Seismic facies and high-resolution biostratigraphic analysis provide a sequence stratigraphic framework for interpreting lateral distribution of sand-prone facies and possible reservoir connectivity in the Ship Shoal 351-358 to Ewing Bank 988 minibasin, offshore Louisiana. The interval of interest is an isochron thick interpreted as a lowstand systems tract deposited in bathyal water depths within an intraslope-minibasin. This basin is approximately 50 km from the age-equivalent shelf/slope break. The isochron thick was deposited between the late Pliocene Discoaster pentaradiatus and early Pleistocene Discoaster brouweri Condensed Sections, as calibrated by data from five wells penetrating this interval within the intraslope basin.

The D. pentaradiatus - D. brouweri sequence consists of the synclinal fill of a salt withdrawal basin forming an isochron thick that thins onto adjacent salt-cored structural highs. This isochron interval was subdivided into four seismic facies and each was calibrated with local well data. Mapped patterns of these seismic facies suggest a network of channel-form systems supplying sediment to depositional lobe-forms within a slope valley. Sandy sediments were transported by gravity-driven processes from the shelf, down slope through a valley and into this local intraslope-minibasin. Upon filling this minibasin, the gravity-driven sediments spilled farther south into the area of the Green Canyon 18 field, where these sandstones are the main reservoir. The mapping of channel-and levee-form patterns suggests that the transport process included turbid flow with consequent overbanking of the channel and levee formation.

Following the above analysis, three wells and two sidetracks were drilled to further test the prospectivity of the area. Rock type and hydrocarbon predictions based on calibrated seismic facies analysis were confirmed by the new wells.

Sandstones occur within the axes of isochron thicks and shales dominate the isochron thins over paleo-highs with gradational facies between. Hydrocarbon accumulation occurs in two settings: 1) channel-fed lobes with blocky log character yield hydrocarbons within areas of structural closure; and 2) overbank facies that contain hydrocarbons in low-resistivity pay both within and outside of structural closure Local abundance peaks of planktonic microfossils provide correlation control between the four depositional lobes of this lowstand deposit. The biostratigraphic data and paleogeographic interpretation suggest that each depositional lobe is separated from the others by mudstone drapes, resulting in discrete partitioning of reservoir sands.

Unnumbered Figure. Gulf of Mexico Gravity Flow Event; from Shanmugam (1993, AAPG Bulletin) artwork Mark Lindsey, Mobil Technology Company.

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