ABSTRACT

Although turbidites are typically associated with deep-water environments, they also can be preserved in shallow shelf settings. Sandy turbidites originating as density underflow currents or from sediment failure events are common in the delta-front region of most fluvial-deltaic systems. This is particularly true of shelf-edge deltas characterized by high sedimentation rates, a common condition in Gulf Coast reservoirs.

Like basinal turbidites, delta-front turbidites are deposited from suspension in a downslope-directed gravity flow. Internally, the sandy reservoir beds exhibit stratification typical of waning flow energy and are commonly normally graded. Core and outcrop examples tend to show a higher percentage of clasts and dispersed organic grains than most deep-water turbidites.

Unlike basinal turbidites, delta-front turbidites are typically nonchannelized. Although individual flows may be directed down gullies or other bathymetric depressions, the resulting deposits tend to be lobate and suggest that reservoir geometry and character is determined mainly be sheet-flow processes. Lateral continuity of delta-front turbidites probably is somewhere between deep-water sheet sands and deltaic channel mouth bar deposits.

Production histories from several Gulf Coast fields show prolific production from vertically stacked delta-front turbidite packages. Although intermixed mass movement deposits can complicate the reservoir succession, they can also improve vertical connectivity. An improved understanding of delta-front turbidites requires development of reservoir models that integrate geological, geophysical, engineering, and production data.

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