In the authors’ recent papers on oil shale chemical composition and geochemical variability, as well natural gamma radiation, the significantly variable layered lithological structure of the up to 90 m thick oil shale (OS) unit of the Muwaqqar Chalk-Marl Formation (MCM), Central Jordan, was described in detail for the first time. In this work, the original results of detailed comparative petrographic and mineralogical studies of the unit and its separate layers are presented.
The studied drill cores represent an area about 73 km2 out of the large Attarat Um Ghudran (AUG, Attarat) deposit Maastrichtian in age. Oil shale with primary depositional structure and texture dominates. Laterally, layers, beds and interbeds of uniform composition and conditions of accumulation continue over the exploration area. Significant layering-dependent variations of chemical and mineral composition in the vertical succession of the oil shale unit occur. Petrographically, the dominating thick finely laminated uniform siliceous-carbonate mudstone (MS) oil shale intercalates with variable intervals of grain-bearing oil shale, in which interbeds of proper mudstone alternate with laminae, lenses and thin interbeds of wackestone (WS). The dominating biogenic compounds are: (i) calcite as < 5 mμ micrite forming the groundmass mudstone, and both micrite of the matrix and > 5 mμ to 1 mm grains – skeletal particles (shells and their broken fragments) in wackestone, (ii) silica as < 5 mμ particles belonging to the groundmass of mudstone and wackestone matrix, (iii) organic matter (kerogen) in the mudstone and matrix of wackestone, (iv) phosphate skeletal fragments in certain interbeds of wackestone and very fine apatite in groundmass. The clay minerals are the only possible terrigenous admixture in certain intervals. In accordance with the negative correlation between CaO and SiO2 the layers of the dominant calcite or silica (quartz, tridymite, cristobalite) occur, whereas in MgO-rich barren interlayers dolomite may prevail and in certain P2O5-enriched beds/interbeds apatite or in Al2O3-enriched layers clay minerals occur. In the vertical succession of the oil shale unit, also quantitative proportions of principal mineral and organic (kerogen) components vary a lot. Distinct thin interlayers of carbonates with low organic matter (OM) and silica contents reflect temporary breaks and imminent recoveries of oil shale accumulation. The data serve for a further assessment and commercial development of oil shale deposits.
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