Sequence stratigraphic interpretation methods for low-accommodation, alluvial depositional sequences: applications to reservoir characterization of Cut Bank field, Montana

摘要

In South Central Cut Bank Sand Unit (SCCBSU) of Cut Bank field, primaryproduction and waterflood projects have resulted in recovery of only 29 % of theoriginal oil in place from heterogeneous, fluvial sandstone deposits. Using highresolutionsequence stratigraphy and geostatistical analysis, I developed a geologicmodel that may improve the ultimate recovery of oil from this field.In this study, I assessed sequence stratigraphic concepts for continental settingsand extended the techniques to analyze low-accommodation alluvial systems of the CutBank and Sunburst members of the lower Kootenai formation (Cretaceous) in Cut Bankfield. Identification and delineation of five sequences and their bounding surfaces led toa better understanding of the reservoir distribution and variability.Recognition of stacking patterns allowed for the prediction of reservoir rockquality. Within each systems tract, the best quality reservoir rocks are stronglyconcentrated in the lowstand systems tract. Erosional events associated with falling baselevelresulted in stacked, communicated (multistory) reservoirs. The lowermost Cut Bank sandstone has the highest reservoir quality and is a braided stream parasequence.Average net-to-gross ratio value (0.6) is greater than in other reservoir intervals. Littleadditional stratigraphically untapped oil is expected in the lowermost Cut Banksandstone. Over most of the SCCBSU, the Sunburst and the upper Cut Bank strata arevalley-fill complexes with interfluves that may laterally compartmentalize reservoirsands. Basal Sunburst sand (Sunburst 1, average net-to-gross ratio ~0.3) has betterreservoir quality than other Sunburst or upper Cut Bank sands, but its reservoir quality issignificantly less than that of lower Cut Bank sand.Geostatistical analysis provided equiprobable representations of theheterogeneity of reservoirs. Simulated reservoir geometries resulted in an improveddescription of reservoir distribution and connectivity, as well as occurrences of flowbarriers.The models resulting from this study can be used to improve reservoirmanagement and well placement and to predict reservoir performance in Cut Bank field.The technical approaches and tools from this study can be used to improve descriptionsof other oil and gas reservoirs in similar depositional systems.