Visualizing a Sub-Salt Field With Image Logs: Image Facies, Mass Transport Complexes, and Reservoir Implications From Thunder Horse, Mississippi Canyon, Gulf of Mexico

Abstract

Thunder Horse is a deepwater asset in the Gulf of Mexico, Mississippi Canyon, with turbidite sandstone reservoirs deposited during the Middle Miocene in the Boarshead Basin. Imaging the reservoirs is difficult because the adjacent salt stock and canopy inhibit seismic imaging, and drilling in deepwater conditions makes coring expensive. Image logs provide an alternate means of high resolution visualization of the reservoir, and through the image facies scheme presented here, reveal the character of sandstone and interbedded mudrock fabric that allows for interpretations of depositional processes. Image logs are interpreted by picking bedding boundaries that represent the dip magnitude and azimuth of the dipping beds. The picked dips follow trends according to the sandstone or mudrock fabric being imaged. Similar dip magnitudes and azimuths in sandstone indicate sediment gravity flows traveling the same direction, like high density turbidity currents. Another pattern in sandstone is dips with similar magnitudes but varying azimuths, indicating that flows are in the same plane but traveling in different directions, like an avulsing channelized system. High variability of dipping beds in mudrock indicates deformation through downslope slumping, and steepening up/shallowing up dip patterns also seen in mudrock indicate slide blocks and folds. Correlating image logs across northwest Thunder Horse reveals that slumps, slides and folds built up mass transport complexes that eroded underlying sandstone in the proximal side of the Boarshead Basin. The mass transport complexes built a 200 m+ thick mounded feature that ponded another sandstone reservoir behind it, locally increasing the reservoir thickness. Previously, all mudrock at Thunder Horse was thought to be planar-bedded shale deposited from hemipelagic settling or low density turbidity currents, but high resolution image logs were the keystone in confirming the deformed, resedimented nature and erosive capability of the mass transport complexes. This level of detail is highly valuable to a sub-salt field where stratigraphic features such as erosion or amalgamation are not always visible through seismic imaging. As a case study this type of high-resolution data has wide applicability to other deepwater, sub-salt reservoirs, as improved depositional interpretations inform reservoir performance and impact future well planning.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017