Genetic Stratigraphy, Stratigraphic Architecture, and Reservoir Stacking Patterns of the Upper Miocene - Lower Pliocene Greater Mars-Ursa Intraslope Basin, Mississippi Canyon, Gulf of Mexico

Lawrence D. Meckel, III, Gustavo A. Ugueto, H. David Lynch, Earl W. Cumming, Ben M. Hewett and Eric J. Bocage: Shell Exploration & Production Company, New Orleans, LA; Charlie D. Winker: Shell Technology EP, Houston, TX; and Brian J. O'Neill: Shell Deepwater Services, New Orleans, LA

Abstract

The late Miocene - early Pliocene sediments of the northern greater Mars-Ursa Basin in the Gulf of Mexico are a record high-resolution (ca. 0.5 Ma) cyclic deposition of deepwater fan lobes (sheet sands) and channelized or amalgamated systems. Local to regional condensed sections bound these couplets. Internally, the sheet sands and channelized systems are separated by a surface of bypass and/or erosion. The fourth-order cycles, or parasequence sets, occur within both "ponded" and "bypass" seismic facies assemblages, third-order packages that have been interpreted to record progressive fill and spill sedimentary dynamics in salt-withdrawal minibasins.

The third-order sequences reflect variations in sediment supply to the greater Mars-Ursa Basin. The rate of deposition of the ponded assemblage (10.0 - 7.0 Ma) is significantly greater than that of the bypass assemblage (7.0 - 4.2 Ma). Not coincidently, the ponded assemblage contains a greater abundance of thick, laterally extensive sheet sands and has a higher composite net-to-gross; however, the bypass facies assemblage contains more amalgamated and bypass channels and has a commensurately lower composite net-to-gross. Couplets in both assemblages display compensational stacking patterns, in which the overlying channel system is best developed where the sheet sand system is thin or not present.

The fourth-order stratigraphy of the basin is controlled by local variations in accommodation space and sedimentation. The formation of genetic parasequence sets depends on variations in four factors: the rate of sediment supply, the sediment caliber, the rate of creation of accommodation space, and the capacity for dispersive sediment transport. Eustatic sea level changes may affect the supply of sediment from the shelf to the slope, but is not a primary factor in the formation of deepwater sequences or parasequence sets.

The condensed sections that bound the parasequence sets are deep marine (pelagic) claystones that drape the topography. They occur at the tops of faunal abundance and diversity peaks, and often correspond to abrupt changes in incremental overpressure. Because they represent periods when the rate of sedimentation is at a minimum in the basin, and the relative creation of accommodation space is at a maximum, basin margins had their highest relief at these times. Several of the surfaces are interpreted to correlate to regional flooding events on the shelf, but the inability to correlate other condensed sections to maximum flooding surfaces suggests that local variations in supply and accommodation controlled sequence formation.

Thick, high net/gross fan lobes that occur above the condensed sections have been deposited as sedimentation rates in the basin increased. They fill paleo-topographic lows, and terminate abruptly against the high-relief basin margins. Deep-water faunal abundances in these intervals are at relative minima due to the influx of sediment, and in some instances, the sands have a biofacies signature indicative of reworking. However, there are not corresponding increases in either terrigenous content or reworked Cretaceous material, suggesting a local source for the sands.

Subtle surfaces separating the sheets and channels are identified by the onset of apparent paleo-bathymetric deepening. Typically, the surfaces are related to intra-basinal system avulsion, caused by decreasing sediment supply. In certain cases, these surfaces document sediment bypass and erosion, caused by infill of available accommodation space by the sheet sands. In either case, they record a significant change in basin conditions.

The channelized systems or amalgamated channel/sheet systems that overlie the "bypass" surfaces are associated with a decreasing rate of sedimentation relative to the rate of creation of accommodation space. Thin, laterally restricted amalgamated channels and sheet deposits may accumulate in the limited accommodation space. If present, erosional channels and bypass scours are back-filled by sand.

This detailed stratigraphy is proposed as a conceptual framework that could help to guide future exploration and development in subsalt areas and deep, untested intervals in the greater Mars-Ursa Basin, and may have implications for other basins worldwide.