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Abstract

Stacked mass transport deposits interbedded with turbidite sandstone reservoirs were characterized in detail from image logs at Thunder Horse, a conventional asset located in the Boarshead mini-basin, lower Mississippi Canyon, Gulf of Mexico. Image logs visualize the borehole wall, allowing bedding boundary information and some sedimentary fabric to be identified. From bedding boundaries, dip azimuths and magnitudes can be calculated, and depositional processes can be inferred from patterns in the dips. Dips in the primary sandstone reservoirs show little variation (< 20°) in structural tilt, indicating continuous deposition by high density turbidity currents prior to major structural deformation and are thus the best proxy for regional structural dip. In contrast, dips in 80% of all mudrocks in this canyon show widely varying dip magnitude and azimuth over small and large scales and are interpreted as having been deposited by slumps, slides, and folds, building up to form mass transport complexes. Only very rarely do we observe mudrock dips that are conformable to the regional dip and strike.

Unconformable bedding contacts at the tops of the sandstone reservoirs in northern Thunder Horse suggest scouring and erosion by overlying mass transport deposits, whereas basal sandstone contacts are in conformance with structural dip, suggesting amalgamation. On seismic data, albeit low-resolution due to the overhanging salt canopy, the mass transport complexes appear to build up into a gradual mound.

Core was acquired in the upper 60 m of the mass transport complex, revealing convoluted and folded bedding capped by highly bioturbated marl, which is thought to represent a Glossifungites hiatal surface resulting from the mass transport complexes building up a topographic high, forcing subsequent debris and turbidite flows to travel around rather than over that location. The topographic high ponded the overlying reservoir behind it, preferentially thickening the sandstone reservoir behind the mound. Thus, mass transport complexes were found to both erode and augment sandstone reservoirs. Biostratigraphic dating of the two reservoirs brackets their deposition as occurring in <400,000 years during the Serravalian, Middle Miocene, probably as a result of the Harang shelf failure.

As a case study this type of integrated, high resolution data (core, image logs, seismic) has wide applicability to other deepwater, subsalt reservoirs, as improved structural and depositional interpretations inform reservoir performance and impact future well plans. Further, recognition of mass transport complexes as agents of reservoir compartmentalization and sandstone ponding allow for more accurate reserve estimation.

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