Petroleum Potential of Mississippi Fan: Exploration Analogs to Flex Trend Reservoirs

Paul Weimer

The Mississippi fan is a large mud-dominated submarine fan over 4 km thick that was deposited in the deep Gulf of Mexico during the late Pliocene and Pleistocene. The fan contains deep-water sediments coeval to the Flex Trend turbidite reservoirs deposited in the continental slope to the north. The fan reflects turbidite deposition in an unconfined basin, in contrast to the confined intraslope basins. Analysis of 19,000 km of multifold seismic data across the fan defined 17 seismic sequences, each characterized by a series of channel, levee, and associated overbank deposits and other mass transport deposits. At the base of nine sequences is a series of seismic facies consisting of mounded, hummocky, chaotic, and subparallel reflections which constitute 10-20% of the sedim nts of each sequence. These facies are externally mounded in cross section and occur in two general regions of the fan. In the upper and middle fan, they occur below channels and are elongate in shape, mimicking the channel's distribution. In the middle to lower fan, they have a fan-shaped distribution, increasing in width downfan. These facies are interpreted to have formed as disorganized slides, debris flows, and turbidites and are informally called mass complexes.

Overlying this basal interval and characteristic of all sequences are well-developed channel-levee systems, which constitute 80-90% of the fan's sediments. Channels consist of high-amplitude, subparallel reflections. Levee sediments have subparallel reflections that have high amplitudes at the base changing upward to low amplitude. The vertical change in amplitude may reflect a decrease in the grain size of the levee sediments. Overbank sediments consist of interbedded subparallel to hummocky and mounded reflections, suggesting both turbidites from the channel as well as slides and debris flows derived both locally and from the slope.

The Mississippi fan has four prospective reservoir intervals and serves as a possible exploration model for Flex trend reservoirs: channel sands with linear trends, unchannelized sands beyond the terminus of the channel downdip that are areally widespread, potentially sandprone levees immediately adjacent to initial channels deposited in some sequences, and limited portions of mass transport complexes. Large structural traps are present where the fan has been deformed by salt and the Mississippi fan foldbelt.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.