McLAUGHLIN, ORLA, Exxon Production Research Company; KEN HOOD, Exxon Exploration Company

Abstract: Provenance Controls on Sediment Composition and Reservoir Quality in Deepwater Depositional Systems - A Global Perspective

An Exxon-developed Geographic Information System technique utilizing surface geology, detrital mineralogies, drainage patterns and topography predicts sediment composition provided to depocenters. These data are used to map regional trends in provenance and lithofacies and thereby infer the paleogeographic distribution of the best reservoir quality sands. Well control and modern river sediments have been used to calibrate and confirm modeled results.This is a quick-look technique for comparing relative provenance risk, and it provides a framework to evaluate reservoir quality in potential Tertiary plays with limited well control. However, this technique has some limitations: its applicability decrease with increasing reservoir age and it does not incorporate the effects of transport distance and weathering.

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. Regional study results indicate the primary control on reservoir quality in West Africa Tertiary deep-water reservoirs is depositional texture. These reservoirs are commonly fairly shallow, unconsolidated and unaffected by diagenetic cements due to the low burial temperatures. Burial compaction plays a minor role in affecting the quality of these reservoirs. The grain textures are controlled by the depositional environment, regional sediment 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 degrees 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 #90923@1999 International Conference and Exhibition, Birmingham, England