Accommodation-Based Controls on Fluvial-Deltaic Reservoir Compartmentalization: Examples from the Oligocene Frio Formation, South Texas

Paul R. Knox

Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining In U.S. fluvial-dominated deltaic reservoirs. To improve this prediction, facies-specific relationships between accommodation trends and sand-body architecture that were established by outcrop studies in the western interior were successfully applied in a study of Oligocene Frio Formation reservoirs along the Vicksburg Fault Zone of South Texas. Compartment architecture and internal heterogeneity were determined for mature upper and lower delta-plain reservoirs within two 4th-order genetic units to document reserve growth potential.

In lower delta-plain settings, low-accommodation conditions at the base of genetic units produce narrow distributary channels that feed many small mouth bars, whereas high-accommodation conditions near the top of genetic units yield broader distributaries and an expansive wave-dominated delta front. In proximal upper delta-plain settings, low accommodation produces few narrow fluvial channels that are commonly internally homogeneous, probably because coarse-grained trough cross-strata predominate, with the finer grained silty upper point-bar deposits being removed by successive scour events during which base level has not risen substantially. In contrast, high accommodation yields many broad internally heterogeneous channels in which high accommodation prevents significant scour and e osion of fine-rained channel-top facies.

This study documents the profound effect that accommodation conditions can have on reservoir compartmentalization and resulting production behavior. Similar studies in other settings are needed to identify these relationships for the entire spectrum of depositional systems.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California