ABSTRACT

The U. S. Department of Energy has identified mature fluvial-deltaic reservoirs as being the highest priority reservoir type for near-term domestic reserve-growth potential. Detailed characterization studies to locate the estimated 15 billion barrels of mobile oil remaining in these reservoirs must focus first on those reservoirs with the greatest potential if they are to be economically successful. In quick-look analyses that estimate reserve-growth potential, stratigraphic heterogeneity is the most difficult factor to evaluate. Through detailed study of Frio Formation upper delta-plain fluvial reservoirs in T-C-B field, south Texas, and by using outcrop observations from the Ferron Sandstone, central Utah, a model has been developed that relates stratigraphic heterogeneity to rate of accommodation during deposition, a feature that varies predictably throughout a depositional cycle.

Upper delta-plain fluvial channelbelts deposited at the base of a depositional cycle, during periods of low accommodation, are narrow, few in number, and relatively internally homogeneous. As a result, they are not effectively contacted by rigid well patterns but can drain large reservoir volumes through individual completions. Because they are commonly isolated within floodplain mudstones, they possess the potential for stratigraphically trapped accumulations away from the structural crest. In contrast, channelbelts deposited at the top of a depositional cycle, during periods of high accommodation, are broad and internally heterogeneous. Despite appearing laterally continuous, they are extensively compartmented by high volumes of low-permeability siltstones and shales, as well as mudclast-rich lag deposits that drape channel boundaries and limit fluid flow at channel-on-channel contacts. AU other factors being equal, these channelbelt types possess the greatest reserve-growth potential because past completions have only contacted small reservoir volumes.

This accommodation-based model for reservoir heterogeneity in upper delta-plain fluvial settings represents the first in a new generation of reservoir models. Similar models developed for reservoirs deposited in the spectrum of depositional settings will improve the effectiveness and increase the use of detailed reservoir characterization studies, resulting in the improved identification and recovery of oil and gas remaining in mature reservoirs throughout the United States.