Marcos Zavala¹, Vielma Miriam², Timothy A. Cross³, Brian Horn⁴, Michael L. Hendricks⁵

(1) PDVSA/ Colorado School of Mines, Golden, CO
(2) PDVSA/Colorado School of Mines, Golden, CO
(3) Colorado School of Mines, Golden, CO
(4) BP-AMOCO, Houston, TX
(5) Hendricks and Associateds INC, Englewood, CO

ABSTRACT: Shoreface/Tidal Couplets in the Upper Almond Formation, Wamsutter, Echo Springs and Rock Springs Fields, South-Central Wyoming

A high-resolution stratigraphic study was conducted on the Upper Almond Formation from the Wamsutter, Echo Springs and Rock Springs fields, Washakie Basin, south-central Wyoming. Two dip and two strike stratigraphic cross sections using 60 core-calibrated well logs define the stratigraphic architecture and facies distributions in the study area. Thirteen small-scale (=7 m thick) progradational/aggradational units comprising coastal plain, shoreface and tidal strata are arranged in alternating landward-stepping and seaward-stepping stacking patterns. Tidal strata occur only on top of shoreface and coastal plain strata of the top landward-stepping progradational/aggradational units, in the position of maximum accommodation:sediment supply (A/S) regime. Otherwise, the progradational/aggradational units consist only of coastal plain and shoreface strata. Usually, tidal strata are coarser (lower and upper medium sand) than shoreface (upper fine) sandstone and are much better reservoirs. Landward and seaward depositional limits of tidal strata apparently are coincident with depositional limits of shoreface strata, but extend up to 18 km landward of the landward depositional limit of the shoreface. The contact between tidal and coastal plain or shoreface strata is sharp, but very low or no relief; there is no obvious indication of scouring or ravinement. The regular occurrence of tidal strata above shoreface and coastal plain strata in the position of maximum A/S indicates these are a genetic couplet. The couplet is explained by a sea-level rise at the end of shoreface progradation that creates a strongly embayed coastline. Wave energy is dampened by the shallow, flooded, flat surface, and tidal currents are strengthened and focused by the embayed coast. The transgression taps directly into the coarser bedload of higher gradient rivers on the flooded coastal plain.

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