ORGANIC PETROLOGY AND COAL-BED GAS CONTENT, DEEP-BASIN GULF COAST COALS, NORTHERN LOUISIANA

To investigate possible relationships between gas content and coal type, maceral modes and vitrinite reflectance were determined for 16 Wilcox Group coal samples cored from 4 coal zones [depths 401-482 m (1,317-1,581 ft); total coal thickness 3.9 m (12.8 ft)] in a coal bed methane test well in Ouachita Parish, Louisiana. Similar determinations were made on cuttings collected from 5 non-cored coaly intervals at depths ranging from 120-223 m (420-730 ft) in the overlying Sparta Sand. The Wilcox samples contain abundant huminite macerals (74-90 vol.%, mmf basis), and lesser amounts of liptinite (5-20%), and inertinite (2-15%). Inertinite and liptinite contents are greater in the upper portions of individual beds within each coal zone; mineral matter is greatest at the base of each bed. The Sparta cuttings include 2 samples with >50% mineral matter; the remaining 3 samples contain mostly huminite (80-93%), and liptinite (6-19%) with trace inertinite (£ 1%). These results are similar to previous maceral content determinations of shallow, mineable coals of the Gulf Coast. Maximum reflectance measurements determined on ulminite B range from 0.29 to 0.39% and are invariant with respect to depth (r²=0.08), whereas equilibrium moisture (r²=0.53) and heating values (r²=0.36) are weakly correlated with depth.

Total gas content (1.28-2.15 cm³/g; 40.98-69.01 scf/ton, daf) of the Wilcox coal samples is more strongly related to depth (r²=0.67) or ash yield (r²=0.58), rather than with type (maceral content). However, a weak positive correlation is noted between the abundance of attrital huminite macerals and total gas content, whether all samples are considered isorank or on a zone-by-zone basis. Furthermore, a similarly weak positive correlation is noted between liptinite abundance and total gas content. Values of effective diffusivity of gas from Wilcox coals show a very narrow range (4.51x10^-4 to 9.26x10^-5 sec^-1) and do not appear to be influenced by maceral type abundance. The limited number of samples in this data set in conjunction with similar maceral assemblages probably prohibits clear determination of relationships between coal type, gas diffusivity, and storage capacities. However, these preliminary results are promising and will be clarified through additional petrographic examination of isorank Gulf Coast coals.