As a component of Phase I and II of the International Weyburn CO2 Sequestration Project, regional seismic investigations are conducted around a 100 km radius of the Midale (Mississippian) reservoir in Southern Saskatchewan. The operator EnCana intends to sequester 20 million tones of CO2 to recover an estimated 130 million bbl of oil in the next 40 years. The objective of the regional study is to document the tectonic, petrophysical and rheological properties of the sedimentary fill which guarantee the permanent storage of CO2 in the region. To achieve the above goals, 2000 kms of industry donated seismic reflection data and over 1000 boreholes and related wireline information, as well as a 15 km2 3D seismic coverage of the reservoir are analyzed. In Phase II, started from 2008, 800 boreholes with 4000 well log data have been added in the central one third area of the Phase-I territory. Eleven geologically recognizable seismic horizons are mapped, from the top of the Cretaceous to the Precambrian basement unconformity. By the integrated analysis of the seismic sections, the regional structural setting of the sedimentary fill and top of the Precambrian are mapped. The relationship between the basement structures and the disturbances, influences of deep epeirogenic movements on the development of the basin are established. The fault/fracture pattern, similarly like in any other intracratonic basin on the world, is extremely intricate. The different 1-5 kilometer blocks reactivated four times. These reactivations can be identified on the seismic sections. The starting point of establishing the three dimensional mapping of the tectonically disturbed zones has been the analysis of the 2D/3D seismic sections. To delineate and connect the different trends in areas that were covered by 2D seismic sections only, other geophysical methods, mapped geological sequences and thickness maps were applied. Included in the investigation is a detailed petrophysical analysis of the rock volume from the surface to basement level. Special attention and a detailed neural network enhanced seismic inversion have been applied to the 40 meter thick top seal of the reservoir. In the investigated area, no large scale regional tectonic elements intersect in the Weyburn field. Small scale structural disturbances (fault with small offsets/local flexures) have been identified in and above the reservoir. However, through the comprehensive analysis of the petrophysical and structural properties of the 40 meter thick cap-rock seal, the long term storage of the CO2 can be estimated with higher probability.


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