KRYSTINIK, LEE F., Krystinik Litho-Logic, Fort Worth, TX, and RICHARD H. MEAD

ABSTRACT: Finding Subtle Traps Using Sequence Stratigraphic and Synsedimentary Tectonic Analysis

The interplay between sedimentary processes and synsedimentary tectonics can produce a broad spectrum of deposits. These deposits vary greatly in response to sediment supply and reactive sea level (often dominated by tectonic uplift or subsidence). This variability offers opportunity for explorationists.

The Campanian upper Almond Formation in southwestern Wyoming contains at least 15 aggradational to backstepping shoreline complexes laid down during a period of net transgression from, along the tectonically active margin of a foreland basin, 72 to 70.5 m. y. ago. Most of these sanding-upward, storm-dominated shorefaces interfinger in the landward direction with oyster-bearing lagoonal mudstone and are interpreted as microtidal to low mesotidal barrier islands. Well-exposed outcrops of the Almond Formation occur within a few kilometers of densely drilled Almond Formation oil fields, making it an excellent subject for analog studies. The Almond Formation can be considered analogous to paludal portions of the Officina Formation in Venezuela.

Reservoir compartmentalization in the Almond Formation occurs at several scales, with the largest scale compartments composed of aggradational to retrogradational sequences, bounded by fourth- or fifth-order sequence boundaries. Almond lowstand shorelines stack aggradationally prior to transgression by areally extensive marine mudstone. Each sequence in the upper Almond is composed of several parasequences that intercalate with lagoonal mudstone to the west and marine mudstone to the east. Highstand systems tracts are poorly preserved, often completely removed by erosion along overlying sequence boundaries, which can be tied to seaward jumps of facies in excess of 30 km. These seaward jumps place fluvial sediment, coal, and lagoonal deposits abruptly over marine mudstone. Fine-scale compartmentalization in the barrier complexes occurs at parasequence boundaries and in association with major subfacies boundaries (barrier margins, tidal inlets, flood-tidal deltas, and wash-over fans).

Depending upon accommodation rate, transgressive erosion (ravinement) can remove much of a barrier succession (5-30 m). Further stratigraphic complexity is induced by synsedimentary faulting and subsidence, which locally enhance preservation of isolated reservoir-quality barrier sandstone units by dropping them below the depth of ravinement.

The degree of preservation of Almond reservoir sandstones is highly dependent upon where deposition occurred relative to active and rapidly growing structural features within the basin. On structural highs (areas of slower subsidence), erosion by combined cycles of fluvial incision and transgressive ravinement can remove 100-200 m of sediment relative to flanking structural lows (higher subsidence areas). Because of these syndepositional structural complexities, "bars" previously considered to be isolated sandstone bodies can be shown to have originated as much larger shoreface deposits that have been erosionally dissected. These erosional remnants are now the focus of active exploration in the basin, and suggest potential for large stratigraphic traps in numerous "mature" basins.

AAPG Search and Discovery Article #90910©2000-2001 AAPG Distinguished Lectures