Prediction of Volume and Spatial Distribution of Reservoir Facies, Tiger Ridge Natural Gas Field, Montana
Laird D. Little
Deterministic predictions of the geometry and distribution of subsurface
reservoir facies is an exploration and production application of numerical
stratigraphic models under development. The models simulate the architecture and
facies distributions of offshore marine to coastal-plain strata as discrete
time-bounded progradational events or genetic sequences. The models predict a
systematic arrangement of genetic sequences in three geometric patterns:
seaward-stepping, vertical
stacking, and landward-stepping. In seaward-stepping
units, identical facies tracts are displaced seaward across sequence boundaries;
in
vertical
stacking, identical facies are vertically superposed; and in
landward-stepping units, identical facies are displaced landward. The models
further predict that sediment deposited during the progradational events is
partitioned systematically, but in varying proportions, into different facies
tracts according to the position of the event within the hierarchical stacking
pattern of genetic sequences.
The Tiger Ridge natural gas field in north-central Montana is productive from
upper shoreface facies of the Upper Cretaceous Eagle Sandstone (Campanian).
Complex production trends within the field include geographic shifts, gaps, and
thickness changes in pay zones. The Eagle has been described as consisting of
three lithostratigraphic members, and complex production patterns have been
explained by diagenetic or structural complications. By contrast, genetic
stratigraphic analysis indicates that the Eagle comprises five to seven genetic
sequences arranged in the predicted geometric stacking pattern. Production
occurs from similar reservoir rock types contained within separate genetic
sequences, and the complex production trends are a predictable function of the
position of the sequence within the stacking pattern. Isopachs of individual
genetic sequences and of reservoir facies indicate that the position and
geometry of reservoir facies tracts are predictable in terms of lateral and
vertical
shifts within the overall genetic stratigraphic framework.
This field study illustrates the increased temporal resolution
attainable by
the method of genetic stratigraphic correlation and documents the ability of the
numerical models to predict the distribution of reservoir facies in the
subsurface.
AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.