Summary

Modelling and simulation of fluid flow in hydrocarbon reservoirs requires dimensional and geometrical constraints in order to define the 3D sandbody architecture of interwell volumes. The John Henry Member of the Cretaceous Straight Cliffs Formation, Kaiparowits Plateau, southern Utah, provides large scale outcrop sections, within a well constrained sequence stratigraphic framework (Shanley and McCabe, 1993). These outcrops were used to collect quantitative data to constrain modelling of interwell volumes of analogous fluvial reservoirs.

The John Henry Member of the Straight Cliffs Formation contains an overall coarsening upwards succession of fluvial and tidally influenced channel bodies in a background of soil complexes and central basin shales. Variation in alluvial architecture through the John Henry Member, from a lower low net to gross system to an upper high net to gross system, affects the connectivity of the individual sandbodies identified at outcrop. Low net to gross units are characteristic of zones of high accommodation space creation, commonly identified as transgressive systems tracts. High net to gross units are characteristic of zones of low accommodation space creation, commonly identified as lowstand or highstand systems tracts. In terms of a reservoir with a similar architecture, as net to gross increases upwards from transgression to highstand, so does the connectivity of the reservoir facies. Lateral connectivity of sandbodies will be controlled by the channel geometries and dimensions.

Data has been collected from a 250m high, 2.5km long section which is broadly perpendicular to regional palaeoflow, with sandbodies ranging in exposed widths between 0.1km to 2.5km, and thicknesses ranging between 1 meter and 35 meters. The 2D alluvial architecture of the John Henry Member taken from calibrated outcrop photomosaics is modelled using STORM fluvial modelling software. Modelling of the outcrop section as a reservoir volume is done using the method of correcting 2D frequency distributions of sandbody dimensions to 3D frequency distributions (Geehan and Underwood, 1993), which are then used to populate the reservoir model. The problem of representing outcrop analogue datasets statistically in the subsurface, through stochastic modelling, is addressed, as well as an analysis of resulting reservoir connectivity in 3D.

Results from this study of the John Henry Member have been compared with contrasting fluvial systems modelled using IRAP-RMS software. These models provide an insight into the factors which control the connectivity between reservoir facies and the size of the reservoir connected volume. Such modelling studies indicate that the primary 3D control on reservoir connectivity is net to gross. Other factors which were shown to influence reservoir connectivity are channel aspect ratios and sinuosity. Variation in channel orientation was shown to affect reservoir connectivity only to a limited extent.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas