Turbidite Reservoir Architecture in the Northern Gulf of Mexico Deepwater: Insights from the Development of Auger, Tahoe, and Ram/Powell Fields
J. W. Kendrick, 2000. "Turbidite Reservoir Architecture in the Northern Gulf of Mexico Deepwater: Insights from the Development of Auger, Tahoe, and Ram/Powell Fields", Deep-Water Reservoirs of the World, Paul Weimer
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The development and early production of several fields is making available new geological, fluid, and pressure data with which to validate, or modify, existing models of reservoir architecture. This paper summarizes the observations and insights from several developed turbidite reservoirs. The reservoirs represent examples from each of several major reservoir architecture types, including layered/amalgamated sheet sands (Auger field); channel-levee deposits (Tahoe Field); and amalgamated channel complexes (Ram/Powell field).
Previous authors (McGee et al., 1994) have described the “S” sand at the Auger field as a layered- and amalgamated-sheet sand deposited within a structurally confined salt minibasin. Production performance reveals that the pressure of the reservoir has declined more slowly than expected, indicating aquifer support from sheet sands that extend across the entire minibasin. Sequential production logs of a downdip well indicate that the reservoir is not watering out in a uniform, bottom-to-top pattern, however. The occurrence of water-bearing zones above oil-bearing ones indicates the presence of effective internal seals within the reservoir that, like the sands, are laterally extensive.
Thin-bedded reservoirs, such as those that produce gas at prospect Tahoe, are interpreted as the overbank portion of channel-levee complexes. The thin-bed reservoirs comprise millimeter to centimeter thick sand beds separated by mudstones of similar thickness. Early concern about the lateral extent of such thin-bedded sands led to pre-development production testing (Shew et al., 1993), which indicated that the beds were laterally continuous over areas exceeding several hundred acres. Pressure profiles in newly drilled wells, however, exhibit stratigraphically varying pressure depletion. Sands in the upper levee facies appear to be laterally continuous, even across the channel. Sands in the lower portion of the reservoir, however, are much less connected. Channel-levee architecture appears to evolve from localized to extensive overbank sedimentation. The thicker beds, which occur at the base of the levee facies, do not appear to be the more continuous.
Most problematic, from a development perspective, are channel-fill reservoirs, such as the Miocene “N” sand at Ram/Powell. The reservoir appears to comprise multiple, partially stacked, laterally migrating sand units deposited within a pre-existing erosional scour; they exhibit large thickness differences, even over short distances. Paradoxically, all wells indicate some degree of pressure communication, but the occurrence of numerous perched water levels implies the existence of multiple, internal reservoir compartments. Partial amalgamation of the two sand members allows fluid communication to occur, but the lateral heterogeneity in sand thickness creates structurally low closures in which water is trapped.
Our observations illustrate a spectrum of turbidite reservoir architectures and their influence on production performance. The elements important to reservoir characterization are often below the limits of seismic resolution. It is valuable therefore to integrate the diverse information that comes from development and production activities in order to develop the reservoir architecture models that will facilitate future planning and decision making.