High Resolution Characterization of Reservoir Fluid and Rock Properties: Integrating Sedimentology and Advanced Geochemical and Nuclear Magnetic Resonance (NMR) Methods – A Case Study from the Athabasca Oil Sands Deposit
M. Fustic 1,2, J. Bryan3, B. Bennett1, H. Huang1, A. Mai3, R. Spencer1, S. Hubbard1, A. Kantzas3, and
1Alberta Ingenuity Centre for InSitu Energy, University of Calgary, Alberta
2NEXEN Inc., Calgary, Alberta
3TIPM Laboratory, University of Calgary, Alberta
Recent studies have demonstrated that the bitumen residing in the Athabasca Oil Sands Deposit (AOSD) shows large variation in hydrocarbon composition and bitumen physical properties (e.g., API gravity ranges 6-11). The hydrocarbon compositional variations are observed on both lateral and vertical scales. In general, the bitumen located towards the base of an oil column, in close proximity to a basal oil-water contact, is usually of a higher viscosity compared to the bitumen residing higher up in the oil column. The variation in physical and chemical properties of the bitumen may show gradients in composition which has been attributed to the effects of biodegradation. Detailed compositional analyses of a number of petroleum columns have shown that physical and chemical property variation in some oil columns may show anomalous behaviour by displaying inverse gradients, and even steps in composition trends that may coincide with geological features.
The core material selected for this investigation consist of an array of geological features such as present day bitumen-water contact, discontinuous (compartmentalized) oil columns, and a range of reservoir facies. A suite of geochemical methods has been employed to characterize the bitumen composition of core samples obtained from AOSD. The data we show that describe the bitumen properties includes viscosity (at 20°C), weight percentage (%) bitumen determined by the Dean Stark method and the level of biodegradation determined from molecular markers by Gas Chromatography – Mass Spectrometry. In addition, we describe the qualitative indication of the fraction of fines present in the rock using low field NMR spectra measurements both from core and logging tools. NMR spectra were also used to infer the viscosity of the bitumen between sampled points. A detailed description of reservoir geology has been compiled from core descriptions, geophysical logs and sedimentological analysis. The integration of geology and bitumen composition shows that compositional gradients displaying anomalous behaviour may be primarily attributed to compartmentalization of the reservoir (i.e. the presence of a barrier preventing bitumen communication) and nature of the present day bitumen-water contact and secondarily to reservoir facies properties such as porosity and lithology. The results obtained indicate that even very closely spaced compartments (<5 m) may contain bitumen with very different chemical and physical properties. Although biodegradation appears to be recognized as the dominant alteration process in the AOSB, we suggest the following geological factors should also be considered: 1) petroleum charging and mixing histories; 2) reservoir rock properties; 3) local variations in conditions (e.g. water composition, lithology) that may lead to different biodegradation pathways.
This information is useful for indicating the range of chemical and physical property variations that may be encountered in oil sands reservoirs. The information may be used to assist in prospect evaluation, and can indicate where potential recovery problems may occur when a resource is evaluated for InSitu operations.
AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada