Dolomite Reservoir Architecture and Pool Characterization of Ordovician Midale Field

Key words: 
• dolomite reservoir /
• zone correlation /
• reservoir prediction /
• low-relief structure /
• pool type

Abstract: The Upper Ordovician Midale Field is located in the northern Williston Basinin southeastern Saskatchewan, Canada.It consists of a serious of low-relief structural traps. The oil source rocks are the same or older stratigraphy in the southern basin center. Thereservoirs are characterized by intercrystalline porosity in the dolomitized matrix with variable amount of vugs and fractures. Based on core examination and density and neutron log analysis, the carbonate reservoirs in the Red River Yeomen Formation can be divided into four zones.Zones1 and 2 are situatedin the upper part of the structural traps and usually bear oil. Although zones3 and 4 are thicker with higher porosity, they usually contain water only because they are located below the spill point of hydrocarbon traps.On a regional scale, however, zone 3 and 4 could be the attractive targets for the future exploration of possible stratigraphictraps where these zones pinch-outlaterally. Thickerzone 1 and 2 are characterized by high-amplitudeandhigh-frequency reflections on seismic profilesbased on examination of log cross-sections, their correspondingwell-linked 3 D seismic profiles,and forward modeling of seismic response. Zone3 and 4, where all four zones are present, can be identified by an additionalpeak beneath the original reservoir reflectionon seismic profiles. The terminationof this additional peak on a seismic profile is, therefore, an indication of pinchout of reservoir zone 3 and 4.This can be used to identify possible stratigraphic traps. The Ordovician oil pools of the Midale area are spatially associated with localized low-relief structuresand presence of the reservoirs zones.The driving mechanism for the Midale pools is interpreted to be a combination ofedge water and bottom waterdrive systems.Based on our analyses of 3D seismic dataand their implication on the stratigraphic and structural evolution in the region, these low relief structures are interpreted as the drape of Red Riverstrata over local Precambrianbasement highs that could have reactivated later. The traps were initiated during the Middle Silurian to the Early Devoniantime and finalized during the Late Cretaceous. Future explorationtargets of stratigraphic trap play are identifiedon thenortheastern wings of the nose structure that is close tohydrocarbon sources.