Michael H. Gardner¹

(1) Colorado School of Mines, Golden, CO

ABSTRACT: The Distinction between Ancient Fluvial and Valley-Fill Sandstone Reservoirs: Semantics or Economics

Because fluvial sandstones form significant reservoirs a mature set of geologic models exists that relates facies, sediment bodies, and geomorphology to reservoir heterogeneity. Most subsurface channelform sandbodies were interpreted as channels when fluvial models were first developed (60s and 70s). Sequence stratigraphy (80s and 90s) reinterpreted many of these as incised valley-fills. Today valley fills represent the favored interpretation for subsurface channelforms. Has the pendulum swung too far? A matter of semantics or are their economic implications to these terms?

The Ferron Sandstone was deposited along the active western margin of the upper Cretaceous seaway. This foreland basin setting recorded high accommodation and sediment supply. Fluvial-deltaics deposited during a 1.8 m.y. period produced a 250-m thick clastic wedge that extends 50-km into the seaway. Ferron channels have been interpreted as distributaries, meanderbelts, and incised valleys. By contrast the Fall River Formation and Muddy Sandstone were deposited on the eastern cratonic margin (low subsidence and sediment supply) of the early Cretaceous seaway. An equivalent thickness of fluvial-deltaic deposits (20-m.y.) form three thin clastic wedges dissected by regional unconformities and associated valley systems that extend at least 500-km across the seaway.

Because Ferron distributary channels do not extend beyond the seaward depositional limit of deltas they fed, channel sandbodies are restricted to the delta. By contrast early Cretaceous valleys extend 100s of kms beyond the seaward depositional limit of deltas they incise. Early Cretaceous valley-fill and related deltaic reservoirs have produced over 3 BBBl of oil. These reservoirs cover a 100,000 km2 area making both their proven production and future exploration area several orders of magnitude larger than Ferron channels.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado