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Abstract: Characterization of Heterogeneity Style and Permeability Structure in Fluvial-Deltaic Reservoirs Deposited in a Low-Accommodation Setting

Shirley P. Dutton, Michael H. Gardner, Brian J. Willis

Because of the worldwide importance of resources in fluvial-deltaic reservoirs, a consortium of companies is funding research at The University of Texas aimed at reservoir characterization of fluvial-deltaic depositional systems. The goals of this industrial associates program are to develop an understanding of sandstone architecture and permeability structure in a spectrum of fluvial-deltaic reservoirs and to translate this understanding into more realistic, geologically constrained reservoir models. Our approach is to quantify the interrelationships among sequence stratigraphy, depositional architecture, diagenesis, and permeability structure through detailed outcrop characterization.

The current project focus is the Lower Cretaceous Fall River Formation, a deltaic system deposited in a low-accommodation setting in South Dakota and Wyoming, USA. The Fall River is exposed around the margins of the Black Hills Uplift and produces oil and gas in the adjacent Powder River basin. Extensive field work established the Fall River as a storm- and tide-dominated deltaic depositional system in which the shallow-water marine deposits were locally removed by valley incision during periods of base-level fall. Valley-fill sandstones, which form the most productive Fall River reservoirs, were deposited during base-level rise and grade upward from low-sinuosity fluvial to estuarine deposits. Permeability structure in marine and valley-fill deposits was determined from >5,000 min permeameter data points in vertical and horizontal transects. Coarse-grained fluvial deposits at the base of valley fills have markedly higher permeabilities, whereas late-stage estuarine deposits have lower permeabilities relative to the valley-fill average. In this low-accommodation setting, unconformity-bounded valley fills are flow units, and pronounced lateral changes in permeability occur where valley fills of different character are juxtaposed. The interpretations of the Fall River that were developed on outcrop are being used to guide characterization of Buck Draw field, a Fall River field with an outstanding suite of cores and reservoir production data.

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