ichnofacies. Micro-CT imaging, spot permeability measurements and petrographic assessments show that permeability distributions are strongly influenced by the location and nature of bioturbation. Spot permeability data taken from core-plugs indicates that the burrow permeability can be up to two orders of magnitude greater than the matrix. Therefore, it proffers a biogenically influenced dual-permeability flow media. Visualization in 3D allowed the identification of a dense, highly interconnected, mostly horizontal and inclined burrow system. These modifications constitute selective fluid flow networks that occur through the imposition of coarser grained sediment in tunnels and shafts in otherwise fine-grained strata. Bulk assessments of permeability based on core-plug data indicate that the harmonic mean of matrix versus burrow permeabilities provides the most accurate estimate of bulk permeability. These modifications result in better resource quality and control the biogenically-enhanced permeability distributions.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California