Abstract: Accommodation Controls on Fluvial-Deltaic Reservoir Architecture

Michael H. Gardner, Brian J. Willis, Mark D. Barton

Hydrocarbon recovery efficiency is controlled by reservoir heterogeneities resulting from geometric arrangements of strata, or "stratal architecture." Traditional reservoir characterization relates depositional systems to stratal architecture. High-resolution sequence stratigraphy of outcrop analogs provides a chronostratigraphic framework for evaluating accommodation conditions of depositional systems. Key stratigraphic surfaces and/or correlative strata define a hierarchy of chronostratigraphic units of different periodicities.

The Upper Cretaceous Ferron Sandstone, an analog to high accommodation and sediment supply fluvial-deltaic reservoirs, comprises an intermediate-term stratigraphic sequence consisting of seven short-term stratigraphic cycles. Each short-term stratigraphic cycle contains fluvial- to storm-dominated shallow-marine deposits laterally replaced by distributary channel deposits. Major unconformities are absent and sandstone bodies exhibiting a characteristic permeability structure, facies association, and stratal geometry, termed depositional elements, are fully preserved. These attributes vary systematically as a function of the elements stratigraphic position making them the most important reservoir unit affecting fluid flow (flow units). These preservational conditions emphasize depositi nal controls on stratal architecture.

The Lower Cretaceous Fall River Formation, an analog to low accommodation fluvial-deltaic reservoirs, comprises an intermediate-term stratigraphic sequence consisting of six short-term stratigraphic cycles. The lower five short-term cycles are unconformity-bounded. Each short-term stratigraphic cycle contains thin tidally influenced, fluvial- to storm-dominated shallow-marine deposits locally incised by valley-fill deposits changing upward from low-sinuosity fluvial to estuarine. Detailed outcrop study of valley fill strata shows unconformities controlling permeability distributions, segregating the reservoir, and juxtaposing low and high permeability strata. This is particularly prevalent in stacked, or nested valley fills which are truncated by multiple, irregular and high-relief un onformities. Hence, in low accommodation fluvial-deltaic strata the most important stratal element affecting fluid flow are unconformity bounded short-term stratigraphic cycles. This contrasts with high accommodation fluvial-deltaic strata where smaller-scale depositional elements comprising individual short-term stratigraphic cycles form flow units.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France