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.