Calibrating Cores to Image Logs in a Turbidite Sequence from Ventura Avenue Field

Jon Schwalbach¹, Michael Glascock¹, Ralph Coldewey¹, Jay Holloway¹, and James Vickery^2
1Aera Energy LLC, 10000 Ming Avenue, Bakersfield, CA 93311
²California State University Bakersfield, 9001 Stockdale Hwy, Bakersfield, CA 93311

Geologists are increasingly called upon to provide detailed descriptions of reservoirs to support secondary recovery programs and optimize reservoir development. High-resolution logging tools such as borehole image logs add significant value to these descriptions because they enable us to interpret reservoir facies and predict reservoir properties. The reliability of these interpretations and the ability to make predictions in offset wells, however, is significantly enhanced when the image logs are calibrated with cores.

A core from the Repetto Formation of the Ventura Avenue Field, Ventura, California, provides this calibration in a turbidite sequence. The cored interval contains at least 3 distinct stratal packages, each characterized by a unique stacking arrangement of facies and resultant reservoir properties. A thin-bedded sequence, dominated by “classical” turbidite facies and interbedded mudstones, contains the best sorted sands with the highest measured permeability. The other two stratal packages are dominantly coarser-grained, “high-density” turbidites. The high-density turbidites have porosity ranges generally similar to the ranges for the thin-bedded facies but are more poorly sorted and have lower permeability. The coarsest-grained (pebble) intervals, as well as a few other intervals in the core, have significant calcite cement, destroying any porosity or permeability that might have previously existed.

The facies packages exhibit significant stratal heterogeneity that would be difficult to describe without high resolution data. Particularly for the thin-bedded intervals, conventional log analysis would likely predict a more pessimistic "net to gross" and poorer reservoir qualities. Additional reservoir complexity is introduced by the presence of fractures, visible in both core and on the image logs. The image logs reveal these fractures are oriented nearly north-south, perpendicular to the anticline axis. A fault with small (cm scale) offset is also evident, and trends east-west, approximately parallel to the fold axis.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California