GUSTASON, EDMUND R., BP Exploration (Alaska), Anchorage, AK

ABSTRACT: Facies Architectural Analysis of Progradational Parasequence Sets, Upper Coal-Bearing Ferron Sandstone, Central Utah: Implications for Reservoir Description

The #1 and #2 deltaic deposits of the upper Ferron Sandstone Member of the Mancos Shale are interpreted as progradational parasequence sets. Component seaward-stepping parasequences display at least two distinct types of upward-coarsening and upward-thickening vertical successions of lithofacies associations or architectural elements, interpreted as strandplain and distributary mouth bar deposits.

Strand-plain parasequences are characterized by an upward-coarsening and upward-thickening succession of sandstone, siltstone, and claystone beds dominated by swell-wave-generated ripple cross-lamination, and storm-wave generated planar parallel lamination and hummocky and swaley cross-stratification. These parasequences display a high degree of lateral continuity of individual beds and architectural elements. Bedding features dip gently up to 1 degrees in a basinward direction.

Distributary mouth bar parasequences also are characterized by an upward-coarsening and upward-thickening succession, but are dominated by massive to normally graded horizontal stratification, current-generated ripple cross-lamination, both high- and low-angle cross-stratification, and rare hummocky cross-stratification. Heterolithic strata grade upward into amalgamated sandstone beds with discrete downstream accreting macroforms, up to a few meters thick, and erosional scours up to a few meters thick and filled with trough cross-stratified sandstone, interpreted as distributary channel deposits. Sandstone beds and architectural elements are laterally discontinuous and display a relatively steep basinward dip (up to 6 degrees).

Inasmuch as each parasequence type displays differences in lateral continuity, connectivity, and dip- and/or strike-oriented attitudes of architectural elements, it is important to identify component architectural elements from vertical successions. This predictive relationship reduces the amount of uncertainty associated with correlation and/or spatial distribution of fluvio-deltaic architectural elements in analogous reservoirs.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.