Modelling Unconventional Gas Traps: Elmworth Field, Alberta, Canada

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Abstract
The Elmworth Field is the largest gas field of the Western Canada sedimentary Deep Basin. Gas found in stratigraphic traps lies downdip from water, with no known permeability barrier between. A two dimensional quantitative analysis of the basin was undertaken in order to simulate such unconventional entrapment. The modelling shows how it is possible to accumulate gas in the low permeability sand reservoirs of the Lower Cretaceous. It appears that the Elmworth basin is a system in dynamical evolution, where gas loss due to migration is partly compensated for by contemporary gas generation. While the on-going gas generation is highly influenced by the recent paleoheat flow, the early migration has been controlled by cementation which, by lowering the permeability, reduces the gas loss with time, so that significant gas trapping can be obtained today. This work presents a set of conditions leading to such unconventional gas accumulations and discusses the parameters influencing the total amount of gas accumulated at present day; characteristics of the sediments, such as porosity and permeability behavior through time, cementation parameters, amount of sediments removed during the Paleocene erosion, or heat flow variation with time, are also bracketed in order that gas accumulations predicted are in agreement with those observed today. The inherent underpressure due to gas saturated formations is also considered.