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Core analysis and correlation to seismic attributes, Weyburn Midale Pool, southeastern Saskatchewan

Publisher – 
Summary of Investigations 2004, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2004-4.1, CD-ROM, Paper A-9, 8p.

Authors –
Nicole M. Pendrigh, Colorado School of Mines, Colorado, U.S.A.

Paper – [pdf]  O&G_SGS_Summary_of_Investigations_2004

Abstract
The Reservoir Characterization Project at the Colorado School of Mines has acquired multi-component time-lapse data at the Weyburn Midale Pool to monitor the CO2 flood for enhanced recovery. Using the time-lapse data, geophysically rendered maps of the pool have been created. Although specific attributes aid in the interpretation of certain shapes or anomalies that may be associated with the CO2 injection, particular questions about the geology remain unanswered. By studying cores and logs, a detailed geological model can be generated that will provide a local focus for correlation to the seismic interpretations.

In the Mississippian Midale Vuggy unit, the vertical and lateral distributions of shoal and intershoal strata show that the shoals are not lithologically interconnected and are separated by mudstones representing intershoal areas. Earlier fracture analysis and first-hand observation of the cores indicate rocks from intershoal areas are more likely to be fractured. Therefore, although intershoal rocks are less porous and permeable, fractures within them may act as fluid conduits from one shoal to another. The contact between the Midale Marly and the Midale Vuggy may have been an exposure surface, which now has drastically different porosity and permeability values that prevent fluid from flowing from one zone to another. Another contributory factor to poor flow is the presence of anhydrite that infills many vugs and cements numerous fractures.

Replacement metasomatic anhydrite commonly fills fractures and permeates outwards into the matrix of the shoal facies in the lower Vuggy.

Preliminary analysis of selected seismic
attributes allows for several possible geological interpretations. For example, analysis of seismically rendered p-wave amplitudes may show a correlation to differences in reservoir quality rocks within the Marly zone; an interval of thinly interbedded limestone and dolostone at one well and a 6.1 m (20 ft) thick layer of massive, bioturbated skeletal dolostone at another well are situated in areas with different p-wave amplitude anomalies. Also, change in p-wave acoustic impedance maps indicates that the injected CO2 is not entering the Marly, but is finding a pathway, possibly through vertical fractures, that allows fluids to migrate downwards in to the underlying Frobisher Beds, a process which may account for oil staining seen below the Frobisher contact. This oil staining is evidence that no basal seal is present at that well.

To integrate geology and geophysics, rock properties known at each well location can be cross-plotted with seismic attributes. If linear trends are produced, a direct relationship between the geology and the specific attribute is inferred. This will lead to more accurate interpretation of ambiguous anomalies.