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Oil Sands Fabric: The Grain Component and Influences on Reservoir Properties

Abstract

The grain component of the oil sands fabric was explored from three Canadian oil sands depositional settings from the McMurray Formation to determine grain morphology and weatherability, along with ‘sourcing grains’ to confirm depositional environments. In reservoir management studies, it is well known that accurate predictions of reservoir quality have a major influence on the success of a recovery program. Inherit characteristics of grains can be directly observed including textural properties (such as grain size, grain shape and sorting) and pore scale properties all which control reservoir properties. 18,000 grains from cores, 6000 from each environment, a transition depositional setting, estuary reservoir proper and the main estuarine reservoir were observed by SEM and results analyzed with an image analysis package. Preliminary results from SEM analysis revealed that grains from the three depositional settings displayed markedly different characteristics. Grains from the transitional depositional setting ranged from coarse to very fine sand (according to the Udden-Wentworth grain scale), although grains were predominately medium and fine sand sized. Whilst grains from the estuary reservoir proper changed to be predominately medium and fine sand sized, as were those from the main estuarine reservoir. These shifts in grain sizes between depositional environments are indicative of sorting differences which greatly influence reservoir characteristics and quality. Weatherability and fracturing of grains that also control reservoir conditions were observed to vary between the three depositional settings. Grains from the transitional depositional setting displayed the least amount of weathering; whereas weathering significantly increased in the estuary reservoir proper and significantly increased again in the main estuarine reservoir. Few grains of the transitional depositional setting were observed to be fractured either along the edges or across grain surfaces. Whilst grain fracturing significantly increased in the estuary reservoir proper, although slightly decreased in the main estuarine reservoir. Fractured and weathered grains can introduce a secondary porosity to the reservoir and impact reservoir productivity. Mechanical grain features observed from the three depositional settings confirmed that sources of the grains are derived from the logged distinct environments.