Integration of Core, Image-Log, and Wireline-Log Data Extends Facies Interpretations in the Upper Cretaceous Olmos Formation of South Texas

Treviño, Ramón H.¹, Robert G. Loucks¹, Julia Gale², Abdelmoniem K. Abdelmoniem³ (1) The University of Texas at Austin, Austin, TX (2) University of Texas at Austin, Austin, TX (3) University of Texas At Austin, Austin, TX

The Olmos Formation is composed of shelf, shoreface, and delta-system sandstones and associated mudstones. Facies architecture has distinct lateral and vertical variations. Cores are essential for documenting specific facies, but cores are rare. Hydrodynamic and biological features down to several inches are apparent and recognizable on image logs calibrated to core. The shelf strata in core are extensively bioturbated, corresponding to the Cruziana ichnofacies (Asterosoma, Schaubcylindrichnus, Zoophycus, Bergaueria, Scolicia, Thalassinoides, Chondrites and Diplocraterion). The bioturbated strata appear as nonlaminated sandstone on the image log. The shoreface strata in core exhibit crossbedding and Ophiomorpha and Macaronichnus trace fossils (Skolithos ichnofaces). The crossbedding and burrows are identifiable on image logs. The deltaic system is characterized by channel and salt-marsh sub-environments. The salt-marsh related facies comprise beds of carbonaceous shale laminae to thin beds of coal, and very fine grained, mud-rich, bioturbated sandstone and dark-gray bioturbated (Psilonichnus) sandy mudstone with common root traces. Channel related facies comprise fine- to medium-grained cross-bedded sandstone, containing up to 50% rock fragments with common shale clasts and minor amounts of detrital coal. The channel-related sandstone is well-laminated. These two sub-environments compose a wave-dominated, deltaic depositional system, and the related facies can be identified on image logs. Image logs are more readily available than cores and, after calibration with cores, can be used to identify facies. In conjunction with wireline logs adjacent to or near image logs, facies can be reasonably predicted, extending facies, depositional systems, systems tract and sequence stratigraphic interpretations over a much larger area.