Lee F. Krystinik1 and Dale A. Leckie2
1 Krystinik Litho-Logic, Fort Worth, TX
2 Nexen Inc, Calgary, AB
The incised valley-fill model has become widely popular over the last two decades, largely to the exclusion of other channelized deposits. Because of sedimentologic variability of incised valley fills, almost any sandstone/mudstone succession can be made to fit into some part of the model.
Paleovalleys may contain braided fluvial sands mixed with eolian dunes, fluvial point bars, bay-head deltas, estuarine muds, tidal point bars, bioherms, flood-tidal deltas, transgressive barriers or marginal to open marine mudstone, among others. An incised valley-fill interpretation triggers a number of model-driven implications. Sequence boundaries are implicit, leading to potentially large exploratory expenditures. How does one distinguish incised valley-fill deposits from other channelized successions and avoid potential economic train wrecks?
There are well defined criteria to identify valley fill successions, but these are not always applied objectively. Key observations include regional stratigraphic observations, the nature and scale of the container, the depth of the incision (greater than one parasequence), the incision surface is linked to a regionally extensive interfluve unconformity, the fill is multistory and the succession records an upward increase in accommodation space.
Careful observations also help differentiate other channelized systems. Distributaries may stack, but are single story fills within their containers. Tidal inlets may occur within the fills of incised valleys, but exist within very different containers. Data from single wells may not adequately differentiate among the options, but outcrop, core, seismic data and image logs provide data that, interpreted without an overriding model in place, can have large economic impact.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005