Reservoir characterization of a 'tight' oil reservoir, the middle Jurassic Upper Shaunavon Member in the Whitemud and Eastbrook pools, SW Saskatchewan

摘要

New horizontal drilling and multi-stage fracture completion technology have proven successful in unlocking hydrocarbons from previously uneconomic reservoirs. In this paper, we investigate the application of this new technology to a medium gravity oil sandstone reservoir to enhance the poor oil recovery within the pool but also to evaluate if the pool boundaries can be expanded. Petrophysical well-logs were integrated with detailed core descriptions, thin sections and core analysis data to provide insight to the reservoir characteristics and factors controlling production, reservoir distribution, depo-sitional environments and lateral extent of depositional fairways for the reservoirs. The middle Jurassic Upper Shaunavon B sandstone is the main reservoir in the studied Whitemud and Eastbrook pools in southwest Saskatchewan. The reservoirs are composed of mixed siliciclastic/car-bonate, shallow marine shoreface deposits which have a strike trend northeast-southwest. Although the pools were discovered in 1953 and 1966 and have cumulative production of 3.3 MMBbls of medium gravity 22 API oil, estimated oil recovery is less than 4%. The Upper Shaunavon B sandstone has two main reservoir facies, shoreface sandstones with permeabilities in the 0.1—10 md range and relatively thin but high permeability (10-1000 md) coquina beds. Until recently, wells targeted mainly the high permeability coquina for its higher production rates and as a means to drain the associated thick, lower permeability shoreface sandstone reservoir. This study evaluates the Upper Shaunavon B shoreface sandstone reservoir quality by examining the reservoir properties, heterogeneity, connectivity, and pore volume of the flow units based on core and petrophysical data. Comparison of production data from wells intersecting both facies of the Upper Shaunavon B versus wells where the underlying high permeability coquina facies is absent allows a perspective on the impact that the coquina has on the recovery factor in both reservoir types. Integration of petrophysical well-logs, detailed core descriptions, thin sections and core analysis data provide insight to the facies interpretation, reservoir distribution and lateral extent of depositional fairways for both units. Reservoir characterization of the lower permeability siliciclastic facies at the Whitemud and Eastbrook pools, coupled with recent advances in horizontal drilling and completion technology show that additional drilling opportunities exist within the pools to improve oil recovery, but also that oil-saturated sandstone reservoirs exist outside the boundaries of the pools. Recent successes with multi-stage fracturing of horizontal wells have confirmed the findings of this study.