Database-Driven Analysis of Geologic Controls on the Geometry of Incised-Valley Fills: Implications for Facies and Sequence Stratigraphic Models

Abstract

Incised-valley fills are important fluvial and shallow-marine reservoir targets; they also serve as reference for exploration of downdip deep-marine sands. Valley morphology strongly controls location and rates of sediment accumulation, valley-fill facies architecture, and the preservation potential of lowstand lithosomes on continental shelves. However, little is known about the geologic controls on valley morphologies and whether valley-fill characteristics (geometry, facies architecture) can be used predictively.

Here, a database-driven statistical analysis has been performed to investigate geologic controls on incised-valley morphometry, based on quantitative data extracted from the published literature and relating to >200 classified valley fills from different physiographic and climatic settings worldwide. Particular focus is given to Late Quaternary analogs, because geologic boundary conditions acting upon these systems can be constrained with confidence. The geometry of valley fills can therefore be related to controlling factors such as direction and rate of sea-level change, drainage-basin size, shelf physiography, substrate lithology, and tectonic setting.

Key results are as follows. (i) The scale of incised-valley fills is generally larger than that of channel-belt sand bodies, including aggradational channel belts deposited on the shelf at lowstand, as assessed in terms of axial thickness, strike-oriented width, and cross-sectional area. (ii) Valley size and shape are governed by both upstream controls, such as river discharge, and downstream controls, including coastal-plain and shelf physiography, shelf width, shelf-break depth and substrate lithology, with drainage-basin area as the primary control. (iii) The distributions of incised-valley dimensions for shelf-break depth < 120 m and shelf-break depth > 120 m are markedly different, but in a manner that contrasts with what is expected on the basis of sequence stratigraphic models that predict distinct incised-valley geometries in relation to Type 1 versus Type 2 sequence boundaries. (iv) Valley-fill dimensions change in variable ways basinward, from coastal-plain to shelf-break, depending on the types of coastal processes operating during their infill.

Some results challenge paradigms embedded in sequence stratigraphic models regarding the relative importance of upstream and downstream controls, and have significant implications for hydrocarbon-reservoir prediction and characterization.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018