Recognition of Estuarine Deposits within Incised Valleys - A Key for Exploration

Keith W. Shanley, Peter J. McCabe, and J. Michael Boyles

Large channel-fill sandstone reservoirs incised into the upper parts of shallow marine parasequences have often been interpreted as distributary channels or tidal inlets cut into coeval shoreface deposits. Over the last few years, many such reservoir-bearing units (especially in the Cretaceous of the Western Interior of the USA, Jurassic of the North Sea, and Miocene of the Gulf of Mexico) have been reinterpreted as the product of incised valleys cut during periods of base-level fall and filled during subsequent rises in base level. At a single outcrop, or within a single well-bore, the differences between these two interpretations may be relatively minor. When a field-wide perspective is taken, there are significant differences in the prediction of sandbody geometry and ateral relationships between these two interpretations. Most distributary channel deposits are single-storey, mudstone-dominated features that have erosion surfaces coeval with adjacent shoreface deposits. Incised valley-fill deposits are commonly multistorey, contain sandstone channel deposits, and show increasing estuarine influence upwards. Because the fill of incised valleys is significantly different in age and lithology to the underlying shallow-marine strata there is often a pronounced difference in reservoir quality across the sequence boundary.

Recognition of estuarine strata within incised valleys has important implications: (1) Incised valleys generally exhibit a complex fill resulting from repeated incision and aggradation. Aggradation within an incised valley reflects an overall long-term base-level rise, however, widespread erosion surfaces within the valley may reflect changes in sediment supply or downdip fluctuations in stratigraphic base level. With continued base-level rise, fluvial deposits become increasingly tidally-influenced as incised valleys evolve into estuaries. Within otherwise continental successions, estuarine deposits are chronostratigraphically equivalent to maximum flooding surfaces in the marine realm. (2) Because the timing of estuarine deposition reflects a balance between sediment supply and rate of base-level rise, estuarine strata commonly mark a pronounced change in fluvial sandbody geometries, from well interconnected below to more isolated above.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California