Sub-Seismic Deformation in Traps Adjacent to Salt Stocks/Walls: Observations From Green Canyon, Gulf of Mexico

Abstract

We use core and image logs to characterize sub-seismic deformation at Heidelberg and Caesar-Tonga Fields, both of which are 3-way traps against salt. Despite the lack of seismically mappable faults in these sub-salt structures, numerous deformation bands (DBs) are observed in Miocene turbidites in core and image logs. Both fields occur in steeply dipping halokinetic sequences (dip magnitudes ~20° to overturned) with similar reservoir properties (high-porosity [20-26%], high permeability [>300 mD] subarkosic sandstones comprising ~90% quartz, ~5% clay, and ~5% other minerals). Microstructural analysis indicates that most of the DBs are cataclasites, with a lesser amount of disaggregation seams. Core calibration with high-resolution oil-based image logs allows interpretation of DBs that were previously unrecognized, although not all DBs are visible. Spatial distributions, orientations, and frequencies of DBs are established from structural interpretation of image logs acquired in wells at different structural positions along the salt-sediment interface of each field. In both fields, a clear strike-parallel conjugate set of DBs is observed. The bisector of the acute angle of the conjugate sets is ~parallel to bedding. At Caesar-Tonga many of the faults with similar orientations as the DB’s show apparent normal displacements, while at Heidelberg, some of the DBs show apparent reverse offset in core. At Heidelberg an additional, and perhaps more important (in terms of flow baffling) DB set strikes at a high-angle to the salt-sediment interface (and bedding). We interpret the strike-parallel DB set to develop in response to flexural slip folding of the beds during salt stock growth, and the strike-perpendicular DB set at Heidelberg to accommodate along-strike changes in the geometry of the salt-sediment interface, possibly related to the presence of a mega-flap. Core permeability measurements indicate that disaggregation seams are approximately an order of magnitude less than the host rock permeability, with ~2-3 orders of magnitude reduction for cataclasites. Bulk effective permeabilities are estimated for each well by incorporating the host sand and DB permeabilities, as well as the reservoir thickness and volumetric fraction of DBs, and then compared to collected pressure and production data. This subsurface data set illustrates the usefulness of collecting detailed sub-seismic structural data from core and image logs for understanding well performance.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018