Geological Reservoir Characteristics of Fine-Grained Turbidite Systems

By

BOUMA, ARNOLD H.

Louisiana State University, Baton Rouge, LA,

SPRAGUE, RONALD A., and KHAN, ARFAN M.

Enterprise Oil Gulf of Mexico, Inc., Houston, TX

Coarse-grained and fine-grained turbidite systems are two end members of the majority of deepwater sand deposits. Their internal characteristics differ considerably, requiring different analytical approaches. Coarse-grained systems are sand-rich, gradually prograde into the basin and become thinner and finer in the downdip direction. Fine-grained systems bypass more than 75% of their sand to the outer fan area.

Characteristics of fine-grained systems are: 1) upper fan (basin slope) with erosional canyon, feeding sediments into the basin. It is the last part filled, comprised of mud-rich deposits; 2) middle fan with leveed channels and leveed portions of distributaries. Net accumulation is slow because of frequent erosion, which removes mud drape and part of the underlying sands. Massive amalgamated sands may locally show preserved muds at bed contacts. Levee deposits often show parallel lamination, cross-bedding, and climbing ripples; 3) lower fan with non-leveed distributaries and oblong, sinuous sheet sands, commonly in communication with each other. Individual layers show turbidite sequence characteristics. Several layers can be stacked with sand-on-sand contact in the axis, becoming mudstone at the margins. Stacking is followed by lateral switching and occasional forward progradation. Muds are found between these packages. Individual sand sheets split into narrow, sinuous fingers in the fan fringe area. Mud deposition in the depressions between sheet sands and their fingers presents a morphology that mimics a progradational, frequently bifurcating channel system. The major deposition takes place during the initial transgressive systems tract period.