Meckel, Lawrence D.¹, David A. Mercer¹ 
(1) Shell Exploration & Production Co, New Orleans, LA 

ABSTRACT: Reservoir Characteristics of 4th-Order Early Lowstand Deposits in the Deepwater Greater Mars-Ursa Basin, Mississippi Canyon, Northern Gulf of Mexico

Stacked late Miocene - early Pliocene sands in the greater Mars-Ursa area, northern Gulf of Mexico, make up a series of fourth- to fifth-order (< 0.5 Myr) genetic stratigraphic sequences. Each sequence is comprised of (1) a lower unit characterized by thick, high net-to-gross, areally extensive fan lobes or sheet sands and (2) an upper unit characterized by thin, variable net-to-gross, laterally restricted channelized or amalgamated systems. The fan lobes overlie marine condensed sections, and were deposited during relative sea-level falls. However, neither erosional surfaces nor correlatable conformities associated with eustatic sea-level lowstands have been identified. 
The fan lobes are often the best reservoirs in the basin. Usually, they are single, laterally continuous, acoustically soft seismic loops, with little evidence of shingling or other complex internal reservoir architectures. However, unlike similar sands in classic Gulf of Mexico Plio-Pleistocene ‘ponded’ seismic facies assemblages, the sheet sands in the Mars-Ursa basin do not exhibit convergent, baselapping seismic geometries, but rather maintain a fairly constant thickness across the basin with tabular or tapering (wedge-shaped) external seismic geometries, finally terminating abruptly against the basin margins. 
Individual sand packages are characterized by single to multiple units with flat bases and blocky to bell-shaped log signatures can be easily correlated between wells (some up to several miles apart), which suggests excellent reservoir continuity on a sub-seismic scale. Maximum net sand thickness of the sheet sands (100s of feet) exceeds all but the thickest of channel axes. Average log-derived stratigraphic net-to-gross ratios are typically higher than in the channels (45-80% N/G vs. 40-65%), though this is not always the case. Detailed core analysis shows that porosity and permeability are not as predictable as other properties: porosities for both channels and sheets range from 18-30%, and sands with similar porosities in sheet and channel environments can have highly variable permeabilities, and vice-versa.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.