Coal petrography and associated stratigraphy in central part of Mae Moh Basin, Changwat Lampang

Abstract

35 coal samples from 3 boreholes in the central part of Mae Moh basin have been investigated for stratigraphy, petrography, and geochemical analyses. Lithostratigraphic of studied boreholes is Na Khaem formation in Mae Moh group, characterized by fine grained sediments with thin to thick coals from bottom to top as Q-, K- and J seams. The Na Khaem formation has been formed by the sedimentation under the alternation of lacustrine and swamp. Na kheam formation is overlaid by Huai Luang formation mainly of fine to coarse grained sediments with thin coal as I seam interbedded, indicated fluviatile environment. Detailed petrographic of individual seam reveals the studied coals consist largely of macerals >70% with small amount of inorganic materials <30%. Huminite group (43-68%) is characterized by gelinite, densinite, and some texto-ulminite. Liptinite group (5-30%) is composed of liptodetrinite, sporinite, cutinite, alginite, and exsudatinite. Inertinite group (4-15%) is fusinite, semifusinite and sclerotinite. The inorganic material (8-30%) is characterized by diatomite, clay mineral, framboidal pyrite, silicate, carbonate, and some volcanic ash in place. Mae Moh coals were plotted in the Diessel's diagram (1986) with TPI (Tissue Preservation Index) and GI (Gelification Index) base on maceral assemblage. The results are low TPI values (0.11-0.52) and moderate to high GI value (3.67-13.50); suggesting coals have originated under limnic condition (lacustrine) of peat formation with vegetation characteristics of marsh and wet forest swamp. Based upon the geochemical analyses, the studied coals can be classified as low rank (lignite A to subbituminous B) and low quality due to high ash and sulphur contents.