A geological/engineering model was constructed for the extended Tertiary Incentive Project (TIP) mile area of Unit A in Bell Creek field, Powder River County, Montana. The model was constructed to develop a methodology for characterizing reservoir heterogeneity for barrier island deposits. This report summarizes the status of the NIPER geological/engineering model as of September 1987. A structure map of Muddy formation sandstones and an isopach map of the reservoir were constructed to delineate the framework of the pay zones. In addition, reservoir quality was differentiated by grouping facies into three classes. Valley incisions and indications of localized faulting have been documented within the barrier-island deposit. Valley fill deposits unconformably overlie portions of the barrier island sequence in the Bell Creek field.