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Abstract

The Alberta Deep basin, situated along the northeastern front of the Rocky Mountain belt, is the deepest part of the Alberta synclinal sedimentary basin. This trough-shaped deep basin, extending across northwestern Alberta and into northeastern British Columbia, covers an area of 65,000 sq km (25,000 sq mi).

Enormous volumes of natural gas have been found in recent years within the thick, clastic Mesozoic sediments which partly fill the deep basin. These sediments exceed 3,100 m (10,200 ft) in total thickness.

Based on detailed geochemical analyses of more than 300 rock samples (mainly cutting samples) from several wells in the Elmworth gas field, information was obtained on the hydrocarbon source strata and the generation and redistribution of hydrocarbons.

The clastic Mesozoic rock section contains numerous shaly zones which are very rich in organic matter, and also a suite of coal strata. This section, containing mainly Type III kerogen, is the ideal gas generator. Maturity is defined as 0.5 % vitrinite reflectance to about 2.0 % in the deeper part of the section. Maturity has also been defined in terms of the “Methylphenanthrene Index” which is based on aromatic hydrocarbons. Apparently the mature section is still in an active phase of hydrocarbon generation. Due to the tightness of the rock, the hydrocarbon transport mechanism seems to be dominated by diffusion processes. The light hydrocarbon distribution patterns observed throughout the wells suggest a dynamic trapping mechanism. Light hydrocarbons are lost at the top of the mature hydrocarbon-generating zone and are replenished in the middle part of the section where rich source rocks are found.

Based on this concept numerical treatment of gas diffusion with finite element computation is presented for well 6-28-68-13W6M of the Elmworth area. Using subsidence curves, time-temperature relationships, maturity-related methane generation data for source-rock intervals, and effective diffusion coefficients for methane (2 × 10−5 to 10−6 sq cm S−1) concentration/ depth curves were calculated as a function of geologic time. The results of the simulation for the present-day status compare remarkably well with the hydrocarbon distribution observed in this well today.

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