ABSTRACT

Mass-transport complexes (MTCs) are one of the most sedimentologically and seismically distinctive depositional elements in deep-water depositional systems. Seismic reflection data provide spectacular images of their structure, size, and distribution, although a lack of borehole data means there is limited direct calibration between MTC lithology and petrophysical expression or knowledge of how they may act as hydrocarbon reservoir seals. In this study, we evaluated the lithological and petrophysical properties and seismic characteristics of three deeply buried (>2300 m below the seabed) Pleistocene MTCs in the northern Gulf of Mexico. We show that (1) MTC lithology is highly variable, comprising a mudstone-rich debrite matrix containing large (4.5-km3), deformed, sandstone-rich blocks; (2) MTCs are generally acoustically faster and are more resistive than lithologically similar (i.e., mudstone-dominated) slope deposits occurring at a similar burial depth; (3) MTC velocity and resistivity increase with depth, likely reflecting an overall downward increase in the degree of compaction; and (4) the lowermost 15–30 m of the MTCs, which represent the basal shear zones, are characterized by relatively high P-wave velocity and resistivity values, likely caused by shear-induced overcompaction. We conclude that detailed analysis of petrophysical data, in particular velocity and resistivity logs, may allow recognition of MTCs in the absence of high-quality seismic reflection data, including explicit identification of the basal shear zone. Furthermore, the relatively thick basal shear zone, rather than the overlying and substantially thicker MTC itself, may form the primary permeability barrier and thus seal for underlying hydrocarbon accumulations.

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