Abstract: Quantitative Sandbody Architecture of the Straight Cliffs Formation, Kaiparowits Plateau of Southern Utah, Applied to the Modelling of Fluvial Reservoirs.
DALRYMPLE, MARK, ANGELA GRUBB and TIMOTHY R. GOOD, Heriot-Watt University, Edinburgh, Scotland.
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 #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah