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Composition and facies variations in mid-cretaceous gates formation coals, Northeastern British Columbia : implications for interpretation of paleo-wetland environments and assessment of coalbed methane characteristics

University of British Columbia

A field and laboratory based study of medium volatile bituminous coals of the mid-Albian Gates Formation in northeastern British Columbia, Canada, was undertaken to determine the nature, controls and implications of coal compositional variation. Differences in lithotype composition reflect microscopic compositional variation. From bright to dull coals, there is a progressive decrease in vitrinite and increase in inertinite. Liptinite is negligible (<1%). Composition and texture affect the methane and carbon dioxide gas adsorption characteristics of the coal. On a mineral-matter-free basis, the amount of methane adsorbed generally increases with vitrinite enrichment. Carbon dioxide surface area generally decreases with increased mineral matter and increases with increased vitrinite content. The increase in adsorption of both methane and carbon dioxide with increased vitrinite concentration is attributable to differences in the pore size distribution of vitrinite (microporous) and inertinite (meso- to macroporous). Methane adsorption isotherms and surface area data indicate that the maceral compositional variations are at least as significant as coal rank in determining coal bed methane characteristics. Compositional differences between lithotypes reflect differences in vegetation, accumulation rate and degree of plant decomposition. Lateral and vertical variation in lithotype composition was controlled by groundwater fluctuations and proximity to active fluvial systems. The coals formed on broad, low relief coastal plains, but were rarely subject to marine inundation as sulphur content is consistently low. The vegetation of the Gates mires is interpreted to have grown in environments ranging from low nutrient, taxodiaceous Stillwater swamps with little or no fluvial influence, to eutrophic, higher energy taxodiaceous alluvial swamps subject to frequent flooding, to marsh environments. The Gates peats were deposited east of the rising Canadian Cordillera. This leeward position resulted in drier conditions than the humid climate normally associated with peat formation. Inertinite formed as a result of fires during periodic drought, and was concentrated by preferential destruction of vitrinite precursors at the peat accumulation stage. Image analysis and point-count petrographic data show that inertinite percentage on average is quite high, and may vary cyclically within seams. Cyclic variation in composition within some of the seams is attributed to climatic variations, subsidence rates or both. Scrub savanna wetlands were one of the most common environments. Scrub savanna peats gave rise to low ash yield, inertinite rich coals. Gates Formation wetlands were maintained by periodic disturbance, and the stratigraphy of the coals reflects short and long term changes in sedimentological, tectonic and climatic conditions. In ombrotrophic settings, fire was fundamental to nutrient cycling and maintenance of hydric conditions, whereas periodic flooding was important in alluvial settings. Constant (or near constant) subsidence associated with deposition in a foreland basin allowed for preservation of the Gates Formation peats in an area which, due to climatic conditions, might not normally give rise to significant peat deposits.

Thesis/Dissertation

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2008-09-10

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http://hdl.handle.net/2429/1781

Geological Sciences

University of British Columbia

UBCV

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