Interpretation of Downhole Measurements, Deformation Analyses, and Lithologic Characterization in the Ursa Basin, Gulf of Mexico

Abstract Integrated Ocean Drilling Program (IODP) Expedition 308 drilled a transect along the Ursa Basin in the eastern Gulf of Mexico for examining how sedimentation, overpressure, fluid flow, and deformation are coupled in passive margin settings. A total of eight holes were drilled in the Ursa Basin, five were cored and three were logged using either logging while drilling (LWD) or wireline techniques, to evaluate the controls on slope stability, understand the timing of sedimentation and slumping, establish the petrophysical properties of shallow sediments, and provide a better understanding of turbidite systems. Overall, the log responses vary for the different lithostratigraphic units and associated regional seismic reflectors. Significant lateral variability in physical properties has been observed on downhole profiles but the acquired data also make bed-to-bed correlation between sites possible, which is valuable for the study of sandy turbidites and studies of regional deformation. In the Ursa Basin sediments, resistivity-at-the-bit images show significant deformation of the overlying hemipelagic drape and distal turbidites that were drilled in these areas. Numerous dipping beds throughout these intervals with dips ranging from 5-55° confirm core observations. Steeply deformed beds, with dips as high as 65°, and folded and faulted beds suggest down slope remobilization as mass-transport deposits. Resistivity images also show evidence of these mass-transport deposits where steep dips and folds suggest the presence of overturned beds within a series of cyclic intervals that we interpret as a succession of sand-silt-mud lamina. Preliminary structural analyses suggest that many of the deformation features trend in an E-W direction with the majority dipping to the north. Introduction IODP Expedition 308 drilled three sites within the Ursa Basin in the eastern Gulf of Mexico for examining how sedimentation, overpressure, fluid flow, and deformation are coupled in passive margin settings [1]. The bathymetry in the Mars-Ursa region is produced by deposition of a thick wedge of Pleistocene sediments originating from the Mississippi River (Fig. 1). In the north-central Gulf of Mexico, the late Pleistocene shelf, shelf-margin, and minibasin turbidite deposits that were sourced from the Mississippi River are termed the Eastern Depositional Complex [2]. These shelf and shelf-margin deposits have been previously described described [3, 4] and the deepwater strata explored during Expedition 308 comprise one component of this system. In this overview, we examine results from wireline loging and logging while drilling data, identify changes in stratigraphy, interpret resistivity image data, and characterize deformation patterns mainly attributed to mass transport deposits (MTDs). We also identify lateral lithological varibilties using log to seismic correlations at each site. Ursa Basin IODP Expedition 308 drilled a total of eight holes in the Ursa Basin, five holes were cored and three were logged using LWD techniques. The main objectives for acquiring logging measurements in this area were to determine the physical properties of the sequences drilled, characterize the changes in stratigraphy, provide information regarding the deformation associated with MTDs, assess drilling conditions when encountering potential overpressured sand intervals, and provide the basis for linking LWD observations and seismic reflectors through time-depth conversions and synthetic seismograms.