BARTON, MARK D., MICHAEL H. GARDNER, and NOEL TYLER, Bureau of Economic Geology, The University of Texas, Austin, TX

ABSTRACT: Predicting Architectural and Permeability Changes in a Distributary Channel-Belt through Base-Level Cycles

Detailed architectural and permeability analysis of distributary-channel facies along the depositional profile of a retrogradational delta cycle in the Ferron Formation, Utah, reveal systematic changes in channel morphology and permeability structure. The fundamental depositional units in Ferron distributary channel-belts are macroforms such as side-attached bars in distributaries. The geometry, connectivity, and stratal diversity of macroforms change as a function of their position within a base-level cycle and their geographic position along thedepositional profile. The dynamic evolution of channel-belt architecture records a complete cycle of base-level fall and rise that corresponds to channel incision, expansion, and aggradation. In upstream reaches, near the landward pinchout of coeval delta-front facies, high-permeability intermediate and late stage macroforms are volumetrically dominant. By contrast, in downstream channel reaches, volumetrically dominant early-stage macroforms exhibit high permeability. Thus, an overall upstream onlap pattern displayed by early- to late-stage macroforms corresponds to observed permeability trends and is predictably related to base-level fall and rise.

Incised into coeval delta-front deposits, early-stage heterolithic macroforms are laterally restricted, poorly interconnected and separated by mudstones. They contain a low diversity of stratal types. These are overlain by poorly interconnected macroforms segregated by thick mudclast lags. These macroforms are heterogeneous and display a higher diversity of stratal types formed by lateral and downstream migration. Late-stage aggradational macroforms are more homogeneous, display a convex-downward geometry, and contain the highest diversity of stratal types. These macroforms interfinger with abandoned channel-fill and crevasse-splay deposits.

Differential preservation of macroform types results in downstream changes in average permeability, continuity, and channel interconnectedness. In a downstream direction, overall permeability and internal continuity increase. Early-stage macroforms average 700 md compared with 300 md in subordinate late-stage macroforms. Internally, early-stage macroforms are relatively homogeneous, displaying uniformly high permeabilities. Upstream, dominant middle and late-stage macroforms display higher stratal diversity and greater overall permeability variation. However, channel interconnectedness decreases in adownstream direction. Early-stage macroforms are poorly interconnected and compartmentalized by extensive mudstones. Upstream, middle and late-stage macroforms are separated by extensive, iscontinuous, low-permeability mudclast lag deposits.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.