Maceral study to determine the desorbed gas content in the Upper Cretaceous coals of the Landázuri area, Middle Magdalena Valley Basin, Colombia

For several decades, some coal petrographic properties have been proposed as important parameters in the methane gas sorption processes. In this contribution, the petrographic variables (Vitrinite Ratio, Inertinite Ratio, the petrographic indexes (Gelification Index, Groundwater Index, Tissue Preservation Index, Vegetation Index, Vitrinite/Inertinite ratio, and the Vitrinite Reflectance were evaluated according to the maceral preservation and were related with the desorbed gas content. Twenty-five coal seams obtained from the drill cores of two wells in the Landázuri Area-Valle Medio del Magdalena basin were analyzed. The coal samples were grouped according to gas content using principal component analysis (PCA). The petrographic results were analyzed by linear regression and multiple regression. The Medium Volatile Bituminous to Low Volatile Bituminous coals from Landázuri 1 are twice as high in gas content that the High Volatile Bituminous A to Medium Volatile Bituminous coals from Landázuri 2. The volume percentage and the preservation degree macerals are related closely to the gas content and the pore's size involved in the sorption process. The Inertinite is the maceral group related with the highest gas content groups in Landázuri (600 SCF-Standard Cubic Feet/ton, 300 SCF/ton), while the other groups show the correspondence with the vitrinite macerals. The syngenetic and diagenetic origin of the pyrite contributes microporosity to the desorption process, while the pyrite epigenetic by its size reduces it. The petrographic indexes reveal that the Upper Cretaceous coals were developed in swampy or lacustrine continental basins- limnic facies.

Bituminous coals on emergent surfaces in an Asbian, lower Carboniferous (Mississippian) limestone succession on the North Wales carbonate platform, UK, and implications for palaeoclimate

Emergent surfaces in the Mississippian (Asbian to Brigantian) carbonate platform succession of North Wales record periods of plant colonisation and peat formation that led ultimately to the local development of coals. Examination of bituminous coals on three emergent surfaces within Cefn Mawr Quarry reveals information on palaeoclimate that is not available from study of the limestones alone. Three coal seams in the Asbian Loggerheads Limestone Formation were identified and the lowest one studied in detail. Vitrinite reflectance data from alternating bands of vitrite and duroclarite microlithotypes, the distribution of pyrite within them, and the sharp contacts between them, suggest that there were abrupt changes in marine influence during the development of the peats that formed the coals. It is inferred that local palaeoclimate alternated between periods of high and low rainfall, the amount of rainfall influencing the extent to which seawater encroached into the peats, with higher rainfall suppressing the ingress of saline waters into groundwater. On the basis of modern peat growth rates, the timescale of the alternation indicated by each duroclarite-vitrite couplet is suggestive of an annual cycle, such as would arise in a monsoonal climate. The low proportion of ash in the three coals, the preservation of internal lamination, the low diversity of spore species in the lowest coal compared with the over- and underlying mudrock, and the presence of rhizoconcretions in palaeokarstic limestone beneath the lowest and highest coals, demonstrate that the peat swamps were isolated from the hinterland and autochthonous. This study demonstrates that a wider application of palynology and coal petrology is an important contribution to the study of marine carbonate successions of any age where terrestrial organic matter, formed during emergence, has been preserved.