The “Above Magenta” Reservoir at Ursa Field: A Process-Response Model to Explain a Classic Wireline Long Singnature
Kevin Schofield, John Serbeck, 2000. "The “Above Magenta” Reservoir at Ursa Field: A Process-Response Model to Explain a Classic Wireline Long Singnature", Deep-Water Reservoirs of the World, Paul Weimer
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In many deep water slope and basin settings, the gamma ray (GR) log profile of a blocky, low gamma unit capped by a “fining upward” trend of increasing GR response back to the shale baseline is commonly interpreted as the signature of a leveed channel system. The “Above Magenta,” the smallest and shallowest of the three principal reservoirs at Ursa Field (located in the Gulf of Mexico some 130 miles southeast of New Orleans), is a classic example of this profile.
The interval has been cored in the Mississippi Canyon (MC) Block 809-1 well. It is characterized by five lithofacies, ranging from clean, structureless fine sands through very fine to fine, laminated sands; carbonaceous very fine sand; finely interbedded very fine sands and mudstones; and structureless gray mudstones. The sand-dominated lithofacies are associated with stacked “blocky” GR responses at the base, representative of at least three sand bodies, each with a different, but internally-consistent sedimentological character. These are in turn overlain by an upward-increasing GR motif, associated with the heterolithic facies.
Within the leveed channel model, these log and core facies are interpreted as a channel fill or lag, overlain by a fining-upward levee. Recent descriptions of the process dynamics of leveed submarine channels (Peakall et al., 2000) suggest that this interpretation is mechanically difficult to justify, as even highly sinuous leveed submarine channels rarely migrate far enough laterally to deposit a full levee profile over a channel fill.
We propose an alternative model for explaining the vertical profile of the “Above Magenta” sand. The model provides a more reasonable explanation for the observed wireline log, core, and seismic character of the sands which are not well accounted for by the leveed channel model. More importantly, it offers a predictive description of the geometry of the reservoir sands, which suggests different connectivity and lateral extent than would be expected from the channel model.
The model proposes that the “Above Magenta” sands are representative of lobate bedforms akin to “HARP” facies (sensu Damuth, 1998) associated with deposition precipitated in the region of the basin-edge by the interaction of gravity currents and the local slope. Different lithofacies in the sands are ascribed to deposition from differing gravity flow regimes associated with deposition from variously through-going or deflected flows.
We predict that in the most active depocenter, in the region immediately adjacent to the “Venus” salt, thickly amalgamated lobate sands will occur, forming a series of laterally-offsetting sheets. These will be separated by thinner intervals of more distal facies, representing periods of relative inactivity when the active depocenter migrated to adjacent lows. Thinner bedded, lower net-to-gross facies will predominate away from the salt-defined edge of the basin to the east and south, where flows are unimpeded and pass through the basin with only minor deposition.
Our model explains the seismically observed increase in thickness towards the “Venus” salt, and the apparent cessation of thick sand deposition to the south. It implies a different geometry for the sands than the channel-fill model. The overlying low net-to-gross package may represent the levee deposits of a channel system which develop in the center of the basin subsequent to the phase of deposition described above, or a period of unconfined flow and relatively slow deposition as the deposystem evolves from sand-rich to sand-poor through the depositional cycle.