Abstract: Provenance Controls on Deepwater Depositional Systems

McLaughlin, Órla - Exxon Production Research Co.; Ken Hood - Exxon Exploration Co.

An Exxon-developed Geographic Information System technique utilizing surface geology, detrital mineralogies, drainage patterns and topography predicts sediment composition provided to depocenters. This provides a framework to evaluate reservoir quality in potential Tertiary plays with limited well control. Well control and modern river sediment have been used to calibrate and confirm modeled results.

Analysis of basins in West Africa, Brazil, North Sea and Gulf of Mexico have documented important links between provenance and reservoir quality of deep-water sandstones. Reservoir quality is strongly controlled by the initial depositional composition of the sandstones, which in turn is controlled by provenance, drainage basin size, climate and relief effects, temporary sediment traps and transport.

Drainage basin size and provenance control sandstone mineralogy and texture. Large drainage areas contribute mature, moderately sorted, typically fine-grained sediments (Amazon, Congo/Zaire, Nile and Niger rivers). Conversely, small drainage systems often contribute immature, poorly sorted sediments (Paraiba do Sul, Cross, Loge and M'Bridge rivers).

Regional analysis of provenance and lithofacies can help constrain where in the basin sands with high reservoir-quality potential may be located. Sandstones derived from quartzose protoliths eroded by the Niger River are more resistant to mechanical compaction and diagenesis and exhibit good reservoir quality. However, at temperatures >90C these sandstones may be prone to quartz cementation due to their fine-grained nature. Sandstones derived from the volcanic terrain in the Cross River drainage area yield ductile lithic grains that are susceptible to mechanical compaction and diagenesis during burial, with abundant detrital clay matrix resulting in poorer reservoir quality.

Applying this approach on a global scale allows further quantification of these key controls, and provides an excellent framework for risking reservoir quality in frontier exploration areas.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil