--> Recognition and Quantification of Heterogeneities in Tidal Backfilled Paleovalley Sandstone Reservoirs, by R. W. Tillman, E. D. Pittman, and E. Kasap; #90986 (1994).

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Abstract: Recognition and Quantification of Heterogeneities in Tidal Backfilled Paleovalley Sandstone Reservoirs

Roderick W. Tillman, Edward D. Pittman, Ekrum Kasap

The sandstone fill of rivers and delta distributaries are ordinarily followed relatively close in time to the formation of their channels, have significant moderate-scale cut and fill within the channels, and have levee building and lateral splays associated with them. Backfills of paleovalleys by tidal processes have virtually none of the above characteristics.

Tidal valley-fill deposits generally have numerous locally continuous horizontal markers. These markers are significant in that they (1) are dominantly clay and fine sand and silts deposited during marine to brackish flooding events, (2) are easily recognizable in core but not on most logs, (3) have very low porosity and permeability, (4) prevent or deter vertical flow between reservoir compartments and where they are discontinuous act as baffles to flow, and (5) are the focus of diagenetic clay formation in immediately adjacent sandstone.

Tidally deposited sandstones, when sourced from the sea, are generally finer grained and contain significantly more detrital (and diagenetic) clay than most fluvial or distributary channel sandstones. The clay occurs as drapes, clasts and "grains." Where clay clasts are abundant, in otherwise high quality reservoir sandstones, the quality of the reservoir is masked on logs by the effect of the clay clasts. Tidal and tidally-reworked valley fills may have significantly lower porosity and permeability values than fluvial deposits.

Multiple fills of valleys occur when successive relative sea-level falls are followed by sea-level rises. Accommodation space for the later-deposited valley-fills may result from destruction of large portions of earlier deposited valley fills by either intense storm-tide or fluvial erosion. This may result in laterally or obliquely adjoining valley-fills. Stacked valley-fills also may result when initial valley-fills do not fill all the accommodation space.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994