Identifying and Scaling Fabrics in Carbonate Rocks – Outcrop and Stratigraphic Approach to Borehole Geophysical Logs

W. Bruce Ward¹, William F. Murphy¹, Richard Nolen-Hoeksema¹, Gary Fleming¹, Beckett Boyd¹, and David Allen^2
1 Earthworks LLC and Theoretical & Applied Geology, Newtown, CT
² Schlumberger, Ridgefield, CT

The science of observation on the outcrop can be reduced to fundamental processes that have been developed since Steno. When these processes are applied to (a) the electrical resistivity image log and (b) the NMR T2-relaxation distribution log in a variety of carbonate settings, critical facies and stratigraphic fabrics are revealed in the borehole that constrain geological rules for scaling between wells.

The electromagnetic logs are particularly useful in high-porosity carbonates occurring in hydrocarbon and water reservoirs, such as those common in the Middle East and Florida, respectively. For a Middle East borehole, the reservoir architecture determined from core can be determined by applying observational and sequence stratigraphy techniques to just the two logs.

Observation of carbonate outcrops, cores, and boreholes (via logs) involve three simultaneous processes: 1) Calibrating instruments (eyes) to textures. 2) Identifying fabrics across scales (mm - km). Fabrics are partly defined by the distribution of particles and pores (e.g., bedding, macrofossils). These are highlighted by outcrop weathering and measured by the electromagnetic logs. The electrical image log is sensitive to fluid content and pores. The MNR log reveals variations in pore-size distributions. 3) Testing hypotheses on how textures are packaged. This includes the principles of cross-cutting relationships and superposition, and models of stratigraphy, facies, diagenesis, structure, and observation method.

The models provide rules that guide extrapolation away from the borehole and interpolation between wells. Fundamental geological relations severely constrain the allowable geometries and scaling relations. These relations and key parameters are observed at the borehole with the electrical image and NMR relaxation logs.