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An Application of Sequence Stratigraphy to Depth-Related Clastic Diagenesis, Cold Lake Oil Sands, East-Central Alberta: Abstract
Sequence stratigraphy is a method of investigating the origin of genetically related sedimentary packages deposited during sea-level fluctuations. A depositional sequence is a stratigraphic unit composed of a conformable succession of related strata bounded at its top and base by unconformities or their correlative conformities. The cyclic deposition of a sequence is controlled by the relative position of sea level at the time of deposition, the tectonism of the continental margin and local variations related to the environment of deposition. Depositional models based on sequence stratigraphy can be investigated by examining chemical, physical and ichnological variations within sediment at various depths from a pre-existing sediment/water interface.
Early diagenetic processes include those which occur during burial to depths of several hundred metres. Early diagenetic changes in clastic strata in marine environments are largely a function of sediment composition. Detrital solids are the major source of diagenetic reactants in clastic marine environments. Reactivity of mineral phases is a function of grain size and thermodynamic mineral stability. Diagenetic reactions can be quantified in terms of depth below a sediment/water interface. Three depth zones of early diagenesis can be identified, based on distinctive chemical reactions: oxidation, sulphate reduction andfermentation. Bacterial processes involving the degradation of organic matter dominate each zone. Various microbial reactions may result in the production of diagenetic pyrite, bicarbonate and methane. The presence of these products may be used to identify early diagenetic depth zones in ancient sedimentary strata provided numerous diagenetic episodes have not occurred.
Sequence stratigraphic concepts are used to interpret a depositional model for Clearwater sediment accumulation in the Cold Lake oil sands. Sequence boundaries between different depositional sequences are used as a reference plane for investigating early diagenetic alteration below a sediment/water interface, relating results to observed physical and ichnological characteristics present in the sedimentary record. This leads to a better understanding of diagenetic processes in the Clearwater Formation, perhaps leading to improvements in current production methods, and ultimately the future recognition and exploitation of new and analogous hydrocarbon reservoirs.
Acknowledgments and Associated Footnotes
1 Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3
2 Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3
3 Department of Geology, University of Western Ontario, London, Ontario N6A 5B7
Copyright © 2009 by the Canadian Society of Petroleum Geologists